Airway Evaluation

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CLINICAL REVIEW

Airway evaluation in obstructive sleep apnea

Boris A. Stuck*, Joachim T. Maurer

Department of Otorhinolaryngology, Head and Neck Surgery, Sleep Disorders Center,

University Hospital Mannheim, 68135 Mannheim, Germany

KEYWORDSObstructive sleepapnea;Mueller maneuver;Sleep endoscopy;Critical closingpressure;Upper airway;Imaging

Summary As the interest in sleep-disordered breathing has increased, variousattempts have been made to assess upper airway anatomy in patients with this rela-tively frequent disorder. The aim is not only to reveal potential differences in upperairway anatomy to better understand origin and pathophysiology of the disease butalso to improve patient management and treatment success. The present review isbased on a systematic literature search with regard to upper airway evaluation insleep-disordered breathing; the articles were selected and discussed in light ofour clinical experiences. Based on clinical assessment including endoscopy duringwakefulness, the value of the Mueller Maneuver, static radiologic imaging tech-niques (X-ray cephalometry, computed tomography (CT) scanning and magneticresonance imaging (MRI)), dynamic scanning protocols (e.g. ultrafast CT or cine

MRI), upper airway endoscopy during sleep and sedated sleep, pressure measure-ments and the assessment of the critical closing pressure are discussed. Each tech-nique itself and its history in the field of sleep medicine are briefly reviewed andproblems of standardization and interpretation are discussed when appropriate.Insights into the pathophysiology of the disease gained with the help of the inves-tigational techniques are presented and the impact of the techniques on patientmanagement is reported. Although all these additional techniques for upper airwayassessment have substantially improved our understanding of sleep-disorderedbreathing, their significance in daily practice is limited. In contrast to the wide-spread use of the Mueller maneuver and sedated endoscopy, convincing data sup-porting their use in terms of treatment outcome are lacking. So far, there is onlyvery limited evidence that selected techniques improve treatment outcome forselected indications. In general, there is not enough evidence that these techniques

are superior to the routine clinical assessment. 2007 Elsevier Ltd. All rights reserved.

Abbreviations: CT, computed tomography; MR, Magnetic resonance; MRI, magnetic resonance imaging; PAS, posterior airwayspace; SNA, angle from the sella to the nasion to the subspinal point; SNB, angle from the sella to the nasion to the supramentalpoint; SDB, sleep-disordered breathing; AHI, apneaehypopnea index; OSA, obstructive sleep apnea.

* Corresponding author. Tel.: 49 621 383 3965; fax: 49 621 383 3827.E-mail address: [email protected](B.A. Stuck).

1087-0792/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.smrv.2007.08.009

Sleep Medicine Reviews (2008) 12, 411e436

www.elsevier.com/locate/smrv
mailto:[email protected]://www.elsevier.com/locate/smrvhttp://www.elsevier.com/locate/smrvmailto:[email protected]


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Methods of airway evaluation

As the interest in sleep-disordered breathing (SDB)has increased, various attempts have been made toassess upper airway anatomy in patients with thisrelatively frequent disorder. From the very begin-ning, researchers and clinicians used a multitude of

different techniques not only to reveal potentialdifferences in upper airway anatomy to betterunderstand the origin and the pathophysiology ofthe disease but also to improve patient manage-ment and treatment success. While the value ofthorough clinical assessment remains indubitable,the value of the Mueller maneuver has been ques-tioned from the beginning. Static radiologicimaging techniques such as X-ray cephalometry,computed tomography (CT) scanning and magneticresonance imaging (MRI) have been used mostly todetect differences in airway anatomy. Dynamicscanning protocols (ultrafast CT or cine MRI e.g.)and multiple pressure recordings have been used togain insights into the mechanism and level ofairway obstruction. Upper airway endoscopy hasbeen inaugurated during sleep and sedated sleep todirectly visualize airway obstruction, and theassessment of critical closing pressures has beenused to quantify upper airway collapsibility.

The present review is based on a systematicliterature search in the National Library of Medi-cine, textbooks, major journals in the field ofsleep medicine and otorhinolaryngolgy, and addi-tional personal sources of the authors. The articles

were selected and discussed in the light of ourclinical experience.

The sections in this article are structured ina comparable fashion. The technique itself and itshistory in the field of sleep medicine are brieflyreviewed and problems of standardization andinterpretation are discussed when appropriate.Potential structural or functional differences inthe upper airway between patients with SDB andhealthy controls are discussed and the insights intothe pathophysiology of the disease gained with thehelp of the investigational technique are pre-sented. Furthermore, the impact of the techniqueon patient management is reported; finally, theauthors summarize the overall potential and clin-ical usefulness of the technique.

Clinical examination and clinical scores

A clinical examination including an endoscopy ofthe upper airway during wakefulness still consti-tutes the basis of every airway evaluation insnorers and obstructive sleep apnea (OSA)

patients. Given the early failures in the surgicaltreatment of these patients, anatomic and staticfindings were the first parameters to be evaluatedin order to improve treatment success. The impactof enlarged palatine tonsils became evident in thesurgical experiences with children. If performedsimultaneously, tonsillectomy was described by

most authors as a positive predictive factor fora successful UPPP,1e7 demonstrating a strongcorrelation between tonsil size and success rate.8

This was also supported by studies investigatingsimple tonsillectomy in OSA patients.9,10 All theother anatomic parameters such as the size of theuvula, the existence of longitudinal pharyngealfolds and so forth did not show any relationship tothe success rate of UPPP if evaluated separately.11

In contrast to the significant influence of enlargedtonsils in palatal obstruction, equivalent clinicalfinding for tongue base obstructions could not bedetected. Woodson and Wooten12 only found hints

that the oropharynx was normal in cases withretrolingual obstruction.

Aware of this dilemma, Friedman et al.13e15

developed a clinical 4 degree staging systemincorporating the tonsil size, the position of thesoft palate, the tongue size and the body massindex (BMI). They reported a success rate of 80%after a solitary UPPP with tonsillectomy in patientswith large tonsils, visible posterior pharyngeal walland a BMI below 40 kg/m2 (defined as stage 1).(According to our own experience these patientsare rarely found among the group of typical OSA

patients.) If the tonsils were small or missing, thetongue was rather large and the BMI was above40 kg/m2 (defined as stage 3), Friedman et al.14

achieved a success rate of only 8%; it was improvedby a simultaneous radiofrequency treatment of thetongue base in addition to UPPP. Furthermore,their anatomical staging system predicted thesuccess rate much better than OSA severitydid.16,17 Performing UPPP with genioglossaladvancement on a series of 44 patients with severeOSA, a study from Taiwan was not able todemonstrate a similarly strong predictive value.But the results of this study are problematicbecause almost every patient belonged to thesame anatomical stage.18 One may argue that thestaging system merely reflects the clinical exami-nation of an experienced sleep physician; never-theless, such a system may be particularly helpfulfor less experienced observers.

Whether there are further predictive anatomicparameters for other surgical strategies has notbeen evaluated to date (except those including theMueller maneuver which will be discussed below).The subjectivity of the assessment and the

412 B.A. Stuck, J.T. Maurer


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variability of the nomenclature of the clinicalfindings are a significant limitation in this context.However, it has been shown that this variabilitycan be reduced by using pictograms.19

The Mueller maneuver

Snoring as well as apneas can be simulated by mostpeople and a direct effect of the Mueller maneuvermay be seen during wakefulness. Thus, snoringsimulation and the effects of the Mueller maneuverhave been used in upper airway evaluation beforesurgical intervention in patients to predict surgicaloutcome and to improve patient selection.20e22 Inaddition, the Mueller maneuver has been per-formed to assess and predict postoperativechanges of the upper airway,23e26 although thevalue of this relatively simple examination hasbeen questioned repeatedly in the past.27

Techniques of the maneuver

In order to be able to compare results betweendifferent investigators and patients as well asbefore and after an intervention, the maneuvershould be performed and documented in a stan-dardized fashion. Regarding the simulation ofsnoring, Herzog et al.28 made a first attempt tostandardize snoring simulations during wakeful-ness. They asked the patients to snore with theirmouth open or during forced inspiration and used

a standardized documentation system which leadto a low interrater variability. For the Muellermaneuver itself, the awake patient is sitting orlying and inspiring maximally with nose and mouthclosed while the pharynx is examined via a flexibleendoscope.5,29e32 The endoscope is placed at thelevel of the supraglottis, the uvula tip and thenasopharynx.

