arachnoid cyst

supratentorial arachnoid cysts need to be distinguished from other cystic malformations in the same region: where aqueductal stenosis is present, the dilated third ventricle appears oval with tapered edges posteriorly, and there will be corresponding ventriculomegaly. in agenesis of the corpus callosum, the enlarged third ventricle is high in location at the level of the lateral ventricles, which themselves will display the characteristic features of that condition. porencephalic cysts are typically asymmetric, but do not exhibit mass effect. intracranial tumours are usually heterogenous in appearance. a cyst of the cavum septi pellucidi may be discerned by its characteristic location. semi lobar holoprosencephaly, in which a dorsal cyst is present, can be determined by examination of the thalami (fused) and cavum septi pellucidi (absent). a vein of galen aneurysm, superior and posterior to the thalami, will have turbulence and flow on using pulsed doppler. infratentorial arachnoid cysts must always be considered when there is enlargement of the cisterna magna (usually >10 mm in the trans-cerebellar plane). the differentiation from the dandy-walker syndrome or variant can be difficult. posterior fossa cysts are less common and tend to be asymmetric. a definite separation of the cyst and fourth ventricle by normal cerebellar vermis will indicate an arachnoid cyst, just as deficiency of the vermis or splaying of the cerebellar hemispheres will prompt the diagnosis of dandy-walker. there may also be other cns malformations seen in association with dandy-walker syndrome which will help in the diagnosis. in cerebellar hypoplasia, the diagnosis may be determined by comparing cerebellar dimensions with gestational age. where communicating hydrocephalus is present, there will typically be generalised subarachnoid accumulation of fluid in addition to ventriculomegaly. in megacisterna magna, no other abnormal findings will be evident.