Two distinct populations of Chiari I malformation based on ...and syringomyelia.24 Furthermore, tonsil impaction at the foramen, in which the anatomical abnormality of the cere - bellar

CLINICAL ARTICLEJ Neurosurg 126:1934–1940, 2017

It has been recognized for many years that some pa-tients with the Chiari I malformation have a congeni-tally reduced posterior fossa volume (PFV). Several radiographic analyses demonstrate, on average, reduced linear and volumetric measurements of the posterior fossa in patients with Chiari I malformation compared with con-trols.1–3,10,18,28 The reduced size of the posterior fossa ap-pears to be most significant at an early age and becomes less disproportionate with increasing age.3,31

It has been postulated that in patients with a dispro-portionally small posterior fossa, the reduced intracra-nial volume is insufficient to accommodate the volume

of the hindbrain, resulting in herniation of the cerebellar tonsils.2,8,15 Many patients with Chiari I malformation, however, do not have a disproportionately small posterior fossa.23,25 In these patients the tonsillar descent cannot be attributed to a confined PFV with too little space for the cerebellum and brainstem, and thus there is no explanation for the pathogenesis of the Chiari I malformation.

In the present series of patients with Chiari I malforma-tion, we observed that in some patients preoperative MRI demonstrated generous subarachnoid spaces above and/or around the cerebellum. This appearance suggested that a limited PFV did not cause the tonsillar herniation in these

ABBREVIATIONS HR = herniated tissues (tonsils) not confined to the posterior fossa compartment; PFC = volume of the tissue contents within the posterior fossa com-partment; PFC + HR = total hindbrain content contained within the posterior fossa and the component that has herniated; PFV = posterior fossa volume.SUBMITTED December 22, 2015. ACCEPTED June 8, 2016.INCLUDE WHEN CITING Published online September 2, 2016; DOI: 10.3171/2016.6.JNS152998.

Two distinct populations of Chiari I malformation based on presence or absence of posterior fossa crowdedness on magnetic resonance imagingDavis G. Taylor, MD, Panagiotis Mastorakos, MD, John A. Jane Jr., MD, and Edward H. Oldfield, MD

Department of Neurological Surgery, University of Virginia Health System, Charlottesville, Virginia

OBJECTIVE A subset of patients with Chiari I malformation demonstrate patent subarachnoid spaces around the cer-ebellum, indicating that reduced posterior fossa volume alone does not account for tonsillar descent. The authors distin-guish two subsets of Chiari I malformation patients based on the degree of “posterior fossa crowdedness” on MRI.METHODS Two of the coauthors independently reviewed the preoperative MR images of 49 patients with Chiari I mal-formation and categorized the posterior fossa as “spacious” or “crowded.” Volumetric analysis of posterior fossa struc-tures was then performed using open-source DICOM software. The preoperative clinical and imaging features of the two groups were compared.RESULTS The posterior fossae of 25 patients were classified as spacious and 20 as crowded by both readers; 4 were incongruent. The volumes of the posterior fossa compartment, posterior fossa tissue, and hindbrain (posterior fossa tissue including herniated tonsils) were statistically similar between the patients with spacious and crowed subtypes (p = 0.33, p = 0.17, p = 0.20, respectively). However, patients in the spacious and crowded subtypes demonstrated sig-nificant differences in the ratios of posterior fossa tissue to compartment volumes as well as hindbrain to compartment volumes (p = 0.001 and p = 0.0004, respectively). The average age at surgery was 29.2 ± 19.3 years (mean ± SD) and 21.9 ± 14.9 years for spacious and crowded subtypes, respectively (p = 0.08). Syringomyelia was more prevalent in the crowded subtype (50% vs 28%, p = 0.11).CONCLUSIONS The authors’ study identifies two subtypes of Chiari I malformation, crowded and spacious, that can be distinguished by MRI appearance without volumetric analysis. Earlier age at surgery and presence of syringomyelia are more common in the crowded subtype. The presence of the spacious subtype suggests that crowdedness alone cannot explain the pathogenesis of Chiari I malformation in many patients, supporting the need for further investigation.https://thejns.org/doi/abs/10.3171/2016.6.JNS152998KEY WORDS Chiari I malformation; posterior fossa volume; syringomyelia; posterior fossa decompression; diagnostic and operative techniques

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Two subtypes of Chiari I malformation

J Neurosurg Volume 126 • June 2017 1935

patients, but rather the impacted tonsils were the result of the cerebellum dropping into the foramen magnum. We hypothesized the following: 1) that these patients would have a smaller fraction of the volume of the posterior fossa occupied by tissue than patients with a developmentally small posterior fossa, 2) that we could identify these two distinct subsets of patients with Chiari I malformation by inspection of their preoperative MRI, and 3) that adult patients with Chiari I malformation may be more likely to constitute the subset of patients lacking posterior fossa crowdedness.

