Physicians frequently bemoan the lack of effective therapeutic interventions in neurology. However, herpes simplex virus (HSV) encephalitis represents a medical success story: mortality in untreated HSV encephalitis is ≈70%—with high-dose intravenous acyclovir it falls to <20%.1

In the UK, acute encephalitis is a rare syndrome. Exact figures are not known despite the requirement by statute for notification of clinical suspicion. It can affect all age groups, with highest incidence in infants and those aged over 65 years. Most cases are sporadic. Recent data from England show that 42% had an infectious aetiology, 21% an immune-mediated cause, whereas 37% had no identifiable cause.2 One-quarter of the cases were caused by acyclovir-sensitive viruses (HSV type 1 and 2; varicella zoster virus). However, the morbidity and mortality was high across aetiologies, including those of unknown cause. Of those with HSV encephalitis, 11% died and at 6 months only 39% had a good outcome (Glasgow outcome score >4). The worst outcome was seen in patients identified retrospectively to have had an antibody-associated encephalitis (voltage-gated potassium channel antibody or N-methyl d-aspartate receptor antibody encephalitis).

How can we improve outcome? For HSV …