- Open Access
Dimensions of the posterior fossa in patients symptomatic for Chiari I malformation but without cerebellar tonsillar descent
© Sekula et al; licensee BioMed Central Ltd. 2005
Received: 26 June 2005
Accepted: 18 December 2005
Published: 18 December 2005
Chiari I malformation (CMI) is diagnosed by rigid radiographic criteria along with appropriate clinical symptomatology. The aim of this study was to investigate the dimensions of the posterior cranial fossa in patients without significant tonsillar descent but with symptoms comparable to CMI.
Twenty-two patients with signs and symptoms comparable to CMI but without accepted radiographic criteria of tonsillar descent > 3–5 mm were referred to our clinic for evaluation. A history and physical examination were performed on all patients. In reviewing their MRI scans, nine morphometric measurements were recorded. The measurements were compared to measurements from a cohort of twenty-five individuals with cranial neuralgias from our practice.
For patients with Chiari-like symptomatology, the following statistically significant abnormalities were identified: reduced length of the clivus, reduced length of basisphenoid, reduced length of basiocciput, and increased angle of the tentorium. Multiple morphometric studies have demonstrated similar findings in CMI.
The current classification of CMI is likely too restrictive. Preliminary morphologic data suggests that a subgroup of patients exists with tonsillar descent less than 3 mm below the foramen magnum but with congenitally hypoplastic posterior fossa causing symptomatology consistent with CMI.
The diagnosis of Chiari I malformation (CMI) is made by radiographic and clinical criteria. Criteria include tonsillar descent 3–5 mm or more below the foramen magnum and compatible symptomatology. Recognized experts in the area of the craniovertebral junction have begun to question whether the radiographic criterion of degree of tonsillar ectopia, albeit useful, may be too restrictive in CMI. A substantial number of patients may be excluded by the current radiographic criteria of CMI [1, 2].
The aim of this study was to investigate the dimensions of the posterior cranial fossa in a subset of patients without significant tonsillar descent (<3 mm) but symptoms comparable to CMI. We quantitatively compared multiple structures in the posterior fossa with normal controls from our practice to assess any recognizable differences. We were then able to compare our measurements with the published normative data and the data from the CMI population.
Number of Patients
Male to female ratio
Duration of symptomsb
The number and percentage of patients in the Chiari-like group with symptoms and signs of Chiari I malformation
Symptoms and Signs
Chiari-like Group (N = 22)
Absent/impaired gag reflex
Lower extremity parasthesia/hyperesthesia
Retro-orbital pressure or pain
An MRI (1.5T) of the craniocervical junction was obtained from each patient prior to arrival in Pittsburgh. We reviewed the images and confirmed tonsillar descent less than 3 mm and the absence of syringomyelia. Patients with basilar invagination and /or platybasia were excluded from the study.
Morphological features of the posterior cranial fossa
Unpaired Student's t test was used to determine the possible differences between group means. A two-tailed P value of less than 0.0057 was considered statistically significant. Because we tested multiple (nine) independent null hypotheses, the P value needed to be lowered to keep the overall P value less than 0.05 .
Quantitative measurements of the posterior cranial fossa
Measurements of the posterior cranial fossa in patients with Chiari-like symptomatology and controls
Measurements depicted in Fig. 2
Chiari-like Group (N = 22)
Control Group (N = 25)
32.95 ± 8.7
43.00 ± 6.6
Supraocciput (IOP to OP)
40.50 ± 5.9
41.80 ± 6.2
18.63 ± 5.3
23.64 ± 4.9
14.00 ± 4.3
19.36 ± 4.7
47.05 ± 4.1
46.40 ± 4.4
47.36 ± 7.9
47.04 ± 4.8
McRae's (B to OP)
43.55 ± 4.9
42.52 ± 5.9
Twining's (DS to IOP)
84.55 ± 7.8
87.32 ± 6.6
Tentorial Angle (a)
41.27 ± 6.5
34.84 ± 7.2
Morphometric studies have confirmed that the clivus and other parameters are hypoplastic along with an increased slope of the tentorium in CMI patients [1, 13]. In the present study, the clivus, basisphenoid, and basiocciput were significantly shorter with a steeper cerebellar tentorium than in the control group. Whether or not downward herniation of the normally developing hindbrain occurs, may depend on the degree of overcrowding in the posterior cranial fossa. While > 3 mm tonsillar herniation may form part of the radiographic picture in CMI patients, it may represent an anachronistic view of CMI. We agree with Milhorat et al. and Nishikawa et al. that the fundamental pathogenic entity in CMI is most likely underdevelopment of the para-axial mesoderm resulting in posterior fossa hypoplasia with CSF flow abnormalities manifested in the adult with varying degrees of tonsillar herniation [1, 11]. We suspect our study population without significant radiographic tonsillar herniation but symptoms compatible with CMI and posterior fossa hypoplasia represents a variant of the CMI.