We prospectively categorized “crowdedness” on in-terpretation of MRI of all Chiari I malformation patients treated by two faculty neurosurgeons at our institution from November 2004 to November 2014 and then mea-sured the volumes of the posterior fossa compartment, the tissues within it, and the posterior fossa tissue including the herniated tonsils. Our observations confirm the exis-tence of two subsets of patients classified as having either a “spacious” or a “crowded” posterior fossa based on re-view of preoperative MRI studies.

MethodsPatients

Following institutional review board approval, we ret-rospectively evaluated 86 patients who underwent poste-rior fossa decompression for Chiari I malformation per-formed by either of two faculty neurosurgeons between November 2004 and November 2014. Inclusion criteria included Chiari I malformation, at least 5 mm of tonsil-lar herniation, age greater than 5 years, and preoperative T1-weighted sagittal MRI studies available for review. Of these, 72 patients met initial inclusion criteria. Exclusion criteria included prior surgery (n = 7), additional posterior fossa pathology (n = 1), or inadequate preoperative sagit-tal T1-weighted MRI for measurement of PFV, including MRI that did not extend to the lateral margins of the pos-terior fossa (n = 14), and 1 case with severe motion artifact limiting accurate measurements. Thus, 49 patients were available for inclusion in the analysis.

Imaging Analysis of Posterior Cranial FossaAll patients underwent routine preoperative MRI in the

standard supine position. Sagittal T1-weighted sequences were evaluated by two neurosurgery residents (D.G.T. and P.M.) and classified as “crowded” or “spacious” based on the presence or absence of patent pericerebellar subarach-noid spaces or tissue compression by the bony elements of the posterior fossa. The midline sagittal images were emphasized in the interpretation. Crowded subtypes dem-onstrated absent subarachnoid spaces or evidence of com-pression of the cerebellum or brainstem, while spacious subtypes demonstrated patent subarachnoid spaces and lacked tissue compression except at the inferior margin of the opisthion, where the tonsillar descent and impaction in the foramen magnum are nearly universal features in Chiari I malformation.

We then used T1-weighted imaging to determine linear and volumetric measurements. Traditional linear measure-

ments were made on midline sagittal MR images as previ-ously described, including the length of the clivus, length from clival tip to tentorial angle, length of tentorium, length from internal occipital protuberance to opisthion, McRae’s line, Twinning’s line, and cerebellar height.1,4,30

Volumetric measurements were made using semiauto-mated OsiriX Imaging Software, an open-source DICOM viewer used extensively in anatomical volumetric measure-ments.14,20,34 The posterior fossa volume (PFV), the volume of the tissue contents within the posterior fossa compart-ment (PFC), and the tissue of the posterior fossa expanded to include the volume of the descended tonsils (PFC + HR) were outlined by free-hand to conform with traditional sagittal linear measurements (clivus, clivus to tentorial angle, tentorium, occiput, and McRae’s line) and were con-tinued laterally along the bony and tentorial boundaries to the lateral-most limit. Others have used similar methods.4

Statistical AnalysisThe Mann-Whitney U-test was used to examine sta-

tistical differences in the linear and volumetric measure-ments and age at surgery between the spacious and crowd-ed subtypes. The presence or absence of syringomyelia among subtype classification was compared using Fisher’s exact test.

ResultsThe most common presenting symptom was headaches

(84%), followed by paresthesias (28%) and sleep apnea (14%) (Table 1). There was no difference between spacious and crowded subtypes with regard to symptoms.

Among the 49 patients, 25 of the posterior fossae were classified as spacious and 20 as crowded (Fig. 1A and B), and in 4 patients there was incongruent categorization be-tween the two evaluators. Thus, 92% of the patients were categorized similarly by both readers inspecting the MR images. Data pertaining to the 4 patients with discordant categorization were not used in the comparisons of the two subsets of Chiari I malformations.

