The Small Posterior Cranial Fossa Syndrome and Chiari Malformation Type 0

Patients showing typical Chiari malformation type 1 (CM1) signs and symptoms frequently undergo cranial and cervical MRI. In some patients, MRI documents >5 mm of cerebellar tonsillar herniation (TH) and the diagnosis of CM1. Patients with 3-5 mm TH have borderline CM1. Patients with less than 3 mm of TH and an associated cervical syrinx are diagnosed with Chiari zero malformation (CM0). However, patients reporting CM1 symptoms are usually not diagnosed with CM if MRI shows less than 3-5 mm of TH and no syrinx. Recent MRI morphometric analysis of the posterior fossa and upper cervical spine detected anatomical abnormalities in and around the foramen magnum (FM) that explain these patients' symptoms. The abnormalities include a reduced size of the posterior fossa, FM, and upper cervical spinal canal and extension of the cerebellar tonsils around the medulla rather than inferior to the foramen magnum, as in CM1. These morphometric findings lead some neurologists and neurosurgeons to diagnose CM0 in patients with typical CM1 signs and symptoms, with or without cervical syringes. This article reviews recent findings and controversies about CM0 diagnosis and updates current thinking about the clinical and radiological relationship between CM0, borderline CM1, and CM1.

Automated summary

Patients with typical CM1 symptoms usually undergo MRI, and a TH of more than 5 mm is diagnosed as CM1, a TH between 3-5 mm is diagnosed as borderline CM1, and a TH of less than 3 mm with an associated cervical syrinx is diagnosed as CM0. However, patients are usually not diagnosed with CM if MRI shows less than 3-5 mm of TH and no syrinx. Recent MRI morphometric analysis has detected anatomical abnormalities that explain these symptoms.