Hearing Loss in Survivors of Neonatal Extracorporal ...Journal of the American Academy of Audiology/Volume 9, Number 1, February 1998 early screening for hearing loss in these new-borns

J Am Acad Audiol 9 : 47-58 (1998)

Hearing Loss in Survivors of Neonatal Extracorporeal Membrane Oxygenation (ECMO) Therapy and High-Frequency Oscillatory (HFO) Therapy Robert E. Lasky* Lori Wiorek' Tim R. Becker~

Abstract

Survivors of extracorporeal membrane oxygenation (ECMO) therapy and high-frequency oscillatory (HFO) therapy during the newborn period were followed to evaluate their hear-ing. Eleven of the 66 ECMO survivors (16.7%) were diagnosed with significant hearing loss after being discharged from the neonatal intensive care unit (NICU) . This rate of hearing loss is consistent with other reports of hearing loss in ECMO survivors . The majority of ECMO survivors with hearing loss developed a bilateral sloping hearing loss ; the high frequencies were more impaired than the low frequencies . The hearing of nine ECMO survivors with hear-ing loss was assessed with auditory brainstem evoked responses (ABRs) in the newborn period prior to discharge from the NICU . Seven of nine ECMO survivors with hearing loss passed their newborn ABR screen . This result is consistent with the results of other researchers . Eight newborns receiving ECMO therapy had congenital diaphragmatic hernias (CDH). Three of these newborns (37.5%) were subsequently diagnosed as having a hearing loss . The com-bination of CDH and ECMO therapy may be a strong predictor of hearing loss . Six HFO survivors with hearing losses were also followed . Their losses were similar to the ECMO sur-vivors with hearing loss, including the progressive nature of the loss (all five of these newborns with neonatal ABR screens passed them only to later be diagnosed with a hearing loss) . A growing body of research indicates that newborns experiencing severe oxygen deprivation are at risk for progressive hearing loss .

Key Words: Auditory brainstem evoked responses, congenital diaphragmatic hernia, extra-corporeal membrane oxygenation, high-frequency oscillatory therapy, high-risk newborns

Abbreviations: ABR =auditory brainstem evoked response, BOA= behavioral observation audiometry, BPD = bronchopulmonary dysplasia, CDH = congenital diaphragmatic hernia, CMV = cytomegalovirus, DPOAE = distortion product otoacoustic emission, ECMO = extra-corporeal membrane oxygenation, HFO = high-frequency oscillatory ventilation therapy, HFV = high-frequency ventilation therapy, IVH = intraventricular hemorrhage, MAS = meco-nium aspiration syndrome, NICU = neonatal intensive care unit, PPHN = persistent pulmonary hypertension of the newborn, RDS = respiratory distress syndrome, SF = sound field, SNHL = sensorineural hearing loss, TEOAE = transient evoked otoacoustic emission, V = vibrotactile threshold, VRA = visual reinforcement audiometry

G

raduates of neonatal intensive care units (NICUs) are at increased risk for hearing loss (American Academy of

Pediatrics, 1982 ; American Speech-Language-Hearing Association, 1991). Estimates of the

incidence of hearing loss in NICU graduates range from 20 to 100 :1000 births versus 1:1000 births in the general pediatric population (Carter and Wilkening, 1991 ; NIH 1993 ; Bess and Par-adise, 1994). In response to their increased risk,

*Neurology Department, The University of Wisconsin-Madison Medical School, Madison, Wisconsin ; tChildren's Hospital of Wisconsin Speech and Hearing Clinic, Milwau-kee, Wisconsin

Reprint requests : Robert E . Lasky, Neurology Depart-ment, The University of Wisconsin-Madison Medical School, H6-528 Clinical Science Building, 600 Highland Avenue, Madison, WI 53792-5132

Journal of the American Academy of Audiology/Volume 9, Number 1, February 1998

early screening for hearing loss in these new-borns has been widely advocated.

