Matthew Ruhl^1
1Department of Chemical Pathology, Pathology Queensland

Nitrous oxide, or “laughing gas”, is a commonly used inhaled anaesthetic, usually as an adjunct to other agents.¹ It also has significant potential for recreational misuse, with sequelae including gastrointestinal, neurological, haematological and psychiatric symptoms.¹
I describe a patient with several issues arising from nitrous oxide misuse. A 51-year-old man presented to emergency via ambulance with a reduced GCS of 14 (E₄V₄M₆) following loss of consciousness, with associated bradycardia and hypotension. This was on a background of right hip osteoarthritis for which he had been self-medicating with 6-8 kg of inhaled nitrous oxide daily for over 7 years. He had paraesthesia in both feet.
Normocytic hyperchromic anaemia was noted on the patient’s full blood count, with haemoglobin of 127 g/L (135-180), mean cell volume of 98 fL (80-100) and mean cell haemoglobin of 33.5 pg (27.0-33.0). Serum vitamin B₁₂ was low, at 59 pmol/L (133-680), and serum homocysteine showed marked hyperhomocysteinaemia of 410 μmol/L (<15), with associated elevated serum methylmalonate of 6.6 μmol/L (<0.40).
These results support a frank vitamin B₁₂ deficiency secondary to chronic massive nitrous oxide misuse. The full blood count results did not show the expected macrocytic anaemia, though this is less common than believed.² The patient was treated with 1 mg cyanocobalamin intramuscularly daily, alongside 1 g methionine orally three times a day, both for 14 days; and 100 mg thiamine orally daily, ongoing, with later resolution of paraesthesia. He declined review by alcohol and other drug services and was referred for hip arthroplasty.