Arnold-Chiari malformation type II is a rare, central nervous system malformation characterized by caudal displacement of the cerebellum, pons, medulla and fourth ventricle through the foramen magnum into the spinal canal, and is typically associated with myelomeningocele. Variable other central nervous system abnormalities might be present (partial or complete agenesis of the corpus callosum, a small fourth ventricle, obstructive hydrocephalus, falx and tentorium defects, and polygyria). Symptoms include hypotonia, apnea with cyanosis, dysphagia, opisthotonus, nystagmus, spasticity, ataxia, and occipital headache. The disease is Chiari malformation type II (Monarch Disease Ontology term MONDO_0008816). Also known as: Arnold-Chiari malformation type 2, Arnold-Chiari malformation type II, Chiari malformation type 2.