For the clinician, consideration of other meningitis causes, such as bacteria, mycoplasma, or fungi, is crucial, since these can have devastating outcomes if left untreated. The physician should also realize that the picture of aseptic meningitis is created not only by infectious agents, but also by chemical irritation (chemical meningitis), neoplasm (meningitis carcinomatous), granulomatous disorders, and any other inflammatory conditions. This discussion, however, will focus on meningitis by viral agents.

Viral pathogen may gain access to the CNS via 2 major routes: hematogenous or neural. Hematogenous is the most common route for penetration of most known viral pathogens. Neural penetration refers to spread along nerve roots and usually is limited to herpes viruses (HSV-1, HSV-2, and varicella zoster virus [VZV] B virus), and possibly some enteroviruses.

Multiple host defenses prevent viral inoculum from causing clinically significant infection. These include local and systemic immune responses, skin and mucosal barriers, and the blood-brain barrier (BBB)

In the US more than 10,000 cases are reported annually, but the actual incidence may be as high as 75,000. Lack of reporting is due to the uneventful clinical outcome of most cases and the inability of some viral agents to grow in culture. According to CDC reports, inpatient hospitalizations resulting from viral meningitis range from 25,000-50,000 each year. An incidence of 11 per 100,000 population per year has been estimated in some reports.

Obtaining accurate international prevalence and incidence of this clinically heterogenous and often benign disease is difficult. Worldwide causes of viral meningitis include enteroviruses, mumps virus, measles virus, VZV, and HIV. Meningitis symptoms may develop in as few as 1 in 3,000 cases of infection by these agents. Studies from Finland have estimated the incidence to be 19 per 100,000 population in children aged 1-4 years. This is in significant contrast to 219 cases per 100,000 population estimated for children younger than 1 year. Japanese B encephalitis virus, the most common pathogen in epidemic viral meningitis worldwide, accounts for more than 35,000 infections annually throughout Asia but is estimated to cause 200-300 times that number of subclinical infections.

The distribution and attack characteristics of some agents, such as arboviruses, which are transmitted by arthropod vectors, show strong geographic variability. Lack of effective vaccination policies in some Third World countries plays a role in the geographic discrepancy of other infectious agents.

Mortality/Morbidity

• Excluding the neonatal period, the mortality rate associated with viral meningitis is less than 1%; the morbidity rate is also low.
• Some controversy exists as to the long-term effects on children, with some studies attributing learning disabilities, neuromuscular impairments, and deafness to viral meningitis. Investigators believe that most of these cases must involve the CNS parenchyma, causing encephalitis or encephalomyelitis. Children may complain of irritability, incoordination, and inability to concentrate for several weeks or longer. Infants with enteroviral meningitis during the first few months of life may have an increased risk of altered language development.
• Physicians must realize that viruses capable of causing meningitis also can cause more serious infections of the CNS as well as other organs. The World Health Organization (WHO) statistical reports from 1997 reported enteroviral meningitis with sepsis as the fifth most frequent cause of neonatal mortality. Complications such as brain edema, hydrocephalus, and seizures can occur in the acute period and are discussed later in this article.

Depending on the type of viral pathogen, the ratio of affected males to females can vary. Enteroviruses are thought to affect males 1.3-1.5 times more often than females. Mumps virus is known to affect males 3 times more frequently than females. Most arboviruses have diverse attack characteristics, affecting both sexes but at different ages

• The incidence of viral meningitis drops with age.
• Neonates are at greatest risk and have the most significant risk of morbidity and mortality.
• In neonates older than 7 days, enteroviruses are the most common cause of aseptic meningitis. Vaccination has greatly reduced the incidence of meningitis from mumps, polio, and measles viruses.
• The incidence during the first year of life is 20 times higher than in older children and adults.
• Some of the arboviruses strike at the extremes of age, with the elderly at greater risk of infection, while mumps and measles peak in the later teenage years.