Cystic lesions of the brain. A classification based on pathogenesis, with consideration of histological and radiological features.

A classification of the existing multitude of cystic lesions of the brain is proposed, which allows an understanding of their genesis and consequent therapeutic implications, as well as their diagnostic characteristics. Essentially, cerebral cystic lesions may be classified into the following categories: Cysts containing CSF-like fluid, which include ex vacuo type cysts, such as leptomeningeal cysts, and cysts following surgical resection; cysts with fluid secreting walls and CSF-like content, such as arachnoid cysts; cysts associated with dysgenesis, for example Dandy-Walker cysts. The ex vacuo cysts increase craniospinal compliance, whereas the other cysts with CSF-like content do not; they are not per se expansive, however, although their occasional location along CSF pathways may cause obstruction and hydrocephalus. Another category includes cysts with a lining of non-neural epithelium like colloid cysts, epidermoid cysts, or craniopharyngiomas. They may increase in size and cause symptoms by compression, although not at the rate of tumour-associated cysts. The cysts associated with gliomas and other tumours have a pathogenesis bearing upon blood-brain barrier impairment and formation of vasogenic oedema. Finally, one may distinguish a category of cysts with infectious origin, such as brain abscesses and hydatid cysts. The cysts with CSF-like contents may be recognised by their magnetic resonance characteristics resembling those of CSF, whereas cysts containing proteinaceous fluid are associated with blood-brain barrier impairment and consequent contrast enhancement. The cysts with a lining of non-neural epithelium exhibit diverse properties of attenuation on computed tomography (CT) and magnetic resonance imaging (MRI), depending on the nature of their cyst contents.