Due to its simplicity the classification accordingto Sher et al.33 has been widely used. In thisclassification, 4 degrees of airway obstruction atthe different levels are defined, ranging fromminimal to complete occlusion. Furthermore, anyvisible obstruction linked to the epiglottis isdescribed. Catalfumo et al.34 described anepiglottic collapse that they found during Muellermaneuver in 11.5% of UPPP failures. Fujita35 clas-sified airway obstruction as isolated palatal (type1), isolated retrolingual (type 2) or combined (type3) according to their predominance. Terriset al.s36 study group integrated the collapse ofthe lateral pharyngeal walls. He compared his ownresults of airway evaluation using the Muellermaneuver with the results obtained by one of his

residents. They found the same degree of collapsein one-third of the patients. Allowing for thedifference of 1 according to the classification ofSher as described above, both investigators agreedupon the classification in 80% of their patients.However, they found a difference of 3 in somecases. There was no systematic error between

resident and specialist. Only Jager et al.

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founda significant correlation of more than 0.8 betweenthe degree of obstruction obtained by Muellermaneuver and MRI.

Recently, the problem of inter-investigatorvariability could be eliminated by the quantita-tive, computer-assisted analysis of digitizedendoscopic recordings of the maneuver.38e42 Horiet al.43 proposed a variation of the Muellermaneuver (sustained forced transnasal inspirationwith the mouth closed) calling it the Bernouillieffect producing maneuver. The significance ofthis variation needs further evaluation.

Nevertheless, taking all the available data intoaccount, the reliability of the Mueller maneuverremains highly questionable and the evaluation ofthe maneuver seems highly subjective and hard toreproduce.

Predicting airway obstruction during sleep

There is some evidence that the sites of obstruc-tion detected with the Mueller maneuver do notreliably reflect the sites of obstruction during

sleep. This could be demonstrated through acomparison with videoendoscopy,12,44,45 multi-channel pressure recordings46 and dynamic MRIduring sleep.47 Furthermore, several different sitesof obstruction during sleep were documented thatcould not be recognized with the Mueller maneuverduring wakefulness.46 Table 1 shows the differentsites of airway obstruction detected with thedifferent methods of airway evaluation accordingto selected examples from the literature.

The impact of body position on the significanceof the Mueller maneuver also remains unclear.30,42

Some of our own (non-snoring) laboratory staff isable to produce varying mechanisms and levels ofobstruction when performing the Muellermaneuver. This active component of the Muellermaneuver is confirmed by investigations concern-ing the critical closing pressure which rangesbetween 10 and 17 mbar in healthy adultsduring sleep.48 During the Mueller maneuver,healthy subjects may produce extreme negativepressures of 80 mbar without any signs ofpharyngeal collapse.42 This clearly demonstratesthe significant differences in upper airway

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Table 1 Distribution of the sites of obstruction detected by different methods of airway evaluation (selected literature

Method Author Diagnosis n Palatal Retrolingual Co

Mueller maneuver Petri et al.5 OSAS 30 8 0 22

Sher et al.33

OSAS 171 101 56 14Skatvedt46 SBAS 20 4 0 4

Sum (mean value %) 221 113 (51%) 56 (25%) 40

Endoscopy during sleep Launois et al.165 OSAS 18 11 2 5Woodson and Wooten206 OSAS 11 5 6 n.d

Sum (Mean value %) 29 16 (55%) 8 (28%) 5 (

Endoscopy under sedation Croft and Pringle169 SBAS 56 25 n. d. 31Pringle and Croft177 SBAS 70 33 9 28Camilleri et al.184 SBAS 25 17 0 8Hessel and de Vries178 SBAS 340 111 8 22Steinhart et al.183 SBAS 306 139 23 13Den Herder et al.182 SBAS 127 65 15 47Quinn et al.174 Snoring 50 35 4 5Marais173 Snoring 168 101 52 13El Badawey et al.179 Snoring 46 8 2 36Abdullah et al.181 Snoring 30 12 0 18Abdullah et al.181 OSAS 89 12 4 71

Sum (mean value %) 1307 558 (43%) 117 (9%) 61

Pressure recordings during sleep Hudgel200 OSAS 9 4 5 0Chaban et al.187 OSAS 10 5 5 0Metes et al.207 SBAS 51 30 7 n.dTvinnereim and Miljeteig203 OSAS 12 6 2 n.dSkatvedt46 SBAS 20 2 5 10

Katsantonis et al.201 OSAS 20 5 4 9Woodson and Wooten206 OSAS 11 8 3 n.d

Sum (Mean value %) 133 60 (47%) 31 (23%) 19

SBASpatients with primary snoring or OSAS; OSASonly patients with OSAS; palatalnasopharynx, tonsils, soft palate and/or lateral pharynhypopharynx; epiglottisexclusively epiglottis; no resulteither the method was not tolerated or the result was not utilizable; n.d.not


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collapsibility during wakefulness and sleep. All thedata given do not support the idea that the resultsobtained by the Mueller maneuver may be trans-ferred to natural sleep.

Predicting surgical success

It has to be questioned to what extent the Muellermaneuver can predict surgical outcome. Variousresearch groups were not able to better predictthe success rates obtained with UPPP when usingthe Mueller maneuver.5,30,31,49 This was also truefor non-apneic snorers.50 Recently, airway changesafter UPPP were first quantified using the quanti-tative, computer-assisted analysis of digitizedendoscopic recordings. Hsu et al.23 could demon-strate a correlation between the postoperativeincrease of the retropalatal area in the supineposition during the Mueller maneuver and thedecrease of the AHI. Nevertheless, they did not

present data demonstrating that the surgicaloutcome of UPPP could be predicted according totheir preoperative findings.23

Aboussouan et al.29 and Sher et al.51 consideredan additional retrolingual collapse during theMueller maneuver as an exclusion criterion fora UPPP because their success rate was only 5% inthese cases. There is only one Chinese studyshowing a comparable success rate for UPPP forpreoperative airway evaluation with the Muellermaneuver and sleep endoscopy.52 Surgery of theepiglottis was always considered and done sepa-

rately if it was seen during the Mueller maneuver.Catalfumo et al.34 could reduce the apnea indexsignificantly in their UPPP-failure patients withlaryngeal obstruction during the Mueller maneuverby partial resection of the epiglottis.

Only Riley et al.21 report on the impact of usingselection criteria based on the Mueller maneuverfor the combination of different surgical proce-dures. In phase 1 of their protocol, an isolated UPPwas only performed if the Mueller maneuver as wellas cephalometry revealed an isolated retropalatalcollapse.21 In cases of an isolated retrolingualcollapse in both examinations they performed

a hyoid suspension and a genioglossal advance-ment. If there was a combined collapse, all surgicalsteps were done simultaneously. According to theirselection criteria, the authors found an isolatedpalatal collapse in 10, an isolated retrolingualcollapse in 6 and a combined collapse in 223 (93%)of their patients. The success rate of this protocolaveraged out at 61% which is higher than Sheret al.51 found in their meta-analysis of isolatedUPPP using Mueller maneuver, somnofluoroscopy orcephalometry (52%) or no specific method (45%) for

airway evaluation. Rileys group did not providedata concerning success rates without usingspecific selection criteria. Even though theyincluded mainly patients with moderate to severeOSA, the clinical relevance of the selection dis-cussed may be questioned as patients with an iso-lated collapse of either palate or tongue base seem

to be rare. This is stressed further by Vicenteet al.53 who found a combined collapse in all 122OSA patients who rejected CPAP.

Significance of the Mueller maneuver

The Mueller maneuver is a safe and simple exami-nation that does not exert relevant strain on thepatient. The reliability of the Mueller maneuver isinsufficient, but may be improved with the imple-mentation of computer-aided evaluation tools.Nevertheless, the results of the Mueller maneuver

cannot be transferred to natural sleep. A hypo-pharyngeal collapse indicates the exclusion ofpatients from UPPP, thus indirectly improving itssurgical success rate. Finally, the Mueller maneuverdoes not facilitate patient selection for the varyingsurgical interventions used in OSA patients.

X-ray cephalometry

Over the years, lateral X-ray cephalometry hasbecome one of the standard diagnostic tools inpatients with SDB, especially with regard to theevaluation of the skeletal craniofacial morphology.Not specifically developed for the field of SDB,imaging techniques and standards for data analysishave been incorporated from the field of maxillo-facial surgery, where it has already been used fordecades.

Being a standard tool for maxillofacial surgeonsand orthopedic surgeons, one focus of X-ray ceph-alometry in SDB has always been the assessment ofdentofacial characteristics before or after the useof mandibular advancement devices or max-illomandibular advancement surgery. The assess-

ment of potential differences in maxillofacial andupper airway soft tissue anatomy has becomeanother focus for lateral X-ray cephalometry.

Providing insights into the pathophysiologyof SDB

Comparing OSA patients with healthy controlsExtensive literature is available comparing upperairway anatomy and dentofacial structures usingX-ray cephalometry between OSA patients and



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healthy controls, both for obese and non-obesepatients as well as for different ethnicities.

