Arnold-Chiari malformation, also known as type II Chiari malformation (type II CM) or classic CM, is a structural defect in the skull base that results in the brainstem and cerebellum extending below the foramen magnum into the cervical spinal canal. Chiari defects are either primary (congenital) or secondary (due to trauma, disease, or infection). They are categorized by severity from grade I (asymptomatic to mildly symptomatic) to IV (most severe). The prevalence of CMs is approximately 1 in 1000 with the majority of cases (type I CM) being asymptomatic. Higher grades of Chiari malformations are associated with more severe symptoms. Chiari I and II malformation are more common in females than in males. The focus of this summary is type II CM.

Type II CM is usually diagnosed in infants and young children. It is almost always associated with myelomeningocele (spina bifida). Manifestations in infants include dysphagia and difficulty feeding, stridor, episodes of apnea, and extremity weakness due to brainstem dysfunction. Manifestations in childhood include progressive hydrocephalus and syringomyelia; scoliosis may occur.

Common complaints in children include severe head and neck pain, occipital headache, blurred vision, dizziness, balance issues, and spasticity. Type II CM often results in the development of a syrinx (a fluid-filled cavity in the spinal cord). More severe symptoms include limb weakness, changes in breathing patterns (apnea), or abnormal eye movements (nystagmus).