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What are Chiari malformations?
What causes Chiari malformations?
How are Chiari malformations classified?
Who is at risk?
What conditions are associated with Chiari malformations?
What are the symptoms of Chiari malformations?
How are Chiari malformations diagnosed?
How are Chiari malformations treated?
What is the prognosis?
What are Chiari malformations?
Chiari malformations are structural defects in the cerebellum, the part of the brain that controls balance. Normally the cerebellum and parts of the brainstem sit in an indented space at the lower part of the skull, above the foramen magnum, which is a funnel-like opening that leads to the spinal canal). When part of the cerebellum is located below the level of the foramen magnum, the anomaly is called a Chiari malformation.
Chiari malformations may develop when the bony space is smaller than normal, causing the cerebellum and brainstem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brainstem may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF) -- the clear liquid that surrounds and cushions the brain and spinal cord -- to and from the brain.
What causes Chiari malformations?
Chiari malformations have several different causes. They can be caused by structural defects in the brain and spinal cord that occur during fetal development, whether caused by genetic mutations or lack of proper vitamins or nutrients in the maternal diet. This is classified as primary or congenital. It can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances, or infection. This is called secondary or acquired Chiari malformation. The former is much more common than the later type.
How are Chiari malformations classified?
Chiari malformations are classified by the severity of the disorder and the parts of the brain that protrude into the spinal canal.
Type I involves the extension of the lower part of the cerebellum, called the cerebellar tonsils, into the foramen magnum, without involving the brainstem. This type, which may not cause symptoms in many individuals, is the most common form of Chiari malformation and is usually first noticed in adolescence or adulthood, often by accident during an examination for another condition.
Type II, also called Arnold-Chiari malformation, involves the extension of both cerebellar and brainstem tissue into the foramen magnum. In addition, the cerebellar vermis (the nerve tissue that connects the two halves of the cerebellum) may be only partially developed or absent. Type II is usually accompanied by a myelomeningocele, which is a form of spina bifida that occurs when the spinal canal and backbone do not close before birth, causing the spinal cord and its protective membrane to protrude through a sac-like opening in the back. A myelomeningocele usually results in partial or complete paralysis of the area below the spinal opening.
Type III is the most serious form of Chiari malformation, where the cerebellum and brainstem protrude through the foramen magnum and into the spinal canal, causing severe neurologic defects. Part of the brain’s fourth ventricle, a cavity that connects the upper parts of the brain and circulates CSF, may also protrude through the hole and into the spinal canal. In rare instances, the protruded cerebellar tissue can cause an occipital encephalocele, a pouch-like structure that protrudes out of the back of the head or the neck and contains brain matter. The covering of the brain or spinal cord can also protrude through an abnormal opening in the back or skull.
Type IV Chiari malformation involves an incomplete or underdeveloped cerebellum -- a condition known as cerebellar hypoplasia. In this rare form of the disorder, the cerebellar tonsils are located further down the spinal canal, parts of the cerebellum are missing, and portions of the skull and spinal cord may be visible.
In the past, it was estimated that this condition occurs in about one in every 1,000 births. However, the increased use of diagnostic imaging has shown that Chiari malformation may be much more common. Complicating this estimation is the fact that some children who are born with the condition may not show symptoms until adolescence or adulthood, if at all. Chiari malformations occur more often in women than in men and Type II malformations are more prevalent in certain groups, including people of Celtic descent.
What conditions are associated with Chiari malformations?
Individuals who have a Chiari malformation often have other related conditions which are listed below.
Hydrocephalus is an excessive buildup of CSF in the brain as a result of blockage of the normal flow of this fluid, resulting in pressure within the head that can cause mental defects and/or an enlarged skull (in infants). Severe hydrocephalus, if left untreated, can be fatal. The disorder can occur with any type of Chiari malformation, but is most commonly associated with Type II.
Spina bifida is the incomplete development of the spinal cord and/or its protective covering. In this disorder, the bones around the spinal cord don’t form properly, leaving part of the cord exposed and resulting in partial or complete paralysis. Patients with Type II Chiari malformation usually have a myelomeningocele, a form of spina bifida in which the bones in the back and lower spine don’t form properly and the spinal cord extends out of the back in a sac-like opening.
Syringomyelia, or syrinx, is a disorder in which a CSF-filled tubular cyst forms within the spinal cord’s central canal. The growing syrinx destroys the center of the spinal cord, resulting in pain, weakness, and stiffness in the back, shoulders, arms, or legs. Other symptoms may include headaches and a loss of the ability to feel extremes of hot or cold, especially in the hands. Some individuals also have severe arm and neck pain.
