Arachnoid cysts of the fourth ventricle - short illustrated review.

Arachnoid cysts are frequent anomalies of the CNS. They are benign lesions within the arachnoid membrane and have been reported to occur in virtually all locations where arachnoid is present. An intraventricular location, however, is rare and occurrence within the fourth ventricle is particularly uncommon. The first report was published in 1979 on a paediatric patient. Since then, only a few further examples have been reported. Most of these patients presented with hydrocephalus. Shunting procedures were performed, but did not afford long-term improvement of symptoms. Definitive treatment consisted of open resection of the cyst-wall. We report a 34-year-old woman with a large arachnoid cyst within the fourth ventricle who suffered from progressive cerebellar dysfunction. MRI showed massive enlargement of the fourth ventricle by an intraventricular arachnoid cyst which contained multiple septations. Complete excision of the cyst was necessary to reinstitute free CSF-flow and was performed via a median suboccipital approach. This report gives an overview of examples published to date and discusses pathogenesis and clinical features of arachnoid cysts in this location as well as operative strategies including neuroendoscopic techniques.