Post-inflammatory arachnoid cyst of the quadrigeminal cistern. Observation of development of the cyst.

Arachnoid cysts of the quadrigeminal cistern are rare. Patients with this lesion are usually normal at birth, and signs of increased intracranial pressure (IICP) develop at about 4 to 12 months of age. However, there has been no report about this mechanism. A case of arachnoid cyst of the quadrigeminal cistern is presented. In this case, the process of the cyst growth was observed during the course of follow-up for shunted hydrocephalus. Symptoms coincided with the growth of the cyst. Therefore, the development of the clinical symptoms was thought to be due to IICP caused by cyst expansion. Our case was complicated by inflammation, something that may offer a key to solving the mechanism of cyst fluid accumulation and the relationship between symptoms and development of the cyst.