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A Chiari malformation is a change in the normal relationships of the structures of the brain and spinal cord at the base of the skull where brain and spinal cord meet. There are four types of Chiari malformations, but really only the first two are seen with any frequency by neurosurgeons and other physicians:
- Type I: The bottom part of the cerebellum (the cerebellar ?tonsils?) protrudes through the skull base into the upper cervical spinal canal
- Type II: The midline structures of the cerebellum along with the brain stem itself protrude downward through the skull base into the spinal canal
- Type III: Portions of the cerebellum protrude out of an opening in the back of the skull area and forms a pouch on the back of the neck
- Type IV: The cerebellum fails to develop completely
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The cause of the type I malformation is unclear. For years, it was thought that these Chiari malformations were congenital and that the patient was born with the malformation. As MRI imaging has become more commonplace, it has been observed that, in some children, the malformation can develop over time and many of us now think that the Chiari malformation develops because of a mismatch between the rate of growth of the skull and the rate of growth of the brain ? so that the bottom of the cerebellum actually gets pushed out the base of the skull into the spine canal over time. There are occasional cases of familial Chiari I (father-daughter, mother-son, etc.) and the malformation also occurs with some frequency in children with syndromes where the bones of the skull fuse together too rapidly, limiting the amount of space in the skull for the developing brain tissue.
In the Chiari II malformation, the cause is felt to be entirely different. These children almost all have a congenital defect of the spinal cord (spina bifida, myelomeningocele) in which the infant is born with an open and unprotected spinal cord at the bottom of the spine. For reasons probably related to the loss of cerebrospinal fluid through the back opening during fetal development, the back of the brain and brain stem do not expand and grow normally and, therefore, significant alterations in the brain and spinal cord structure referred to above are indeed present at birth.
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Chiari I:
One of the primary symptoms of this condition is headache. The headache has a typical or ?classical? presentation ? it occurs or is aggravated by any activity which temporarily increases pressure in the brain, is usually fairly brief in duration, and is located in the back of the head. Very young children may appear to be in discomfort in the midst of vigorous play; they will stop their activities, lie down or rest, and then resume what they had been doing moments before. Older children may notice head pain with cough or sneeze, bending over, strenuous physical activities, or straining to have a bowel movement. Occasionally, the head pain is long lasting, and not as typical as that described above.
One of the more common presentations of this condition is an unexplained curvature of the spine: scoliosis. Scoliosis will occur in Chiari malformation because of the development of syringomyelia (build-up of fluid inside the spinal cord) in these patients. The reasons for the development of this process continue to be debated, but many of us believe that it is due to interference with normal circulation of fluid around the brain and spinal cord. In brief, the fluid which bathes and protects the brain and spinal cord is constantly made inside the brain and has to pass outside the brain beneath the cerebellum to circulate over the brain and spinal cord. If there is a tight Chiari malformation at the skull base, this fluid cannot circulate normally and then gets diverted into the center of the spinal cord. When the fluid builds up and causes syringomyelia, patients develop a spine curvature because of the harmful effect of the fluid?s pressure on the nerve cells inside the spinal cord which can weaken the spinal muscles in an asymmetric manner leading to unequal pull of the muscles on the spinal column. If the fluid pressure distends the spinal cord enough, other functions of the spinal cord can get affected as well; leading to arm or hand weakness, numbness, balance disturbances, and so on.
Chiari II:
The symptoms of a Chiari II malformation may be quite different. Although patients may have headache just like the Chiari I patients and develop syringomyelia-like complaints, more frequently they have problems with the function of the nerves that leave the brain stem because of their downward displacement with the rest of the brain stem or because of direct pressure. Typical symptoms include weakness of vocal cords, swallowing difficulties, breathing irregularities, and other obviously serious changes in the function of nerves of the throat, face, and tongue.
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Although the diagnosis can be inferred from skull x-rays and CT scans, the most definitive and accurate test is an MRI of the brain. There are varying degrees of seriousness of the malformation and families should understand that radiologists will make this diagnosis as soon as they see any downward displacement of the cerebellum that exceeds 5 to 6 millimeters. The diagnosis of Chiari I malformation is frequently made in the absence of any demonstrable clinical symptoms and it is a very frequent incidental finding. If there are symptoms of headache, other neurological dysfunction, or a syringomyelia is detected, the patient should be assessed by a neurosurgeon to determine if surgery should be carried out for the malformation.
