Deficiency of vitamin B12 often causes anemia, but its most severe and potentially irreversible effects are related to nervous system lesions.
The neurologic symptoms may present in the course of a few weeks, initially with slight ataxia and numbness and tingling in the lower extremities, but may progress rapidly to include spastic weakness of the lower extremities. Complete paraplegia may occur, usually only later in the course of the disease.
With prompt vitamin replacement therapy, clinical improvement occurs; however, if complete paraplegia has developed, recovery is poor.
On microscopic examination, vitamin B12 deficiency leads to a swelling of myelin layers, producing vacuoles that begin segmentally at the midthoracic level of the spinal cord in the early stages.
With time, axons in both the ascending tracts of the posterior columns and the descending pyramidal tracts degenerate.
While isolated involvement of descending or ascending tracts may be observed in a variety of spinal cord diseases, the combined degeneration of both ascending and descending tracts of the spinal cord is characteristic of vitamin B12 deficiency and has led to the designation of the disorder as subacute combined degeneration of the spinal cord.