NICU newborns with respiratory difficulties seem to be at extra risk for hearing impairment . Advances in surfactant replacement therapy and mechanical ventilation have dramatically increased survival rates in newborns with res-piratory disease (Goldsmith and Karotkin, 1996a; Martin and Fanaroff, 1997). Nevertheless, some newborn lungs may be so impaired that an exchange of oxygen and carbon dioxide ade-quate to support life cannot be maintained with traditional therapies. For these newborns, extra-corporeal membrane oxygenation (ECMO) may be necessary for survival (Short, 1994 ; Ichiba and Bartlett, 1996 ; Torosian et al, 1996). ECMO involves diverting blood from the neonatal heart and lungs to an external bypass circuit. Fluids and heparin (for anticoagulation) are added to the blood, its temperature is maintained, and it is passed through a membrane lung where oxy-gen and carbon dioxide are exchanged before it re-enters the newborn's body. This procedure is expensive, technically complex, and labor inten-sive . Complications resulting from the proce-dure have been reduced but remain a concern (Short, 1994 ; Upp et al, 1994; Ichiba and Bartlett, 1996 ; Torosian et al, 1996). As of July 1995, a reg-istry of over 10,000 ECMO newborns had been established (ECMO Registry Report of the Extra-corporeal Life Support Organization : Interna-tional Summary, 1995). The overall survival rate for that registry was 81 percent. Follow-up data of ECMO survivors concerning neurologic and sensory outcomes are incomplete but increasing (Short, 1994 ; Torosian et al, 1996 ; Stork, 1997). Neurologic and sensory morbidity is elevated in ECMO survivors but may be no higher than expected for very sick newborns . The independent contribution of ECMO therapy to that morbidity is unclear.

Other respiratory therapies that are less invasive and resource intense than ECMO have been developed to address respiratory problems unresponsive to the more traditional therapies (Hodson and Truog, 1994 ; Goldsmith and Karotkin, 1996b) . High-frequency ventilation (HFV) is one promising approach (Slutsky, 1988 ; Lunkenheimer et al, 1994 ; Kinsella and Abman, 1996 ; Mammel and Boros, 1996). HFV involves small-volume gas exchanges at very rapid rates. Its advantage is the delivery of an adequate volume of air for the exchange of vital gases with lower airway pressures and, therefore, less risk of barotrauma . High-frequency oscillatory ventilation (HFO) therapy is one of three current

approaches to HFV In HFO therapy, small vol-umes of air oscillate about a mean airway pres-sure (maintaining lung volume) at rates of several hundred to several thousand cycles per second. The oscillations set up currents in the newborn's airways, resulting in a simultaneous axial inflow of fresh air (a bias current) and an outflow of air from the lungs in the margins of the pathway (Harris and Wood, 1996). Evalua-tion of the costs and benefits of HFO are incom-plete; however, HFO therapy may eliminate the need for ECMO and reduce the risks of pul-monary leaks and bronchopulmonary dysplasia in some newborns (Lunkenheimer et al, 1994 ; Kinsella and Abman, 1996 ; Mammel and Boros, 1996).

Survivors of ECMO are at risk for hearing loss (Hofkosh et al, 1991; Schumacher et al, 1991; Cheung et al, 1996). Less is known con-cerning the outcome of HFO survivors. Reported hearing loss in ECMO survivors ranges from 4 to 57 percent (4% by Schumacher et al, 1991 ; 14% by Cheung et al, 1996 ; 21% by Hofkosh et al, 1991 ; and 57% of newborns with congenital diaphragmatic hernia by Lund et al, 1994). Of special concern are reports of late-onset hearing loss in hypoxic newborns, including those receiv-ing ECMO and HFO (Nield et al, 1986 ; Walton and Hendricks-Munoz, 1991; Konkle and Knightly, 1993; Kawashiro et al, 1994 ; Lund et al, 1994; Kaga et al, 1995, 1997 ; Cheung et al, 1996). Lund et al (1994), Kaga et al (1995, 1997), and Cheung et al (1996) have reported that some ECMO and HFO therapy survivors devel-oped significant hearing loss after passing a neonatal auditory brainstem evoked response (ABR) screening. Of the nine ECMO survivors Cheung et al followed who developed sensori-neural hearing loss (SNHL), seven had been screened as newborns and six passed those new-born screens. Lund et al reported that 2 of their 14 newborns who developed a hearing loss passed a newborn hearing screen. Lund et al did not indicate whether the other 12 newborns had been screened in the newborn period . Kaga et al identified six ECMO and four HFO newborns whose hearing loss was detected between 4 months and 3 years of age.

