Arnold-Chiari malformation

**Definition:** Group of congenital structural defects of the posterior fossa and hindbrain causing downward displacement of cerebellar tissue (± brainstem) through the foramen magnum. Classically attributed to a small or underdeveloped posterior fossa, which crowds the cerebellum and brainstem.

**Classification (high-yield types):** - **Type I:** Caudal displacement of the cerebellar tonsils ≥5 mm below the foramen magnum. Usually isolated. Often presents in adolescence or adulthood, and may be found incidentally on MRI. - Associated with **syringomyelia** (cervical syrinx), which causes a cape-like loss of pain and temperature sensation with preserved touch and vibration (dissociated sensory loss) and hand muscle atrophy or weakness. - Symptoms: suboccipital headache worsened by Valsalva, cough, or straining. Also neck pain, dizziness, and lower cranial nerve or brainstem signs (dysphagia, sleep apnea, downbeat nystagmus). - Associations: skeletal anomalies (basilar invagination, Klippel-Feil syndrome), and a possible link to idiopathic intracranial hypertension. - **Type II (classic "Arnold-Chiari"):** Herniation of the cerebellar **vermis and tonsils**, plus a **caudally displaced medulla and a kinked or elongated fourth ventricle**. Presents at birth or in infancy. - Nearly always associated with **lumbosacral myelomeningocele** and **hydrocephalus** from aqueductal stenosis or obstruction of CSF outflow. - Features: beaked tectum, lacunar skull (Lückenschädel), small posterior fossa, and a low-lying torcula. - Infant symptoms: stridor, apnea, dysphagia, aspiration, and vocal cord paralysis from brainstem and cranial nerve compression. - **Type III:** Rare and severe. Occipital or high cervical encephalocele containing cerebellar tissue (± brainstem), with a high mortality rate. - **Type IV:** Cerebellar hypoplasia without herniation. This type is rarely used in current classification.

**Pathophysiology:** Type II is thought to result from CSF leakage through the open neural tube defect in utero. Reduced distending pressure leads to a small posterior fossa, which then forces the hindbrain downward. Failure of neural tube closure is linked to **folate deficiency**, so maternal folic acid supplementation lowers risk. Folic acid is recommended before conception and in early pregnancy. Valproate and carbamazepine are also associated with neural tube defects.

**Diagnosis:** - **MRI** of the brain and cervical spine is the gold standard. It shows tonsillar descent and any syrinx. - Prenatal clues: elevated maternal serum AFP, and ultrasound showing the "lemon sign" (frontal bone scalloping) and "banana sign" (cerebellar curving).

**Management:** - Asymptomatic Type I: observation. - Symptomatic Type I: **posterior fossa decompression** (± duraplasty). - Type II: surgical closure of the myelomeningocele and a **ventriculoperitoneal shunt** for hydrocephalus. Decompression is considered for brainstem compromise.

**Boards pearls:** - Chiari II + myelomeningocele + hydrocephalus in a newborn is the classic vignette. - Chiari I + syringomyelia in a young adult with a cape-like sensory loss is the other classic vignette. - **Dandy-Walker malformation** is a different posterior fossa anomaly: cystic dilation of the fourth ventricle, vermian agenesis or hypoplasia, an enlarged posterior fossa, and hydrocephalus. Don't confuse it with Chiari, where the posterior fossa is small. - Syringomyelia damages the anterior white commissure (spinothalamic fibers crossing), so pain and temperature are lost first.