When Confusion Isn't Confusion: Anterior Thalamic Infarction Mimicking Encephalitis

A 51-year-old man presented with sudden-onset confusion, impaired recall of familiar names, and isolated left eye ptosis without headache, fever, focal neurological deficits, or loss of consciousness. He was fully conscious on arrival, the Glasgow Coma Scale score was (GCS 15/15) with normal motor and sensory examination, and initial laboratory tests and CT brain imaging were unremarkable apart from low vitamin B12 and vitamin D levels, elevated HbA1c, and raised homocysteine. Empirical acyclovir and Pabrinex were commenced for suspected encephalitis, but although his confusion improved by day four, persistent memory impairment necessitated further evaluation. Differential diagnoses included encephalitis, transient global amnesia (TGA), and stroke; however, TGA was considered less likely due to the prolonged course. Magnetic Resonance Imaging Head later demonstrated a focal T2/FLAIR hyperintensity with restricted diffusion in the left anterior thalamus, confirming an acute thalamic infarct, after which antiplatelet therapy and a statin were initiated, and an outpatient stroke workup was arranged. During admission, he developed stage 3 acute kidney injury secondary to acyclovir, which resolved with treatment cessation and intravenous fluids. At discharge, confusion had resolved, but memory deficits remained, with significant cognitive improvement noted at three-month follow-up despite ongoing reduced concentration. This case highlights the diagnostic challenges of acute cognitive dysfunction without overt focal deficits and underscores the value of early neuroimaging in identifying central causes of confusion.