In a recent controlled study from Riha et al.,54

cephalometric characteristics of OSA patientswere compared with those of their siblings. Theanalysis revealed a significantly longer distancefrom the hyoid bone to the mandibular plane in the

siblings affected by SDB (in addition, in dentatebrothers they also found a shorter mandibularcorpus in OSA patients). While the study designwas somewhat innovative by minimizing intra-individual differences with the help of siblings, theresults confirmed those of previous studies,particularly with regard to the significance ofhyoid bone position.

Differences in craniofacial anatomy betweenOSA patients and controls have been demonstratedby numerous other authors. The concrete resultsare often difficult to compare, as the authors oftennot only use different landmarks and parameters

but also sometimes rather complex calculatedindices and ratios to describe the differences theyfound, therefore, the following results can only bea selection of the data available. Reporteddifferences were a longer soft palate,55,56 reducedminimum palatal airway widths,55 increasedthickness of the soft palate,56e59 differences ina calculated craniofacial score (the sums of quar-tile points for distance from sella to nasion andfrom hyoid bone to mandible),60 increasedpharyngeal length,56 a retroposition of themandible59,61 or the maxilla,62,63 micrognathia,64

an increased mid-facial height,62

and differencesin hyoid bone position.56,57,61e66 Ingman et al.67

reported no differences in naso- and hypophar-yngeal soft tissues but a significant narrowing onthe velopharyngeal level.

Yu et al.61 demonstrated that the differenceswere more pronounced in non-obese patients,concluding that craniofacial changes play a domi-nant role in this subgroup. Ito et al.68 also concludedthat the reported differences were morepronounced in the non-obese group. Tangugsornet al.69 reported more pronounced aberrations ofthe cervico-craniofacial skeletal in non-obesepatients, while obese patients had more abnor-malities in the upper airway soft tissue morphology,head posture and the position of the hyoid bone.The predominant skeletal abnormalities in non-obese patients compared to the predominant softtissue aberrations in obese patients were alsoreported by Sakakibara et al.70 in 1999 and byFerguson et al.71 in 1995.

Specific studies were performed with patientsfrom Asia, taking the differences in maxillofacialappearance of this ethnic group into account. Hsu

et al.72 used a modified technique of calibratedcephalometric analysis by using a calibratedcatheter that was inserted into the upper airwayto increase the accuracy of their measurements in106 south-east Asians. They could demonstratethat patients with OSA had a longer lower-faciallength, a narrower skull base, a shorter and

receding mandible, smaller posterior airwayspaces, a longer and thicker soft palate, a longertongue length anddagaindmore inferiorly dis-placed hyoid. According to Li et al.,73 the morepronounced differences in craniofacial morphologyin Asians may contribute to the fact that Asian OSApatients are often non-obese. Cakirer et al.74

demonstrated that brachycephaly is associatedwith an increased apneaehypopnea index (AHI) inwhites but not in African-Americans. The reporteddifferences in craniofacial morphology betweendifferent ethnic groups complicate the interpre-tation and the comparison of cephalometric

studies.In addition to the use of a calibrated catheter as

described above, other modifications of cephalo-metric analysis were used, such as cephalometryat the end-expiratory phase and during the Muellermaneuver,25 but have not (yet) become a routineprocedure.

Correlation to disease severityVarious authors have attempted to correlate theirfindings in X-ray cephalometry with the severity ofthe underlying SDB.

In a recent investigation, Hou et al.75

reportedthat the aberrations in craniofacial morphologythey found in OSA patients (basically confirmingthe characteristics mentioned above) were morepronounced in patients with severe OSA in theirgroup of Chinese patients. Especially mandibularbody length, craniofacial extension and sellaehyoid distance were predictive variables for theAHI. Yucel et al.76 demonstrated that differencesin hyoid bone position and a soft palate thicknesswere more frequent in the subgroup of patientswith severe OSA. Other authors, such as Bates andMcDonald,77 Kubota et al.,78 Naganuma et al.32 andYoung et al.79 have confirmed the observation thatselected anatomic variables were more prevalentin or predictive for severe OSA, Young and McDo-nald79 again underlining the effects of hyoid boneposition. Dempsey et al.80 demonstrated that innon-obese patients and in patients with narrowupper airway dimensions, four cephalometricdimensions were the dominant predictors of AHI,accounting for 50% of the variance. Rose et al.81

question the diagnostic relevance of X-ray cepha-lometry for OSA, as they found no direct



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correlation between skeletal cephalometric find-ings and OSA severity; nevertheless, they alsoreported a correlation with hyoid bone position.

X-ray cephalometry and oral appliances

One of the dominant indications for performing X-

ray cephalometry has been the treatment with oralappliances. Especially with regard to the evaluationof potential predictive parameters for treatmentsuccess and dental side effects, X-ray cephalom-etry has been the standard diagnostic tool.

As early as 1995, Mayer and Meier-Ewert,82 twoof the fathers of the treatment with oral appli-ances in Europe, have looked for cephalometricpredictors of treatment success. In a group of 30OSA patients they reported that specific cephalo-metric variables were indeed predictive for thetherapeutic effect, such as a narrow oropharynx(posterior airway space (PAS) equal to or less than

3.4 mm), a narrow angle from the sella to thenasion to the supramental point (SNB), a widerangle from the sella to the nasion to the subspinalpoint (SNA) and a short uvula. Other authors haveconfirmed the existence of predictive cephalo-metric parameters, especially in relation to hyoidbone position and oropharyngeal airway dimen-sion.83e87 A smaller airway on the oropharyngeallevel is associated with good treatment success.Nevertheless, the problems related to differentnomenclature and selection of airway parametersdescribed above remain. Only Battagel et al.88

reported no identifiable cephalometric features topredict favorable response to the treatment.Although posttherapeutic follow up is not part

of this review article, it should be mentioned thatX-ray cephalometry has regularly been used toassess long term dental and occlusal side effects oforal appliances.89e98

Evaluating the effects of therapeuticintervention

Predicting the results of UPPPX-ray cephalometry has also been evaluated withregard to potential predictive parameters forpostoperative results of UPPP alone or in combi-nation with other approaches.

Liu et al.18 investigated UPPP in conjunctionwith genioglossus advancement in a case series of44 patients and concluded that the only predictiveparameter was the AHI. Boot et al.99 also could notfind any cephalometric parameter predictingsuccess of UPPP. In their retrospective study of 43patients with OSA, Woodson and Conley100 alsocould not find a skeletal measurement predicting

the outcome of UPPP in the entire study group.Nevertheless, in the subtype of patients withoutretrognathia, selected parameters were predictiveof response.100 In addition to the AHI, the absenceof retrognathia was another predictive parameterin a study of Millman et al.101 in 2000. Doghramjiet al.30 also concluded that a preoperative ceph-

alometry cannot be reliably used to predict orenhance surgical success. Overall, the predictivevalue of X-ray cephalometry for UPPP remains atleast questionable.

X-ray cephalometry and maxillofacial surgeryLateral X-ray cephalometry is a standard tool in thepreoperative evaluation of the craniofacial skel-etal anatomy before maxillomandibular advance-ment surgery. It can be regarded as a mandatoryprocedure and its value is not questioned.102

X-ray cephalometry in patient management

X-ray cephalometry has provided substantialinsights into the pathophysiology of OSA, demon-strating significant craniofacial characteristicsassociated with this disease. Although the resultsare not easy to compare, specific cephalometriccharacteristics have been repeatedly mentioned asa risk factor for OSA, such as e.g., a thick and longsoft palate, a retroposition of the maxilla ormandible and especially the more inferiorly posi-tioned hyoid bone. Although there are parametersthat correlate with the severity of the disease, thenecessity of an assessment of the nature andseverity of the disorder by nocturnal sleep testingremains. There are cephalometric parameters thatare correlated with favorable results of mandibularadvancement by oral appliances. Nevertheless, asno cephalometric parameter exists that wouldreliably rule out treatment success and as thetreatment is not invasive, at least the authorswould routinely offer oral appliances for OSApatients as long as the clinical examination (suffi-cient protrusion, satisfactory dental status, limitedobesity) supports their use. This may explain why X-

ray cephalometry has not become a routineprocedure in the diagnostic work-up of OSA as longas maxillomandibular surgery is not planned.

CT scanning

Compared to lateral X-ray cephalometry, CTscanning significantly improves soft tissue contrastand allows precise measurements of cross-sectional areas at different levels as well as three-dimensional reconstruction and volumetric



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assessment. Fast scanning times and relativelyquiet scanning conditions even allow a dynamicassessment of the airway during a respiratory cycleas well as measurements during natural sleep.Nevertheless, ionized radiation remains problem-atic and the vast majority of the authors used CTdata merely for a structural assessment during

wakefulness.

Techniques and standards

Despite its widespread use in airway assessment inpatients with SDB, no standardized scanningprotocol exists for this indication and the nomen-clature of the soft tissue structures is not uniform.Each study group seems to apply its own scanningprotocol and its own definition of the differentstructures described.