Many questions concerning the increased risk for hearing loss associated with ECMO and HFO therapies are unanswered . The variability in reported results reflects the small sample sizes involved, the variety of factors associated with hearing loss in those samples, and differ-ences in procedures . Prospective, multicenter collaborative studies are needed. The present

48

Hearing Loss in ECMO and HFO Survivors/Lasky et al

study reports the results of ECMO survivors from Children's Hospital of Wisconsin. In addi-tion, the present study reports the results from a small sample of survivors of HFO therapy. As new respiratory therapies are developed, the sequelae of survivors of those therapies need to be determined .

METHOD

Cases

Infants receiving ECMO therapy at the Children's Hospital of Wisconsin NICU received a newborn hearing screening and were scheduled for regular hearing assessments as part of their medical follow-up . This study reports audio-metric results of 66 ECMO infants whose hear-ing status was evaluated at least through the first 6 months postnatally. Included in those 66 newborns was one child assessed as normal hearing by ABRs in the newborn period and at 1 year of postnatal age. That child also had nor-mal transient evoked otoacoustic emissions (TEOAEs) at 1 year of age. However, it was not possible to elicit behavioral responses to sounds other than startle responses to loud sounds at 1 and 2 years of age. A central auditory deficit was expected but not confirmed due to the age of the child. For the purposes of this study, that child was considered to have normal hearing.

This study also reports the audiologic his-tory of six survivors of HFO therapy diagnosed with a hearing loss after being discharged from the newborn nursery. These HFO therapy sur-vivors experienced the same protocols evaluat-ing hearing as the ECMO survivors both in the NICU and at follow-up.

Procedures

When the newborns were healthy and in stable condition, a hearing screen was scheduled in the NICU just prior to discharge. The initial hearing screen consisted of either an ABR screen and/or a TEOAE (more rarely a distortion prod-uct otoacoustic emission [DPOAE]) screen . Which assessment a newborn in the sample received was determined primarily by when they were born . Initially, ABRs were used to screen all newborns . More recently, in accordance with National Institute of Health (NIH) recommen-dations (NIH, 1993), newborns were initially screened with TEOAEs. If they failed that screen, ABRs were assessed .

ABRs were recorded on a Biologic Traveler. Both ears were assessed . Clicks produced by 100-psec electrical pulses were presented at a rate of 19.1/sec at two levels, loud (at least 70 dB nHL) and soft (35 dB nHL) . Responses were recorded until a clear response was identified or until 1000 responses were averaged . The laten-cies for Waves I, III, and V had to be within nor-mative limits for a pass . The TEOAE screen was recorded on an IL088 system . Clicks pro-duced by 100-psec electrical pulses were pre-sented at a rate of 50/sec in the nonlinear presentation mode . The target stimulus level was 80 dB SPL. Responses were recorded until a clear response was identified (a reproducibility of 80% or better at 1, 2, 3, and/or 4 kHz) or until 256 responses (replicate sets of four clicks) were averaged . The DPOAEs were recorded with an IL092 system . The low (fl) and high (f2) fre-quency primaries were presented at 70 dB SPL with a f2/fl ratio of 1 .22. The assessments were conducted in the NICU.

