The arachnoid is a thin transparent web-like membrane just inside the dura matter (the tough outer membrane around the brain and spinal cord). The spinal fluid compartment, known as the subarachnoid space, is just inside the arachnoid.
The arachnoid membrane is made of two layers that are normally stuck together. During development, a split can occur in the arachnoid and a blind pouch can develop between the two layers. Called an arachnoid cyst (AC), this pouch can stay unchanged or slowly grow in size.
Arachnoid cysts can develop in any area part of the subarachnoid space including in the posterior fossa (the back compartment of the skull). A common site for posterior fossa ACs is behind the cerebellum (retrocerebellar).
Posterior fossa ACs can cause pressure on the cerebellum and brain stem. If they are large enough, they can also cause the cerebellar tonsils to herniated through the foramen magnum. When this occurs, the resulting symptoms are the same as with the Chiari I malformation and can include headache, dizziness, vertigo, visual disturbances, and poor coordination among other symptoms.
Diagnosis
On MRI, the diagnostic test of choice, ACs have sharply defined borders and fluid identical to CSF (see the retrocerebellar cyst in the image above). Intravenous (IV) contrast is administered to make sure that the cyst is not due to a tumor. Arachnoid cysts are benign and do not enhance (brighten) with IV contrast.
Syringomyelia
Just as in the Chiari I malformation, obstruction of spinal fluid flow at the at the foramen magnum can result in the development of a spinal cord syrinx. In 2005, my colleagues and I reported a case of a 38-year-old woman with achondroplasia (a disorder of bone growth) who developed neurological symptoms from a retrocerebellar arachnoid cyst (image above). The cyst cause a ‘secondary’ Chiari malformation (the Chiari type herniation is secondary to the cyst) and syringomyelia.
Her symptoms and the syrinx resolved following surgery which included removing of portions of the cyst wall (fenestration) and decompression of the foramen magnum.
In the article (
full-text available online), we also reviewed the previous literature on arachnoid cysts associated with syringomyelia.
Treatment
Arachnoid cysts that cause intractable symptoms are treated with surgery. Surgery may be by a craniotomy approach as preformed in the case discussed above, or in some cases by an endoscopic approach. Some arachnoid cysts are treated with a shunt, although this may not be as successful.
Following surgery, the size of the empty-looking area on the MRI may not change much since the cyst has been there for many years and the affected brain tissue may be underdeveloped.
The goal of surgery in cases with herniation of the tonsils is to remove the obstruction at the foramen magnum, improve spinal fluid flow, and relieve compression of the neurological tissues. The outcome following surgery for arachnoid cysts is generally good.
John Oro’, MD................................................................................
Reference
Bauer AM, Mueller DM, Oro JJ.?Arachnoid cyst resulting in tonsillar herniation and syringomyelia in a patient with achondroplasia. Case report.?Neurosurg Focus. 2005 Nov 15;19(5):E14. Review.
Chiari Times is not able to respond to specific patient inquiries.
For patient inquiries, please see one of the sites listed
in the
Organizations section.
Copyright © 2007-2008 CyberMed, LLC
1 Comment
The arachnoid membrane is made of two layers that are normally stuck together. During development, a split can occur in the arachnoid and a blind pouch can develop between the two layers. Called an arachnoid cyst (AC), this pouch can stay unchanged or slowly grow in size.
Arachnoid cysts can develop in any area part of the subarachnoid space including in the posterior fossa (the back compartment of the skull). A common site for posterior fossa ACs is behind the cerebellum (retrocerebellar).
Posterior fossa ACs can cause pressure on the cerebellum and brain stem. If they are large enough, they can also cause the cerebellar tonsils to herniated through the foramen magnum. When this occurs, the resulting symptoms are the same as with the Chiari I malformation and can include headache, dizziness, vertigo, visual disturbances, and poor coordination among other symptoms.
Diagnosis
On MRI, the diagnostic test of choice, ACs have sharply defined borders and fluid identical to CSF (see the retrocerebellar cyst in the image above). Intravenous (IV) contrast is administered to make sure that the cyst is not due to a tumor. Arachnoid cysts are benign and do not enhance (brighten) with IV contrast.
Syringomyelia
Just as in the Chiari I malformation, obstruction of spinal fluid flow at the at the foramen magnum can result in the development of a spinal cord syrinx. In 2005, my colleagues and I reported a case of a 38-year-old woman with achondroplasia (a disorder of bone growth) who developed neurological symptoms from a retrocerebellar arachnoid cyst (image above). The cyst cause a ‘secondary’ Chiari malformation (the Chiari type herniation is secondary to the cyst) and syringomyelia.
Her symptoms and the syrinx resolved following surgery which included removing of portions of the cyst wall (fenestration) and decompression of the foramen magnum.
In the article (full-text available online), we also reviewed the previous literature on arachnoid cysts associated with syringomyelia.
Treatment
Arachnoid cysts that cause intractable symptoms are treated with surgery. Surgery may be by a craniotomy approach as preformed in the case discussed above, or in some cases by an endoscopic approach. Some arachnoid cysts are treated with a shunt, although this may not be as successful.
Following surgery, the size of the empty-looking area on the MRI may not change much since the cyst has been there for many years and the affected brain tissue may be underdeveloped.
The goal of surgery in cases with herniation of the tonsils is to remove the obstruction at the foramen magnum, improve spinal fluid flow, and relieve compression of the neurological tissues. The outcome following surgery for arachnoid cysts is generally good.
John Oro’, MD
................................................................................
Reference
Bauer AM, Mueller DM, Oro JJ.?Arachnoid cyst resulting in tonsillar herniation and syringomyelia in a patient with achondroplasia. Case report.?Neurosurg Focus. 2005 Nov 15;19(5):E14. Review.
Chiari Times is not able to respond to specific patient inquiries.
For patient inquiries, please see one of the sites listed
in theOrganizations section.
Copyright © 2007-2008 CyberMed, LLC