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In the unknown case a retrocerebellar arachnoid cyst is seen with assymetric compression of the cerebellar hemispheres. An additional case of a supratentorial arachnoid cyst extending from the left ambient cistern to the calcarine fissure, with mild compression of the left occipital lobe is seen.
Arachnoid cysts are intracranial, extra-cerebral mass lesions, the
walls of which are composed of a vascular collagenous membrane lined
with flattened arachnoid cells. They either may be congenital or acquired.
The acquired cysts are more common. Congenital arachnoid cysts are
likely formed by the maldevelopment of the leptomeninges and are located
between the pia-arachnoid layers. The intervening space between the
pia and arachnoid is covered by fine trabeculae and there is the potential
for arachnoid cysts to occur where there is abnormal splitting of
the cell layers. Cerebrospinal fluid then becomes entrapped and the
space created enlarges to form the arachnoid cyst. The cyst may become
walled off with a “ball-valve communication with the subarachnoid
space and thus may grow progressively. The majority of arachnoid cysts
are supratentorial and 50% are located in the middle cranial fossa.
The most common site for the formation of cysts is the middle cranial
fossa (50–65%), but they may also be found in the quadrigeminal
cistern (5–10%), suprasellar cistern (5–10%), cerebral convexity
(5%) or posterior fossa (5–10%).
When large, they may cause symptoms by compression of the adjacent
brain. They are occasionally complicated by hemorrhage, either within
or adjacent to the cyst. Middle cranial fossa arachnoid cysts are
known to be associated with hypoplasia of the adjacent temporal lobe.
Arachnoid cysts in other sites such as the quadrigeminal cistern or
suprasellar cistern may cause hydrocephalus, owing to compression
of the adjacent structures and impairment of drainage of cerebrospinal
fluid. However, they do not cause brain maldevelopment, and, if treated
before irreversible brain damage occurs, a good outcome may be anticipated.
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Arachnoid cysts must be distinguished from other supratentorial cysts that include choroid plexus cysts, porencephalic (schizencephalic) cysts, cystic tumors, midline cysts associated with agenesis of the corpus callosum, dorsal cysts of holoprosencephaly, and arteriovenous malformations (vein of Galen aneurysms). Arachnoid cysts create a mass effect and never communicate with the lateral ventricle, whereas porencephalic cysts create no mass effect and usually communicate with the lateral ventricle. A cyst in the quadrigeminal plate cistern may simulate the appearance of a vein of Galen aneurysm. However, no Doppler flow is seen in arachnoid cysts while high-velocity signals return from a dilated Galen vein. Choroid plexus cysts are easily discriminated by location. Cystic tumors (teratomas) usually have relatively large amounts of disorganized solid tissue associated with the cystic component, which is a feature that is not shared with arachnoid cysts. Distinction of arachnoid cysts from cysts seen in callosal agenesis and holoprosencephaly is based on the lack of associated brain maldevelopment seen invariably in these other disorders. In agenesis of the corpus callosum, there is superior extension of the roof of the third ventricle in 80% of cases. This extension can be sufficiently marked almost to fill the whole length of the interhemispheric fissure, and a greatly dilated third ventricle appears to be the cause of most so-called interhemispheric cysts associated with dysgenesis of the corpus callosum. It is important to separate out enlarged third ventricles from true arachnoid cysts, as the long-term prognosis of these patients will differ. Pure interhemispheric arachnoid cysts are rare and dysgenesis of the corpus callosum is a separate congenital malformation that has a different long-term prognosis.
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Professor of Radiology, Obstetrics, Gynecology and Reproductive Science University of California Medical Center, San Francisco, California |
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