The diagnosis of bacterial meningitis is usually not difficult in an immunocompetent individual. Viral meningitis (which is far more common than bacterial meningitis), subarachnoid hemorrhage, chemical meningitis (following lumbar puncture, spinal anesthesia, myelography, or rupture of a intracranial lesion containing irritative material, e.g., craniopharyngioma or dermoid), and tuberculous, leptospiral, sarcoid, and fungal meningoencephalitis, and allergic-immune reactions, for example, due to certain medications, enter into the differential diagnosis as well, as discussed in later sections.
Overwhelming sepsis itself, or the multiorgan failure that it engenders, may cause an encephalopathy but if there is meningitis, it is imperative, in deciding on the choice of antibiotics, to identify it early. The same can be said for the confused alcoholic patient. Too often, the symptoms are ascribed to alcohol intoxication or withdrawal, or to hepatic encephalopathy, until examination of the CSF reveals meningitis. Although this approach undoubtedly results in many negative spinal fluid examinations, it is preferable to the consequence of overlooking bacterial meningitis.
A number of nonbacterial meningitides must be considered in the differential diagnosis when the meningitis recurs repeatedly and all cultures are negative. Included in this group are Epstein-Barr virus (EBV) infections; Behçet disease, which is characterized by recurrent oropharyngeal mucosal ulceration, uveitis, orchitis, and meningitis; Mollaret meningitis, which consists of recurrent episodes of fever and headache in addition to signs of meningeal irritation (in many cases caused by herpes simplex, as discussed in Chap. 32); and the Vogt-Koyanagi-Harada syndrome, in which recurrent meningitis is associated with iridocyclitis and depigmentation of the hair and skin (poliosis and vitiligo). The CSF in these recurrent types may contain large numbers of lymphocytes or polymorphonuclear leukocytes but no bacteria, and the glucose content is not reduced (see discussion of Chronic Persistent and Recurrent Meningitis in Chap. 32). These recurrent syndromes rarely present in the fulminant manner of acute bacterial meningitis but sometimes they do, and the CSF formulas can be similar, including a reduction in glucose concentration. Rarely, a fulminant case of cerebral angiitis or intravascular lymphoma will present with headache, fever, and confusion in conjunction with a meningeal inflammatory reaction.
Bacterial meningitis is a medical emergency. The first therapeutic measures are directed to maintain blood pressure and treating septic shock (volume replacement, pressor therapy). A premium is then placed on choosing an antibiotic that is known both to be bactericidal for the suspected organism and is able to enter the CSF in effective amounts. Treatment should begin while awaiting the results of diagnostic tests and may be altered later in accordance with the laboratory findings. Whereas penicillin formerly sufficed to treat almost all meningitides acquired outside the hospital, the initial choice of antibiotic has become increasingly complicated as resistant strains of meningitic bacteria have emerged. The selection of drugs to treat nosocomial infections also presents special difficulties.