A first step was a two-dimensional assessment ofthe upper airway of healthy controls and patients

with OSA, comparing two-dimensional structuressuch as soft palate and tongue dimensions as well ascross-sectional areas. Furthermore, 3D techniqueswere used to assess volumes of soft tissue struc-tures and airway spaces.103e108 As early as 1987ultrafast or dynamic CTwas inaugurated in this fieldto evaluate dynamic changes of the upper airwaydimensions during respiratory cycles109,110 and wasfurther used in the following years.76,111e114 Whilethe vast majority used CT imaging during wakeful-ness, several authors also used scanning protocolsunderhypnotic relaxation,115 sleep,110 sleep during

apneas110

and also used direct comparisonsbetween wakefulness and sleep.109


Comparing OSA patients with healthy controlsThe majority of published data points to potentialdifferences in upper airway structures anddimension between OSA patients and healthycontrols or snorers. The various studies obtainedcomparable results. In general, the upper airway isdescribed as smaller in apneic patients compared

to controls,113 especially with regard to the ret-ropalatal region.103 Cross-sectional areas werefound to be significantly narrower in affectedpatients.116e118 Inversely, retropalatal tissue wasdescribed as being greater in OSA patientscompared to controls119 and larger tongue and softpalate dimensions and volumes were found.59,120

Schwab et al.113 have pointed out the differencesin upper airway configuration with an anterioreposterior configuration (in contrast to the hori-zontal configuration in healthy subjects with the

major axis in the lateral direction)da result that isin line with data obtained from MRI.

Correlation to disease severityDifferent authors have described anatomic condi-tions that reflect the severity of the disease andhave correlated their measurements with poly-somnographic data. Lowe et al.59 reported that inhis group of 80 patients with OSA, a high apneaindex was associated with large tongue and softpalate volumes. In a study with three-dimensionalCT scan, a significant correlation of the retro-palatal space and its lateral diameter with therespiratory disturbance index was documented ina large group of 194 patients with SDB.104 Voset al.108 used a combination of the smallest cross-sectional area, the upper airway resistance andthe BMI to predict the severity of OSA. In theirdynamic CT study Yucel et al.76 described a nar-rower cross-sectional area and a thicker soft

palate in severely affected patients compared topatients with only mild-to-moderate OSA.

Mechanisms and sites of airway obstructionAvrahami et al.115 compared the cross-sectionalareas of 23 adult patients with severe OSA in theawake state and during hypnotic relaxation.According to the presented results, airways weresmaller during relaxation than during wakefulness.With the help of dynamic and ultrafast CT, furtherinsights into airway obstruction were gained. Inaddition to the fact that the naso- and oropha-

ryngeal airways were smaller in OSA patientscompared to weight-matched controls, Galvinet al.111 could show an increased collapsibility inaffected patients compared to healthy controls.During a respiratory cycle in 25 awake patientswith OSA, substantial changes in cross-sectionalareas were seen in patients with SDB, the velo-pharyngeal segment being the narrowest and mostcollapsible region.114 These results were essen-tially confirmed later, showing that patients withsevere OSA have significantly narrower cross-sectional areas at the velopharyngeal level76 (seeFigure 1).


Effects of therapeutic intervention have beenassessed with the help of CT, mostly with regard tothe treatment with oral appliances and surgicalintervention. Gale et al.121 investigated the airwayenlargement achieved with an oral appliance in 32OSA patients. Although the results were obtainedin awake patients in the supine position, a



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statistically significant increase in the minimal

pharyngeal cross-sectional area was described.The results were confirmed in a more sophisticatedstudy 5 years later with cine CT.112 Whilea decrease in the diameters at the retropalatal andretroglossal level was seen during apnea, thesecross-sectional areas were significantly enlargedwith the help of the appliance. Sam et al.demonstrated that a non-adjustable oral appliancecan increase overall airway volume.122

With regard to surgical treatment effects, it hasbeen demonstrated that the upper airwayincreases after mandibular distraction osteo-genesis in children106 and after maxillomandibularadvancement in adults.123 Even more data isavailable for the effects of UPPP and its modifi-cations. Shepard and Thawley124 demonstratedthat UPPP more than doubled the upper airwaycross-sectional area in his 23 patients with OSA andhe describes specific appearances that were morelikely to lead to surgical success. Langin et al.125

also concluded that a postoperative oropharyngealenlargement seen in pharyngeal CT measures isassociated with a good outcome in UPPP whileRyan et al.107 demonstrated that patients withsmaller upper airways respond well to UPPP.

Finally, Li et al. demonstrated that an extendeduvulopalatal flap led to a significant increase inretropalatal space in their 15 patients with OSA.105

CT scanning in patient management

A significant number of the above mentioned andcited authors have concluded that CT scanningdoes play or will play a major role in themanagement of patients with SDB. To givea selection, it has been stated that CT can play

a role in evaluation of OSAS and indications for

surgery,126

that the outcome of UPPP may bepredicted,120 or the surgical treatment will bemodified with the help of CT scanning.110 Incontrast to these statements, CT scanning has notbecome part of the routine assessment of patientswith SDB, especially not with regard to surgicaltreatment selection. Beneficial effects on treat-ment selection and thereby treatment outcomehave been postulated repeatedly but could not bedemonstrated to date.

MR imaging

Compared to lateral X-ray cephalometry or CTscanning MRI offers various advantages, such asexcellent soft tissue contrast, three-dimensionalassessments of tissue structures and lack ofionized radiation. The advantages with regard tothe lack of ionized radiation have made MRimaging the imaging technique of choice in theassessment of children with SDB.

Techniques and standards

Concerning their scientific or clinical use in thecontext of SDB, routine imaging techniques wereinitially applied following various protocols used ineveryday clinical practice. For patients sufferingfrom SDBdboth children and adultsdit wasattempted to determine anatomical preconditionsand peculiarities for SDB. In this research, compar-isons with healthy controls have been utilized,measuringtwo-dimensionaldistances and diametersof the upper airway or its related structures.127e131

In addition to simple two-dimensional assessment,

Figure 1 Upper airway narrowing during tidal breathing as assessed with CT scanning: (A) cross-sectional image ofa patient at the level of uvula in tidal breathing and (B) the significant narrowing at the same level in forced expirationis seen. The region of interest (white line) was used to assess total cross-sectional areas in each image (reproducedwith permission from Yucel et al.76).



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researchers began to evaluate volumes of either softtissue structures such as the tongue, the adenoids,the soft palate or the pharyngeal walls132e134 orthe remaining compromised or non-compromisedairway spaces.132,135 To obtain three-dimensionaldata, volumes were either calculated based oncross-sectional areas and slice thickness133,136 or

established by various computerized models.

134,137

Ultimately, ultrafast or dynamic imaging was used tovisualize dynamic motion of the upper airway toassess upper airway collapse or differences in upperairway motion between patients with SDB andhealthy controls.127,134,138e144 Subjects were eithermeasured during wakefulness39,137 or during wake-fulness and sleep127,142,145; children were routinelyscanned under sedation.138,139,141,146 Sleep inadults was either pharmacologically induced143 orspontaneous.47,140

Only a small number of authors have attemptedto establish distinct protocols for MRI of the upper

airway in SDB136,137,147; the results of themeasurements were either validated witha phantom137 or tested for variability in repeatedmeasures over time and with different investiga-tors.136 Validation and standardization of thisimaging paradigm seems essential; nevertheless,in contrast to, e.g., lateral X-ray cephalometry,hardly any consensual standards exist for thisindication.


