False Localization of a Ruptured Intracranial Dermoid Secondary to Subarachnoid Spread of Cyst Contents.

A 34-year-old right-handed female presented to the emergency department with a worsening headache, neck stiffness, intermittent abnormal sensation, and right arm weakness. Shortly after arrival, she had a generalized tonic-clonic seizure. A noncontrast head computed tomography scan revealed a right-sided, low-attenuating, lobulated mass ipsilateral to her arm symptoms. Magnetic resonance imaging revealed an abnormal signal throughout the subarachnoid space and increased fluid-attenuated inversion recovery sequence signal contralateral to the mass. This presentation suggested a false localization sign of sensory and motor disturbance ipsilateral to the mass likely caused by cyst rupture and sebum spread contralateral, causing cortical irritation (evidenced by increased fluid-attenuated inversion recovery sequence signal). During mass resection, sebum was visible throughout the subarachnoid space. The patient had an uneventful recovery from surgery and has been seizure free since the resection with steady improvement of symptoms. This case highlights the importance of avoiding cyst rupture of dermoid cysts.