Herpes simplex virus (HSV) encephalitis is a life-threatening consequence of HSV infection of the central nervous system. Although HSV encephalitis is rare, mortality rates reach 70 per cent in the absence of therapy and only a minority of individuals return to normal function. Antiviral therapy is most effective when started early, necessitating prompt diagnosis.

A case of atypical HSV encephalitis is reported. The appearance of a strong headache followed by impairment of consciousness and hypertone of arms and legs complicated the post-operative course in a 33-year-old patient who underwent surgical removal of an acoustic neuroma. Several brain magnetic resonance imaging (MRI) and computed tomography scans performed in the first week after onset of symptoms of infection did not establish a proper diagnosis. Diffusion-weighted MRI detected brain abnormalities on the fourth day after onset of symptoms, and polymerase chain reaction identification of HSV 1 DNA confirmed the diagnosis. A positive prognosis was achieved due to the decision to start specific, high-dose antiviral therapy based on clinical suspicion, before a firm diagnosis was established.