Tethered spinal cord syndrome occurs when the spinal cord attaches itself to the bony spine. This progressive disorder causes abnormal stretching of the spinal cord and can result in permanent damage to the muscles and nerves in the lower body and legs. Children who have a myelomeningocele have an increased risk of developing a tethered cord later in life.
Abnormal spinal curvatures are also common among patients with syringomyelia or Chiari malformation Type I. Two types of spinal curvature can occur in conjunction with Chiari malformations: scoliosis (a bending of the spine to the left or right) and kyphosis (a forward bending of the spine). Spinal curvature is seen most often in children with Chiari malformation whose skeleton has not fully matured.
Chiari malformations may also be associated with certain hereditary syndromes that affect neurological and skeletal abnormalities, other disorders that affect bone formation and growth, fusion of segments of the bones in the neck, and extra folds in the brain.
What are the symptoms of Chiari malformations?
Many persons with a Type I Chiari malformation do not have symptoms and may not know they have the condition. Patients with other types of the disorder may complain of neck pain, balance problems, muscle weakness, numbness or other abnormal feelings in the arms or legs, dizziness, vision problems, difficulty swallowing, ringing in the ears, hearing loss, vomiting, insomnia, depression, or headache that are worsened by coughing or straining. Hand coordination and fine motor skills may also be affected. Symptoms may change for some patients, depending on the buildup of cerebrospinal fluid and the resulting pressure on the tissues and nerves. Adolescents and adults who have Chiari malformations but no symptoms initially may develop signs of the disorder later in life. Infants may have symptoms from any type of Chiari malformation and may have difficulty swallowing, irritability when being fed, excessive drooling, a weak cry, gagging or vomiting, arm weakness, a stiff neck, breathing problems, developmental delays, and an inability to gain weight.
How are Chiari malformations diagnosed?
Many people with Chiari malformations have no symptoms and their malformations are discovered only during the course of diagnosis or treatment for another disorder. The physician will perform a physical exam and check the patient’s memory, cognition, balance (a function controlled by the cerebellum), touch, reflexes, sensation, and motor skills (functions controlled by the spinal cord). One of the following diagnostic tests may also be ordered:
In addition to a neurologic exam, imaging modalities such as computed tomography (CT) or magnetic resonance imaging (MRI) can be used in the diagnosis of Chiari malformations and its associted conditions.
How are Chiari malformations treated?
Some Chiari malformations are asymptomatic and do not interfere with a person’s activities of daily living. In other cases, medications may ease certain symptoms, such as pain.
Surgery is the only treatment available to correct functional disturbances or halt the progression of damage to the central nervous system. Most patients who have surgery see a reduction in their symptoms and/or prolonged periods of relative stability. More than one surgery may be needed to treat the condition.
Posterior fossa decompression surgery is performed on adult patients with this disorder to create more space for the cerebellum and to relieve pressure on the upper spinal column. Surgery involves making an incision at the back of the head and removing a small portion of the bottom of the skull to correct the irregular bony structure. A related procedure, called spinal laminectomy, involves the surgical removal of part of the bony roof of the spinal canal (the lamina) to increase the size of the spinal canal and relieve pressure on the spinal cord and nerve roots.
The surgeon may also make an incision in the dura (the covering of the brain) to examine the brain and spinal cord. A graft may be added to the dura to create more space for the flow of CSF. The neurosurgeon may also use a procedure called electrocautery to shrink the cerebellar tonsils. This surgical technique involves destroying tissue with high-frequency electrical currents, but often does not cause any long-term sequelae.
Infants and children with myelomeningocele may require surgery to reposition the spinal cord and close the opening in the back.
Hydrocephalus may be treated with a shunt system that travels underneath the skin from the head to the abdomen and drains excess CSF from the brain into the abdominal cavity where it is absorbed. Similarly, the surgeon may open the spinal cord and insert a shunt to drain a syringomyelia, although in some cases the syrinx resolves spontaneously over the following several months, without the need for surgical intervention, when the Chiari malformation is repaired.
Following proper surgical repair of Type I Chiari malformation, most patients will improve or see a complete resolution of their symptoms over the following few months. On occasions some patients will have recurrence of their symptoms at some point after surgery and this may be due to formation of scar tissue at the site of the repair, leading to blockage of proper CSF flow. In such situations, further surgery may be needed to remove the scar tissue.
