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Encephalitis and Its Mimics in Critical Care
John J. Halperin
Mount Sinai School of Medicine, Atlantic Neuroscience Institute, Overlook Hospital, Summit, New Jersey, U.S.A.INTRODUCTION
A critically ill patient presents to the emergency department with a low-grade fever and
altered mental status. Is this a brain infection? Is it a stroke? Is it the “toxic metabolic”
encephalopathy so commonly seen in patients who are septic, hypotensive, hypoxic, or
otherwise severely compromised? How far must one go to exclude the possibility of a central
nervous system (CNS)-damaging process? How does one most rationally approach this all too
frequent occurrence? This chapter will attempt to provide a framework to address these
frequent and challenging questions.
The neurologist’s approach to the patient with impaired nervous system function is firmly
rooted in the classic clinical approach of characterizing the disease process in space and time.
Although technology, including magnetic resonance imaging (MRI) and electroencephalography
(EEG), can augment the neurologic examination, much of the necessary information can be
quickly ascertained by performing a limited but directed bedside assessment.
A basic premise of clinical neurology is that brain infections, like other CNS-damaging
processes, cause constant or progressive impairment of specific neurologic functions related to
the location of the responsible CNS damage. In contrast, many systemic illnesses will cause
impairments that wax and wane in both time and space—deficits may appear focal, but
improve, only to be followed by transient impairment of other functions. Simple and repeated
clinical assessments of brainstem function, and of specific cortical functions such as language,
memory, and vision, can raise or lower the index of suspicion for a primary CNS process.
BRAIN INFECTION OR NOT?
A key conceptual first step is to differentiate among three distinct entities—encephalopathy,
meningitis, and encephalitis. All may initially present in strikingly similar fashion, with
systemic symptoms accompanied by changes in level of alertness and cognitive function.
Encephalopathy is by far the most common of the three, and, from a neurologic perspective,
the most benign. Although the word can be defined to include any abnormality of brain function,
it is most commonly used to describe alterations of consciousness and cognition in response to
systemic disorders, without necessarily any underlying structural brain damage. Common causes
of “toxic metabolic encephalopathy” include hyper- or hypo-glycemia, hyponatremia, hypoxia,
hyperthermia, sepsis, and organ system failure such as significant renal or hepatic insufficiency.
The unifying theme of these disorders is that, by altering the brain’s physiologic milieu, they alter
brain function. Although all can result in nervous system damage if sufficiently severe or
prolonged, each can cause transient neurobehavioral changes that are completely reversible.
Meningitis refers to inflammation in the subarachnoid space—the fluid-filled space that
surrounds the brain and spinal cord. Infections that remain limited to this space, such as viral
meningitis, are benign, though unpleasant. “Aseptic meningitis” (a term dating to a time when
only bacterial pathogens could be readily identified) typically causes severe headaches and
systemic symptoms but rarely has serious sequelae. Changes in neurologic function are
generally nonfocal and simply reflect the fact that the patient is ill and very uncomfortable.
Bacterial meningitis, on the other hand, can be devastating. This infection starts in the
subarachnoid space, but bacteria then invade and damage the arteries and veins passing into
the adjacent brain, and invade the brain directly. It is this vascular-related damage, combined
with focal cerebritis and abscess formation, as well as the effects of having a purulent exudate
in the subarachnoid space obstructing cerebrospinal fluid (CSF) flow, that lead to severe
neurologic sequelae. This, in combination with the systemic effects of the bacteremia, can
result in a lethal outcome.