Chiari malformation is an abnormality in the lower part of the brain called the cerebellum. There are several different forms. In normal anatomy, the cerebellar tonsils are located just above this line called the foramen magnum. But in an individual with Chiari, the tonsils hang below the line (herniates) into the spinal canal. The degree to which the tonsils extend can vary tremendously.

The most common symptom of Chiari malformation is a headache, which begins at the back of the head (neck) and radiates upward.Visual problems such as nystagmus (involuntary eye movements), double or blurred vision may occur. Balance difficulties, vertigo and dizziness also may be present. Some people may have cranial nerve compression.

An operation may be recommended. This is referred to as a posterior fossa decompression. The surgeon makes more room in the back of the head by removing small pieces of the skull bones. This reduces compression of the brain stem and allows the tonsils to move back into their natural position. The specific surgical techniques will vary among surgeons; no consensus yet exists on the best variation on this surgical procedure.

How are they classified?

CMs are classified by the severity of the disorder and the parts of the brain that protrude into the spinal canal.

Type I involves the extension of the cerebellar tonsils (the lower part of the cerebellum) into the foramen magnum, without involving the brain stem. Normally, only the spinal cord passes through this opening. Type I — which may not cause symptoms — is the most common form of CM and is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition. Type I is the only type of CM that can be acquired.

Type II, also called classic CM, involves the extension of both cerebellar and brain stem tissue into the foramen magnum. Also, the cerebellar vermis (the nerve tissue that connects the two halves of the cerebellum) may be only partially complete or absent. Type II is usually accompanied by a myelomeningocele — a form of spina bifida that occurs when the spinal canal and backbone do not close before birth, causing the spinal cord and its protective membrane to protrude through a sac-like opening in the back. A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening. The term Arnold-Chiari malformation (named after two pioneering researchers) is specific to Type II malformations.

Type III is the most serious form of CM. The cerebellum and brain stem protrude, or herniate, through the foramen magnum and into the spinal cord. Part of the brain’s fourth ventricle, a cavity that connects the upper parts of the brain and circulates CSF, may also protrude through the hole and into the spinal cord. In rare instances, the herniated cerebellar tissue can cause an occipital encephalocele, a pouch-like structure that protrudes out of the back of the head or the neck and contains brain matter. The covering of the brain or spinal cord can also protrude through an abnormal opening in the back or skull. Type III causes severe neurological defects.

Type IV involves an incomplete or underdeveloped cerebellum — a condition known as cerebellar hypoplasia. In this rare form of CM, the cerebellar tonsils are located further down the spinal canal, parts of the cerebellum are missing, and portions of the skull and spinal cord may be visible.

Another form of the disorder, under debate by some scientists, is Type 0, in which there is no protrusion of the cerebellum through the foramen magnum but headache and other symptoms of CM are present.