Myeloneuropathy from nitrous oxide abuse: unusually high methylmalonic acid and homocysteine levels.

A 23-year-old patient developed diffuse paresthesias and sensory loss. He had mildly reduced serum vitamin B12 (B12) concentration with unusually high levels of methylmalonic acid (MMA) and homocysteine and no evidence of B12 malabsorption. Following parenteral B12 administration, his neurological deficit promptly resolved and B12 and MMA levels normalized, but elevated levels of homocysteine persisted. One year later, he admitted to inhaling nitrous oxide (N2O). After halting N2O abuse his homocysteine level normalized. This case demonstrates the importance of serum homocysteine level measurements in cases of suspected N2O toxicity [corrected].