OSA in childrenEspecially in children, extensive research has beendone with regard to the pathophysiology of pedi-atric OSA. Fricke et al.129 compared children withor without persistent OSA after tonsillectomy andadenoidectomy and demonstrated that enlargedlingual tonsils were present in those children withpersistent disease; this was especially true forchildren with Down syndrome. Arens et al.132

found significant differences in upper airwaystructures between children with and childrenwithout OSA. An airway restriction was detected inthe vicinity of both the adenoids and the tonsils(especially where adenoids and tonsils overlap)and an enlarged soft palate was found in theaffected group. Nevertheless, the airway restric-tion is not limited to these areas but seems tooccur throughout the initial two thirds of the upperairway.132,135 These results were confirmed byFregosi et al.,128 additionally demonstratinga close dependency between the frequency ofrespiratory events and the size of the tonsils andthe soft palate. Furthermore, the upper airway

narrowing was more pronounced in those childrenwith a high number of respiratory events comparedto the less affected group. Differences inmandibular dimensions between patients with andwithout OSA, however, could not be detected, atleast not in children without apparent craniofacialabnormalities.148

With regard to dynamic airway evaluation, morepronounced fluctuations in airway area during tidalbreathing138 and significant differences in airwaymotion (static patent or dynamic patent vs.intermittent collapsed or static collapsed)141 weredemonstrated in children with OSA compared tocontrols. Shott and Donnelly149 investigated chil-dren with Down syndrome with persistent airwayobstruction after tonsillectomy and adenoidec-tomy and could confirm multiple sites of airwayobstruction in this severely affected group of OSApatients. As the source of obstruction, recurrentadenoid tissue, glossoptosis, soft palate collapse,

hypopharyngeal collapse and enlarged lingualtonsils were described.149

OSA in adultsIn adults, potential differences in upper airwayanatomy and structure have been described withstatic and dynamic imaging. As early as 1989,authors pointed out the significance of pharyngealfat deposits in patients with OSA.150 In theircomparative trial, Horner et al.150 concluded thatin patients with OSA more fat is present in theareas surrounding the collapsible segment of the

pharynx. In more recent publications, morespecific sites of fat deposition have been described(especially anterolateral to the upperairway) evenin non-obese patients with OSA.151 While otherauthors described alterations of the lingualmusculature in OSA,152 in the study of Do et al.133

there was only a weak trend for larger tongues inpatients with SDB (tongue size was independentfrom AHI and showed a better correlation with BMIand neck circumference). In contrast, Schwabet al. could demonstrate an increased risk of OSAin patients with increased tongue volume; Iida-Kondo et al.153 described a significantly highertongue volume in relation to the oral cavity volumein patients with OSA compared to controls. Otheranatomical conditions associated with SDB were anelliptic horizontal cross-sectional area of thepharynx with the long axis oriented in the sagittalplane131 and large volumes of the lateral pharyn-geal walls and total soft tissue surrounding theupper airway.134 In a recent publication of Okuboet al.,154 the anatomic appearance of themandible and soft tissue structures of the upperairway in Japanese patients with OSA was



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compared to controls. While the authors did notfind any significant differences in tongue volume,soft palates or pharyngeal walls, they pointed outspecific anatomic factors of the mandible in OSApatients, such as a wider mandibular divergence

and a smaller mandibular internal length.154

With regard to dynamic imaging, two mainaspects have been the focal point of interest: first,to learn more about the dynamics of upper airwayobstruction and second, to detect the level ofobstruction with regard to potential surgical ornon-surgical therapeutic interventions.

Numerous authors have demonstrated that themechanism and level of airway obstruction can bevisualized by MRI,39,127,142,143 even under naturalsleep.47 Figure 2 demonstrates an example ofupper airway obstruction during natural sleep.

The fact that patients with OSA presentmultiple sites of pharyngeal abnormality wasdemonstrated by Suto et al.143 as early as 1993;nevertheless, the authors pointed out that thelevels of airway obstruction during wakefulnessdid not match those levels found during sleep.Jager et al. used fluoroscopic MRI in patients withOSA and healthy controls and demonstrated thatthe entire length of airway obstruction can bevisualized on MR images.37 Furthermore, theyrevealed significant differences between the twogroups in terms of degree of obstruction. In 2001,

Ikeda et al.142 compared patients with OSA andhealthy controls during wakefulness and sponta-neous sleep. While no pharyngeal airway nar-rowing was seen in the healthy patients,a significant narrowing was seen in the OSA

patients already during wakefulness, but evenmore so during sleep. Ciscar et al.127 essentiallyconfirmed these findings; moreover, they coulddemonstrate that apnoeic patients have a morecircular occlusion, underlining the relevance ofthe lateral pharyngeal walls in the pathogenesisof airway obstruction.

Several authors have speculated that detectingthe site of airway obstruction with the help ofdynamic MRI may be useful in the determination oftherapeutic, especially surgical, interven-tion.142,144,145 Nevertheless, data demonstratingthat the detection of the sites of airway obstruc-tion with this method is indeed beneficial in patientselection for surgery are lacking. This is especiallytrue with regard to a potential improvement insurgical outcome.


Finally, MRI has been used to assess the effects orside effects of various therapeutic interventions,including surgical and non-surgical strategies.

Figure 2 Complete pharyngeal collapse as detected with dynamic MRI during natural sleep. Dynamic single sliceimages of a 45-year-old man during an apnea period. The first images show the complete naso-, oro- and hypo-pharyngeal obstruction, the arrows mark collapse of the different pharyngeal regions (reproduced with permissionfrom Schoenberg et al.47).



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With regard to conservative treatment, Sanneret al.155 evaluated the effects of mandibularadvancement devices in 13 patients with OSA withultrafast MRI and demonstrated that the use of sucha device can prevent pharyngeal obstruction ina subgroup of patients, at least during wakefulness.They concluded that ultrafast MRI together with

the Mueller maneuver while wearing the devicemay be predictive of the success of a treatmentwith a mandibular advancement device in OSA.Although Gao et al.156 also reported potentialanatomic criteria as assessed with MRI forsuccessful treatment with mandibular advance-ment devices, MRI has not become a routineprocedure in the management of patients desig-nated for a treatment with a mandibular advance-ment device.

With regard to surgical treatment, MRI hasbeen used to assess potential effects of surgeryon upper airway anatomy. In radiofrequency

surgery, imaging has been used to visualizeimmediate postoperative effects on the softtissue of the soft palate157 and the tonguebase.158 Especially the latter study had practicalconsequences insofar as the extent of the lesionscreated by radiofrequency surgery were corre-lated with energy application and concreterecommendations for the use of this technique atthe tongue base could be made. A standardizedprotocol has furthermore been used to studyanatomic changes at the upper airway afterradiofrequency surgery of the tongue base159 and

after hyoid suspension.160

In both studies, nochange in upper airway anatomy could be detec-ted during wakefulness, indicating that theseinterventions work more via functional changes inupper airway collapsibility than via an enlarge-ment of the upper airway.

Latest developments have led to a computa-tional model of the human upper airway based onsignal averaging of MRI.161 Based on this model andwith the help of the finite element method,various surgical interventions have been simulatedand the effects of these simulated interventions onupper airway mechanics and collapsibility havebeen assessed, offering new possibilities in thedevelopment and improvement of surgical andnon-surgical treatment.

MRI in patient management

Static and dynamic MRI has substantially improvedour understanding of the pathophysiology of SDB.With the help of this method, significant differ-ences in upper airway anatomy and structure have

been detected between patients with SDB andhealthy subjects, and relevant insights have beengained in terms of the mechanisms and levels ofairway obstruction. Nonetheless, MRI has notbecome a standard procedure neither in thediagnostic work-up for patients with SDB nor in themanagement of the disease in terms of surgical or

non-surgical treatment.A number of issues remain unresolved: MRIduring sleep (especially spontaneous sleep) ispossible but not easy to perform and measure-ments during wakefulness or induced sleep are,to a certain extent, artificial or may simply notreflect clinical conditions. Furthermore, theresults of MRI, even when performed duringsleep, can only provide information concerninga short period of time and are limited to thesupine position. For routine clinical application,the limited availability and the associatedcosts are additional limiting factors. Although

MRI has substantially improved our understandingof SDB, it has not yet become a part of routineclinical evaluation of patients with thiscondition.

Videoendoscopy during spontaneoussleep

As early as 1978 the first report about video-endoscopic recording of the pharynx and larynxduring sleep was published. Borowiecki et al.162

described a palatopharyngeal collapse at the endof expiration and directly before inspiration inpatients with OSA. They described differentdegrees of airway obstruction, often associatedwith a medialization of the lateral pharyngealwalls. Snoring sounds during arousals wereattributed to the soft palate and the lateralpharyngeal walls. As there was no treatmentother than tracheotomy available at this time forthose patients, patient selection was not anissue.

Today videoendoscopy during spontaneous sleepis performed in order to improve patient selectionfor the different treatments available and mayalso be performed in combination with overnightsleep recordings.163e168 Because videoendoscopyduring spontaneous sleep allows the assessment ofthe upper airway during different sleep stages andlacks the side effects of sedating drugs, thismethod may be considered superior to video-endoscopy under sedation. However, sleep video-endoscopy is scarcely performed as it usuallyrequires nightly measurements and puts additionalstrain on both patient and doctor.



