A case of acyclovir neurotoxicity presenting with atypical cerebrospinal fluid findings

An 82-year-old man with a history of end-stage renal disease presented with progressively worsening confusion and somnolence for the past 4-5 days. The patient was diagnosed with herpes zoster by his primary care physician 5 days ago and was started on a course of valacyclovir 1 g three times a day (dose not adjusted for renal impairment).A lumbar puncture was performed and cerebrospinal fluid (CSF) studies revealed 37 white blood cells (WBCs)/hpf (100% monocytes), protein 64 mg/dL and glucose 52 mg/dL. He was started on ceftriaxone, ampicillin and acyclovir. MRI of the brain was done and was unremarkable. Acyclovir-induced encephalopathy was high on differential, but his CSF findings were concerning for viral encephalitis. Nonetheless, all antimicrobials were discontinued and he was scheduled for a 5-hour dialysis session. The very next day, he showed immense improvement and eventually recovered completely. CSF PCR tests for both herpes simplex virus and varicella zoster virus came back negative.