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During surgery, the neurosurgeon attempts to relieve the pressure on the structures at the base of the skull and reestablish normal circulation of the fluid in the area. Because the configuration and pressure of the bone is one of the most important causes of the compression, the neurosurgeon removes about an inch-high block of bone from the lowest part of the skull at the skull base to widen the room for the structures there; the back of the first spinal vertebra is virtually always removed as well. Removing this bone will expose the dura (the firm covering of the brain and spinal cord tissues here) and many neurosurgeons will then use an ultrasound probe to visualize these tissues to see if they have been decompressed enough by just the bone removal alone. Unfortunately, because the dura usually very tightly adheres to the bone over it and has the same configuration, we have found in our practice that just taking the bone off alone rarely deals with the pressure beneath and the dura has to be opened and widened out by a patch of some type to increase the room for the fluid to circulate around the area. We will often reduce the bulk of the lower part of the cerebellum by shrinking the tissue with electrical forceps; this maneuver causes no neurological damage and effectively reduces the bulk of this otherwise non-essential cerebellar tissue. Finally, if the patient has a syringomyelia, we inspect the opening under the cerebellum where the cerebrospinal fluid normally exits to make sure that it is now clear of scar and that fluid can circulate normally through the area. In certain cases, a small tube made of silastic may be placed into this opening, like a doorstop, to keep it open.
Surgery for a Chiari II malformation is carried out in a slightly different fashion because of the radical changes in the shape of the bones in the back of the head in these children and because the anatomy of the underlying malformation is vastly different. But the over-all essential goals of the procedure are the same ? to relieve the focal pressure on the neural tissues at the base of the skull and to reestablish normal fluid circulation through and around the area. The surgery for the Chiari II malformation is more difficult to carry out and the normal anatomy more difficult to discern.
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The child should be examined by a neurologist or neurosurgeon to determine that there are no abnormalities on neurological exam and to also assess the severity of the Chiari malformation that has been detected. If the malformation is minimal and there are no symptoms, we may recommend a follow-up evaluation by MRI and examination in a few years or no further follow-up at all. If the malformation is pronounced and the child is young, follow-up may be more frequent, both clinically and by MRI. In other words, this is an individualized decision based on why the study was done, what the child?s examination is like, and how severe the malformation is on the MRI study.
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If there is a sizeable syrinx present when a Chiari malformation is diagnosed, we almost always recommend surgery. Enlargement of the syrinx may lead to neurological deficits that often cannot be reversed. If scoliosis is present, its progression to a curve greater than 35 degrees may mean that spinal fusion will become necessary. We try to get surgery done before curves worsen in order to lessen the likelihood of the need for a spinal fusion later on.
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Our data show that the syrinx will be virtually eliminated in 50% of patients, substantially reduced in another 45%, and stabilized in the remainder. No operation is always successful, but this is one of the most successful operations that pediatric neurosurgeons do and we can be optimistic with the patients and families about its outcome.
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This is an individualized decision and many of our surgeons use different materials. The most frequently used material, alloderm tm, is a processed skin matrix from which the cells have been removed; leaving behind a network of collagen which becomes incorporated into the patient?s own tissues. It has the advantage of sealing the dural opening well and being easy to sew to and can be trimmed to whatever size the surgeon needs. In certain patients, we may use their own tissues: harvesting tissue from the outer layer of their skulls (pericranium) or from supportive tissue of muscle (fascia) ? these tissues have the disadvantage of creating scar and promoting bone growth; sometimes a disadvantage in young children, but the advantage of being from the patient?s body.
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Our published data indicate that this is a rare event, but will more frequently occur in the child who is operated on under the age of 6 in whom the skull bone can regrow and recompress the area with time or in whom there are other associated causes of the malformation, as in some type of skull fusion syndromes.
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Depending on what needs to be done, the surgery and operating time can take from 2 to 4 and half hours. The children are in the hospital an average of 4 and one-half days, including the day or surgery, but this is also variable: depending on the patient, their state of health, and any other associated medical conditions.
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The children are usually awake and talking right after the operation, but will have headache, neck pain, and nausea for usually 48 hours after the surgery. We keep the children comfortable with medications to deal with each of these problems and the children usually begin to eat and get out of bed one to two days after the operation.
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No head protection is needed, since there are strong and thick muscles at the base of the back of the neck that protect the area. Many of us allow our postoperative patients to participate in all activities including football, lacrosse, soccer, etc.
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We have children with this condition on our surgical schedule every week and the staff here ? including anesthesia, nursing, and neurosurgical personnel? are totally familiar with the procedure and its pre- and post-operative management.
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The typical Chiari I malformation patient virtually never has a tethered spinal cord and we do not believe that such surgical explorations are warranted in the vast majority of patients. In children with Chiari II malformations, however, there are often many other causes of the typical Chiari symptoms which merit investigation including assessment of ventricular shunt function (often placed in these children early in life to treat hydrocephalus), evaluation of true tethering in these children with previously repaired spinal cord malformations in their lower backs, etc.
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If you have any questions, please contact us for more information.
R. Michael Scott, M.D.
Edward R. Smith, M.D.
Liliana C. Goumnerova, M.D.
Joseph R. Madsen, M.D.
Mark R. Proctor, M.D.
Children's Hospital Boston
Department of Neuroseurgery
300 Longwood Avenue
Boston, MA 02115
Phone
617-355-6008
Email
Michael Scott
Edward Smith
Liliana Gourmnerova
Joseph Madsen
Mark Proctor
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Chiari Malformation
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