Cases of EEE at Boston Children’s Hospital in Boston, Massachusetts, during 1970–2010 were identified by searching hospital records and records of the Massachusetts Department of Public Health. Institutional review board approval was obtained.

Case-patients were defined as children with illness compatible with encephalitis and with laboratory confirmation of EEEV infection (14). EEE was confirmed by the William A. Hinton State Laboratory Institute, Department of Public Health, Boston, on the basis of at least 1 of the following: isolation of EEEV or demonstration of specific viral antigen or genomic sequence from cerebrospinal fluid (CSF) or brain tissue; detection of virus-specific IgM in CSF by antibody-capture ELISA; >4-fold increase in acute-phase versus convalescent-phase serum antibodies; or detection of EEEV-specific IgM in CSF by ELISA, followed by confirmation of virus-specific neutralizing antibodies by plaque-reduction neutralization assay.

A records search revealed a total of 15 cases; medical records were available for 14. Partial records from the Massachusetts Department of Public Health were available for patient 2. Patient 1 has been described (15). We defined the illness prodrome as the time between initial nonspecific symptoms and the first major neurologic symptom, such as headache with photophobia, stiff neck, altered mental status, or seizures. We used the Pediatric Cerebral Performance Category Scale (PCPC) to classify outcomes, which were determined on the basis of the clinical condition of the patient at the time of hospital discharge (16). Normal, mild, and moderate disabilities were classified as favorable outcomes (PCPC scores 1–3); severe disability, coma, and death were classified as unfavorable outcomes (PCPC scores 4–6).

Radiographs were reviewed by a neuroradiologist who was aware of the EEE diagnosis but blinded to the specifics of the patient’s history and previous radiographic and pathologic findings. Lesions seen on radiographs were scored on a scale of 0–3 for the degree of T2 fluid-attenuated inversion recovery enhancement (0, no lesion; 1, 2, and 3, minimal, moderate, and severe enhancement, respectively).

Postmortem CNS specimens from 2 patients were analyzed by histopathologic and immunohistochemical examination. Control CNS tissues were postmortem specimens from patients with other forms of encephalitis.

The Mann-Whitney rank-sum test was used to compare the length of prodromes for patients with favorable outcomes with those for patients with unfavorable outcomes; p<0.05 was considered statistically significant. Pearson’s correlation was used to determine relationships between continuous variables.

During 1970–2010, a total of 19 cases of EEE in children in Massachusetts and New Hampshire were reported to the Centers for Disease Control and Prevention; our case series includes 15 of those 19 cases. Thirteen of the patients were from Massachusetts and 2 were from New Hampshire. The median age was 5.3 years (range 0.5–14.7 years). Medical care was sought for all patients during August, September, or October. The clinical features, diagnostic results, treatments, and outcomes for each patient are described in Table 1.

Of the 15 patients, 4 (27%) died, 5 (33%) had severe neurologic sequelae, 2 (13%) had mild to moderate deficits, and 4 (27%) fully recovered. The median duration of hospitalization was 16.0 days. The median duration of hospitalization for patients who died was 6.6 days.

Fever was reported for all patients on or near day 1 of hospitalization. Signs and symptoms of meningoencephalitis, such as seizures, headaches, nausea, vomiting, and stiff neck, were observed for all patients (Table 2). Of the 10 patients who had seizures, 5 had complex partial seizures, 3 had generalized tonic–clonic seizures, and 2 had both types of seizures.

The prodrome phase lasted >3 days for 7 patients and <3 days for 8 patients (median 2 days, range 0–8 days) (Table 1). Patients with a longer prodrome typically reported a few days of nonspecific symptoms, such as fever, headache, and nausea followed by slower progression of neurologic symptoms, including photophobia, stiff neck, altered mental status, and seizures. In contrast, patients with a shorter prodrome often had rapidly declining mental status and seizures. Of the 15 patients, 13 (87%) became stuporous or comatose during the first 3 days of hospitalization (median 2 days). The remaining 2 patients became delirious but remained responsive.

CSF samples from all patients had neutrophilic pleocytosis, and 12 (86%) had elevated levels of protein (Table 3). Lumbar puncture was performed a median of 2.0 days (range 0–8 days) after symptom onset. Of 14 patients, 13 (93%) had an elevated peripheral leukocyte count on or before day 1 of hospitalization, and 8 (57%) had peripheral neutrophilia.

Electroencephalogram results were available for 14 patients. A pattern of generalized slowing of brain electrical activity was observed in 13 patients; subclinical status epilepticus was noted for 1 of these patients, and epileptiform activity was noted for 5. No correlation could be drawn between electroencephalogram findings and outcome.

