Eastern Equine Encephalitis (EEE) Workup

Because of the numerous organisms that can produce signs and symptoms similar to those of eastern equine encephalitis (EEE), clinical diagnosis is difficult. Likewise, laboratory confirmation is challenging because it requires either specific serologic findings or virus isolation in brain tissue or cerebrospinal fluid (CSF). If the possibility of EEE is considered early, recovery of the virus from serum during the prodromal phase is possible; however, isolation from either blood or CSF is often difficult.

Neuroimaging studies (eg, computed tomography [CT] and magnetic resonance imaging [MRI]) may play an important role in the early identification of EEE virus and are routinely performed in patients with central nervous system (CNS) symptoms. Of note, most radiographic changes resolve in those patients who recover.

Relatively recent advances in imaging show that previous neuroradiographic manifestations of EEE were not precisely defined. Early studies (not entirely sensitive) revealed a predilection for the thalamic nuclei and the basal ganglia ^[6] ; however, these changes are also common in infections with Japanese encephalitis, measles, mumps, echovirus 25, conjunctivitis, cyanide poisoning, and carbon monoxide (CO) poisoning.

Brain biopsy is rarely indicated and is simply a last resort for diagnosis.

Few laboratory test abnormalities are particular to EEE. The following are abnormal findings:

• Leukocytosis has a 95% prevalence rate, with a median of 15,000 cells/µL. It is predominantly neutrophilic, but bands or lymphocytosis may not be visualized.

• Hyponatremia often develops, but the exact relationship between the virus directly responsible and a secondary syndrome of inappropriate secretion of antidiuretic hormone (SIADH) has not been outlined.

Blood cultures reveal nothing in this particular disease but may be performed if suspicion of bacterial infection is high.

Biochemical assays are valuable for EEE diagnosis. With early suspicion, obtain sera at 2- to 3-day intervals. A potential drawback is the slow turnaround time for these test results. Additionally, VecTest antigen assays and Vero cell plaque assays have been in use for arthropod surveillance programs and have also been effective in human diagnosis. ^[7]

Potential assays for isolation include the following:

• Enzyme-linked immunosorbent assay (ELISA) is used to detect immunoglobulin M (IgM), primarily during convalescent stages or prolonged courses.

• ELISA may be used to detect antiarboviral immunoglobulin G (IgG), which has similar results to the neutralization assay (current use primarily as an adjunct to the IgM ELISA). ^[8]

• Serum hemoagglutinin-inhibition titer of at least 1:320 is most common.

• Complement fixation titer of at least 1:128 primarily occurs in convalescing patients.

• Immunofluorescence titer of at least 1:256 is uncommon.

• Neutralization assay titer of at least 1:160 is common.

Obtain a lumbar puncture (LP) as soon as possible when EEE is strongly suspected. Assess the CSF for elevated opening pressures and obtain a complete blood count (CBC) with differential, Gram stain, glucose, protein, bacterial culture, viral culture, fungal culture, acid-fast bacillus, India ink stain, and Venereal Disease Research Laboratory (VDRL) test.

Increases in protein and protein concentration in the CSF (approximately 100 mg/dL) are common. The CSF red blood cell (RBC) count may be elevated. The CSF white blood cell (WBC) count may be elevated (initial WBC count, 500-2000/µL; median, 600/µL; neutrophils predominate). Hypoglycorrhachia is not present.

Viral culture

Previously, the recovery of EEE was limited because only a few facilities had the resources to amplify the virus. Recent studies indicate excellent growth of the virus recovered from patient CSF in A549 and MRC-5 cell cultures, which are mediums that virology laboratories routinely use to recover adenovirus, herpes simplex virus (HSV), and enterovirus. ^[9]

Polymerase chain reaction

EEE-specific TaqMan reverse transcriptase polymerase chain reaction (PCR) analysis is used as a final alternative to analyze the various organisms known to cause encephalitis. It is hoped that this analysis will provide rapid diagnosis in the future. ^[10]

Recent studies are more promising for PCR because they indicate that it is more accurate than serologic diagnostics, which have a 10% likelihood of yielding positive virus results. Other current advantages include the ability to target antiviral therapy, the ability to reduce the need for brain biopsies, and the ability to increase the speed of diagnosis (obtaining the panel can occur in 72 hours).

The current limitation is that this study likely requires a state or national effort, which may not be available for EEE.

CT scanning

Perform CT scanning to monitor the evolution of lesions or to determine primary areas of disease. The most common finding is a lesion of the basal ganglia. Lesions vary in size and may exhibit a secondary mass effect with edema.

A CT scan may reveal areas of punctate hemorrhage, focal edema with a mass effect, poorly marginated lesions, or interventricular hemorrhage. In elderly patients, the findings can mimic early infarction or they may be nonspecific, which is common in elderly patients. Occasionally, meningeal enhancement may also be present, indicating a subarachnoid hemorrhage or meningitis.

MRI

MRI is often sensitive to early changes secondary to EEE, and it may help achieve a prompter diagnosis in as many as 60% of patients. Compared with CT scanning, MRI is more sensitive and reveals more abnormalities with increased detail. Use a T2-weighted image for optimal observation of the lesions, which appear as areas of increased signal intensity.

The most commonly affected areas of the CNS include the basal ganglia (unilateral or asymmetric with occasional internal capsule involvement) and thalamic nuclei. Other areas include the brain stem (often the midbrain), periventricular white matter, and cortex (most often temporally).

MRI findings are abnormal in all comatose patients, and normal MRI findings may indicate the need to consider another diagnosis.

MRI also provides critical information for differentiating EEE from herpes simplex encephalitis. EEE is common in the basal ganglia, whereas herpesvirus is not. If attenuation is present in the basal ganglia, herpesvirus tends to occur laterally, whereas EEE has a medial preponderance.

EEG

Electroencephalography (EEG) often reveals a generalized slowing and disorganization of the background. Later, this process is followed by epileptiform activity that may range from isolated discharges to gross seizure activity.

The perikaryon and dendrites are primarily affected and demonstrate evidence of cytoplasmic swelling, eosinophilia, and nuclear pyknosis. Occasionally, mature viral particles are observed in extracellular spaces. The brain is grossly edematous, and inflammation is evident both parenchymally and perivascularly.

Perivascular inflammation, vasculitis, thrombi, neurolysis, neuronophagia, and demyelination may be observed. The predominance of neutrophils in the inflammatory cell type is particularly important.