The word "meningitis" means an inflammation of the meninges, which are the soft tissues that surround the brain and spinal cord. These tissues can be irritated by bacteria, viruses, fungi, cancers, and even some medications like ibuprofen.
A meningitis caused by bacteria is called septic meningitis. This is a medical emergency, and calls for immediate treatment with intravenous antibiotics. Fortunately, this type of meningitis is less common than aseptic meningitis.
Aseptic meningitis simply means a meningitis not due to bacteria, which leaves a large number of other potential causes. Most of the time, an aseptic meningitis isn't life threatening. If the brain tissues also become inflamed (an encephalitis), the situation is more serious. Some forms of meningitis, like that caused by herpes simplex, have a higher risk of becoming an encephalitis. Because meningitis and encephalitis are so closely related, some physicians use the word "meningoencephalitis" to describe the diseases simultaneously.
What Are The Symptoms of Aseptic Meningitis?
The classic findings of meningitis are fever, a stiff neck (nuchal rigidity), and headache. Other signs include nausea, vomiting and worsening of headache with light (photophobia). Children who are old enough to talk may complain of headache or nausea.
If meningitis always caused all three of those symptoms, diagnosing meningitis would be easy. Unfortunately, the situation is often not so straightforward. In mild cases, the classic findings of a rigid neck may not be obvious. Making a diagnosis is especially hard in children, who may be too young to describe a headache. Infants may have fever and other signs of being generally sick, such as rash, diarrhea, or not eating well.
What Usually Causes Aseptic Meningitis?
The most common cause of aseptic meningitis is a viral infection. In fact, sometimes the terms aseptic meningitis and viral meningitis are used interchangeably, although in actuality other things like an allergic reaction or fungus can also cause an aseptic meningitis. Most of us have probably had a mild aseptic meningitis in the past, such as a headache when we have the flu. However, aseptic meningitis can also come in more serious forms, some of which can be lethal.
The viruses that usually cause meningitis in children include a family called enteroviruses. This viral family causes about 90% of all viral meningitis. This family of viruses also usually causes a rash, nausea, vomiting, and some respiratory symptoms, as well as the muscle aches that we frequently associate with feeling ill (myalgias). Only a bit more than one half of children older than one or two years have nuchal rigidity. While most people recover without too much difficulty, one variant called enterovirus 71 is particularly nasty, and can cause cranial nerve palsies, paralysis and pulmonary edema.
Another family of viruses known for causing meningitis is the herpes simplex virus family (HSV). Most of us think of this virus as being a sexually transmitted disease, but in fact it can be spread through other means as well. Usually our immune systems keep HSV from causing serious trouble, but when HSV gets out of control, it is a very serious neurological emergency. Encephalitis is common, which can result in seizures and neurological deficits like weakness, numbness and confusion. Many people with HSV encephalitis die even if they receive treatment. Without treatment, the mortality rate is even higher. Fortunately, HSV usually causes classic symptoms like fever, stiff neck, and headache, making it easier to recognize and treat as soon as possible.
Arboviruses are a virus family carried by mosquitoes and ticks. Usually these forms of meningoencephalitis are fairly mild, with some extreme exceptions. The virus that causes St. Louis encephalitis ranges from a mild flu-like meningitis to a fatal illness. La Crosse encephalitis often causes seizures and focal neurologic signs. West Nile virus can also cause a wide range of disease, including paralysis and coma, especailly in older adults. In contrast, western equine encephalitis causes more severe symptoms in children than adults, including seizures.
Many other viruses can cause an aseptic meningitis. Infants may become infected by human parechovirus (HPeV), which can cause meningoencephalitits and even paralysis. The most common symptoms, however, are just irritability, fever, and rash. Young infants may also suffer from congenital lymphocytic choriomeningitis virus, which can cause severe neurological problems in addition to meningitis. The rabies virus usually causes an encephalitis without meningitis, but may cause a meningitis as well. Mumps meningitis is rare now due to vaccinations, though occasionally it may still be seen. Fortunately, meningitis in mumps is both rare and relatively harmless.
How Do Doctors Diagnose Aseptic Meningitis and Encephalitis?
If there is any concern that someone may have a serious form of meningitis or encephalitis, doctors will usually start antibiotics right away, even before finishing their diagnostic work-up. Because some forms of meningoencephalitis are so deadly, even waiting a few extra hours for tests to come back could potentially be disastrous.
The first step is to ensure no bacterial meningitis is present, which would require certain antibiotics to be quickly started. Ultimately, the best way to diagnose meningitis is with a lumbar puncture. In this procedure, a needle is slipped between the bones of the back into a sac of cerebrospinal fluid (CSF) well below where the spinal cord actually ends. Doctors then look for signs of inflammation, such as more white blood cells in the CSF than expected. A gram stain is used to look for bacteria. Glucose and protein are measured as well. If glucose is too low, it may be because additional cells are gobbling up that sugar.
If there are any signs of concern for elevated intracranial pressure, such as severely altered mental status, a head CT will often be done first to ensure that a lumbar puncture doesn't cause a pressure shift. If pressure below the brain is removed, the increased pressure within the skull can shift the brain through the small hole through which the spinal cord exits, which can result in paralysis and death.
Blood tests will be drawn to look for signs of infection outside the nervous system as well. The blood will be cultured to see if any bacteria grow.
If there are any signs or symptoms suggestive of a particular virus or bacteria, tests can be run to single those infectious agents out specifically. For example, due to the severity of HSV infections, for example, tests are often run to ensure that no HSV is present in the cerebrospinal fluid after a lumbar puncture.
Nonviral Causes of Aseptic Meningitis
Certain kinds of bacteria do not behave in the manner that we would expect of the germs that cause septic meningitis. For example, in most cases of septic meningitis, the glucose is abnormally low. However, the glucose may still be normal in cases of Lyme disease, leptospirosis, ehrlichiosis, and syphilis.
Other infections decrease the glucose, but do not appear in the normal gram-stain used to look for bacteria. Special tests are needed in order to investigate for meningitis caused by these organisms. Fungal infections, tuberculosis, and listeria are examples of these kind of less immediately obvious causes of meningitis that can avoid detection on standard CSF analyses.
Some drugs can cause an inflammation of the meninges. The most common drug to do this is probably an over-the-counter nonsteroidal anti-inflammatory drug (NSAID) such as ibuprofen. Antibiotics like trimethoprim-sulfamethoxazole, which is commonly used to treat urinary tract infections, can also cause aseptic meningitis.
Rotbart HA. Viral meningitis. Semin Neurol 2000; 20:277.
John Attia; Rose Hatala; Deborah J. Cook; et al.Does This Adult Patient Have Acute Meningitis? JAMA. 1999;282(2):175-181.