Postdischarge hearing was assessed in the Speech and Hearing Clinic, which is in the same medical complex as the Children's Hospital NICU. Screening tympanometry was conducted to assess middle ear pathology. Behavioral audiometry depended on the developmental level of the child. Initially, behavioral observa-tion audiometry (BOA) was conducted. When the child was capable of localizing sound from a speaker, visual reinforcement audiometry (VRA) was conducted. When the child's response to sound could be operationally conditioned and the child would tolerate earphones, play audiome-try was the assessment method of choice . Finally, conventional audiometry was conducted when the child was capable of it . Diagnostic and screen-ing ABRs and TEOAEs were conducted when behavioral methods were unproductive and to resolve specific diagnostic issues .

Testing was conducted by ASHA-certified audiologists following recommended procedures (ASHA, 1989 ; Joint Committee on Infant Hear-ing, 1991).

RESULTS

0 f the 66 ECMO survivors with audiomet-ric follow-up at 6 months of age or older, 14

(21.2%) had one or more evaluations consistent with hearing loss . Eleven (16.7%) of these sur-vivors were diagnosed with a hearing loss at their most recent evaluation . Evaluations indi-cating only middle ear pathology were excluded from these results. Twelve of these 14 survivors


Case

Table 1 Newborn

Diagnosis

Outcomes of ECMO Survivors

ABR Screen

with Hearing Loss

ABR Response Level (dB nHL)

1 MAS Failed L and R 2 MAS, cardiac abnormalities Failed L and R 3 MAS Failed L and R L - NR (70)

R-70 4 PPHN Failed L and R L-80

R-80 5 MAS, hypoxia, grade IV Passed L and R

IVH, hydrocephalus, CMV 6 PPHN, RDS Passed L and R 7 RDS Passed L and R 8 MAS, grade IV IVH Passed L and R 9 CDH Passed L and R

10 CDH Passed L and R 11 Respiratory problems Passed L and R 12 CDH Passed L and R 13 CDH, candida fungus 14 Collapsed lung

TEOAE Screen

Failed L and R

Failed L and R

MAS = meconium aspiration syndrome ; PPHN = persistent pulmonary hypertension of the newborn; IVH = intraventricular hemorrhage ; CMV = cytomegalovirus, RDS = respiratory distress syndrome, CDH = congenital diaphragmatic hernia ; L = left, R = right; NR = no response .

were screened for hearing in the NICU prior to discharge (Table 1) . Also included in Table 1 are brief descriptions of the primary medical diag-noses of these 14 newborns while in the NICU.

The first four children listed in Table 1 failed their NICU hearing screen . For all four chil-dren, their neonatal hearing screen consisted of an ABR. Case 5 passed an ABR screen in the NICU but failed a TEOAE screen . Higher false positive rates have been associated with OAE in comparison to ABR hearing screens. False neg-ative rates are considered to be very low for both assessments. Thus, case 5 would be con-sidered to have normal assessed hearing in the newborn period (NIH, 1993). Cases 6 to 12 passed their neonatal ABR screen but were eval-uated as having a hearing loss after being dis-charged from the NICU. Cases 13 and 14 were not screened for hearing in the newborn period because they were discharged before a screen could be conducted.

The audidmetric follow-up results from these 14 children are presented in Table 2 . Many of these children had multiple visits at the time of this report . For those children, the results of their first as well as their most recent assessments are presented. Three of these 14 children were assessed as having normal inner ear and retro-cochlear function (at least no SNHL) at their most recent assessment .

Case 1 had abnormal tympanograms at 5 years of age, suggestive of middle ear pathology.

That pathology explains the mildly elevated air-conduction but normal bone-conduction behav-ioral thresholds assessed during the same clinic appointment. This child was included with the other survivors with assessed hearing loss because of failing to pass a neonatal ABR screen (see Table 1) . Case 2 also failed a neonatal ABR screen but was assessed with normal auditory thresholds at 21/2 years of age. Cases 1 and 2 seemed to be free of cochlear and/or retrocochlear pathology. Failure on the neonatal ABR screen by cases 1 and 2 may be considered false posi-tives. The explanation for those false positives is uncertain. Their evoked responses may have been disrupted in the newborn period when they may have experienced transient hearing losses . Alternatively, recording error may have been involved .

