The Small Posterior Cranial Fossa Syndrome and Chiari Malformation Type 0

Area of Science:

• Neurology
• Radiology
• Neurosurgery

Background:

• Chiari malformation type 1 (CM1) is diagnosed with >5 mm tonsillar herniation (TH).
• Patients with CM1 symptoms but <3-5 mm TH and no syrinx are often undiagnosed.
• Chiari malformation type 0 (CM0) is diagnosed with <3 mm TH and a syrinx.

Purpose of the Study:

• To review recent findings and controversies regarding CM0 diagnosis.
• To update understanding of the clinical and radiological relationship between CM0, borderline CM1, and CM1.

Main Methods:

• Review of recent MRI morphometric analyses of the posterior fossa and upper cervical spine.
• Analysis of anatomical abnormalities in and around the foramen magnum (FM).

Main Results:

• Reduced posterior fossa, FM, and upper cervical spinal canal size are observed.
• Cerebellar tonsils extend around the medulla, not inferior to the FM.
• These findings explain CM1 symptoms in patients without significant TH.

Conclusions:

• Some neurologists/neurosurgeons diagnose CM0 in patients with CM1 symptoms, regardless of syrinx presence.
• Morphometric findings support CM0 diagnosis in cases previously considered borderline or undiagnosed CM1.