We report a case of an interhemispheric cyst complicated by subdural hematoma in a 16-year-old boy. He was diagnosed with a right interhemispheric cyst on magnetic resonance imaging (MRI) of the head during examination for an afebrile seizure at 14 years of age. Approximately one year later, a head MRI was performed to investigate the cause of headaches. The interhemispheric cyst was found to have decreased in size, but subdural effusion continuing from the cyst at the right convexity was observed. Two months later, following a severe headache, a head MRI showed hemorrhage at the place of effusion in the subdural space. Additionally, displacement of the brain parenchyma was observed. Emergent drainage of the subdural hematoma was performed, and the patient was diagnosed with chronic subdural hematoma. In this case, the head MR images taken over time showed the development of subdural effusion from the interhemispheric cyst, followed by the development of a subdural hematoma. Slight trauma is known to cause subdural hematoma in patients with arachnoid cysts; upon diagnosis, it is important to inform patients of the precautions to prevent an increase in intracranial pressure and response to the symptoms of increasing intracranial pressure.
