Chiari malformation is considered a congenital condition, although acquired forms of the condition have been diagnosed. In the 1890s, a German pathologist, Professor Hans Chiari, first described abnormalities of the brain at the junction of the skull with the spine. He categorized these in order of severity; types I, II, III and IV. The term "Arnold-Chiari" was latter applied to the Chiari type II malformation. These malformations, along with syringomyelia and hydromyelia, two closely associated conditions, are described below.
The cerebellum controls the coordination of motion and is normally located inside 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 (and behind the brainstem). All of these structures are located just above the foramen magnum, the largest opening at the base of the skull through which the spinal cord enters and connects to the brainstem.
The prevalence in the general population has been estimated at slightly less than one in 1000. The majority of these cases are asymptomatic. Chiari malformations are often detected coincidently among patients who have undergone diagnostic imaging for unrelated reasons.
This malformation occurs during fetal development and is characterized by downward displacement by more than four millimeters, of the cerebellar tonsils beneath the foramen magnum into the cervical spinal canal. This displacement may block the normal pulsations of CSF between the spinal canal and the intracranial space. This form of Chiari malformation may be associated with syringomyelia/hydromyelia. It is diagnosed more commonly in adolescents or adults.
Anomalies of the base of the skull and spine are seen in 30-50 percent of patients with Chiari I malformation. These include
Many people with Chiari I malformation have no symptoms. However, any of the following symptoms may occur, alone or in combination. Some of the symptoms are related to the development of a syrinx (a fluid filled cavity in the spinal cord).
This malformation is characterized by downward displacement of the medulla, fourth ventricle and cerebellum into the cervical spinal canal, as well as elongation of the pons and fourth ventricle. This type occurs almost exclusively in patients with myelomeningocele. Myelomeningocele is a congenital condition in which the spinal cord and column do not close properly during fetal development, resulting in an open spinal cord defect at birth. Other abnormalities associated with myelomeningocele include hydrocephalus, cardiovascular abnormalities, imperforate anus as well as other gastrointestinal abnormalities and genitourinary abnormalities.
The symptoms associated with a Chiari II malformation can also be caused by problems related to myelomeningocele and hydrocephalus. These symptoms include
This malformation includes a form of dysraphism with a portion of the cerebellum and/or brainstem pushing out through a defect in the back of the head or neck. These malformations are very rare and are associated with a high early mortality rate or severe neurological deficits in patients that survive. If treatment is undertaken, then early operative closure of the defect is necessitated. Hydrocephalus, which is commonly present, must also be treated through shunting.
Additional severe birth defects are often present, which may require extensive treatment. Infants with Chiari III malformation may have life-threatening complications.
This malformation is the most severe form and the rarest. The cerebellum fails to develop normally. There may be other associated malformations of the brain and brainstem. Most babies born with this malformation do not survive infancy.
When CSF forms a cavity or cyst within the spinal cord, it is known as syringomyelia or hydromyelia. These are chronic disorders involving the spinal cord, and may be expanding or extending over time. As the fluid cavity expands, it can displace or injure the nerve fibers inside the spinal cord. A wide variety of symptoms can occur, depending upon the size and location of the syrinx. Loss of sensation in an area served by several nerve roots is one typical symptom, as is the development of scoliosis.
Syringomyelia can arise from several causes. Chiari malformation is the leading cause of syringomyelia, although the direct link is not well understood. It is thought to be related to the interference of normal CSF pulsations caused by the cerebellar tissue obstructing flow at the foramen magnum.
This condition can also occur as a complication of trauma, meningitis, tumor, arachnoiditis or a tethered spinal cord. In these cases, the syrinx forms in the section of the spinal cord damaged by these conditions. As more people are surviving spinal cord injuries, increased cases of post-traumatic syringomyelia are being diagnosed.
Hydromyelia is usually defined as an abnormal widening of the central canal of the spinal cord. The central canal, a very thin cavity in the middle of the spinal cord, is a remnant of normal development.
There are several tests that can help diagnose and determine the extent of Chiari malformation and syringomyelia, listed most common to least commonly ordered.
Magnetic resonance imaging (MRI): A diagnostic test that produces three-dimensional images of body structures using magnetic fields and computer technology. It can provide an accurate view of the brain, cerebellum and the spinal cord, is very good at defining the extent of malformations, and distinguishing progression. The MRI provides more information than the CT scan when analyzing the back of the brain and spinal cord, and is usually the preferred test. It can also evaluate the extent of fluid blockage and neural movement at the foramen magnum using CSF flow studies.
Computed tomography scan (CT or CAT scan): A diagnostic test that creates an image by computer reconstruction of x-rays; it is particularly good at defining the size of the cerebral ventricles and showing an obvious blockage. It is most useful for evaluating bony anomalies at the base of the skull and the cervical canal. It is less effective for analysis of the posterior fossa contents or the spinal cord
Sleep study: Involves sleeping overnight in an room where they can monitor breathing, snoring, oxygenation and seizure activity to determine if there is any evidence of sleep apnea.
Swallowing study: Fluoroscopy (XRays) used to watch the internal swallowing process to determine if there is an abnormality suggestive of lower brainstem dysfunction.
Brainstem auditory evoked potential (BAER): An electrical test to examine the function of the hearing apparatus and brainstem connections. This is used to determine if the brainstem is working properly.
Myleogram: An X-ray of the spinal canal following injection of a contrast material into the CSF space; can show pressure on the spinal cord or nerves due to malformations. This test is performed less frequently now.
Treatment of Chiari malformations and syringomyelia is very dependent on the exact type of malformation, as well as progression in anatomy changes or symptoms.
Chiari I malformations that are asymptomatic should be left alone (this involves the majority of Chiari malformations). There is no indication for "prophylactic" surgery on these. If the malformation is defined as symptomatic, or is causing a syrinx, treatment is usually recommended.
Chiari II malformations are treated if the patient is symptomatic, and physicians have determined that there are no complications from hydrocephalus. In some patients, consideration of a tethered cord is also explored. In many infants who become symptomatic from a Chiari II malformation, the symptom onset and progression are severe and rapid, and this requires an urgent or emergency approach.
Surgical treatment of these malformations depends on the type of malformation. The goal of surgery is to relieve the symptoms or stop the progression of the syrinx or symptoms.
There are many different types/extent of surgery based on the extent of compression or other anomalies. Chiari I malformations may be treated surgically with only local decompression of the overlying bones, decompression of the bones and release of the dura (a thick membrane covering the brain and spinal cord) or decompression of the bone and dura and some degree of cerebellar tissue resection. Occasionally some patients will also require cervical spinal fusion.
Chiari II decompression is treated similarly, but is usually restricted to decompressing the tissues in the spinal canal and leaving the back of the skull alone.
The goals of Chiari surgery are
The benefits of surgery should always be weighed carefully against its risks. Although some patients experience a reduction in their symptoms, there is no guarantee that surgery will help every individual. Nerve damage that has already occurred usually cannot be reversed. Some surgical patients need repeat surgeries, while others may not achieve symptom relief.
This information is provided by the American Association of Neurological Surgeons for educational purposes only and not intended to serve as medical advice. For specific medical concerns, consult with a neurosurgeon.