Herpes Simplex virus encephalitis is an infection of the brain parenchyma by either HSV-1 or HSV-2. Typically, beyond the neonatal period, nearly all herpes encephalitis cases are due to HSV-1, whereas HSV-1 or HSV-2 may be the disease causing organism in neonates. HSV-1 is the most common cause of viral encephalitis worldwide; it is amongst the most important viral encephalitidies, as it is associated with greater than 70% mortality if untreated, and it is the only viral encephalitis that has effective therapy supported by clinical trials.
The clinical syndrome is usually characterized by an acute onset of headache, fever and focal neurological signs, such as: altered mental status and level of consciousness, memory disturbances, focal cranial nerve findings, ataxia, focal seizures, aphasia, dysphasia, and altered behaviour or personality changes. Other neurological findings are associated with HSV encephalitis, such as hypomania, Klüver-Bucy syndrome (KBS), Guillain-Barré syndrome (GBS), and urinary/fecal incontinence, amongst others.
Acutely in immunocompetent adults, there is typically asymmetrical and bilateral involvement of the limbic region of the brain (i.e. temporal lobes, orbitofrontal region, insula and cingulate gyrus) and inferolateral frontal lobes with sparing of the basal ganglia on MRI. In immunocompromised adults, a more diffuse involvement can be seen, and is more likely to involve the brainstem. In children, extralimbic involvement is more common than in adults, specifically in the parietal lobe. The sparing of the basal ganglia is an essential in encephalitis, as it can help differentiate this from an MCA infarct.
On T1 imaging, there may be general edema in the affected region, as well as hyperintense signals if a subacute hemorrhage has occurred. On T1C+ imaging, there is usually no enhancement; however, later in the disease course, there is altered signal and gadolinium enhancement within the temporal lobe and may show involvement of the insula, orbitofrontal cortex or cingulate gyrus. On T2 imaging, hyperintensity of cortex and white matter are suggestive of HSV encephalitis. DWI/ADC is more sensitive than T2-weighted imaging, and we can see restricted diffusion due to cytotoxic edema, but it is less intense than infarction.
References:
I. Herpes simplex encephalitis: A review. Levitz, Robert E. Heart & Lung: The Journal of Acute and Critical Care, Volume 27, Issue 3, 209-212.