Disclosures: Evan Arbit: No financial relationships or conflicts of interest

Case Description: Patient reported one month of numbness in her bilateral lower limbs that she attributed to working long hours as a waitress. In the week before presentation the sensations became more dysesthetic involving her upper limbs and trunk with increasing gait instability and multiple falls. Physical exam was notable for ataxic gait and absent vibratory and proprioceptive sensation of bilateral lower extremities.

Setting: Community teaching hospital

Patient: A 20-year-old female with recreational use of nitrous oxide Assessment/

Results: Initial serum vitamin B12 level was 254pg/mL, was reported as normal per the testing lab’s reference range (190-950pg/mL). The patient was prescribed gabapentin 200mg TID and noted only slight improvement of dysesthesias. MRI revealed abnormal T2 signal hyperintensity in the posterior and lateral columns of the cervical and thoracic spinal cord, consistent with subacute combined degeneration. Serum levels of homocysteine and methylmalonic acid were elevated to 111mol/L (normal 1 – 15mol/L) and 3.8mol/L (normal 0 – 0.4mol/L). The patient was treated with intramuscular cyanocobalamin and bilateral ankle-foot orthoses to mitigate her sensory ataxia.

Discussion: Nitrous oxide interferes with the bioavailability of vitamin B12 by oxidizing the active, reduced form of cobalt contained in the B12 molecule. Vitamin B12 is crucial for the conversion of homocysteine to methionine, for maintenance of myelin sheath. Dysfunction of this system can lead to serious neurologic complications including subacute combined degeneration of the spinal cord. This case illustrates that absolute serum B12 concentration alone is not a reliable indicator of deficiency. Methylmalonic acid and homocysteine are substrates that rely on functional vitamin B12 for their metabolism, and elevated levels can identify cases of functional B12 deficiency

Conclusion: Subacute combined degeneration can lead to devastating neurological impairments, but may be reversible if diagnosed and treated in a timely fashion. It should be considered and investigated appropriately, particularly in young patients with known substance abuse histories.

Level of Evidence: Level V

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/subacute-combined-degeneration-with-normal-serum-vitamin-b12-concentration-in-a-young-woman-with-frequent-recreational-use-of-nitrous-oxide-a-case-report/