A 56-year-old right-handed man presented with a three-hour episode of anterograde amnesia and disorientation. He drove ten miles to a horse riding club where he joined his daughter for an hour and drove back home. His family noted that he was confused on return and had repeatedly asked his daughter whether they had been horse riding as planned. He had no memory of driving or spending time at the club.
The patient has no history of epilepsy, head trauma, migraine, or cerebrovascular disease. He was not taking regular medications. He is an ex-smoker having given up smoking 20 years ago.
Neurological examination including cognitive function was unremarkable on presentation.
1.5 Tesla magnetic resonance imaging (MRI) of the brain performed four days later revealed right hippocampal restricted diffusion (figure 1). Routine blood tests, EEG, Holter monitor, carotid Doppler, and transoesophageal echocardiogram were normal, and the toxicology screen was negative. A repeat MRI two months later showed complete resolution of signal abnormality across all the MRI sequences including T2 and fluid attenuation recovery (FLAIR).
WHAT IS THE DIAGNOSIS?