Giant retrocerebellar cyst with obstructive hydrocephalus
The child presented by delayed mental and physical development with gradual increase in the head circumference and bulge of the anterior fontanel with sunset appearance of both eyes, attacks of vomiting .
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Cisterna magna appear huge in size and filled with CSF, it is seen mainly filling the right side of the posterior fossa with marked compression of the right cerebellar hemisphere displacing it anteriorly and to the left side, the 4th ventricle is markedly attenuated with moderate dilatation of both lateral ventricles, preserved cerebral mantel and no CSF seepage mostly related to the placed VP shunt.
Cisterna magna is usually subjected to CSF entrapment forming arachnoid cyst, in such situation it will be named giant or mega cisterna magna, gradual accumulation of CSF if the cyst has valve-like mechanism, this will cause progressive enlargement of the cyst with compressive sequelae on the posterior fossa structures, here the 4th ventricle is the weakest structure, so that hydrocephalic changes will result, placement of shunt tube inside the obstructed ventricle will relieve its dilatation not the cause of obstruction, sometimes the tube placed inside the cyst itself or surgical excision if clinical improvement is not achieved, cine CSF flowmetry can assess the valve mechanism, if positive results obtained then the cyst must be removed.
Dandy-Walker spectrum includes also malformation, variant, Blake's pouch and should be differentiated from giant cisterna magna; in malformation and variant, there is vermian aplasia or hypoplasia, while in Balke's pouch there is displacement of the vermis by the ballooned inferior medullary velum as a result of failed fenestration of outlet foramina.