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Arachnoid Cyst

Retrocerebellar arachnoid cyst(*) with assymetrical compression of cerebellum
Supratentorial arachnoid cyst (arrow)


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.

Differential Diagnosis:

Dandy-Walker Malformation in two fetuses. Enlarged 4th ventricle (*) and vermian agenesis (arrow)

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.


A study by Pierre-Kahn et al evaluated the follow up of 54 fetuses presenting with arachnoid cysts in the second and third trimesters. They reported a good prognosis in 88% of the cases in terms of behavior, neurological development, and intelligence at 4 years. Nine pregnancies were terminated because they were associated with other brain anomalies. Some reports describe complete resolution of the cysts 10 and cysts rarely progress postnatally.


Filly RA Ultrasound evaluation of the fetal neural axis In Ultrasonography in Obstetrics and Gynecology, Ed Peter W. Callen, WB Saunders and Co., Philadelphia, 3rd Edition, 1994

Rafferty PG, Britton J, Penna L, Ville Y. Prenatal diagnosis of a large fetal arachnoid cyst. Ultrasound Obstet Gynecol 12:358–361, 1998

Pierre-Kahn A, Hanlo P, Sonigo P et al. The contribution of prenatal diagnosis to the understanding of malformative intracranial cysts: state of the art. Childs Nerv Syst. 16:619-26, 2000

Bretelle F, Senat MV, Bernard JP et al First-trimester diagnosis of fetal arachnoid cyst: prenatal implication Ultrasound OB-Gyn 20:400-2, 2002

Diakoumakis EE, Weinberg B, Mollin J: Prenatal sonographic diagnosis of a suprasellar arachnoid cyst. J Ultrasound Med 5:529, 1986

Mack LA, Rumack CM, Johnson ML: Ultrasound evaluation of cystic intracranial lesions in the neonate. Radiology 137:451, 1980

Oliver LC: Primary arachnoid cysts. Br Med J 1:1147, 1958.
Starkman SP, Brown TC, Linell EA: Cerebral arachnoid cysts. J Neuropathol Exp Neurol 17:484, 1958.

Williams B, Guthkelch DL: Why do central arachnoid pouches expand? J Neurol Neurosurg Psychol 37:1085, 1974.

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California