The subarachnoid space covers the brain and spinal cord. A subarachnoid hemorrhage (SAH) occurs when there is bleeding into this space. A subarachnoid hemorrhage can be life threatening. The most common cause of bleeding is a rupture of an aneurysm. An aneurysm is a focal area of swelling of an artery in the brain. An aneurysm can be congenital (present from time of birth) and may not cause any symptoms unless it bleeds. A tendency to form aneurysms can be an inherited trait. Other causes of SAH include head trauma and arteriovenous malformation (AVM, a tangle of veins and arteries that is a variation of normal anatomy and is prone to rupture).