demonstrate extraordinarily bizarre behavior, they almost always retain orientation and
memory. Testing these simple functions usually will be very helpful.
Many disorders other than infections can produce focal brain damage—strokes and
tumors being the most common. Differentiating between these disorders and infections should
usually be straightforward, based on the clinical context. Stroke usually has a virtually
instantaneous onset and causes abnormalities related to the specific blood vessel involved.
Tumors typically cause symptoms that develop insidiously (over weeks or longer) and are not
usually accompanied by systemic symptoms of infection.
Often, most challenging are epileptic disorders. If there is no past history of epilepsy, and
if no motor seizure activity was witnessed, these can be particularly perplexing. Post-ictal
confusional states usually clarify themselves by resolving over minutes to hours. However
non-convulsive status epilepticus, in which part of the brain seizes continuously but with no
corresponding motor activity, can result in a patient with profoundly altered cognitive
function, but with a cause only identifiable by EEG monitoring. Although, as in patients with
brain tumors, these patients do not typically have systemic symptoms of infection, assuming
that this excludes encephalitis can be dangerous—not all patients with encephalitis have
systemic signs at the onset, and encephalitis can present as non-convulsive status!
INFECTIOUS ENCEPHALITIS
All encephalitides, regardless of cause, share several key characteristics—all are inflammatory
processes involving the substance of the brain, resulting in at least a transient alteration of
brain function, but ultimately potentially causing irreversible CNS damage. All are potentially
devastating and much-feared diseases—think of rabies or “sleeping sickness” as just two
examples. On the other hand, most of the viruses that can cause encephalitis cause many more
asymptomatic infections than symptomatic ones, and typically even among patients with
symptomatic infection only a small subset develops neuroinvasive disease (2).
The initial presentation of these infections is often unimpressive—typically much less
dramatic than that of meningitis, where infection of the brain lining causes severe pain,
sensitivity to light and sound, and reflex protective neck stiffness. The meninges and cortical
blood vessels have nociceptive receptors, so inflammation is painful; the brain itself has no
nociceptors. Fever, often low grade, is common—but less so in the very young, the elderly, and
the immunocompromised. Neurologic changes are often initially limited to subtle alterations
of consciousness or cognition—easily confused with the mild changes typically seen as a
nonspecific result of systemic infection. Specific etiologic agents may cause more specific
symptoms. Enteroviruses and listeria often cause prominent associated gastrointestinal
symptoms. Some arboviruses similarly can present with gastrointestinal (GI) or other
nonlocalizing symptoms.
Most CNS bacterial infections do not need to be considered further in this discussion, as
affected patients generally present acutely toxic with little doubt about the diagnosis. However
some bacteria, typically more slow growing ones that elicit a less dramatic immunologic
response, cause much more indolent CNS infections—typically spirochetes, listeria, and
mycobacteria.
Specific Encephalitides
A consideration of the specific infections (Table 1) that cause encephalitis should begin with
those that are most treatable—spirochetoses, mycobacteria, and herpes viruses—all of which
cause meningitis with varying degrees of parenchymal brain involvement. Consideration
should next turn to disorders with significant prevalence—the arboviruses and most
specifically West Nile Virus. Finally, there is a broad array of other agents that must be
identified—if for no other reason than for epidemiologic recognition and prevention of
additional victims (e.g., rabies).
Bacterial Brain Infections
Tuberculosis
Worldwide, tuberculosis (TB) remains a significant public health problem, particularly in the
less developed world. In the United States, it occurs primarily in patients who have emigrated
Encephalitis and Its Mimics in Critical Care 155