Sensory ataxia
B12 / functional B12 deficiency
TractsDorsal columns + lateral corticospinal
MRIDorsal-column “inverted V” sign
LabsB12 <200 + ↑MMA (most specific) + ↑homocysteine
MimicCopper deficiency (with normal B12)
Progressive myelopathy in a young “whip-it” user? Suspect nitrous oxide — functional B12 deficiency.
Key clinical features
- Dorsal columns: ↓ vibration / proprioception, sensory ataxia, positive Romberg
- Lateral corticospinal: spasticity, hyperreflexia, Babinski
- MRI dorsal-column “inverted V” sign
- Can occur WITHOUT anemia
- N₂O inactivates methionine synthase → functional B12 deficiency
- Also: cognitive decline, neuropathy, depression
◆ Signature clue
Young patient, recreational nitrous oxide, progressive myeloneuropathy → check B12, MMA, homocysteine. If B12 is normal, copper deficiency mimics the identical picture.
Diagnosis
B12 <200 pg/mL; ↑MMA (most specific) + ↑homocysteine. MRI dorsal-column “inverted V.” Check copper / ceruloplasmin if B12 is normal.
Management
IM cobalamin repletion; stop nitrous oxide. If copper-deficient: replace copper + remove the zinc source.