Case 12 passed a neonatal ABR but had elevated thresholds from 6 through 10 months of postnatal life (a flat, moderate to severe hear-ing loss). That child had normal tympanograms, but bone-conduction thresholds were not as-sessed . At 10 months, case 12 was assessed as having normal air-conducted thresholds . That result was repeated three more times over the next 21/2 years. The early elevated thresholds may be considered a false positive associated with behavioral testing. Again, the explanation for those elevated thresholds is uncertain. They could reflect a transient hearing loss or assess-ment error.

50


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With one exception, hearing loss was diag-nosed either in the NICU (cases 1-4 failed their ABR screen prior to discharge from the NICU) or by their first formal postdischarge assess-ment . The first formal postdischarge assess-ment occurred no later than 12 months after birth. The one exception was case 8, who was first formally diagnosed with a hearing loss at 4 years of age (see Table 2) . Case 8 had 15 dB HL 500- and 4000-Hz thresholds at 1 year of age and thresholds of 30 dB HL at those same frequen-cies at 2 years of age. Both assessments were by VRA. The audiologist rated the quality of the thresholds recorded at both visits as "good." The two frequencies assessed were the only fre-quencies that could be assessed given the child's cooperation.

For 3 of the 11 children with a hearing loss at their most recent assessment, their last assessment was within the first year and a half of postnatal life (6 months for case 13, 12 months for case 3, and 17 months for case 5) . Conse-quently, a characterization of their losses was less certain than for the other eight children whose most recent assessments were between 21/2 and 6 years of age. For the eight children with assessments between 21/2 and 6 years of age, their losses were high-frequency sloping losses . They were bilateral with the exception of case 6, whose only loss (70 dB HL) was at 4000 Hz for the left ear. The magnitude of the losses ranged from moderate to profound .

In addition to the ECMO survivors, seven survivors of HFO ventilation (no ECMO) were diagnosed with a hearing loss by their NICU hearing screen or by a postdischarge hearing evaluation . (Some of the ECMO survivors also had HFO ventilation ; for example, two newborns

developed grade IV intraventricular hemor-rhages while on ECMO and, consequently, were switched to HFO therapy.) Table 3 presents the newborn results for these children . Case 21 was not screened during the newborn period . Case 15 failed a TEOAE screen for the left ear but passed for the right ear. These results were replicated 1 month after the first TEOAE screen . The other five passed their neonatal ABR screen .

The audiometric follow-up results from these seven children are presented in Table 4. Case 15's thresholds at 9 months of age were 20 dB HL at 1000 Hz and 15 dB HL at 2000 and 4000 Hz as assessed by VRA. Thus, this child's better ear may have been normal . These results are con-sistent with a unilateral hearing loss implicated by the TEOAE results. The losses for the other children tended to be bilateral and sloping (best for the lowest frequencies) . Generally, they were similar in shape and magnitude to those of the ECMO survivors. Case 17 was first formally diagnosed with a hearing loss at 3 years of age. However, the parents indicated that they were concerned with case 17's speech at 18 months, noting regression by 24 months of age. The other four children with hearing loss were identified by their first birthday.

An alarming percentage of children with hearing loss in this study were assessed as nor-mal hearing in the newborn period . Because the false negative rate in screening high-risk new-borns is so low (less than 1% according to Jacob-son et al [19901), the hearing losses of the 12 children (5-11 and 16-20) in this study who passed a neonatal ABR screen probably developed after the neonatal ABR was assessed . Thus, the majority of losses in the cases in this study were progressive and undetected prior to discharge.

