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Neuroradiology Case of the Week

Case 50

Ravinder Sidhu MD, Lawrence Buadu MD, PhD,
and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 60-year-old female presented with history of headache and seizure.

Radiological Findings: Axial T1 and T2-weighted MR images revealed large area of abnormal high T2 signal in right temporal lobe (Fig.1A&B). Post-contrast MR images showed large irregular areas of enhancement in the right temporal and hippocampus region. Smaller areas of increased T2 signal were also seen in left temporal region (Fig. 2). Diffusion weighted images were unremarkable.

Diagnosis: Herpes encephalitis

Discussion:  Herpes simplex encephalitis is one of the most common viral infections. Herpes encephalitis seen in infants is caused by herpes simplex virus II (genital herpes) whereas herpes encephalitis seen in children and adults is caused by herpes encephalitis type I (oral herpes).
    Clinical symptomology may be non-specific, such as headache, fever, mental deterioration, and seizures. Diagnosis may be difficult due to variable presentation. Early diagnosis is essential as the prognosis is dependent on early treatment. With the advent of MR and diffusion imaging, it is easier to detect the lesions at an early stage. However, diagnostic proof is the detection of viral DNA by polymerase chain reaction in the cerebrospinal fluid [1].
    Herpes simplex virus type I has particular predilection for the limbic system with localization of infection to temporal lobes, insular cortex, subfrontal area, and cingulated gyri. It is usually unilateral initially, however sequential bilateral involvement is highly suggestive of the disease. Histopathologically, herpes infection is a fulminant necrotizing meningoencephalitis associated with edema, necrosis, hemorrhage, and encephalomalacia [2].
    Non-contrast enhanced CT may show ill-defined low attenuation areas with patchy or gyriform enhancement on contrast administration. MR is more sensitive than CT scan in detecting early lesions. Typical early findings include gyral edema seen as low intensity signal on T1-weighted MR images and high signal intensity in the temporal lobe/cingulated gyrus on T2-weighted MR images. The signal abnormality often extends into the insular cortex and spares the putamen. Post-contrast studies may show mild enhancement. Areas of patechial hemorrhage may also be seen. On delayed MR imaging (1-2 weeks after the onset of infection) there may be wide spread abnormalities with involvement of the contralateral temporal lobe. Significant contrast enhancement as well as subacute hemorrhage may be well appreciated. Delayed sequale includes encephalomalacia, atrophy, and dystrophic calcification [3].
    Recently, diffusion imaging has been seen to be far more superior to conventional MR imaging in early detection of lesions. In a study by Heiner [4], diffusion imaging was seen to have a higher sensitivity in picking up early subtle findings of herpes encephalitis as compared to conventional MR imaging [4].
    To conclude, bilateral mesotemporal high intensity lesions on T1WI and FLAIR MR images in a patient with clinical syndrome of encephalitis is considered to be a classic finding for herpes infection. However, close but rare mimickers are neurosyphilis and primary central system lymphoma [5]. In such a diagnostic dilemma, polymerase chain reaction for viral DNA from the cerebrospinal fluid is the mainstay examination.

References:

1. Ohta K, Funaki M, Tanaka M, Suzuki N. Early cerebellar involvement on diffusion-weighted magnetic resonance images in herpes simplex encephalitis. J Neurol 1999; 246:736-8.
2. Jordan J, Enzmann DR: Encephalitis. Neuroimaging Clin. N. Amer. 1997; 1:17-38.
3. Osborn AG. Infection, white matter abnormalities, and degenerative diseases. Diagnostic Neuroradiology. St. Louis; Mosby, 1994; pp 694-696.
4. Heiner L, Demaeral Ph. Diffusion- weighted MR imaging findings in a patient with herpes simplex encephalitis. European Journal of Radiology 2003; 45:195-198.
5. Bash S, Hathout GM, and Cohen S. Mesiotemporal T2-weighted hyper intensity: Neurosyphilis mimicking herpes encephalitis. Am J Neuroradiol 2001; 22:314-316.