The difficulties with vitamin B12.

A 22-year-old woman presented with progressive sensory ataxia and optic neuropathy. Previous investigation by her general practitioner had found a low serum vitamin B12, which had been corrected with oral supplementation. Neurological investigations showed raised plasma homocysteine and methylmalonic acid towards the upper limit of normal with a low serum vitamin B12 MRI showed an extensive cord lesion in keeping with subacute combined degeneration of the spinal cord. We treated her with high dose parenteral vitamin B12 and she has made a partial recovery. We discuss the management of patients who present with neurological manifestations of vitamin B12 deficiency; highlighting the fact that parenteral replacement is needed in such cases, even if the serum vitamin B12 level appears to be normal. We also discuss ancillary investigations that should be performed in patients with suspected vitamin B12 deficiency.