[Red Flag] Very sudden onset headache (thunderclap headache) [Possible causes] Brain bleed (subarachnoid hemorrhage, hemorrhage into mass lesion, vascular malformation), pituitary apoplexy, mass (especially in posterior fossa) [Diagnostic tests] Neuroimaging, lumbar puncture if computed tomography is negative
A bleed in the brain irritates the meninges which causes pain. Pituitary apoplexy (bleeding or impaired blood supply to the pituitary gland at the base of the brain) is often accompanied by double vision or visual field defects, since the pituitary gland is right next to the optic chiasm (eye nerves).