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Videoendoscopy under sedation

Impact of videoendoscopy under sedation onsleep, breathing and snoring

Videoendoscopy under sedation also makes itpossible to visualize the site and mechanism of

snoring and pharyngeal obstruction in patientswith SDB. Therefore, it is mandatory that snoringand airway obstruction can be provoked inaffected patients and that neither the endoscopeitself nor the drugs used for sedation disturb orinfluence breathing patterns, snoring or airwayobstruction during sedation. At first video-endoscopy during sedated sleep was described inchildren and 1 year later in adults using mid-azolam.169,170 Sadaoka et al.171 could demonstratethat during a 3-h videoendoscopy under sedationwith midazolam only the longest apnea and the

portion of REM sleep showed statistically signifi-cant differences compared to natural sleep inpatients with suspected sleep-related breathingdisorders. On the other hand, Jones et al.172 foundsignificantly different snoring sounds when acous-tically analyzing a few snores generated duringsedation endoscopy and comparing them to snoresobtained during natural sleep. These results mayindicate that the generation of apneas and snoringsounds follow different mechanisms and that vid-eoendoscopy during sedation is only partiallyequivalent to videoendoscopy during spontaneoussleep. Furthermore, it has to be mentioned that

videoendoscopy during sedation is usually per-formed for 10e15 min due to practical consider-ations, and not for 3 h as described by Sadaokaet al.171

Not in all patients endoscopy under sedatedsleep succeeds in inducing existing breathingdisorders, namely snoring and obstructiveapnoeas, and on the other hand, snoring may beprovoked even in healthy patients. According tothe current literature, snoring and airwayobstructions can be observed and induced in 79e95% of all manually sedated patients.169,173e175 In

a cohort study using propofol, Marais

173

detectedsnoring sounds in 45% of 126 healthy, non-snoringcontrols. He described the sound as being lessintense but displaying the same pattern as found insnorers. When titrating propofol with target-controlled infusion, all 53 snorers did snore reli-ably at a plasma level of 8 mg/ml whereas nota single control person did at the same plasmalevel, amounting to a sensitivity and specificity of100%.176 Therefore, target-controlled infusion withpropofol seems superior to manual titration.

Description of findings

The patterns of snoring and airway obstructionthat can be observed during videoendoscopy undersedation are multiform. Pringle and Croft177 werethe first to standardize the findings according totheir data obtained in a large series of patients.

Currently, different classifications coexist and,ultimately, none of them are feasible. Theydistinguish either between an isolated or a multi-segmental obstruction,178 or they are modifica-tions of the classification according to Pringle andCroft comprising the epiglottis.173,179 The classifi-cation according to Catalfumo describing theposition of the epiglottis during the Muellermaneuver was transferred to sleep video-endoscopy by Golz et al.180 in 2000. All theremaining authors do not classify their findings butenumerate the various mechanisms and anatom-ical sites of snoring and obstruction.174,181 The

obstructive patterns are described as beingcircular, antero-posterior and latero-lateral at thelevel of the soft palate, the tonsils, the tonguebase and the epiglottis. An involvement of thelatter is found in less than 1% to up to 40%.173e175,180,181 Abdullah et al.181 mentioned a combi-nation of as many as five different concomitantsites of obstruction in primary snorers and even sixin sleep apnea patients. An isolated site of obstruc-tion was found in only 15% of the OSA patients.

Impact on clinical decision making

Even if a sleep-like status can be achieved, ifsnoring sounds and obstructions can be initiatedreliably and documented in detail during video-endoscopy under sedation, the additional time andeffort are only justified under the condition thatthe success rate of surgery for OSA and primarysnoring can be improved with the help of thismethod. In other words, the question is whethervideoendoscopy under sedation has a substantialimpact on clinical decision making and treatmentselection.

Hessel and de Vries178 have established a flow-chart for the therapeutic proceedings, but limit itto the enumeration of all the various operationsavailable at the site of major obstruction. In clin-ical routine a large tongueddefined by a modifiedMallampati score of 3 or 4dis usually considereda negative predictive parameter for a successfulUPPP. However, den Herder et al.182 could notdemonstrate a correlation between video-endoscopy under sedation (retrolingual obstruc-tion) and Mallampati index (tongue size),



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questioning treatment selection based on Mal-lampati index only. Nevertheless, the combinedscore has not yet been compared with video-endoscopy under sedation.

Pringle and Croft177 compared their results ofthe Mueller maneuver to those obtained by vid-eoendoscopy under sedation in a group of 50

patients. Based on the Mueller maneuver, 25patients would have been selected for UPPP.However, 11 (44%) of those patients showeda substantial hypopharyngeal collapse undersedation, which would have meant excluding themfrom UPPP. On the other hand, eight of 25 (32%)patients who were excluded from UPPP due to theMueller maneuver had isolated palatal vibrationsor obstructions under sedation, making themsuitable for UPPP. In this regard Steinhart et al.183

examined 324 patients with suspected OSA ina waking state with Mueller maneuver as well asunder sedation with propofol. They found a signif-

icant increase in airway collapse during sedatedendoscopy at the palate (64.3 vs. 80%) and tonguebase (32.3 vs. 59.7%) compared to clinical exami-nation during wakefulness. Among the pool ofpatients who did not have a relevant obstructionduring Mueller maneuver (n78), isolatedobstructions were found at the palate in one-third,at the tongue base in 14% and at both levels in 50%during sedated endoscopy. In a study of Camilleriet al.,184 four of 27 patients scheduled for UPPPwere excluded after videoendoscopy under seda-tion due to tongue base collapse.

Impact on the success rate

Surprisingly enough, no prospective data is avail-able to date comparing success rates of surgicalintervention with and without the use of video-endoscopy under sedation. This is particularlyconfusing as there have been numerous advocatesof sedated endoscopy presenting data and videoson countless patients with SDB, but they have notyet been able to demonstrate its usefulness withregard to surgical outcome. In the study ofCamilleri et al.184 mentioned above, no superioritywas seen with regard to success rates compared tohistoric controls, despite using sedated endoscopy.Yet an improved success rate was found in thosepatients who did not even show the slightestinvolvement of structures other than the palate.Hessel and de Vries185 retrospectively reviewed 48snorers and 88 sleep apnea patients after UPPP. Inthose patients where the soft palate was leastinvolved in airway collapse during preoperativesedated videoendoscopy, the outcome was supe-rior compared to the others. In another

retrospective analysis of 55 sleep apnea patientsafter UPPP, the same investigators did not findsignificantly different success rates for differentsites of obstruction as revealed by videoendoscopyunder sedation.3

Videoendoscopy under sedation and the role

of the epiglottis

According to our own experience videoendoscopyunder sedation or in sleep is particularly helpful indetecting or excluding a possible glottic or supra-glottic obstruction, most often described asa posterior movement of the epiglottis duringinspiration.34 In 27 adult patients with epiglotticcollapse during sleep videoendoscopy, Golzet al.180 found a reduction of the AHI from 45 to 14after partial epiglottectomy. The publication doesnot reveal whether the same result could havebeen achieved by basic clinicaleendoscopicexamination or Mueller maneuver. Videoendoscopyunder sedation or during sleep may be particularlyhelpful in cases of laryngeal collapse and failuresof standard therapy.

Significance of videoendoscopy duringsedation

Videoendoscopy under sedation is able to initiatesnoring and upper airway obstruction duringa short period of induced sleep, mostly restricted

to the supine position. The severity of the under-lying disorder appears comparable to naturalsleep, although snoring sounds seem different andthe short examination time is a significant limita-tion. The classification of findings can be reducedto isolated obstruction at the palate, the tonguebase or the epiglottis or to combinations of these.There are subtle hints that videoendoscopy undersedation may change the indication for a limitednumber of surgical interventions. Nevertheless,there is not enough evidence to date that thisprocedure improves the outcome of snoring andsleep apnea surgery.

Multi-channel pressure measurements

Changes in inspiratory pressure in the upper airwayduring obstructive events can be measured withcatheters. To assess airway obstruction, differentmeasuring points meaning different pressuretransducers can be used from the nasopharynxthrough the oro- and hypopharynx down to theesophagus. Initially, pressure transducers were



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used mainly to investigate the mechanisms ofairway obstruction in general; nowadays researchis focused on the diagnostic potentials comparedto standard polysomnography and on the assess-ment of obstruction levels and its impact on theoutcome of sleep apnea surgery.

To provide useful results, catheters must be

tolerated during sleep without a significant alter-ation of sleep or breathing. Furthermore, theposition of the measuring points needs to be stableduring the entire investigation, as there is no visualcontrol after the initial positioning. Finally, theimpact on sleep apnea surgery must be assessed asdiscussed above.

Tolerability of the pressure catheters

Initially, pressure recordings in the field of SDBwere performed with balloon and open cathe-ters.186 Chaban et al.187 used stationary balloon

catheters for the esophagus (10 cm long) and thenasopharynx but added a catheter with a built-inpressure transducer (micro-tip catheter) to beslowly pulled through the entire upper airway. Herecorded pressure changes during single apneas ateach different site. Esophageal balloon cathetersirritated the patients significantly, leading to anincrease of alpha waves and thus objectivelydestroying the microstructure of sleep.188 Thereliability of the results of micro-tip catheters wasshown during wakefulness and sleep for theesophagus by Panizza and Finucane189 and for the

pharynx by Tvinnereim et al.190

Chervin andAldrich191 and Skatvedt et al.192 presented well-designed studies demonstrating that catheterswith no more than 2 mm diameter did not alter thesleep structure of patients suspected of SDB andthe results obtained with or without catheters inplace did not differ significantly. In a study ofOeverland et al.193 with 799 patients 3% rejectedthe placement of the catheter and 1% refusedfurther measurement during the night, while 96%tolerated the procedure. From our experienceswith numerous available catheters, the two cath-eters developed by Skatvedt and Tvinnereim are

associated with the least discomfort for thepatients due to the small diameter and softmaterial.

