In suspected encephalitis the treatable diagnosis is herpes simplex, outcome depends heavily on how quickly aciclovir starts, and the two things that most often delay it are waiting for imaging and waiting for the lumbar puncture.
If a patient has fever with altered consciousness, new seizures, or focal neurology with confusion, start intravenous aciclovir at 10 mg/kg eight-hourly as soon as the possibility is raised. Imaging and cerebrospinal fluid can follow; a normal early MRI does not exclude the diagnosis and neither does an early PCR, which can be negative in the first 72 hours. Where suspicion is high and the first PCR is negative, repeat it rather than stopping treatment.
Two practical points. Dose by ideal body weight in obesity and adjust for renal function, because aciclovir nephrotoxicity is common and largely preventable with adequate hydration and a sensible infusion rate. And do not let a plausible alternative diagnosis — alcohol withdrawal, sepsis, a post-ictal state — close the question early; those are exactly the patients in whom herpes encephalitis is found late.
- Start intravenous aciclovir on suspicion, before imaging and lumbar puncture results
- A negative early cerebrospinal fluid PCR does not exclude the diagnosis — repeat it
- Dose by ideal body weight and renal function, and hydrate to prevent crystal nephropathy
- A normal early MRI is not reassurance; changes may take days to appear
- Keep the diagnosis open in patients whose confusion has an easier explanation
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