Chiari Malformation

Diagnosis Definition

• Chiari malformations are a group of disorders with varying degrees of hindbrain herniation through the foramen magnum
• Chiari I malformation usually results from an abnormally small posterior fossa
• The diagnosis is based on patient’s symptoms, neurological exam, and MRI findings
• Displaced cerebellar tonsils may put pressure on the brainstem and spinal cord, block CSF flow, and result in clinical signs and symptoms
• Most patients are asymptomatic but can experience headache, dizziness, muscle weakness, numbness, problems with balance and coordination, and nystagmus
• Surgical management for symptomatic cases not controlled by medication includes osseous decompression of the foramen magnum and, often, upper cervical laminectomies with duraplasty

Imaging Findings

• MRI is the most common test used to diagnose Chiari I malformations and shows low, compressed cerebellar tonsils displaced 5 mm or more below the foramen magnum (normally only the spinal cord passes through this opening); this is referred to as “tonsillar pegging”
• Syringomyelia (fluid-filled cavity or syrinx within the spinal cord) associated with hydromyelia (dilatation of the central canal by cerebrospinal fluid), called syringohydromyelia, is present in 23%-80% of cases; CSF pulsation studies may have predictive value for syringohydromyelia and surgical success

KEY IMAGES

Pearls

• Type I Chiari malformations are the only type that can be acquired; most occur during fetal development but they can occur later in life when an excessive amount of cerebrospinal fluid is drained away because of injury, infection, or exposure to toxic substances
• Conditions that can mimic Chiari I malformations include long-standing compensated hydrocephalus, pseudotumor cerebri with chronic tonsillar herniation, and intracranial hypotension secondary to chronic CSF leak (these latter patients will not benefit from a foramen magnum decompression as will patients with Chiari I malformation as they require repair of the CSF leak)

References

1. Elster AD, Chen MY. Chiari I malformations: clinical and radiologic reappraisal. Radiology Published online May 1, 1992 https://doi.org/10.1148/radiology.183.2.1561334
2. Rozenfeld M, Frim DM, Katzman GL, Ginat DT. MRI findings after surgery for Chiari malformation type I. American Journal of Roentgenology 2015; 205:1086-1093