RESUMEN
Arachnoid cysts are benign space-occupying brain lesions that contain cerebrospinal fluid. Most cases are congenital in origin, caused by failed fusion of the arachnoid membrane early in fetal development. Cases are often incidentally detected on neuroimaging; however, rarely patients present with neuropsychiatric manifestations when cysts expand and cause a midline shift, compression of nearby brain tissue or cerebrospinal fluid compartments or both. We report a case of a 56-year-old woman with no past history or family history of psychiatric illness who developed acute-onset right-sided weakness, depressive symptoms, and other neuropsychiatric deficits. A diagnosis of organic mood disorder caused by an arachnoid cyst was made. Her symptoms and neuropsychiatric deficits remitted after cyst marsupialisation by open craniotomy. Therefore, it is important to investigate the organic aetiology in elderly patients who present with simultaneous mood disorder and cognitive dysfunction.