Bilateral retrocerebellar arachnoid cysts exerting mass effect and associated with cerebellar tonsillar ectopia in an otherwise healthy adult.

Rarely, midline or unilateral posterior fossa arachnoid cysts (ACs) exert local mass effect resulting in the symptoms and signs of cerebellar and brainstem dysfunction. These cysts are sometimes seen in conjunction with cerebellar tonsillar ectopia (TE), although the relationship between these two entities is unclear. Bilateral ACs in the posterior fossa are virtually unprecedented. We describe the case of a 33-year-old man with a history of multiple minor head injuries observed to harbour asymptomatic, bilateral cerebrospinal fluid-density collections over the cerebellar hemispheres. Six years later, he presented with headaches, limb paraesthesias, and drop attacks. Computed tomography, magnetic resonance imaging, and operative findings during burrhole drainage of the lesions showed bilateral posterior fossa ACs, with associated cerebellar TE of 11 mm. The cysts partially recurred, necessitating reopening of the burrholes, after which the patient's symptoms resolved entirely. We then discuss the challenges in diagnosing this unusual case, the relationship between AC and TE, and the role of minor head injury in the symptomatic progression of AC.