[A case of subacute combined degeneration with normal serum vitamin B12 level]

A 40-year-old woman was admitted to our hospital because of pancytopenia with megaloblastic anemia. Two months later she complained of rapidly progressive gait disturbance and numbness in the distal part of limbs. She also told that her hair had turned totally gray in the third decade. Neurologically, mental state, cranial nerves and cerebellar functions were normal. Superficial sensations were impaired below the lower thoracic level and deep sensations were completely lost in the lower limbs. Moderate weakness was found in the lower limbs, symmetrically. Deep tendon reflexes were diminished in the upper limbs and absent in the lower limbs. Babinski's reflex was positive bilaterally. MR images of the spinal cord showed hyperintensity in the posterior column below the thoracic cord. Although the serum level of vitamin B12 was within normal range, serum homocysteine level was elevated markedly. Under the diagnosis of subacute combined degeneration (SCD) due to possible vitamin B12 deficiency, the treatment with intravenous injections of 500 micrograms/day of mecobalamin was undertaken. Muscle strength and sensory impairment improved progressively and she became able to walk with a cane. The coloration of her gray hair was also noted. After treatment, pancytopenia and megaloblastic anemia also markedly improved. Vitamin B12 became high in serum concentration and the serum level of homocysteine became normal. These clinical and laboratory findings support the diagnosis of SCD with normal serum level of vitamin B12 in our case, suggesting that the level of vitamin B12 in serum does not always correlate with that in tissue and, therefore, SCD should not be excluded just only by the reason of normal serum vitamin B12 level.