Posterior fossa dimension and volume estimates in pediatric patients with Chiari I malformations.

INTRODUCTION: Chiari I malformations (CMI) involve pathological hindbrain abnormalities reported to be correlated with a hypoplastic posterior fossa. CMI was traditionally characterized by the downward herniation of the cerebellar tonsils with a descent of 5 mm or more below the foramen magnum. The fullness of the cisterna magna and CSF flow at the level of the cervicomedullary junction have been shown to be more useful in selecting symptomatic patients for surgical decompression. The present study calculates posterior fossa dimension and volume estimates in pediatric patients using magnetic resonance imaging. The combination of neuroradiological and clinical findings is used to re-examine the criteria used for diagnosis and treatment of pediatric CMI patients.

MATERIALS AND METHODS: A retrospective chart review was conducted on patients who were admitted to the Division of Neurosurgery of the Children's hospital of Eastern Ontario between 1990 and 2007. Clinical and radiological assessments were performed on all patients. Posterior fossa volumes (PFV) and intracranial volumes (ICV) were measured from sagittal head magnetic resonance imaging scans using the Cavalieri method.

RESULTS: Sixty-one CMI patients were identified. There were 32 male and 29 female patients with a mean age of 10 years (range: 8 weeks-18 years). Thirty-four (55%) of these patients were symptomatic with scoliosis (38%), suboccipital headaches (29%), and motor/sensory deficits (26%) being the most prominent symptoms. The mean PFV/ICV ratio for all the CMI patients (0.110) was found to be statistically smaller than that of the control patients (0.127, p=0.022). Mean PFV/ICV ratios for asymptomatic and symptomatic CMI patients were found to be similar for children aged 0-9 years (p=0.783) but different for children aged 10-18 years (p=0.018).

DISCUSSION: Mean PFV values were found to be smaller in pediatric CMI patients than control patients; this complements earlier studies in adults and supports the present theory concerning the pathophysiological mechanism of CMI. Subtle morphometric differences among asymptomatic and symptomatic patients aged 0-9 years stress the importance of monitoring asymptomatic patients for the onset of symptoms in their adult years. Symptom development in CMI is likely multifactorial and is much more extensive than the degree of cerebellar tonsillar herniation.