Table 3 Newborn Outcomes of High-Frequency Oscillatory Hearing Loss

Ventilation Therapy Survivors with

TEOAE ABR TEOAE Response

Case Diagnosis Screen Screen Frequencies

15 MAS, PPHN Failed L L - NR Passed R R - 2, 3, 4 kHz

16 CDH Passed L and R 17 MAS, BPD, PPHN Passed L and R 18 MAS Passed L and R 19 MAS Passed L and R 20 MAS, IVH Passed L and R 21 Cardiac surgery

MAS = meconium aspiration syndrome ; PPHN = persistent pulmonary hypertension of the newborn; CDH = congenital diaphragmatic hernia; BPD = bronchopulmonary dysplasia ; IVH = intraventricular hemorrhage ; L = left, R = right .


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Newborn ABRs and TEOAEs collected in an NICU can be difficult to accurately interpret. To ensure that the waveforms were correctly inter-preted, they were independently reviewed by three professionals experienced in recording and interpreting neonatal ABRs. Although mis-interpretation could account for some of the results, it is unlikely to explain all of them .

DISCUSSION

T he hearing loss in this study is consistent with other reports of hearing loss in ECMO survivors (Hofkosh et al, 1991 ; Schumacher et al, 1991 ; Lund et al, 1994; Cheung et al, 1996). Eleven of 66 ECMO survivors (16.7%) were diag-nosed with significant hearing loss after being discharged from the NICU. This incidence is in the middle of the values reported in the litera-ture . It is quite close to the rate of 14.3% reported by Cheung et al (1996) . Their study was very sim-ilar in terms of protocol to the present study.

Lund et al's (1994) 57 percent incidence of hearing loss differs from the other rates reported. Unlike other studies, all of Lund et al's study cases had congenital diaphragmatic hernias (CDH) in addition to ECMO therapy. CDH ECMO survivors may be at increased risk for hearing loss (they are at increased risk for sur-vival; ECMO Registry Report of the Extracor-poreal Life Support Organization : International Summary [1995]). Riley et al (1995) and Cheung et al (1996) have also reported that ECMO sur-vivors with CDH had a greater incidence of hearing loss than non-CDH ECMO survivors. In the present study, three of eight (37.5%) CDH ECMO newborns were diagnosed with a hear-ing loss . Additionally, a CDH newborn who received HFO therapy and was followed audio-logically had a hearing loss . The combination of CDH and ECMO (and, possibly, HFO therapy) may be a strong predictor of hearing loss, as sug-gested by Lund et al (1994) and others (Riley et al, 1995 ; Cheung et al, 1996).

The majority of ECMO survivors with hear-ing loss seem to develop a bilateral sloping hear-ing loss ; high frequencies are more impaired than low frequencies. This result characterized both the present study and Cheung et al's (1996) study, the two studies that reported audiograms for each case . After the first year and a half of postnatal life, bilateral, high-frequency losses characterized the losses of all but one of the cases assessed in the present study and all of the cases in Cheung et al's study. The exception was a unilateral loss (left ear, 4 kHz, the highest

frequency assessed). Further testing will indi-cate whether this ECMO survivor is truly an exception. It is also possible that the etiology of hearing loss in this case may be unrelated to ECMO therapy. Three children in this study who were less than 11/z years of age at the time of testing had flat losses . Individual ears could not be assessed due to their age. As they mature and testing becomes more accurate, their assessed losses may become bilateral, high-fre-quency losses .

The two studies with the most complete data (Cheung et al [ 1996] and the present study) indi-cate that a progressive loss that is not apparent in the newborn period may be the norm rather than the exception for ECMO survivors who develop hearing losses . Cheung et al reported that six of seven ECMO survivors with hearing loss passed their newborn ABR screening. The sev-enth survivor failed a newborn ABR screen but the results were consistent with a conductive loss (all waves delayed) . That newborn passed a subsequent ABR screen prior to discharge from the NICU at 48 weeks corrected age . Similarly, in the present study, seven of nine ECMO sur-vivors with hearing loss passed a newborn ABR screen . (Two other survivors were not screened in the newborn period.) Hearing loss in both studies was identified rather soon after dis-charge, generally within the first year of life . However, Cheung et al (1996) reported three ECMO survivors whose hearing impairment did not become evident for several years. There was one ECMO survivor with a similar history in the present study. Additional follow-up of ECMO survivors is warranted. It is unclear at this point whether more survivors will develop hearing loss as they age. Furthermore, it is unclear whether the losses of those survivors identified with hearing loss will change with age .