Reliability of measuring points

Multi-channel pressure catheters require a reliablepositioning of the measuring points in order toattribute the site of obstruction to the anatomi-cally defined segment of the airway. Verse et al.194

demonstrated with the help of lateral X-ray

cephalometry that the distance from the nostril tothe vertebral bodies, the epiglottic tip and thehyoid bone varies significantly between individ-uals. Therefore, most investigators choose theoropharyngeal sensor as a reference to be placedunder visual control at the free edge of the softpalate. Skatvedt195 reported that the sensor was

found exactly where it had been placed theevening before after one night of measurement.Taking this into account, the level of obstruction isusually described as an upper or lowerobstruction, meaning an obstruction at the level ofthe soft palate or the level of the tongue base. (Anisolated collapse at the level of the epiglottiscannot be assessed with this method.)

Assessment of obstructive events withmulti-channel pressure recordings

Pressure catheters also can be used to measure

nasal and pharyngeal airflow. This implies thatthey are suitable to assess increased respiratoryeffort as well as the AHI. Tvinnereim et al.196

demonstrated that the absolute number ofobstructive and mixed apneas during sleep can beassessed with these catheters with minimal devi-ation from the results obtained by poly-somnography. The overall sensitivity, specificityand the negative predictive value for the detectionof the different types of apneas and hypopneasreached 85e100%.197 In a blinded investigation,Reda et al.198 found a correlation of 0.97 between

the AHI assessed by pressure sensors and thermis-tors used for polysomnography. When addingactimetry to the pressure measurements todistinguish between sleep and wakefulness thecorrelation can be improved even further, as theAHI is based on total sleep time in polysomno-graphic recordings.199 The data available indicatesthat multi-channel pressure recordings are suit-able to assess the severity of SDB.

Assessing the sites of airway obstruction

Hudgel et al.186 were the first to describe thetypical pattern of obstructions at different locali-zations with the help of one measuring pointplaced at different levels of the upper airway.They already pointed out that it was difficult toidentify a combined collapse at different levels ofhypo- and oropharynx.200 Katsantonis et al.201

postulated that a short segment obstruction dis-played a high pressure gradient between twoneighboring sensors. In contrast a long segmentobstruction would show pressure gradientsextending over two or more sensors.202



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Skatvedt195 used a catheter with six sensors,placing the third at the free edge of the softpalate. He found a collapse extending over morethan one segment in 13 of 20 patients with SDB. Inone case the segments were not even neighboringand in seven of 20 patients the site of obstructionchanged during the night. In another study he

found a combined collapse of the upper and loweroropharynx in half of his patients.46 Tvinnereimand Miljeteig203 published their data using a 5-sensor catheter, placing the second sensor at thefree edge of the soft palate. They describedpressure oscillations of high frequency beingsuperimposed on the pressure swings of normaland obstructed breathing and suspected that thosemight be soft tissue vibrations due to snoring. Inaddition, Tvinnereim et al.202 calculated a distri-bution of the obstructive events assessed duringa 3-h recording. Over 90% of the apneas originatedfrom the middle oropharynx while the remaining

events originated more caudally.

Reliability of the determination of the siteof obstruction

Investigations concerning night-to-night variabilityof the distribution of the obstructive sites showedthat the predominant site of obstruction can bereproduced during the second night in 72% of thecases.Resultswere best reproduced in patients withan apnea index above 5 or with more than 75%

palatal events.204

Rollheim et al.205

compared thepatterns of obstruction as obtained in the hospitalwith a recording at home. Although the mean AHIwassignificantly higherin thehospital than at home,the occurrence of palatal obstructions did not differbetween both recordings. In patients who had lessthan 40% or more than 60% palatal obstructions inthe first recording, this relationship was reproducedin 90% of the cases during the second recording.

Skatvedt found that in 15 of 20 cases the site ofobstruction assessed by Mueller maneuver differedfrom the site obtained by multi-channel pressurerecordings. In 12 cases the Mueller maneuvermissed a site of obstruction that was clearlydetectable by manometry.46 Multi-channel pres-sure recordings and videoendoscopy during sleepdid not produce identical results either. Woodsonand Wooten12 examined 22 patients with severesleep apnea and described significant differencesbetween both methods. In another investigation 11UPPP failures with persistent severe sleep apneahad a tongue base obstruction in 67% of the casesas assessed by videoendoscopy versus 17% asassessed by manometry.206

Impact on the success rate of surgery for SDB

Metes et al.207 were the first to publish dataconcerning the impact of pharyngeal pressuremeasurements on the success rate of surgery.They used a catheter with one measuring pointonly which was pulled through the pharynx and

placed at several sites along the upper airway inorder to record several obstructive events at eachsite. The obstruction they found in this way per-sisted in eight of 12 patients after UPPP. Thesuccess rate of UPPP did not differ betweenpatients with predominant palatal or tongue baseobstruction. The evaluation of only a limitednumber of obstructive events as well as theselection of severe sleep apnea patients mayhave limited the results. Skatvedt et al.208

selected 16 patients with different degrees of SDBand predominant palatal obstruction detected bymulti-channel pressure transducers for laser-

assisted uvulopalatoplasty and reporteda decrease in the AHI from 18.6 to 6.4. While therate of upper obstructions dropped from 90% to8.8% of all apneas, the number of upper hypo-pneas was reduced only minimally. Osnes et al.209

compared the efficacy of UPPP in patients withpredominantly transpalatal and subpalatalobstructions. After UPPP, transpalatal apneas andhypopneas were reduced by 81% whereas sub-palatal events only dropped by 42%. The successrate in patients with transpalatal obstruction wassignificantly higher than in those with subpalatal

obstruction. Supporting the usefulness of pressurecatheters, Ng et al.210 described a significantlybetter response of mandibular advancementsplints in OSA patients with a predominantlysubpalatal collapse compared to those witha palatal collapse.

Multi-channel pressure recordings seem to besuperior to single-channel pull-through techniquesin predicting surgical success of soft palatesurgery.

Significance of multi-channel pressure

recordings

Pressure catheters with a small diameter of notmore than 2 mm are well tolerated and have onlyminimal impact on sleep quality and airwayobstructions. The pressure curves allow a reliabledetection of respiratory events. Positioning of themeasuring points by means of pharyngeal inspec-tion seems to be sufficiently accurate for theevaluation of the palatal airway segment. Upperand lower obstructions can be detected. In



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principle, combined obstructions are more difficultto recognize and are therefore rarely mentioned.There is no data supporting that obstructions atthe hypopharynx and the epiglottis can bediscriminated reliably. The distribution of theobstructions can be determined for the entirerecording period. The data available supports the

idea that the success rate of soft palate surgerycan be improved when using multi-channel pres-sure recordings for patient selection.

Critical closing pressure

The severity of SDB is usually described by the AHI,representing the number of upper airwayobstructions during sleep. Nevertheless, it has tobe kept in mind that the AHI simply describes thefrequency of upper airway obstructions, not theseverity of the pharyngeal collapse itself.

Furthermore, measuring the severity of upperairway collapse is believed to be important whenestimating the forces needed to overcome theseobstructions or to maintain upper airway stability.Schwartz et al.211 and Smith et al.212 firstdescribed the airway collapsibility, coining theterm Starling resistor. The model posits thatthe nasal and tracheal airway remain constantlyopen as they consist of rigid tubes, whereas thepharynx behaves as a collapsible segment wherethe airway obstructions occur. Airflow and airway

pressure are assessed using a nasal CPAP devicewith integrated pneumotachograph and pressuresensor. The CPAP device must be able to producepositive as well as negative pressure levels ifneeded. A pressure-flow diagram can be drawn atdifferent levels of airway pressure and a regres-sion line can be calculated. Smith defined the

critical closing pressure (Pcrit) as being the upperairway pressure when the regression line iscrossing the zero line, indicating that airflowcompletely stops (see Figure 3). Gleadhill et al.,213

Gold and Schwartz48 and Gold et al.214 demon-strated that patients with obstructive apneas havea Pcrit clearly above zero; in patients withobstructive hypopneas it is slightly negative, inUpper Airway Resistance Syndrome Pcrit rangesbetween 2 and 6 mbar, in simple snorers iteven drops further to 3 to 12 mbar, whereas innormal controls Pcrit it is on average below8 mbar. Therefore, any successful therapy

should result in shifting Pcrit to values more nega-tive than 8 mbar. By measuring Pcrit it wasbelieved that patients surgical success may bepredicted as patients with Pcrit levels close tonormal might be better candidates for surgicalinterventions than those with a positive Pcritbecause only minimal improvements of Pcrit arenecessary to eliminate SDB. On the other handthese favorable patients will mainly present withhypopneas and respiratory arousals during poly-somnography but not with apneas.

