The Small Posterior Cranial Fossa Syndrome and Chiari Malformation Type 0

Abstract

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.

Keywords: Chiari malformation type 0; Chiari malformation type 1; cerebellar tonsil herniation; morphometric analysis; small posterior cranial fossa; syringomyelia.

Figure 1

Flow diagram of the search strategy.

Figure 2

Types of CM0 without syringomyelia. The McRae line (white line) is shown on the T1-weighted sagittal images (left panels). Axial images at McRae’s line are shown (right panels) on T1-weighted (3) and T2-weighted (1,2,4) images. Adult symptomatic CM0-only patients with tonsillar herniation ≤2 mm (panels (1–3)) and borderline CM1-only patients with tonsillar herniation 3 mm (4). All patients demonstrate short bones (CL and SO < 40 mm), crowdedness of the PCF, tight foramen magnum, and CM1-like clinical manifestations: transient localized suboccipital cough-related headaches (1) or constant and transient suboccipital headaches (2–4), truncal ataxia, vertigo, dizziness, and upper motor neuron signs. The sagittal images in (1,3) (left panels) also show spinal canal narrowing at the level of the odontoid.

Figure 2

Types of CM0 without syringomyelia. The McRae line (white line) is shown on the T1-weighted sagittal images (left panels). Axial images at McRae’s line are shown (right panels) on T1-weighted (3) and T2-weighted (1,2,4) images. Adult symptomatic CM0-only patients with tonsillar herniation ≤2 mm (panels (1–3)) and borderline CM1-only patients with tonsillar herniation 3 mm (4). All patients demonstrate short bones (CL and SO < 40 mm), crowdedness of the PCF, tight foramen magnum, and CM1-like clinical manifestations: transient localized suboccipital cough-related headaches (1) or constant and transient suboccipital headaches (2–4), truncal ataxia, vertigo, dizziness, and upper motor neuron signs. The sagittal images in (1,3) (left panels) also show spinal canal narrowing at the level of the odontoid.