The mean posterior fossa volume (PFV) (± SD) was similar between the patients with spacious and crowed subtypes (179.5 ± 19.1 cm3 vs 176.9 ± 17.9 cm3; p = 0.33) (Fig. 2). Likewise, between subtypes there was no signifi-cant difference in the volume of the tissue contents within the posterior fossa compartment (PFC) or the total hind-brain volume (tissue volume within the posterior fossa + the volume of the herniated tonsils; Fig. 2). In contrast, when the volume of the posterior fossa compartment was related to the volume of the posterior fossa tissue in indi-vidual patients, there were significant differences between the spacious and crowded subtypes in the fraction of the PFV occupied by tissue and in the ratio of the total hind-brain tissue compared with the PFV (p = 0.001 and p = 0.0004, respectively) (Fig. 3). This demonstrates that pa-tients with the spacious subtype had significantly less of their posterior fossa compartment occupied by tissue (Fig. 3 upper) and significantly less hindbrain tissue for a given posterior fossa compartment volume compared with pa-tients with the crowded subtype (Fig. 3 lower).


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There were no statistically significant differences in linear measurements between subgroups. Note, however, that reduced length from the clival tip to the tentorial an-gle in crowded subtypes trended toward significance (p = 0.07). The mean age (± SD) at surgery in the spacious group (29.2 ± 19.3 years) versus the crowded group (21.9 ± 14.9 years) strongly trended toward significance (p = 0.08). Syringomyelia was present in 43% of patients overall but was more prevalent in the crowded than the spacious sub-type (50% vs 28%, p = 0.11; Fig. 4). These results are sum-marized in Table 2.

DiscussionWhile initially attributing “alterations in the cerebel-

lum” to hydrocephalus, Chiari later hypothesized, and several other studies later confirmed, that in some patients insufficient bone growth contributes to the development of tonsillar herniation.1,2,10,16,18,30 This mechanism has also been demonstrated in Cavalier King Charles Spaniels, whose selective breeding produced a small posterior fossa that is commonly associated with Chiari malformation and syringomyelia.24 Furthermore, tonsil impaction at the foramen, in which the anatomical abnormality of the cere-bellar tonsils is identical to the abnormal tonsil anatomy in idiopathic Chiari I malformation, also occurs with posteri-or fossa tumors27 or supratentorial masses, such as tumors or chronic subdural hematomas,17,26 and has been reported in instances of CSF diversion with intracranial hypoten-sion.7,13 Moreover, the anatomical abnormalities (pointed, abnormally shaped and inferiorly displaced tonsils, dorsal cervicomedullary hump) associated with Chiari I malfor-mation consistently resolve in the months after successful decompressive surgery.12 Finally, the neural anatomy of the hindbrain has recently been analyzed and shown to be normal.8 As initially shown by a series of studies at the National Institutes of Health, it is the systolic impaction of the tonsils in the foramen magnum that produces the typi-cal abnormal anatomy of the tonsils, regardless of the etiol-ogy of the tonsil herniation.11,12,19,33 The identical anatomi-cal abnormality of the cerebellar tonsils, whether it occurs with a Chiari I malformation, posterior fossa tumor, supra-tentorial tumor, or chronic subdural hematoma, or whether it is a result of limited PFV, and that the anatomical ab-normality consistently reverses with treatment, as it does with craniocervical decompression that provides free, pul-satile movement of the CSF across the foramen magnum, provide considerable evidence that the abnormality of the cerebellar tonsils that occurs with a Chiari I malformation is an acquired rather than congenital malformation, as has been argued previously.6,32,33

Furthermore, not all patients with Chiari I malforma-tion have a reduced PFV, and as shown in the current study, many of these patients with the “spacious” subtype have patent subarachnoid spaces in the posterior fossa, often above or behind the cerebellar vermis, that is clearly vis-ible, producing a pattern on MRI that is distinct from their crowded counterparts and constituting a spacious subtype that has previously not been recognized as a distinct entity. Although the authors of several volumetric studies have reported a reduced average posterior fossa volume in the

general Chiari I malformation population compared with controls, review of the data consistently demonstrates that a substantial portion of the patients have either a normal PFV or normal ratios of the PFV to intracranial volume when compared with control subjects without Chiari I malformation.3,18,25,28 It is likely that many of these patients with a normal PFV or normal ratios would be in the “spa-cious” subtype described here but were previously un-recognized as such because the focus of the investigators was a comparison of volumetric measurements of Chiari I malformation patients to the general population, rather than a qualitative difference in MRI appearance or com-parisons between patients with Chiari I malformation. Ad-ditionally, the existence of this subtype helps to reconcile other reports of normal PFV among patients with Chiari I malformation.23,29 The discrepancy between reports may be due to differences in the proportion of spacious versus crowded subtypes within each report’s cohort.

