[Neurosyphilis: important differential diagnosis of herpes simplex encephalitis].

Diagnosis of herpes simplex encephalitis in the acute stage is based on clinical symptoms (nonspecific prodromi, neuropsychological deficits, epileptic seizures) in combination with typical CSF abnormalities (lymphomonozytic pleocytosis) and MR imaging abnormalities assumed to be typical for herpes simplex encephalitis (increased fluid-attenuated inversion recovery and T2 hyperintensities in the mesiotemporal lobe region). Definite diagnosis of herpes simplex encephalitis is based on positive polymerase chain reaction in the CSF, usually available some days after hospital admission. Suspected herpes simplex encephalitis requires immediate treatment with acyclovir. Bacterial encephalitis caused by spirochetes may present with similar features but requires different treatment. This should therefore be considered in the differential diagnosis of herpes simplex encephalitis. We report a young patient with neurosyphilis whose correct diagnosis could be made only several days after beginning specific treatment.