B12

A 31 year-old woman underwent lumbar laminectomy two months prior to presentation. Anesthesia included nitrous oxide. The day after surgery, she experienced ascending numbness beginning in her feet and spreading up her legs over one month, followed by numbness in her hands and arms, and then onto her chest. One week prior to presentation she developed unsteadiness of gait, requiring a walker 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. Pin prick was felt as sharp everywhere. Muscle power was normal, deep tendon reflexes were preserved, and plantar responses were flexor.

B12 Deficiency and Combined Systems Degeneration: (Left) T2-weighted sagittal MRI; (Right) T2-weighted axial MRI. Note the large area of bright signal in the posterior aspect of the cord. Upon close inspection of the axial scan (right), note that the abnormality almost exactly matches the distribution of the posterior columns. Laboratory testing revealed a serum vitamin B12 level of 40 pg/mL (normal level > 200), markedly elevated homocysteine and methylmalonic acid, along with the presence of intrinsic factor blocking antibodies. 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 causing loss of parietal cells in the stomach leading to impaired B12 absorption. Classically, the posterior columns and corticospinal tracts of the spinal cord are affected, and hence the name combined systems degeneration. It can also occur in strict vegetarians. In addition, B12 deficiency can be provoked in patients with borderline low B12 levels at baseline who are exposed to nitrous oxide, often used during anesthesia, as in this case.