Arnold-Chiari malformation is a complex congenital malformation of the hindbrain that is often associated with myelomeningocele. The deformity is characterized by downward displacement of the medulla, fourth ventricle, and cerebellum into the spinal canal. The pons and fourth ventricle are elongated, which is believed to be the result of the relatively small posterior fossa. It is the most common, serious malformation of the posterior fossa. Arnold-Chiari is also known as Chiari II malformation (CM II).

Recent estimates are that the frequency of this malformation in the United States is approximately 1:1000.

This congenital anomaly is a complex entity that involves the skull, dura, brain, spine, and spinal cord. The association with myelomeningocele is almost 100%. Several theories have been advocated to explain the development of the Arnold-Chiari malformation.

The various theories can be classed into two schools of thought. The first ascribes the malformation to mechanical factors, and the second finds the cause to be abnormal embryologic development. The “mechanical forces school” uses theories of hydrodynamics and traction. The “embryologic school” includes theories on developmental arrest, primary dysgenesis, small posterior fossa, hindbrain overgrowth, neuroschisis, and neurulation abnormalities. One of the simpler and widely accepted theories suggests that the condition arises when a cerebellum of normal size or proportion develops in a posterior fossa that is abnormally small with a low tentorial attachment.

Arnold-Chiari Malformation. Comparison of (A) normal brain and (B) Arnold-Chiari type II malformation.