ArnoldChiari

Arnold-Chiari malformation, commonly referred to as Chiari malformation, is a group of structural defects in which the hindbrain extends into the upper spinal canal through the foramen magnum. The condition results from crowding at the skull–spinal junction and can be associated with hydrocephalus, syringomyelia, and other neural axis anomalies. Modern terminology typically uses Chiari malformation types I–IV, with Arnold-Chiari historically applied to some of the more severe forms.

Classification distinguishes four main types. Chiari I involves downward displacement of the cerebellar tonsils at least 5 mm below the foramen magnum, often presenting in adolescence or adulthood with headaches and neck pain. Chiari II features downward herniation of the cerebellar vermis and brainstem with typically accompanying myelomeningocele and hydrocephalus. Chiari III is a high-occurrence associated with occipital or posterior fossa defects and encephalocele, while Chiari IV denotes cerebellar hypoplasia. These distinctions reflect severity and associated anomalies rather than a single shared pathogenesis.

Symptoms vary by type and severity. Chiari I may cause occipital headaches worsened by coughing or bending, neck pain, dizziness, balance problems, and sometimes facial numbness or swallowing difficulties. Chiari II and other forms often present in infancy or early childhood with more pronounced neurological deficits, syringomyelia, hydrocephalus, and airway or feeding challenges.

Diagnosis relies mainly on magnetic resonance imaging (MRI), with additional imaging as needed to assess syringomyelia or hydrocephalus. Treatment ranges from observation for mild, asymptomatic cases to surgical decompression for those with significant symptoms or neural compression. Posterior fossa decompression, sometimes with duraplasty, aims to relieve crowding and restore CSF flow.

The prognosis is variable. Many patients experience symptom relief after surgery, though some deficits may persist. The condition was named for late 19th‑century descriptions by Arnold and Chiari, with terminology evolving toward a more specific type-based framework.