Subarachnoid haemorrhage (SAH) is an uncommon type of stroke caused by bleeding into the space around the brain (the subarachnoid space, found between the arachnoid and pia mater).
SAH classically produces a thunderclap headache - patients report a sudden-onset headache that is the worst headache of their life.
SAH can be traumatic (the most common cause), caused by head injury, or be spontaneous, typically due to a ruptured berry aneurysm.