Chiari malformation type III (CM-III)

What it is

Chiari malformation type III (CM-III) is a rare and severe form in which hindbrain structures (such as the cerebellum, and sometimes the brainstem) herniate into a sac — an encephalocele — at the back of the skull or upper neck. It is among the most serious forms of Chiari malformation.

Understanding Chiari malformation type III (CM-III)

In CM-III, brain tissue protrudes through a defect at the back of the skull or high cervical spine into a sac (encephalocele), often along with other significant brain abnormalities and hydrocephalus. It causes severe, often life-threatening neurological problems that appear early in life, and children affected may have developmental difficulties and seizures. CM-III is very rare and requires highly specialized care; treatment often involves surgery, and the outlook is serious. Because of its severity, it is identified in infancy. The care team approaches these difficult situations with honesty and compassion, supporting the child and family. CM-III is distinct from the far more common and usually milder type I.

Why it matters

Understanding CM-III helps families see that it is a rare and severe form, very different from the common type I, requiring specialized, compassionate care — and that its severity and early onset distinguish it sharply from milder Chiari malformations.