LAUSR

© Author(s) (or their employer(s)) 2022. No commercial reuse. See rights and permissions. Published by BMJ. A woman presented with rapidly progressive flaccid tetraparesis, sensory ataxia, urinary retention and Lhermitte’s phenomenon that had developed over 3 days. Her symptoms had started within 24 hours of finishing a recreational nitrous oxide (N2O) binge during which she inhaled 200×125 mL N2O canisters over 72 hours. There was no history of prior N2O use. Examination showed weakness in the legs and arms (especially severe in hip and knee flexors), areflexia, bilateral extensor plantar responses, severely impaired proprioception and absent vibration sensation in the legs. She was bedbound and required hoist transfers. MR scan of cervical spine showed features of combined degeneration, with an inverted Vshaped T2 hyperintensity in the posterior columns of the cervical cord (figure 1). Investigations showed haemoglobin 128 g/L (normal 115–155), mean corpuscular volume 103 fL (80–99), low serum B12 117 pmol/L (170–800) and elevated methylmalonic acid 14.50 μmol/L (normal less than 0.40). She was not known to have B12 deficiency before presentation, but we suspected preexisting B12 deficiency secondary to metformin use for polycystic ovarian syndrome and intentional weight loss of 20 kg using a regular diet over the