In milder cases, in the first hours or day of the illness, there may be no abnormalities of the spinal fluid, and the patient may erroneously be thought to have migraine or a headache induced by a systemic infectious illness. Microorganisms cannot be demonstrated by conventional smear or culture but may be detected by polymerase chain reaction, the last of these techniques being employed largely in cryptic cases. As a rule, the glucose content of the CSF is normal; but infrequently, mild depression of the CSF glucose (rarely below 25 mg/dL) occurs with the meningitis caused by mumps, HSV-2, lymphocytic choriomeningitis, or VZV.
Aseptic meningitis is a common occurrence, with an annual incidence of approximately 20 cases per 100,000 population (Beghi et al; Ponka and Pettersson). Most are caused by viral infections. Of these, the most common are from enterovirus—mainly echovirus and Coxsackie virus. These make up 80 percent of cases in which a specific viral cause can be established. HSV-2 is perhaps next in frequency in adults, followed by varicella, HIV, mumps in children, lymphocytic choriomeningitis (LCM), HSV-1, and adenovirus infections. The remainder are due to a variety of agents, including EBV (infectious mononucleosis), cytomegalovirus (CMV,) leptospira, and the bacterium Mycoplasma pneumoniae (see Chap. 31), influenza virus, and in some parts of the world, tick-borne encephalitides and Borrelia, including Lyme (Kupila et al). During local outbreaks anywhere, the dominant agent is usually one of the arboviruses in the family of Flaviviruses.
It is also recognized that infection with HIV may present as acute, self-limited aseptic meningitis with an infectious mononucleosis-like clinical picture. While HIV has been obtained from the CSF in the acute phase of the illness, seroconversion occurs only later, during convalescence from the meningitis (see The HIV-Acquired Immunodeficiency Syndrome (AIDS)). HSV-2 and HSV-1 have been isolated from the CSF of patients with recurrent bouts of benign aseptic meningitis (Mollaret meningitis), but this finding has not been consistent (Steel et al). As discussed in Chap. 44, it is now considered that a virus, particularly HSV-1, also underlies many cases of what has been traditionally considered idiopathic Bell’s palsy.
Two other aspects of the virology of aseptic meningitis should be noted: first, in most published series of cases from virus isolation laboratories, a specific cause cannot be established with conventional testing in one-third or more of cases of presumed viral origin; second, most agents capable of producing aseptic meningitis also sometimes cause encephalitis. Of course, as most cases of aseptic meningitis are self-limited and relatively benign, extensive testing for the etiology is unnecessary.
Differential Diagnosis of the Cause of Viral Meningitis
Clinical distinctions between the many viral causes of aseptic meningitis cannot be made with high reliability, but useful clues can be obtained by attention to certain details of the clinical history and physical examination. Inquiry should be made regarding recent respiratory or gastrointestinal symptoms, immunizations, past history of infectious disease, family outbreaks, insect bites, contact with animals, and areas of recent travel. The presence of a local epidemic, the season during which the illness occurs, and the geographic location, are other helpful data.