Chiari

Chiari malformation, commonly referred to as Chiari, describes a group of structural defects at the craniovertebral junction characterized by downward displacement of the cerebellar tonsils and sometimes other hindbrain structures through the foramen magnum. The condition is categorized into four main types, Chiari I through IV, which vary in severity and associated abnormalities.

Chiari I involves downward displacement of the cerebellar tonsils, typically five millimeters or more below the foramen magnum, and may be asymptomatic or cause headaches, neck pain, dizziness, balance problems, swallowing difficulties, or facial numbness. Onset is often in adolescence or adulthood. Chiari II, or Arnold-Chiari malformation, is usually linked with myelomeningocele and other spinal and brainstem abnormalities and can present at birth with hydrocephalus and more extensive neurological impairment. Chiari III and IV are rare and involve more severe congenital anomalies, including occipital encephaloceles or cerebellar hypoplasia.

Etiology is multifactorial, involving crowding of the posterior fossa, abnormal skull and brain development, and sometimes accompanying spinal defects. Chiari can occur in isolation or as part of broader syndromes; syringomyelia and hydrocephalus may accompany certain types and contribute to symptoms.

Diagnosis relies primarily on magnetic resonance imaging (MRI) of the brain and craniocervical junction, which documents tonsillar herniation, brainstem position, and CSF flow. Cine MRI can assess CSF dynamics and help guide management.

Treatment is individualized. Many individuals with mild or asymptomatic Chiari do not require intervention. Symptomatic Chiari I or associated syringomyelia may be treated with posterior fossa decompression surgery, sometimes with duraplasty, to enlarge the foramen magnum and improve CSF flow. Prognosis varies with type and severity; some patients improve after surgery, while others have persistent symptoms.