Our study demonstrates categorization of patients with Chiari I malformation based on MRI interpretation and confirmed by objective volumetric ratios representing the degree of crowdedness of the posterior fossa compart-ment. No single value delineated the fossae as crowded or spacious; rather, the clinical interpretation of scans based on the presence of subarachnoid spaces and cerebellar compression was the principal determinant of classifica-tion and was supported by volumetric ratios. Four patients demonstrated incongruence of categorization between the interpreters. Some degree of incongruence is to be expect-ed, and when compared with objective measurements of the discordant scans, the PFC/PFV of these 4 patients fell within the region of overlap between crowded and spacious subtypes. Most importantly, there was 92% congruency between interpreters, demonstrating reasonable reproduc-ibility and reliability of interpretation of the MR images.

In addition to reduced PFV, the crowded subtype dem-onstrates a strong trend toward an increased prevalence of syringomyelia. Association of a syrinx with the crowded subtype is further supported by Sgouros et al., who noted that patients with syringomyelia and Chiari I malforma-tion had smaller posterior fossa volumes and smaller ra-tios of the posterior fossa to total intracranial volume than

TABLE 1. Presentation of patients with spacious and crowded subtypes of Chiari I malformation

Signs & Symptoms

No. of Patients (%)Total

(n = 49)Spacious

PF (n = 25)Crowded

PF (n = 20)

Headache 41 (83.7) 21 (84) 17 (85)Paresthesias 14 (28.1) 7 (28) 6 (30)Weakness 2 (4.1) 2 (8) 0Dysphagia 3 (6.1) 1 (4) 2 (10)Sleep apnea 7 (14.3) 5 (20) 3 (15)Ataxia 1 (2) 1 (4) 0Vertigo 3 (6.1) 0 2 (10)Asymptomatic w/ syringomyelia 3 (6.1) 3 (12) 0

PF = posterior fossa.




FIG. 1. Note the clear differences in the absence or presence of subarachnoid space unoccupied by tissue in the posterior fossa of crowded and spacious subtypes of Chiari I malformation. Upper: Midline sagittal MR images of illustrative patients with the crowded (left) and spacious (right) subtypes of Chiari I malformation. The arrows point to the limited and prominent CSF space above the cerebellar vermis in the crowded and spacious type, respectively. Lower: Midline sagittal MR images of three typical patients with the spacious (upper row) and crowded (lower row) subtypes.


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patients with Chiari I malformation without syringomy-elia.25 Though our findings did not reach statistical signifi-cance with respect to subtype and incidence of syringo-myelia, the strong trend toward significance supports the need for further research into the association of posterior fossa crowdedness with syringogenesis.

The clinical importance of our findings, if any, remains to be discovered. Our study was not designed to determine causal relationship among the subgroups but to confirm the presence or absence of the subgroups herein identi-fied. Regardless of etiology, effective treatment of Chiari I malformation occurs when the obstruction of the foramen magnum is relieved and normal pulsatile CSF flow across

the foramen magnum is restored.11,12,19 It is reasonable then to hypothesize that any mechanism that resolves the ob-struction of normal CSF pathways will result in resolution of pathology, regardless of crowded or spacious subtype. Surgery limited to bony decompression or a partial-thick-ness dural incision effectively treats most children with Chiari I malformation.9 However, surgery without open-ing the dural incision has not been as successful in adults.5 It remains to be determined if clinical outcomes of the crowded and spacious subtypes, as determined via MRI, will respond similarly to the more limited surgery.

Of particular interest is the mechanism by which pa-tients with the spacious subtype develop and maintain a Chiari I malformation. The presence of the crowded sub-type, as identified in this study and supported by studies demonstrating reduced PFV in some patients with Chiari I malformation, suggests that “crowdedness” likely plays some role in, and indeed may alone be sufficient to cause, obstruction of the free, pulsatile movement of CSF across

FIG. 2. Scatter plots showing the posterior fossa volume (179.5 ± 19.1 cm3 vs 176.9 ± 17.9 cm3; p = 0.33) (A), tissue volume in the posterior fossa (150.0 ± 16.8 cm3 vs 155.1 ± 17.7 cm3; p = 0.17) (B), and total hindbrain volume (tissue volume in the poste-rior fossa + volume of the herniated tonsils) (153.0 ± 16.7 cm3 vs 157.8 ± 17.8 cm3; p = 0.20) (C) of patients in the spacious and crowded subtypes, respectively. Note the trend toward greater tissue volume and less posterior fossa volume in patients with the crowded subtype, although not significant.

