Of the common viral encephalitides, this is perhaps the gravest and is by far the most common. HSV encephalitis occurs sporadically throughout the year and in patients of all ages and in all parts of the world. About 2,000 cases occur yearly in the United States, accounting for approximately 10 percent of all cases of encephalitis (a rate of 2 to 4 per million population per year). Between 30 and 70 percent were in the past fatal, and many patients who survive are left with serious neurologic abnormalities, more so in the era before effective antiviral treatment. It is almost always caused by HSV-1, which is also the cause of the common herpetic lesions of the oral mucosa; only rarely, however, are the oral and encephalitic lesions concurrent. The type 2 herpesvirus may also cause acute generalized encephalitis, usually in the neonate and in relation to genital herpetic infection in the mother. Type 2 infection in the adult more typically causes an aseptic meningitis and sometimes a polyradiculitis or myelitis, again in association with a recent genital herpes infection. Exceptionally, the localized adult type of encephalitis is caused by the type 2 virus and the diffuse neonatal encephalitis by type 1.
The symptoms, which evolve over several days, are in most cases like those of any other acute encephalitis—namely, fever, headache, seizures, confusion, stupor, and coma. In some patients, these manifestations are preceded by symptoms and findings that betray the predilection of this disease for the inferomedial or lateral portions of the frontal and temporal lobes and the insula. These symptoms and findings include olfactory or gustatory hallucinations, temporal lobe seizures, personality change, bizarre or psychotic behavior or delirium, aphasia, and hemiparesis. Although several seizures at the onset of illness are not an uncommon presentation, status epilepticus is rare. Disturbed memory function can often be recognized, but usually this becomes evident only later in the convalescent stage as the patient awakens from stupor or coma. Hemorrhagic swelling and herniation of one or both temporal lobes through the tentorial opening may occur, leading to coma during the first few days of the illness, a poor prognostic sign.
The CSF is typically under increased pressure and almost invariably shows a pleocytosis (range 10 to 200 cells/mm3; infrequently greater than 500 cells/mm3). The cells are mostly lymphocytes, but there may be a significant number of neutrophils early on. In a few cases, 3 to 5 percent in some large series, the spinal fluid was normal in the first days of the illness, only to become abnormal when reexamined. The hemorrhagic nature of brain tissue destruction with this infection is infrequently reflected in the spinal fluid. In fact, in only a minority of cases, red cells, sometimes numbering in the thousands but usually far fewer, and xanthochromia are found. The protein content is increased in most cases. Rarely, the CSF glucose levels may be reduced to slightly less than 40 mg/dL, creating confusion with tuberculous and fungal meningitides. The cerebral imaging appearance, giving highly characteristic features, is discussed later in the section on “Diagnosis.”