Further investigations revealed:

• Vitamin B12 : 279pg/mL
• Homocysteine : 58 nmol/mL
• MMA :1.59 nmol/mL

Further social history reveals nitrous oxide abuse, she used about 5-10 canisters of nitrous oxide per week. She is negative for HIV and other investigations were unremarkable.

A clinical diagnosis of myelopathy due to nitrous oxide toxicity was made.