Mycoplasma Pneumonia
Most patients with mycoplasma pneumonia have prominent headaches (18); however actual
CNS involvement, or even alteration of cognitive function or alertness, is quite rare. When
encephalitis does occur there are few specific features. Diagnosis is generally by measuring
either cold agglutinins or specific antibody titers. Prognosis is generally excellent.
Viral Brain Infections
Herpes Simplex Encephalitis
Human herpes viruses, similar to polioviruses, differ from many other encephalitis-causing
viruses in that they have just one host—humans. Because of this it is at least theoretically
possible to eliminate these pathogens entirely—primarily through effective vaccines. While
sufficiently potent vaccines are not yet available for herpes simplex, this strategy has
eliminated smallpox and hopefully will eliminate polio in the not too distant future.
Unfortunately, this approach cannot eliminate the innumerable other viruses, such as West
Nile and rabies, which are zoonoses, existing in multiple species. Even with successful
vaccination, the best that can be hoped for with zoonotic infections is temporary protection of
the immunized individuals, not permanent elimination of the virus and therefore the disease.
Herpes virus is the most commonly identified agent of sporadic encephalitis (19). Herpes
simplex virus (HSV) 1 and 2 are ubiquitous; following initial infection, primarily via the
mucous membranes, the virus generally establishes permanent residence in the innervating
dorsal root ganglion neurons. Periodically the virus will migrate back down the axon, causing
a recurrent cutaneous eruption. A similar mechanism is thought to underlie HSV1 encephalitis.
The sensory neurons of the trigeminal nerve, which innervate the lips, also innervate the
meninges of the middle and anterior cranial fossa. Experimentally, reactivating virus can be
shown to migrate centrally, affecting the medial temporal and frontal lobes, the primary site of
involvement in herpes simplex encephalitis.
HSV1 encephalitis is potentially a devastating illness with mortality approaching 90% in
the pretreatment era. Initial presentation can be as a nonspecific febrile prodrome with
headaches. Often mild personality changes are noted for a few days. Two important (and
probably interrelated) functions of the medial temporal lobes are olfaction and memory. Early
manifestations of this necrotizing, localized infection often consist of focal seizures manifest as
olfactory hallucinations and perceptions of de ́ja`vu or jamais vu. Often a diagnosis is not made
until the patient has a generalized or at least focal motor seizure.
The diagnosis should be considered in a previously healthy individual with abrupt onset of
altered mental status and fever; headache is present in most. Clinically evident seizures are a
presenting symptom in up to half. Since other brain infections can be clinically similar, confirmatory
testing is necessary. Imaging, particularly MRI scans, classically will demonstrate changes in the
medial temporal lobes, though this may take a few days to be evident. EEG can show paroxysmal
periodic discharges—but again usually only after several days. CSF examination is the most
helpful—although cases have been reported in which CSF is initially normal; typically it shows a
modest lymphocytic pleocytosis witha significant number of erythrocytes, and mild hypoglycor-
rhachia. Most importantly, CSF PCR for herpesviruses is highly sensitive and specific.
Speed is of the essence in treating HSV encephalitis—there is a much higher probability of
successful outcome if treatment is initiated when the patient is awake and unimpaired than if it
can only be started when the patient is comatose (20). Therefore it is common practice to perform
an MRI and lumbar puncture rapidly, initiate treatment immediately, and then stop treatment if
PCR and other testing do not support the diagnosis. Treatment consists of acyclovir 10 mg/kg
every 8 hours for 21 days. Its major complication is renal toxicity; this risk can be decreased with
aggressive hydration. However the requisite fluid load can be somewhat problematic since
patients with HSV encephalitis frequently develop significant hyponatremia and significant
cerebral edema, both requiring fluid restriction. Most patients require anticonvulsants. The role of
steroids is unclear, without substantial evidence supporting their use.
Other Herpes Viruses
Neurologic complications used to accompany about 1 of every 10,000 cases of chickenpox (19).
With widespread vaccination, this is now rarely seen. Cytomegalovirus can cause
Encephalitis and Its Mimics in Critical Care 159