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Chapter 27: Viral Infections of the Nervous System

Kiran Thakur; James M. Noble

ACUTE VIRAL ENCEPHALITIS

General Considerations

Encephalitis is defined by the presence inflammation of the brain in association with clinical evidence of neurologic dysfunction. Of the pathogens reported to cause encephalitis, the majority are viruses. More than 100 viruses, in various families, may cause encephalitis. Viruses can cause encephalitis through acute primary infection or through a parainfectious or postinfectious immune-mediated response. Based on retrospective International Classification of Disease data, an estimated 263,352 encephalitis-associated hospitalizations occurred in the United States from 1998 to 2010; this corresponds to an average of 20,258 encephalitis-associated hospitalizations per year. Herpes simplex virus type 1 (HSV-1), arthropod-borne viruses (arboviruses), West Nile virus, and enteroviruses are the most common causes in adults. Table 27–1 outlines the epidemiology and clinical features of selected viruses that cause encephalitis.

Table 27–1.Viral encephalitis: epidemiology of selected causes.
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Table 27–1. Viral encephalitis: epidemiology of selected causes.
Virus Time of Year Geographic Distribution Frequency (US) Clinical Findings
Herpesviruses
HSV-1 Any Worldwide 1000/y; most common cause of sporadic encephalitis in United States

Predilection for orbitofrontal and temporal lobes

Personality changes, cognitive impairment, focal neurologic deficits (aphasia, quadrantanopsia, hemiparesis), and seizures common

Up to 30% mortality and high morbidity even among patients treated with acyclovir

Cases of chronic and recurrent encephalitis reported
HSV-2 Any Worldwide Uncommon

Usual cause of encephalitis in neonates

In adults, presentation is similar to HSV-1 but course is usually milder

Meningitis, radiculitis, and myelitis are more likely complications in adults
CMV Any Worldwide Rare

Acute or subacute onset

Retinitis, polyradiculitis, myelitis, or multifocal neuropathy may accompany encephalitis

Usually occurs in setting of systemic infection; therefore, serum PCR amplification for CMV DNA should be positive

Immunocompromised are at risk (eg, AIDS; see Chapter 28)
EBV Any Worldwide Rare

May be accompanied by optic neuritis, myelitis, polyradiculitis, or cerebellitis

Cerebellitis and meningitis are more common complications
VZV Any Worldwide Rare

Encephalitis, myelitis, or cerebellitis are rare

Infarcts from small- or large-vessel CNS vasculitis are much more common, occurring during or within weeks of zoster rash

Elderly or immunocompromised (eg, AIDS) are at risk
HHV-6 Any Worldwide Unknown

Course seems to simulate HSV-1

Most cases reported in transplant recipients receiving immunosuppressive therapy
Arthropod-Borne Viruses
Eastern equine Mosquito season Atlantic and Gulf Coasts 0–15/y

Fulminant onset with predilection for basal ganglia and thalamus

High morbidity, mortality, and risk of neurologic sequelae

Children and elderly are at higher risk
Western equine Mosquito season Western US 0–40/y

Milder symptoms than with other arbovirus infections

Low morbidity and mortality

Children appear to be at highest risk
St Louis Mosquito season Entire US 2–250/y; occasional epidemic

Meningitis is more common in children; encephalitis in older adults

Usually abrupt onset with spectrum of symptoms from mild to severe

Tremor and myoclonus, reflecting predilection for basal ganglia involvement

Myopathy is common
California serotypes (La Crosse) Mosquito season Eastern and central US 30–160/y

Meningitis is probably ...

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