Rapidly progressing de novo arachnoid cyst in an adult patient

Intracranial arachnoid cysts are thought to be primarily congenital lesions that are commonly found incidentally on brain imaging. Other postulated pathophysiological mechanisms include meningeal infections, trauma, or iatrogenic causes (e.g. post neurosurgical procedures). The spontaneous appearance of these cysts is very rare. In this report we present the first case of an arachnoid cyst occurring spontaneously in a 54 year-old man presenting with progressively worsening headaches with subsequent imaging showing a new arachnoid cyst at the left cerebellopontine angle (CPA). Serial MRI tests revealed the cyst to be expanding rapidly, coinciding with worsening of the patient’s headaches, tinnitus, and an ataxia. Thus, the decision was made to offer surgical intervention. While arachnoid cysts in adults are primarily thought to follow a stable course in most cases, the rapid progression in this case suggests that close monitoring with serial imaging may be warranted.