500

450

400

350

300

250

200

150

100

50

0-2 -1 0 1 2 3 4 5 6 7 8

PN (cm H2O)

VImax(ml/s)

Supine

Side

Figure 3 Representative pressureeflow relationships in the lateral recumbent and supine positions for one subjectin non-REM sleep. Maximal inspiratory airflow (VI max) vs nasal mask pressure (PN) is illustrated for lateral recumbent(open circles) and supine (closed circles) positions with corresponding regression lines and confidence interval.Critical closing pressure is represented by the PN at which airflow becomes zero in each body position (reproducedwith permission from Boudewyn et al.215).



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The initial method was later simplified andabbreviated. Boudewyns et al.215 could demon-strate that Pcrit was 2 mbar lower in the lateralposition compared to the supine position (seeFigure 3). Furthermore, sleep stages did not haveany impact on Pcrit.

Gender differences were found in OSA patients,

with men having a more positive Pcrit than women,independent of BMI216 whereas healthy men andwomen had the same Pcrit.

217

Impact of treatment on Pcrit

Schwartz et al.218 could show that substantialweight loss resulted in an improvement of Pcritfrom 3.1 to 2.4 mbar and a concomitant reduc-tion of the AHI. In another study6 they investigatedthe effect of UPPP on Pcrit in 13 patients. Theyfound a significant decrease for the entire group.

Even though a complete normalization of Pcrit from0.8 to 7.3 mbar could be demonstrated in thesubgroup of responders (in contrast, an insignifi-cant change from 1.1 to 0.6 mbar in the subgroupof non-responders occurred) they could not findany preoperative predictor of response. An iden-tical operation had individually varying effects onPcrit. Due to the difficult and extensive measure-ment of Pcrit Huang et al.

219 developed a compu-tational model using finite elements in order tosimulate the effect of upper airway surgery inOSA.219 They could show that stiffening the

tongue base or the palate or resecting parts of thelatter results in an improvement of Pcrit. Howeverthere are no clinical studies validating theseresults.

In summary, the assessment of the criticalclosing pressure is an important tool in theinvestigation of upper airway patency. It is thegold standard for the measurement of the overallcollapsibility of the pharynx and is especiallyuseful in research projects. Up to now, there is nodata concerning the benefit of a preoperativeassessment of Pcrit for a better prediction ofsuccess of upper airway surgery. Together withthe relatively complex assessment of Pcrit thismay explain why Pcrit measurements have notbecome a routine procedure in clinical testingso far.

Summary

The various techniques of airway evaluation pre-sented in this review have significantly increased

our insight into the pathophysiology of SDB.Nevertheless, potential benefits with regard topatient management or the superiority oversimple clinical assessment remains underdiscussion.

The role of routine clinical and endoscopicevaluation of the upper airway is unquestioned

especially in terms of patient management andtreatment selection. Although the subjectivity ofthe assessment and the variability of the nomen-clature of the clinical findings are a significantlimitation, clinical assessment is inevitable espe-cially before surgical intervention or treatmentwith mandibular advancement devices. Over thelast years, different staging systems have beeninaugurated that may especially be helpful for lessexperienced physicians and for standardizationpurposes. Nevertheless, whether the use of thesestaging systems is superior to simple clinicalassessment of experienced observers remains to

be seen.Since snoring as well as apneas can be simulated

by most people and direct effects of the Muellermaneuver may be seen on upper airway behaviorduring wakefulness, this technique has been usedin upper airway evaluation before surgical inter-vention to predict surgical outcome and toimprove patient selection. Nevertheless, the reli-ability of the Mueller maneuver remains highlyquestionable and the evaluation of the maneuverseems highly subjective and hard to reproduce.Furthermore, the data available do not support the

idea that the results obtained by the Muellermaneuver may be transferred to natural sleep. Anisolated hypopharyngeal collapse (although rare)during the Mueller maneuver indicates clinicalfailure with UPPP. Nevertheless, the Muellermaneuver does not facilitate patient selection forthe varying surgical interventions used in OSApatients.

X-ray cephalometry has demonstrated signifi-cant craniofacial characteristics associated withSDB. Specific cephalometric characteristics havebeen repeatedly mentioned as a risk factor forOSA; especially the more inferiorly positionedhyoid bone. There are parameters that correlatewith the severity of the disease and with favorableresults of mandibular advancement by oral appli-ances, but no cephalometric parameters exist thatwould reliably rule out treatment success with oralappliances. Therefore, X-ray cephalometry has notbecome a routine procedure in the diagnosticwork-up of OSA as long as maxillomandibularsurgery is not planned.

Both scanning techniques (CT and MRI) haverepeatedly demonstrated three-dimensional upper



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airway narrowing in children and adults with SDB.Dynamic investigations have been used to visualizeand localize upper airway obstruction and havesubstantially improved our three-dimensionalunderstanding of the pathophysiology of thedisease. Beneficial effects on treatment andpatient selection (particularly for surgical inter-

vention) have repeatedly been postulated but havenot been demonstrated to date. Furthermore, theresults of these scanning techniques, even whenperformed during sleep, can only provide infor-mation concerning a short period of time andare limited to the supine position. For routineclinical application, the limited availability, theassociated costs and the associated ionized radia-tion at least for CT scanning are additional limitingfactors.

Videoendoscopy during spontaneous sleep

allows the assessment of the upper airway duringdifferent sleep stages and lacks the side effects of

sedating drugs. However, sleep videoendoscopy isscarcely performed as it usually requires nightlymeasurements and puts additional strain on bothpatient and doctor. Videoendoscopy under seda-tion is able to initiate snoring and upper airwayobstruction during a short period of induced sleep,mostly restricted to the supine position. Theseverity of the underlying disorder appearscomparable to natural sleep, although snoringsounds seem different and the short examinationtime is a significant limitation. There are subtlehints that videoendoscopy under sedation may

change the indication for a limited number ofsurgical interventions. Nevertheless, there is notenough evidence to date that this procedureimproves the outcome of snoring and OSAsurgery.

Pressure recordings with catheters are rela-tively well tolerated and have only minimal impacton sleep quality and airway obstructions. Thepressure curves allow a reliable detection ofrespiratory events and upper as well as lowerobstructions can be detected. There is no datasupporting that obstructions at the hypopharynxand the epiglottis can be discriminated reliably.The distribution of the obstructions can be deter-mined for the entire recording period. The dataavailable support the idea that the success rate ofsoft palate surgery can be improved when usingmulti-channel pressure recordings for patientselection.

The assessment of the critical closing pressureis the only tool to directly measure upper airwaycollapsibility. Up to now, there is no data con-cerning the benefit of a preoperative assessmentof critical closing pressure for a better prediction

of success of upper airway surgery. Togetherwith its relatively complex assessment this mayexplain why these measurements have notbecome a routine procedure in clinical testing sofar.

Although all these additional techniques forupper airway assessment exceeding simple clinical

assessment have substantially improved ourunderstanding of SDB, their significance in dailypractice is limited. In contrast to the widespreaduse of the Mueller maneuver and sedated endos-copy, convincing data supporting their use in termsof treatment outcome is lacking. So far, there isonly very limited evidence that selected tech-niques improve treatment outcome for selectedindications. In general, there is not enoughevidence that these techniques are superior to theroutine clinical assessment.

Practice points

The significance of the different techniquesfor the evaluation of the upper airway insleep-disordered breathing are presented withregard to the clinical management:

Clinical examination including rigidendoscopy is essential, staging systems

may be useful in the hands of less experi-enced observers.

The Mueller maneuver may be performedbefore considering UPPP, as an isolatedhypopharyngeal collapse indicates clinicalfailure, but the overall value is limited.

There is no convincing indication forroutine X-ray cephalometry as long asmaxillomandibular surgery is not planned.

There is no indication for routine evalua-tion with radiologic scanning techniquessuch as CT or MRI.

Videoendoscopy under sedation or duringsleep may be particularly helpful in casesof laryngeal collapse and failures of stan-dard therapy.

Pressure recordings may help improvingsuccess rates of palatal surgery, butobstructions at the hypopharynx and theepiglottis cannot be discriminated reliably.

Apart from scientific approaches, there isno data supporting the routine assessmentof the critical closing pressure.



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Acknowledgment

We want to thank Mr. J. Wich-Schwarz, Ph.D., forhis editorial assistance.

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Research agenda

The following main questions remain withregard to the clinical use of upper airwayevaluation in sleep-disordered breathing:

Various techniques (especially video-endoscopy and pressure measurements)provide information with regard to themechanism and level of airway obstruc-tiondbut do they really cha

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Airway Evaluation