Subacute combined degeneration with high serum vitamin B12 level and abnormal vitamin B12 binding protein. New cause of an old syndrome.

Subacute combined degeneration of the spinal cord due to vitamin B12 deficiency invariably has been associated with a low serum vitamin B12 level. We describe a young man who presented with a unique syndrome of subacute combined degeneration associated with high serum vitamin B12 level, low red blood cell vitamin B12 level, and an abnormal plasma vitamin B12-binding protein. Uptake of cobalamin by his leukocytes in vitro was inhibited by his own but not by normal control plasma. Intensive hydroxocobalamin (vitamin B12) treatment was associated with clinical and electrophysiologic recovery accompanied by normalization of mean corpuscular volume, red blood cell vitamin B12 level, plasma homocysteine, and urinary methylmalonic acid. The subacute combined degeneration was probably precipitated by treatment with folic acid as the significance of his high serum vitamin B12 level was not apparent when he first presented with megaloblastic anemia 3 years earlier. To our knowledge, this is the first example of neurologic disease associated with high serum vitamin B12 level and provides further evidence that sometimes a serum vitamin B12 level may not be a reliable guide to vitamin B12 deficiency.