Chiari Malformation

Chiari malformation is an anomaly that encompasses four separate development malformations. While these malformations may present at any point in life, they have a very particular set of associated symptoms. These malformations along with syringomyelia, a distinct yet closely associated condition, are described below.

Anatomy

The cerebellum is normally located at the base of the skull, in what is referred to as the posterior fossa. Usually, the cerebellum is composed of two lateral halves, or hemispheres, and a narrow central portion between these hemispheres, known as the vermis. Along the under surface of the hemispheres, there are two small protrusions called the tonsils. The fourth ventricle is a space filled with cerebrospinal fluid (CSF) located in front of the cerebellum that communicates with the central canal of the spinal cord. All of these structures are located inside the skull above the foramen magnum, which is the largest opening at the base of the skull through which the spinal connects to the brain. The cerebellum normally controls coordination of movement.

Types of Malformations

Syringomyelia: refers to a cavitation, or hollowing out, of the center of the spinal cord. While "syringomyelia" is commonly associated with Chiari malformations, the use of this word is not entirely appropriate. More often the spinal cord cavitation is actually a hydromyelia. Hydromyelia, in contrast to syringomyelia, is an expansion of the spinal canal secondary to increased spinal fluid pressure within the canal. With hydromyelia, there is no loss of normal spinal cord tissue but rather compression from the inside of the cord. In either case, spinal cord function can be significantly affected.

Type I Chiari malformations: are characterized by downward displacement of the tonsils of the cerebellum through the foramen magnum into the upper portions of the spinal canal. This form of Chiari malformation is associated with hydromyelia and usually presents in adolescents or adults. In adolescents, the main symptoms are due to this hydromyelia - a flexed paralysis of the arms, loss of pain and temperature sensation of the upper torso and arms, as well as loss of muscle strength in the hands and arms. Adults typically complain of an occipital headache felt at the base of the skull that is made worse by coughing. They also experience neck and arm pain, sometimes in a cape-like distribution.

Type II Chiari malformations: are characterized by displacement of the cerebellar vermis, tonsils, the fourth ventricle, and the medulla, which is a section of the brainstem, through the foramen magnum. Type II, also known as Arnold-Chiari malformation, is more highly associated with hydromyelia than Type I Chiari malformations and is almost always associated with myelomeningocele. A meningomyelocele is a congenital failure of the closure of the spinal cord and column in the lumbar region during fetal development. Other associated abnormalities include hydrocephalus (increased fluid pressure within the skull), cardiovascular abnormalities, imperforate anus as well as other gastrointestinal abnormalities, and genitourinary abnormalities. Symptoms of this malformation are usually obvious and present shortly after birth and may include brief periods of cessation of breathing; depressed gag reflex; involuntary, rapid, downward eye movements; and loss of arm strength.

Type III Chiari malformations: consist of displacement of the cerebellum and portions of the brainstem into a meningocele. In this case, the meningomyelocele involves the occiput or cervical spine. The tissue surrounding the brain and spinal cord protrude into the meningocele.

Type IV Chiari malformations: are associated with an underdeveloped cerebellum. Both Type III and IV Chiari malformations are rare.

Cause

While the cause of Chiari malformation is not exactly known, several potential explanations exist. Most theories indicate that an increase in pressure above the cerebellum will result in its displacement. The cerebellum is forced downward into the foramen magnum not unlike a cork into the neck of a bottle. Magnetic resonance imaging (MRI) allows physicians to accurately view the cerebellum and the associated structures and determine what form and to what extent the malformation has progressed.

Surgical Treatments

There are many ways to treat Chiari malformations, but all require surgery. Surgical therapy for Chiari malformation consists of either local decompression or diversion of CSF pressure. Decompression is performed under general anesthesia and consists of removing the very base of the back of the skull as well as the back of the first few vertebrae to the point where the cerebellum descends. This effectively widens the opening of the foramen magnum to provide more space for the brainstem, spinal cord, and descended cerebellar components. The dura mater, a thick membrane surrounding the brain and spinal cord, is also opened. A tissue graft is often spliced into this opening to provide even more room for the unimpeded passage of CSF. Occasionally, the cavity within the spinal cord resulting from hydromyelia can be drained with a diverting shunt tube. This tube can be directed from the spinal cord cavity to either the chest wall or the abdominal wall. These procedures can be be done together or at separate sittings.

Copyright© 1998-2003; American Association of Neurological Surgeons /

Congress of Neurological Surgeons