Posterior fossa cystic lesions--magnetic resonance imaging manifestations.

Cystic lesions of the posterior fossa remain a controversial subject as to clinical classification and diagnosis, especially for those in combination with other intracranial abnormalities. During the period of November 1985 to June 1991, 16 patients with cystic lesions of the posterior fossa were retrospectively reviewed on neuroradiological evaluation with MR images. The patients were 9 males and 7 females aged from 5 days to 15 years old (medium 1 year old). They were classified into three groups as Dandy-Walker cyst (group A, 5 patients), mega cisterna magna (group B, 4) and retrocerebellar pouch or cyst (group C, 7). Nine patients had associated intracranial anomalies, holoprosencephaly in 3 (group A, 3), dysgenesis of the corpus callosum in 4 (group A, 2; group C, 2), and occipital meningoceles in 2 (group C, 2). Hydrocephalus or ventricular enlargement was found in 10 patients (group A, 5; group B, 2; group C, 3). Surgical treatment (cyst-peritoneal shunt or cystectomy) was performed for 10 patients with clinical symptoms due to cyst. Postoperatively, clinical symptoms due to increased intracranial pressure or hydrocephalus improved in 7 patients. For long-term results, the cyst was reduced in 1 of 5 patients of group A and 2 of 5 of group C. Most of our patients have a poor prognosis because of the associated intracranial anomalies or atrophy of the cerebellum. Multiplanar MR images may provide sufficient evidence for the diagnosis of posterior fossa cysts, especially in the case of rotation or upward displacement of the cerebellar vermis. However, the cyst membrane, and the communication of fluid between the cyst and the cistern, cannot be demonstrated on MR images. Further investigation on these subjects and indications for surgical intervention is highly necessary.