Chiari malformations represent a range of abnormalities and probably a heterogeneous grouping of disorders. Although severe hindbrain maldevelopment, a primary neural anomaly, serves as a useful explanation for Chiari II and III malformations, much evidence supports the theory that CMI is primarily a mesodermal developmental abnormality . Marin-Padilla et al. first observed that the posterior cranial fossa is hypoplastic in Chiari I malformation [5, 6]. Multiple morphometric studies have since implicated overcrowding of a normally developed hindbrain by an underdeveloped occipital endochondrium of the posterior cranial fossa in the development of CMI [1, 3, 8, 10, 11, 13, 15]. Overcrowding of the hindbrain and resulting displacement of CSF likely contributes to the array of symptoms seen in CMI.
With advances in imaging over the past 20 years, tonsillar ectopia, as it is largely used today, is a poor sole criterion for diagnosis. Barkovich et al. demonstrated that fourteen percent of normal control patients had tonsils below the foramen magnum and one in 200 normal control patients had tonsils projecting 5 mm or more below the foramen magnum by MR imaging . Further, extent of tonsillar herniation in CMI has never been satisfactorily correlated with severity of symptoms. In the largest series to date, Elster et al. reviewed MR images from 12,226 patients and found a large percentage (31%) of patients with tonsils herniated 5 mm or more below the foramen magnum were asymptomatic . Milhorat et al. identified 15 patients in his large series of 364 symptomatic patients who did not fit the radiographic definition but had 'MRI evidence of hindbrain overcrowding and CINE-MRI demonstrated abnormalities of CSF velocity/flow' .
Volume assessment was not performed in this study because complete MRI sections of the posterior fossa were unavailable in a fraction of the patients. As a follow-up to this study, we intend to include volume analysis of the posterior fossa as well as phase-contrast cine MRI in another group of patients, which would lend more support to our hypothesis. Phase-contrast cine MRI [18, 19] was obtained in a few patients but was largely unobtainable due to insurance restrictions. Lastly, no study to date has compared degree of tentorial slope vs. tonsillar herniation. Conceivably, if the tentorium were more accommodating to an increasingly compressed hindbrain, there would be less tonsillar herniation in select individuals.
At the present time, we do not recommend utilizing the above morphometric findings as criteria for surgery on patients without >3 mm tonsillar herniation. Observation is suggested. We believe this study is an initial step in the discernment of a possible subgroup of patients without significant tonsillar descent but signs and symptoms otherwise consistent CMI. As our understanding of CMI evolves, the radiological criteria for diagnosis may need to be modified to accommodate these patients who have minimal tonsillar descent but significant posterior fossa hypoplasia and resulting symptomatology.
The radiological definition of CMI is likely too restrictive. Our preliminary morphological data suggests that a subgroup of patients exists with tonsillar descent less than 3–5 mm below the foramen magnum but with congenitally hypoplastic posterior fossa and resulting CSF flow abnormalities causing symptomatology consistent with CMI. Further studies are required to better delineate this population and validate its existence. In lieu of such studies, surgical restraint is imperative.
We appreciate the assistance of Shelley Birgelen, Rick Kortyna, and Diane Cantella with the preparation of this manuscript.
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