The ages of detection of progressive losses in ECMO survivors are similar to the ages of detection of progressive losses in children with persistent pulmonary hypertension of the new-born (PPHN) . Kaga et al (1997) have reported follow-up data from 23 children with PPHN. All of these newborns passed a neonatal ABR screen. Eight of them developed a progressive hearing loss identified between 6 months and 3 years of age (some of these newborns received ECMO therapy) . This age range may reflect time con-stants characterizing cochlear degeneration to hypoxic insult during the newborn period .

An alternative explanation for the progres-sive nature of the hearing loss in ECMO sur-vivors is that ABRs were unreliably interpreted


in those newborns . This explanation seems unlikely given the low false negative rates reported for newborn ABR screening (less than 1% according to Jacobson et al [19901). In addi-tion, in Cheung et al's (1996) study and the pre-sent study, early postnatal behavioral results for several cases indicated normal hearing before losses were detected later in life. Nevertheless, measurement reliability for subgroups of high-risk newborns may differ significantly from those reported in the literature . Waveforms in some infants are of unusual morphology. Fur-thermore, some infants tend to be difficult to test, resulting in noisy, often ambiguous, waveforms. Although ABRs are "objective" assessments, their interpretation generally involves a sub-jective judgment as to the presence of the ABR waveform. (Recognition algorithms such as those used by some ABR screening instruments are exceptions .) Criteria for OAE response identifi-cation are not firmly established. Behavioral assessments in infancy also depend on the audi-ologist's judgment . The reliability of newborn and infant assessments, particularly in specific high-risk subgroups, is an important issue deserving research attention.

In the present study, one ECMO survivor who was subsequently diagnosed with a hear-ing loss failed a newborn OAE screen but passed a newborn ABR screen . Because OAE false pos-itive rates (failure rates reported for OAE screen-ing range between 4.9% and 78% for studies reviewed by Robinette [ 19941 and Bess and Par-adise [19941) are probably higher than ABR false negative rates (less than 1% according to Jacob-son et al [19901), current audiometric practice would interpret this result as consistent with normal hearing. OAE assessment is a relatively recent addition to audiometric testing. As pro-cedures and interpretation have improved and become standardized, the false positive rates have fallen (Robinette, 1994). Some newborns may have sufficient hearing to have ABRs at the lowest screening levels but may have sufficient cochlear pathology that OAEs are absent .

Although there is a growing literature impli-cating ECMO therapy in hearing loss, little data exist regarding the hearing status of sur-vivors of HFO therapy (a study by Kaga et al [1995] is an exception) . This, in part, can be explained by the recent introduction of this therapy into clinical practice . The follow-up results from a small sample of HFO therapy sur-vivors in this study suggest that HFO survivors are probably also at increased risk for hearing loss . The losses of these HFO therapy survivors

were similar to those of ECMO survivors with hearing loss . Furthermore, their losses were frequently undetected by neonatal hearing screens. Specifically, all five newborns on HFO therapy who were screened with an ABR in the newborn period passed that screen and were assessed with a hearing loss post discharge from the NICU. Kaga et al (1995) reported that four HFO newborns in their nursery developed a hearing loss after being discharged from the newborn nursery. The small sample sizes cau-tion against over-interpretation ; however, fur-ther study of the audiologic sequelae of HFO therapy is warranted.

