Chiari malformation

Chiari malformations result from a mismatch between the size of the posterior fossa and its neural contents, causing downward displacement of cerebellar structures through the foramen magnum. Type I (most common) involves isolated tonsillar herniation, often asymptomatic but can cause occipital headaches exacerbated by coughing/straining, neck pain, and syringomyelia (due to disrupted CSF flow) leading to a cape-like distribution of sensory loss and upper extremity weakness. It's associated with basilar invagination and Klippel-Feil syndrome. Type II is more severe, involving herniation of the cerebellar vermis, fourth ventricle, and brainstem, almost universally associated with myelomeningocele and hydrocephalus (due to aqueductal stenosis or obstruction of CSF outflow), presenting in infancy with brainstem dysfunction (stridor, apnea, dysphagia). Type III (rare) involves cerebellar herniation into a high cervical/occipital encephalocele—associated with high mortality. Type IV involves cerebellar hypoplasia (some classifications now exclude this as a true Chiari malformation). Diagnosis is via MRI showing tonsillar position relative to foramen magnum and evaluation for associated syrinx or hydrocephalus. Management ranges from observation to posterior fossa decompression surgery for symptomatic cases, especially with progressive syringomyelia or brainstem compression. Type II often requires myelomeningocele repair and ventriculoperitoneal shunting for hydrocephalus.