To describe the MRI characteristics of Herpes Simplex Virus (HSV) Central Nervous System (CNS) infection.

HSV CNS infection is known to predominantly involve temporal lobe, but the incidence of extra-temporal involvement is not well described, and the clinical significance of different MRI findings is unclear. 

We reviewed records of patients with HSV CNS infection by querying acyclovir use in a clinical registry of intravenous acyclovir therapy at a tertiary medical center from January 2010 until September 2018. Diagnostic criteria for HSV CNS infections were either intrathecal presence of viral DNA or intrathecal synthesis of anti-viral antibodies with clinical signs of CNS involvement. MRI were reviewed for findings on susceptibility-weighted imaging (SWI), diffusion-weighted imaging, T2 and T1 sequence with and without contrast. We reported imaging characteristics associated with poor outcome defined as a modified Rankin scale (mRS) of more than 3 at hospital discharge.  

We identified 35 patients (median age 60, interquartile range[IQR] 44-70, 23% male) comprising of 23(66%) encephalitis (20 HSV-1, 3 HSV-2), 6 (17%) meningitis (4 HSV-2, 2 HSV-1), 5(17%) ischemic stroke (2 HSV-1, 3 HSV-2 ), 1(3%) intracerebral hemorrhage (HSV-1). Most common presentations are encephalopathy and lethargy (N=24, 69%) followed by headache (N=21, 60%), fever (N=20, 57%) and seizure (N=13, 37%). Of 29(83%) patients with abnormal brain MRI (unilateral=20, bilateral=8, isolated pachymeningeal enhancement=1), temporal lobes were involved in 25 patients (86%), 13(45%) had involvement of extratemporal areas including frontal lobe(N=9, 32%), occipital lobe (N=2, 7%), subcortical (N=7, 24%), infratentorial(N=4, 13%). Lesions seen on MRI are T2 hyperintensity(N=27, 93%), restricted diffusion (N=21, 72%), SWI lesions (N=6, 21%) and contrast enhancement (N=16, 55%). Patients with restricted diffusion lesions (p=0.003), extra-temporal and bilateral involvement (p=0.03 and 0.04 respectively) were found more likely to have unfavorable outcome.  

Extra-temporal involvement is common in patients with HSV CNS infection, it is associated with unfavorable outcomes.