A 56-year-old teetotaller man with hypertension and gout presented with a week duration of painless worsening diplopia on a background of loss of weight and appetite, generalised lethargy and weakness for 1 year. On examination, he was noted to be hypothermic and tachycardic with generalised muscle wasting. Proximal myopathy, lower limb fasciculations and areflexia, restricted bilateral eye abduction and nystagmus were observed. Blood investigations demonstrated compensated lactic acidosis, acute kidney injury and leucocytosis. A nerve conduction study showed severe length-dependent axonal sensorimotor polyneuropathy. This was a diagnostic dilemma until an MRI brain revealed symmetrical signal abnormality and enhancement in the periaqueductal area indicative of Wernicke’s encephalopathy, caused by thiamine deficiency from poor nutrition. Beriberi, also caused by thiamine deficiency, accounted for his tachycardia, polyneuropathy, areflexia, hypothermia and biochemical abnormalities. Both beriberi and Wernicke’s encephalopathy are medical emergencies, which were treated with intravenous thiamine to good effect.