Chapter 53

• Herpes simplex virus (HSV) is the most common cause of fatal sporadic encephalitis. Early recognition and early treatment with acyclovir significantly improve outcome.
• A diagnosis of encephalomyelitis should be suspected in patients with fever, headache, behavioral changes, unexplained focal neurologic signs, seizures, or altered mental status without an obvious alternative explanation.
• Focal neurologic abnormalities are demonstrable either clinically or with neurodiagnostic evaluation in 90% of cases of HSV encephalitis (HSVE).
• If focal neurologic signs are present, computed tomography or magnetic resonance imaging should be performed before lumbar puncture. Diagnostic imaging is helpful in ruling out other treatable causes of altered mental status and also may identify patients with increased intracranial pressure in whom lumbar puncture may be dangerous.
• Unless contraindicated, an early lumbar puncture should be performed. This measure is especially important to rule out bacterial meningitis.
• The cerebrospinal fluid (CSF) is almost always abnormal in encephalomyelitis, usually demonstrating increased leukocytes, generally between 10 and 500 cells per microliter. The CSF glucose value is usually normal, and the protein content is usually normal or slightly elevated.
• Although a completely normal CSF does not rule out encephalomyelitis, it should heighten suspicion for a toxic or metabolic encephalopathy.
• Treatment with acyclovir must be commenced early when HSVE is suspected clinically. Polymerase chain reaction detection of HSV in CSF is the gold standard for the diagnosis of HSVE. However, a negative result on polymerase chain reaction does not always exclude HSVE and does not, by itself, provide justification for the termination of acyclovir treatment.
• Arboviruses are the most common cause of encephalitis worldwide. Their frequency varies dramatically, depending mainly on the season and geographic locale. West Nile virus has recently emerged as an important cause of encephalitis in North America.
• Although viruses are the most common cause of encephalomyelitis, nonviral causes should be carefully ruled out because they are often readily treatable.
• The threat of bioterrorism has increased the likelihood that unusual organisms may be etiologic agents.
• Intensive supportive care is indicated in patients with encephalitis because these patients may make remarkable recoveries even after prolonged unconsciousness.

Encephalitis and myelitis refer to inflammation of the brain and spinal cord, respectively. These processes may occur together, often with meningeal involvement; hence, the terms meningoencephalitis and meningoencephalomyelitis. Encephalomyelitis may result from direct invasion of a microorganism into the central nervous system (CNS) or from an autoimmune process that results in demyelination; the latter process is often triggered by a recent vaccination or infection. Up to 20% of all encephalitides result from autoimmune reactions; they have been variously termed postinfectious,parainfectious (when they occur simultaneously with an infection), and postvaccinial (when they occur after a vaccination). Regardless of the pathogenesis, the clinical manifestations and differential diagnosis of these processes overlap to such a degree that they may be considered together.

There are a multiplicity of etiologies for encephalomyelitis1 (Table 53-1). Unfortunately, most patients do not present with symptoms and signs pathognomonic of a ...

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