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34-year-old woman underwent lumbar laminectomy two months prior to presentation. Anesthesia included nitrous oxide. The day following surgery, she experienced progressive numbness beginning in the feet and spreading “like a stocking” up her legs over one month. The numbness subsequently involved her hands and spread up the arms, onto her chest and down her body. One week prior to presentation she developed unsteadiness of gait progressing to the point that she required a walker and assistance to walk even a few steps. On examination she had profound loss of joint position and vibration sensation in all extremities and a severely ataxic gait. Light touch anywhere on the limbs elicited a dysesthetic sensation. Pin prick was felt as sharp everywhere, but elicited a poorly localized tingling sensation. Muscle power was normal, deep tendon reflexes were preserved, and plantar responses were flexor.
(Left) Sagittal T2-weighted MRI; (Right) Axial T2-weighted MRI. Note the large area of increased signal in the posterior aspect of the cord. However, upon close inspection of the axial scan (right), note the abnormality almost exactly matches the distribution of the posterior columns. Serum vitamin B12 was 41 pg/mL, homocysteine was 46.4 µmol/L, methylmalonic acid was 7589 nmol/L, and intrinsic factor blocking antibody was present. Her hematocrit and mean corpuscular volume were normal. She was treated with intramuscular injections of cyanocobalamin and her symptoms immediately stabilized. Within one month she was able to walk with a cane but had persistent dysesthetic sensory symptoms over her entire body and limbs sparing her face and neck. Homocysteine and methylmalonic acid levels returned to normal. B12 deficiency most often occurs as an autoimmune disease with loss of parietal cells in the stomach leading to impaired B12 absorption. B12 deficiency can also occur in strict vegetarians. Also of importance, patients with borderline low B12 levels can have B12 deficiency provoked if exposed to nitrous oxide (NO2), often used during anesthesia.
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