Lessons of the month: Nitrous oxide-induced functional vitamin B12 deficiency causing subacute combined degeneration of the spinal cord

We discuss the case of a 22-year-old man who presented with paraesthesia, reduced sensation and weakness in his limbs. Examination was in keeping with a myeloneuropathy. Initial investigations including vitamin B-12 were unremarkable but magnetic resonance imaging of the spinal cord showed subacute combined degeneration. The patient reported heavy recreational use of nitrous oxide, which can cause functional deficiency of vitamin B-12 with neurological sequelae. A diagnosis of functional vitamin B-12 deficiency was made and confirmed by an elevated methylmalonic acid level. The patient received intramuscular hydroxocobalamin and made a good recovery following rehabilitation. Nitrous oxide use is prevalent and can have significant health effects. Many adverse effects are mediated through inactivation of vitamin B-12 and can be detected by elevated homocysteine and methylmalonic acid levels. Early identification and prompt treatment are important to support neurological recovery.