Subacute Combined Degeneration of the Spinal Cord: A Consequence of Recreational Nitrous Oxide Use.

A 21-year-old female presented to the hospital with worsening bilateral lower extremity weakness and sensory changes in the distal extremities following chronic nitrous oxide (N2O) abuse. Laboratory and radiographic results were suggestive of subacute combined degeneration of the upper cervical and thoracic spinal cord in the setting of a normal vitamin B12 level of 374 pg/mL with an elevation in methylmalonic acid to 1.14 mcmol/L. She was diagnosed with a relative B12 deficiency and treated with supplemental vitamin B12, resulting in an improvement in symptoms. This case highlights the importance of considering relative vitamin B12 deficiency as a diagnosis in the setting of nitrous oxide use, regardless of measured vitamin B12 level.