Post-Traumatic Subarachnoid Hemorrhage
In the setting of trauma, there are three common types of hemorrhage—bleeding—inside the skull but outside the brain.
By far the most common is post-traumatic subarachnoid hemorrhage (ptSAH). This is not the severe form of subarachnoid hemorrhage associated with rupture of an aneurysm (another subject entirely and not discussed further at the moment), but is generally a more diffuse and benign form. PtSAH occurs when the brain is shaken and the tiny vessels stretching across the arachnoid space are disrupted, spilling the blood within into the CSF.
A digression. There are three membranes over covering the brain and spinal cord. These are, from the most superficial to the deepest:
 The dura mater. The toughest and most external—it is usually sutured closed at the conclusion of brain surgery. This is a leathery and very tough membrane, nearly impermeable to infection. It is a very effective physical barrier.
 The arachnoid mater. A wispy, spider web-like membrane too thin to take sutures and under which courses the spinal fluid in the so-called—and all important—subarachnoid space.
 The pia mater. A thin membrane which fits the surface of the brain and spinal cord like a skin or glove. Only a few cells thick. It provides surgeons a nice plane of dissection when moving through the brain.
PtSAH is quickly stopped and, except in severe closed head injury, not an issue. This is a very common form of brain trauma, perhaps present in many cases of otherwise minor concussion.
At times it is entirely asymptomatic, with the patient and the physician entirely unaware that any bleeding has occurred. At other times, it may lead to some irritation of the cortical surface, with the small possibility of a seizure.
PtSAH is rarely anything to worry about and does not require surgery. The bleeding is instantaneous at the time of injury, stops immediately, and rarely recurs.