Background: Arachnoid cysts are congenital or acquired structures found within the brain and are rarely symptomatic for adults. The literature documenting enlarging arachnoid cysts in adults is also discussed.
Observations: An elderly woman presented with acutely worsening headaches, photophobia, cognitive function, and a seizure-like episode. The patient had a known arachnoid cyst with a decade of radiographic stability, which was now idiopathically enlarging. The patient had a previous history of traumatic brain injuries but no reported trauma around the time of presentation. Due to the severity of midline shift and symptomatology, the decision was made to treat the patient surgically with fenestration and shunting. She recovered well postoperatively.
Lessons: During the workup for a symptomatic elderly patient, enlargement of a previously asymptomatic arachnoid cyst should remain on the differential until specifically ruled out, even in the absence of recent trauma. While rare, enlarging arachnoid cysts result in neurological findings and impact the quality of life for patients.
Keywords: arachnoid cyst; enlarging arachnoid cyst; fenestration and shunting; symptomatic arachnoid cyst.