A 25-year-old woman presented with profound weakness and altered sensation. She had undergone a laparoscopic Roux-en-Y gastric bypass 5 months previously for obesity, with a preprocedural weight of 192.8 kg and a body mass index (BMI) of 68 kg/m².

Following the gastric bypass she developed nausea and vomiting such that she drastically reduced her food intake. She was unable to adhere to the advice to have small meals up to six times per day and she did not take the nutritional supplements prescribed apart from regular vitamin B₁₂ injections. She therefore rapidly lost weight. A stricture of her gastrojejunal anastomosis was found which was thought to be causing her symptoms and so it was dilated.

Prior to the stricture dilatation she had had a 2-month history of bilateral tingling and numbness in her legs. This continued to worsen in the 2 weeks following the dilatation with ascending numbness and weakness in her legs and then her arms, such that she became unable to stand or even sit unsupported. There was, however, no associated bowel or bladder involvement.

She had had depression in the past but on admission was not taking any regular medications.

On admission she weighed 140.8 kg. Her BMI was 45 kg/m². Her general examination was unremarkable apart from a pruritic, annular rash with a scaly border on the dorsal aspect of both hands (figure 1). She reported that had been present for 8 weeks.

There was no cranial nerve abnormality or neck weakness. She had marked limb weakness that was more pronounced distally than proximally, with a Medical Research Council (MRC) total score of 38 (22 for the arms and 16 for the legs). Her reflexes were absent and plantar responses were flexor. Pinprick sensation was absent below her elbows and below the T5 …