FIG. 3. When the tissue volume is related to the posterior fossa volume (PFV) in individual patients, there are significant differences between the spacious and crowed subsets. The mean ratio of the volume of the tissue contents within the posterior fossa compartment (PFC) (hindbrain within the posterior fossa) to PFV (A) is 0.838 vs 0.873 (p = 0.001) and the ratio of the volume of the total hindbrain (PFC combined with the volume of the herniated tonsils [PFC + HR]) to the PFV (B) is 0.854 vs 0.891 (p = 0.0004).

FIG. 4. The fraction of patients with syringomyelia was greater in the crowded than the spacious subtype, although the difference was not statistically significant (p = 0.1).




the foramen magnum. The pathogenesis of the develop-ment of the Chiari I malformation in the spacious subtype, however, is unclear. Several reports indicate that an “ac-quired” Chiari I malformation can occur due to spontane-ous CSF leak, CSF hypovolemia, lumbar CSF diversion, or intrathecal hypotension.6,7,13,16,21,22 We detected no correla-tion with any of these risk factors in our patients and are, therefore, unable to attribute pathologic development of spacious subtypes to any particular cause. Regarding the mechanism by which Chiari I malformation is sustained in spacious subtypes, we have no evidence, and thus any ex-planation is speculative. As stated in the discussion above, it is the systolic impaction of the tonsils in the foramen magnum that produces the abnormal anatomy with Chiari I malformation and tonsil herniation from other causes. One possibility is that the repetitive and forceful nature of the downward movement of the brainstem and tonsil impaction into the foramen magnum during the cardiac cycle is too severe for the natural compliance of the cer-ebellum to free itself from the foramen magnum. Further-more, during diastole, the retrograde flow of CSF upward through the foramen magnum in a spinal to cranial direc-tion occurs predominantly at the ventral aspect of the cord due to the severity of the dorsal obstruction, which may maintain the dorsal displacement of the tonsils against the opisthion and sustain the malformation.

ConclusionsOur study identifies two subtypes of Chiari I malforma-

tion, crowded and spacious, that can be easily distinguished on MRI. Earlier age at surgery and presence of syringomy-elia appear to be more common in the crowded subtype, al-though the differences did not reach statistical significance. The presence of the spacious subtype suggests that dispro-portion between the volumes of the hindbrain/cerebellar tissue and the posterior fossa compartment alone cannot explain the pathogenesis of Chiari I malformation in many patients, supporting the need for further investigation.

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TABLE 2. Radiographic data of spacious and crowded subtypes of Chiari I malformation

Variable Total Spacious Crowded Incongruent p Value

No. of patients 49 25 20 4Age at surgery (yrs) 25 29 22 17 0.08Tonsillar herniation (cm) 1.5 1.6 1.3 1.4 NSClivus (cm) 4.2 4.1 4.2 4.2 NSDistance from tip of clivus to tentorial angle (cm) 5.5 5.6 5.3 6.05 0.07Tentorium (cm) 5.1 5.1 5.2 5.0 NSSupraocciput (cm) 4.0 4.0 3.8 3.9 NSMcRae’s line (cm) 3.5 3.4 3.5 3.6 NSTwinning’s line (cm) 8.4 8.3 8.4 8.6 NSLength of cerebellum to foramen magnum (cm) 5.5 5.4 5.5 5.6 NSSyrinx present (%) 21 (43) 7 (28) 10 (50) 4 (100) 0.1

NS = not significant.


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DisclosuresThe authors report no conflict of interest concerning the materi-als or methods used in this study or the findings specified in this paper.

Author ContributionsConception and design: Oldfield, Taylor. Acquisition of data: Taylor, Mastorakos. Analysis and interpretation of data: Old-field, Taylor. Drafting the article: Taylor. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Oldfield. Statistical analysis: Oldfield. Administra-tive/technical/material support: Taylor. Study supervision: Old-field.

CorrespondenceEdward H. Oldfield, MD, Department of Neurological Surgery, University of Virginia Health Systems, Box 800212, Charlottes-ville, VA 22908. email: [email protected].


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Two distinct populations of Chiari I malformation based on ...and syringomyelia.24 Furthermore, tonsil impaction at the foramen, in which the anatomical abnormality of the cere - bellar