An Overview of Chiari Malformation Type I
& Arnold Chiari Syndrome
There are four types of Chiari Syndrome. The only one that would present beyond birth is type I. Type II is also known as Arnold Chiari Syndrome. Type II is associated with myelomeningeocele and these patients are recognized at birth. Myelomeningeocele patients present at birth with their lower spinal cord out-pouched at the lumbar spine (meningeocele). Types III and IV are severe and not survivable.
Chiari type I was thought to occur in 1 of every 1000 births but with the more ubiquitous use of cervical MRI scans; this disorder appears to be more common. Generally, females outnumber males three to one. Many patients with the Chiari malformation are totally asymptomatic and the disorder is only discovered by imaging studies related to other cervical conditions.
This is really a congenital disorder of the formation of the posterior fossa of the skull. The occipital bone forms the deep “cup” that contains the cerebellum. In Chiari syndrome, the bony cavity that forms is too small to contain this part of the brain and subsequently causes compression of the cerebellum. The parts of the cerebellum that can “squirt out of the way” are the cerebellar tonsils, structures that are almond shaped and found on the sides of the cerebellum. These tonsils lie against the brain stem and are forced into the only exit available, the foramen magnum.
The cerebellar tonsils are pushed into the foramen magnum and crowd out the spinal cord. This crowding can create spinal cord compression, brain stem compression, obstruction of the flow of cerebral spinal fluid (CSF) down the spinal canal and a “water hammer” type effect that can cause an enlargement or syringomyelia of the central canal.
Basilar invagination is a condition associated with occipital bone malformation where the base of the skull “melts” over the upper cervical spine. The top of C2 (the odontoid) then protrudes into the foramen magnum, crowds the brainstem and cerebellum causing similar symptoms as the Chiari malformation.
Compression of the Lower Brain Stem (Pons and Medulla)
When the cerebellar tonsils are pushed into the foramen magnum, the lower brain stem and cranial nerve nuclei can be compressed. This includes the Hypoglossal nerve (CN XII which is a pure motor nerve). The Hypoglossal nerve innervates the tongue and compression affects swallowing and speech. Facial pain or numbness can occur from compression of the Trigeminal nerve (CN V). Problems with breathing can occur from compression of the Medulla and problems with sleep can occur due to compression of the Pons. Sleep apnea can be a result.
General Chiari Malformation Symptoms
The Chiari malformation without syringomyelia can cause neck pain and occipital (base of the skull) headaches with coughing, exertion or sneezing. This is due to the increased intrathecal pressure associated with the ValSalva maneuver (holding the breath and contracting the diaphragm-associated with the above actions). The sneeze increases the congestion at the CSF bottleneck at the foramen magnum, which increases base of skull pain. Weakness, numbness, balance problems and hoarseness can also occur. Possible eventual loss of bowel and bladder control can occur eventually in some cases.
Cerebrospinal Fluid Circulation Blockage/Syringomyelia
One of the major problems with Chiari syndrome is the blockage of cerebrospinal fluid (CSF) at the foramen magnum. The choroid plexus inside the lateral ventricles of the brain produces cerebrospinal fluid. CSF then passes though the third ventricle, the aqueduct of Sylvius and then the forth ventricle. CSF then passes out of the skull through the foramen magnum and into the subdural space of the spinal cord. The CSF finally travels down the spine to the sacrum where it is reabsorbed by the arachnoid granules in the sacrum.
Chiari syndrome causes the cerebellar tonsils to block the flow of CSF through the foramen magnum like the cork on a Champaign bottle. Since the pressure increases in the brain, this pulsatile fluid has to go somewhere. One of the pathways still open is the very thin but still patent central canal of the spinal cord.
The persistent production of CSF and the CSF pressure acts as a “water hammer” and can cause dilation of this canal. This dilation is called a syrinx and the cyst that forms can damage the central spinal tracts of the cord. It is estimated that 2/3 of patients with Chiari syndrome will eventually develop a syringomyelia.
Syringomyelia slowly causes a variety of symptoms. Noted are a loss of skin sensitivity to hot and cold, a loss of skin sensation in a “shawl” or “cape” distribution, numbness of the hands and feet, muscle weakness and spasticity.
Physical Examination findings will depend upon the brain and spine structures affected due to the compression. Difficulty with speaking and breathing can be related to brain stem compression. Upper spinal cord compression will cause long tract signs (hyperreflexia, Hoffman’s sign, clonus and Babinski’s reflex). Cerebellum compression will cause ataxia, dysmetria and incoordination, The CSF block will cause symptoms of upper neck pain and headaches with coughing or sneezing.
Non-Surgical Treatment of Chiari Malformation
As noted before, many patients have no symptoms and this malformation is found as an incidental finding. If there are mild symptoms only and there are no cranial cysts or syringomyelia, medications can be used such as non-steroidal anti-inflammatories. Impact and contact sports probably should be avoided.
Surgical Treatment of Chiari Syndrome
Surgery is the treatment for severe Chiari malformation and MRI findings. The problem as noted earlier is crowding of the cerebellum with a too tight foramen magnum. The surgery is a posterior fossa decompression surgery. The back rim of the foramen magnum is removed with a small part of the back of the skull. The dura typically will be constricted here and will need to be widened. This is performed by incising (opening) the dura, letting it gap and then placing a patch over the open hole (similar to widening the waistline on a pair of pants). Occasionally, the back part of the ring of C1 will also need to be removed (C1 laminectomy) to make more room for the cord and cerebellar tonsils.
For additional resources on Chiari malformation type I, or to discuss Arnold Chiari Syndrome with a spine specialist and neck doctor, please contact the Vail, Aspen, Denver and Grand Junction, Colorado office of Dr. Donald Corenman.