Central nervous system (CNS) infections are not uncommon in the neurocritical care unit (NCCU). This chapter reviews the most common causes of CNS infections and discusses the diagnosis and management of these life-threatening illnesses.

ENCEPHALITIS

Owing to the almost overwhelming number of possible etiologies involved, the care and diagnosis of a patient presenting with acute or subacute progressive encephalopathy poses a challenge particular to the neurocritical care environment. A clinical definition of the infectious encephalitis syndrome involves two criteria initially put forth by the California Encephalitis Project:

1. ▪ Encephalopathy marked by > 24 hours of depressed or altered metal status, lethargy, or personality change necessitating hospitalization.

2. ▪ One or more of the following findings: fever, seizures, focal neurologic deficits, cerebrospinal fluid (CSF) pleocytosis, abnormal electroencephalogram (EEG), and abnormal CNS imaging.

In almost two-thirds of cases (62%), patients matching this description remain undiagnosed. Certainly, a confident diagnosis of an infectious cause is the exception rather than the rule, occurring in only 13% of cases.

VIRAL CAUSES OF ENCEPHALITIS

Herpes Simplex Virus Type 1

Background

HSV-1 is the most common cause of fatal sporadic viral encephalitis. Through mechanisms not completely understood and believed to involve host inflammatory reaction, primary invasion or reactivation of the virus quickly results in areas of hemorrhagic necrosis. Untreated, it has a fatality rate exceeding 70%. Morbidity and mortality of herpes simplex encephalitis (HSE) is greatly reduced by early treatment, defined as antiviral therapy initiated within 24 hours of symptom onset, or before the onset of coma (defined as a Glasgow Coma Scale [GCS] > 8).