A Chiari malformation is a structural defect in the cerebellum, the part of the brain that controls balance and motion. Another name for CM is tonsillar brain herniation, in which the brain is squeezed across structures in the brain in response to raised intracranial pressure, traumatic injury, brain tumor, or intracranial hemorrhage.
In CM, the lower part of the brain is forced through the foramen magnum, which translates from the Latin as "big hole." A number of structures pass through the foramen magnum, including the lower brain stem, intracranial arteries, membranes, and ligaments.
There are four types of CM, each with a different cause and prognosis. While some are completely asymptomatic and discovered only incidentally to another condition, others are life-threatening.
In Type I Chiari malformation is the most common. Here, the lower part of the cerebellum, the cerebellar tonsils, protrude through the foramen magnum into the spinal cord. This believed to be caused by an abnormally small posterior fossa, a small space in the skull next to the cerebellum and brain stem.
Although this lesion is present from birth, people often do not develop symptoms until adult life. Headaches and neck pain are the most common.
Individuals with Type I Chiari malformation sometimes develop syringomyelia, hollow spaces within the spinal cord that become filled with cerebral spinal fluid, causing damage to the spinal cord. Symptoms may include pain, stiffness, muscle weakness, bowel or bladder dysfunction, or paralysis.
Treatment of Chiari Type I depends on the seriousness of the symptoms. Headaches may be treated with analgesics. If other, more serious, symptoms are present, surgery may be attempted to decompress the lesion.
Type II, or classic Chiari malformations, involve both cerebellar and brain stem tissue extrusion through the foramen magnum. The cerebellar vermis, which connects the two halves of the cerebellum, may also be incomplete or even absent. Type II is usually accompanied by a type of spina bifida that occurs when the backbone and spinal canal fail to close during fetal development. The term Arnold-Chiari malformation applies specifically to Type II lesions and is named after the two surgeons who first described it.
Symptoms of Type II may include head and neck pain, dizziness and balance problems, difficulty swallowing, visual problems including nystagmus, tinnitus and hearing loss, numbness and tingling in the arms and legs. Headaches may be made worse by sneezing, coughing, or bending over. Some people experience nausea and vomiting.
This is the most serious form of CM. Here, the brain stem and cerebellum are extruded through the FM into the spinal cord. The fourth ventricle, a brain cavity that contains CSF and connects with the upper part of the brain, may also be forced down into the spinal cord. The prognosis for Type III Chiari malformation is poor.
Type IV is a rare form of Chiari malformation characterized by an underdeveloped or incompletely formed cerebellum. In this condition, the cerebellar tonsils are in their normal location, but parts of the cerebellum are missing, revealing parts of the skull and spinal cord.
Serious forms of brain herniation may present with a type of abnormal body position called decorticate posturing, in which the fingers, wrists and elbows are flexed and the legs extended and rotated inward. Glasgow Coma Score is usually between three and five. One or both pupils may be completely dilated and unresponsive to light. Patients may also vomit as a result of compression of vomiting centers in the medulla.