Eastern equine encephalitis

Eastern equine encephalitis (EEE) is one of many viral encephalitides and results from infection with the eastern equine encephalitis virus.

Most patients have non-specific viral prodromal symptoms for approximately one week, including fevers, headache, nausea and vomiting, and malaise 1. This is followed by the emergence of neurological features, such as confusion, focal neurological signs, generalised or focal seizures, meningism, and somnolence 1. These neurological features rapidly progress and the patient's condition typically deteriorates within days to become stuporous or comatose 1.

EEE is caused by the eastern equine encephalitis virus (EEEV), a single-stranded RNA alphavirus 1,2. Mosquitos and wild birds, especially in the east and Gulf coasts of the Unites States of America, are reservoirs for the virus, which is then spread by mosquitoes 1,2. Importantly, only approximately 5% of patients with EEEV infection develop EEE 2.

Asymmetric bilateral basal ganglia and thalami involvement is classical regardless of CT or MR imaging 1-6. Less commonly, the internal and external capsules, brain stem, periventricular white matter, cortex, and leptomeninges may also be involved 1-6.

Lesions may be subtle on CT, but are most often hypodense involving the basal ganglia and thalami. In large lesions, local mass-effect may also be appreciated 1,2.

MRI is the imaging modality of choice and is nearly always abnormal in patients with EEE 1,3. The most striking feature are asymmetric high T2-weighted signal intensities classically affecting the basal ganglia and thalami 1-6. It is thought that these lesions represent a combination of oedema, inflammation, and ischaemia 1,3. As with CT, in large lesions, local mass-effect may also be appreciated 1-6.

Uncommonly, and as aforementioned, there may be additional involvement of the adjacent internal and external capsules, the brainstem (especially the midbrain), the periventricular white matter, the cortex (especially the medial temporal lobes), and the leptomeninges (see leptomeningeal enhancement) 1-6. Particular involvement of the internal and external capsules, visualised as linear T2-weighted regions of high signal, has emerged as a sign that may be unique to EEE compared to other viral encephalitides, and has been described as the ‘parentheses sign5.

In regards to other MR sequences, all of these lesions regardless of location may demonstrate low signal on T1 with variable contrast enhancement, and usually demonstrate iso-intense diffusion signal on DWI 2,6.

There is no specific antiviral treatment available for EEE, and thus supportive management (e.g. anti-epileptics, corticosteroids) is encouraged 1. Mortality is thought to be between 50-75%, however surviving infection with EEEV does confers lifelong immunity 1,3.

EEE was first recognised in the United States of America in horses from Virginia, Delaware, New Jersey, and Maryland during an equine epizootic in 1933 7,8. The first human cases followed during a 1938 outbreak in Massachusetts, when there were 25 deaths from 34 cases 7.

Other infectious causes that can cause a similar imaging pattern include the Flavivirus encephalitides:

Furthermore, other causes of increased T2 signal in the basal ganglia should also be considered, such as:

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rID: 55231
Synonyms or Alternate Spellings:
  • Eastern equine encephalitis (EEE)
  • Eastern encephalitis
  • Eastern equine encephalomyelitis
  • Triple E

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