“Parenthesis sign” in Eastern equine encephalitis

A 65-year-old man from the Northeastern United States presented in the summer with headaches, fever, and severe encephalopathy. Cerebrospinal fluid (CSF) demonstrated lymphocytic pleocytosis and hyperproteinorachia. Neuroimaging is shown in [Figures 1] and [2]. Serum and CSF showed positive immunoglobulin M (IgM) antibodies to Eastern equine encephalitis virus (EEEV).

Figure 1 T2/fluid-attenuated inversion recovery (FLAIR) axial MRI shows bilateral, relatively symmetric hyperintensities within the basal ganglia, insulae, subcortical white matter, and thalami, as well as in the inferior right frontal lobe. Marked linear hyperintensities are present in the external capsules (arrows), and parts of the internal capsules (arrowheads) bilaterally. This finding, known as the “parenthesis sign”, may help differentiate between Eastern equine encephalitis and other encephalitides.

Figure 2 T2/FLAIR hyperintensities extend into the mesial temporal lobes, midbrain, periaqueductal gray, and dorsal pons. Additionally, a dilated perivascular space is noted (asterisk).

EEEV is a mosquito-borne alphavirus. Although rare, it can cause severe arboviral encephalitis, with mortality >33%.[1] Magnetic resonance imaging features such as the “parenthesis sign” ([Figure 1]) can help distinguish between Eastern equine encephalitis and other encephalitides.[2] Although most publications on EEEV originate from North America, EEEV and other related alphaviruses also circulate in the Caribbean, Central America, and South America.[3] [4] [5]

Conflict of Interest

The authors have no conflict of interests to declare.

Authors' Contributions

MBS: case concept and design, literature review, manuscript writing; ACA: interpretation of data, manuscript revision for intellectual content; MV: case concept and design, acquisition of data, figure editing, manuscript revision for intellectual content.

• References

• 1 Morens DM, Folkers GK, Fauci AS. Eastern Equine Encephalitis Virus - Another Emergent Arbovirus in the United States. N Engl J Med 2019; 381 (21) 1989-1992
• 2 Nickerson JP, Kannabiran S, Burbank HN. MRI findings in eastern equine encephalitis: the “parenthesis” sign. Clin Imaging 2016; 40 (02) 222-223
• 3 Corrin T, Ackford R, Mascarenhas M, Greig J, Waddell LA. Eastern Equine Encephalitis Virus: A Scoping Review of the Global Evidence. Vector Borne Zoonotic Dis 2021; 21 (05) 305-320
• 4 Silva ML, Galiza GJ, Dantas AF. et al. Outbreaks of Eastern equine encephalitis in northeastern Brazil. J Vet Diagn Invest 2011; 23 (03) 570-575
• 5 Gil LHVG, Magalhaes T, Santos BSAS. et al. Active Circulation of Madariaga Virus, a Member of the Eastern Equine Encephalitis Virus Complex, in Northeast Brazil. Pathogens 2021; 10 (08) 10

Publication History

Received: 06 February 2022

Accepted: 20 March 2022

Article published online:
04 October 2022