Meningitis is inflammation of the layers of tissue that cover the brain and spinal cord (meninges) and of the fluid-filled space between the meninges (subarachnoid space).
The brain and spinal cord are covered by three layers of tissue called meninges (see Biology of the Nervous System: Viewing the Brain). These layers are the dura mater (outermost), arachnoid membrane (middle), and pia mater (innermost). Between the arachnoid membrane and pia mater is the subarachnoid space. This space contains cerebrospinal fluid, which flows through the meninges, fills the spaces within the brain, and helps cushion the brain and spinal cord.
Meningitis is most often caused by infection with microorganisms such as bacteria, viruses, or fungi. However, certain drugs and disorders that are not infections occasionally cause meningitis (called noninfectious meningitis). These disorders include sarcoidosis, Behçet syndrome, brain cancer, and cancers that have spread to the meninges, including leukemia and lymphoma.
Meningitis often appears suddenly (called acute meningitis). Sometimes it develops over a period of several days to a few weeks (called subacute meningitis). If it lasts 4 weeks or longer, it is considered chronic. It can recur after it has seemed to disappear (called recurrent meningitis).
Meningitis can be classified by what causes it (bacteria, viruses, or something else) or by how quickly it develops (acute, subacute, or chronic). But it is usually classified as one of the following:
Aseptic meningitis, often used to refer to viral meningitis, actually refers to meningitis caused by anything other than the bacteria that usually cause acute bacterial meningitis. Thus, aseptic meningitis can include meningitis caused by viruses, disorders that are not infections, drugs, or other organisms (such as the bacteria that cause Lyme disease or syphilis).
Different types of meningitis may cause different symptoms. Also, symptoms differ in how severe they are and how quickly they develop. However, all types cause the following:
People may also appear sluggish or unresponsive.
Doctors often suspect meningitis based on symptoms. But because meningitis can be serious, doctors do a spinal tap (lumbar puncture—see Symptoms and Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Spinal Tap) to confirm the diagnosis and identify the cause. The sample of cerebrospinal fluid is withdrawn and sent to a laboratory to be examined, analyzed, and cultured (to grow microorganisms). A sample of blood may also be withdrawn and cultured.
Treatment depends on the cause. If doctors suspect that meningitis is caused by bacteria, they start antibiotics immediately because bacterial meningitis progresses rapidly and is life threatening.
General measures may help relieve symptoms. When meningitis is mild, drinking plenty of fluids, resting, and taking over-the-counter (OTC) drugs can help relieve fever and pain. If meningitis is severe, people are admitted to the hospital for treatment.
Last full review/revision February 2013 by John E. Greenlee, MD