In the central nervous system, the subarachnoid space, also called the subarachnoid cavity, is the area in the brain between the arachnoid membrane, which is the middle of three membranes covering the surface of the brain, and the pia mater, which is the deepest protective membrane covering the brain. Strengthening rod-like fibers known as fibrous trabeculae cross through the subarachnoid space to connect the arachnoid membrane to the pia mater, and cerebrospinal fluid fills the cavity to flow around the brain. The subarachnoid space also contains the blood vessels which supply the brain and spinal cord with blood and oxygen. This cavity helps to cushion the brain to protect it from injury, and continues down the spinal column along with the arachnoid membrane.
As part of the blood-brain barrier, the subarachnoid space helps to protect the brain from many blood-borne infections and some neurotoxins. Most viruses and some bacteria are able to permeate this barrier, however, and treatment for these infections is difficult as antibodies and medication are often unable to pass through the barrier to fight the infection. Inflammation and hypertension, as well as exposure to radiation, seem to make the blood-brain barrier more permeable, and this can sometimes be helpful in treatment of brain infections. Nanotechnology may hold the key to treatment of brain infections and other brain diseases in the future, as medications may be developed with particles small enough to pass through the barrier.
A subarachnoid hemorrhage is a specific type of brain injury in which blood enters into the subarachnoid space. This is commonly caused by head trauma, ruptured aneurysm, or ruptured arteriovenous malformation (AVM). Initial symptoms can include a sudden headache – which is generally described as the worst headache of a person's life – stiff neck, nausea, vomiting, blurry vision, loss of consciousness, or seizures. Complications from subarachnoid hemorrhage range from stroke to seizures and even death.
When a patient is diagnosed with a subarachnoid hemorrhage, treatment should be immediate in order to ensure the best prognosis possible. Surgery and medication may be used to stop the bleeding, but severe hemorrhages carry a 50 percent initial death rate, and those who survive are likely to experience a delayed death or permanent brain damage. Risk factors for subarachnoid hemorrhage include head injury, high blood pressure, use of blood thinners, previous history of aneurysm, smoking, heavy alcohol use, hemorrhagic disorders, and disorders of the connective tissues in the body.