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Chiari malformation is considered a congenital condition, although acquired forms of the condition have been diagnosed. Chiari malformations are often detected coincidently among patients who have undergone diagnostic imaging for unrelated reasons.
Types of Malformations
Type I Chiari Malformation
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.
Abnormalities of the base of the skull and spine are seen in 30-50 percent of patients with Chiari I malformation. These include:
Type I Chiari Malformation Symptoms
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).
Type II Chiari Malformation
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 (spina bifida). 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 (fluid in the brain), cardiovascular abnormalities (heart problems), imperforate anus (incomplete or no anus) as well as other gastrointestinal abnormalities.
Type II Chiari Malformation Symptoms
The symptoms associated with a Chiari II malformation can also be caused by problems related to myelomeningocele and hydrocephalus. These symptoms include:
Type III Chiari Malformation
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.
Syringomyelia / Hydromyelia
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 developing, 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.
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. 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. Surgery is only contemplated if the patient is significantly affected by symptoms. There are several surgical approaches advocated by different centers.
After having operated on hundreds of patients over 20 years, we have refine our approach as follows:
We have found that the aforementioned approach has given the best overall results in terms of symptoms resolution. The goal is to adequately reestablish the flow of cerebrospinal fluid through the skull base and into the spinal canal. Our research (published) shows that there is marked elevation of the brain pressure in the posterior fossa and that only by enlarging the skull cavity, can the pressure be normalized.
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. 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. Decompression is performed under general anesthesia. It consists of removing the back of the foramen magnum and often the back of the first few vertebrae to the point where the cerebellar tonsils end. This provides more space for the brainstem, spinal cord, and descended cerebellar components.
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 divert the fluid from inside the spinal cord to outside the cord, or be directed to either the chest or abdominal cavity. These procedures can be done together or separately.
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 goal of Chiari surgery is:
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.