Six of the 9 initial head computed tomography (CT) scans were abnormal. The initial CT scan was performed at a mean of 1 day (range 0–2 days) after the onset of neurologic symptoms. Lesions were most commonly found in the basal ganglia, cerebral cortex, and thalamus (Table 4 and Figure 1, panel A). These abnormalities were often subtle and initially were underreported, particularly the cortical lesions.

Magnetic resonance imaging (MRI) was performed for 12 patients; all images showed evidence of abnormalities. The initial MRI was performed a mean of 2 days (range 1–4 days) after onset of neurologic symptoms. MRI results most commonly revealed abnormalities in the meninges, the cerebral cortex, basal ganglia, and thalamus (Table 4 and Figure 1, panels B–D). Lesions were most apparent on fluid-attenuated inversion recovery, diffusion-weighted imaging, and T1-weighted postgadolinium sequences, and they were more evident on MRI than CT images. The abnormalities seen on MRI or CT colocalized with areas of inflammation and EEE virus seen in pathology specimens (Table 5), but the location and intensity of the abnormalities seen on images did not correlate with clinical outcomes.

All patients received empiric antimicrobial therapy, and most received acyclovir for empiric coverage of herpes simplex encephalitis. Patients also received antiepileptic medications; immunomodulatory agents, including intravenous immunoglobulin, steroids, cyclophosphamide, and interferon-α2b; and treatments for increased intracranial pressure. Intracranial pressure was monitored for 3 patients (Table 1). No patient underwent intraventricular drain placement or a decompressive craniectomy.

To identify variables associated with clinical outcomes, we compared clinical, laboratory, and imaging findings for patients with favorable outcomes with those for patients with unfavorable outcomes. The length of prodrome was associated with clinical outcome at the time of discharge (Figure 2). The prodrome for patients with a favorable outcome was significantly longer than the prodrome for patients with an unfavorable outcome (median 5 vs. 2 days; p = 0.002). The 8 patients for whom the prodrome lasted <2 days had a poor outcome; all 4 deaths occurred in this group. For these 8 patients, the relative risk was 6.0 for an unfavorable outcome. Of the 7 patients for whom the prodrome lasted >3 days, 6 had a favorable outcome, including 4 patients who fully recovered.

Outcome was not associated with any other variable, including patient age, seizure, headache, photophobia, quantity of cerebrospinal fluid pleocytosis, total protein in the cerebrospinal fluid, serum sodium level, or leukocyte count in the blood. Moreover, no significant association was observed between the patient’s clinical outcome and the therapy received.

Follow-up information was available for 10 of the 11 surviving patients. Duration of follow-up ranged from 3 months to 20 years (median 14.5 months). Patient outcomes at follow-up visits were improved (40%) or unchanged (60%) since the time of discharge. Of the 4 patients with severe deficits at hospital discharge, 3 showed improvement at the follow-up visit (Table 1). Patient 3 progressed from being comatose to being able to communicate nonverbally by 30 months after hospital discharge. Patient 4 recovered speech, comprehension, and the ability to self-feed by 18 months after hospital discharge. Patient 6 progressed from being nonvocal and having limited spontaneous movements at discharge to speaking in full sentences by 21 months after hospital discharge. These results demonstrate the opportunity for substantial cognitive and motor rehabilitation in patients with EEE.

Tissue sections from the postmortem brain specimens of patients 10 and 12 demonstrated severe meningoencephalitis. Gross findings were notable for marked, diffuse cerebral edema. There was prominent acute and chronic perivascular inflammation within the cortex, thalamus, basal ganglia, and brainstem (Figure 3, panels A, B).

A multifocal, patchy distribution of EEEV-stained tissue was observed in specimens from patients10 and 12. In the cerebral hemispheres, clusters of infected cells were predominantly in the gray matter. There were distinct, punched-out islands of pallor in the thalamus and the basal ganglia, representing areas of tissue rarefaction and damage (Figure 3, panels C, E). These regions corresponded to areas demonstrating EEEV infection (Figure 3, panels D, F and Table 5). On the basis of cell morphologic appearance and immunohistochemical staining for neuronal cell marker NeuN, neurons appeared to be the predominant cell population infected by EEEV (Figure 4, panels A–C).

The anatomic location of EEEV, inflammatory infiltrates, neuronal death, and rarefied tissue corresponded to the location of the abnormalities seen on MRI (Table 5). These findings provide direct evidence that the abnormalities seen on neuroimaging represent virus-induced inflammatory changes.