Oxygen deprivation may be responsible for the increased risk of hearing loss in ECMO and HFO ventilation therapy newborns . Shirane and Harrison (1987) have reported progressive degeneration of hair cells following hypoxia in chinchillas . Not only is the incidence of hearing loss greater in newborns experiencing significant pulmonary pathology such as PPHN (Carter and Wilkening, 1991; Kaga et al, 1997), but newborns experiencing respiratory distress have been reported to develop hearing loss after hos-pital discharge (Konkle and Knightly, 1993 ; Kaga et al, 1997). New respiratory therapies may save the lives of newborns suffering severe oxygen deprivation; however, a side effect may be a progressive hearing loss in some of those newborns . It is unclear whether insult to the auditory system had occurred by the time res-piratory therapy was initiated or whether that insult was exacerbated during therapy. In a vul-nerable state as a consequence of oxygen depri-vation, otologic stresses such as further oxygen deprivation, metabolic acidosis, noise, ototoxic drugs, and other insults may cause pathology at levels that would not damage a healthy auditory system (Salamy et al, 1994). It is also unclear whether the progressive deterioration reported can be prevented.

Details concerning the progressive nature of hearing loss in ECMO and HFO survivors have important implications . Progressive losses that are not apparent in the newborn period under-mine the value of newborn hearing screening. Passing a newborn hearing screen may cause a false sense of security, delaying the age that a progressive loss is diagnosed and managed. Chil-dren at risk for progressive losses need to be fol-lowed closely, and their families need to be counseled regarding the significant risk of acquired hearing loss . These steps have been taken at Children's Hospital of Wisconsin with good success . The efficacy of newborn screening

56


depends, in part, on identifying newborns at risk for developing a hearing loss after hospital discharge. Identifying risk indicators for pro-gressive losses should be a high priority for those concerned with newborn screening.

It should be noted that prior to the intro-duction of ECMO and HFO ventilation therapy, abnormal ABRs have been reported in newborns who experienced perinatal asphyxia (Galambos and Despland, 1980 ; Kileny et al, 1980 ; Salamy et al, 1980 ; Sanders and Amlie, 1981 ; Hecox and Cone, 1981; Stockard and Stockard, 1981; Stockard et al, 1983). Furthermore, Kileny et al (1980), Sanders and Amlie (1980), and Stockard et al (1983) noted improvement in the ABR with time in some of these newborns . Perlman et al (1983) noted a similar improvement following neonatal hyperbilirubinemia . Two newborns in the present study (cases 1 and 2) failed their new-born ABR screens only to be later assessed with normal hearing. Difficult recording conditions in the newborn period may explain some of these results. Nevertheless, these results, in conjunc-tion with reports of progressive disorders, indi-cate that the audiologic sequelae of oxygen deprivation in the newborn period are varied and not well understood at this time . Research is needed .

A variety of conditions that result in oxygen deprivation may be associated with hearing loss in ECMO and HFO survivors. There is a need for sensitive risk indicators of hearing loss in NICU survivors. Not surprisingly, as medical management of high-risk newborns changes so will the concept of who is at risk. Studies con-ducted even relatively recently may not gener-alize to current medical practice . This is related to a more general need for long-term, high-qual-ity follow-up of medical interventions such as those associated with neonatal intensive care . It is only by careful follow-up that the costs and benefits of medical care can be evaluated. Details of individual patient histories are likely to become increasingly important in determining risk of hearing loss .

Further research concerning the etiology of hearing loss in ECMO and HFO ventilation therapy newborns is needed . Nevertheless, a growing body of research suggests that some newborns experiencing severe oxygen depriva-tion are at risk for progressive hearing loss . This study indicates that ECMO survivors and survivors of HFO therapy are likely to be among those at increased risk . The hearing of these infants should be carefully monitored after they are discharged from the NICU.

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Hearing Loss in Survivors of Neonatal Extracorporal ...Journal of the American Academy of Audiology/Volume 9, Number 1, February 1998 early screening for hearing loss in these new-borns