An unusual cause of headache

A 64 year old woman presented to the emergency department with a history of sudden onset severe generalised headache after waking, associated with nausea. She had no medical history. On examination, the pain had not eased but she appeared alert with no other discernible features. She underwent a computed tomography scan of the head (fig 1). What is the diagnosis, and are further investigations warranted? Fig 1 Axial unenhanced computed tomography section at the level of the pons Perimesencephalic subarachnoid haemorrhage …