Vitamin B-12, a co-factor used by enzymes involved in fatty acid metabolism, helps form myelin sheaths, which surround nerves. Deficiency of B-12 causes these enzymes to not function properly. Consequently, unusual fatty acids with branched chains accumulate in the myelin around nerves and demyelination occurs, causing nerve damage, or neuropathy.
B-12 Deficiency and Feet
B-12 deficiency causes neuropathy manifested by numbness and tingling, or paresthesias, according to "Office Practice of Medicine." Numbness and tingling occurs at the tips of the toes or balls of the feet. Progressively, tingling spreads up the lower legs. Sensory loss occurs over both feet, ankle reflexes are lost and weakness of moving toes upward occurs, which is best seen in the big toe.
Vitamin B-12 deficiency-induced neuropathy occurs with low normal levels of B-12.
B-12 Deficiency and Legs
As neuropathy worsens, it becomes more severe in the legs than the arms. The sensory deficit distribution follows a stocking distribution, meaning that if a patient with B-12 deficiency puts on a stocking, it would cover the area involved.
B-12 Deficiency and Fingers and Hands
By the time tingling reaches the upper shin, tingling occurs in the tips of the fingers. The sensory deficit distribution follows the pattern of a glove.
B-12 Deficiency and Subacute Combined Degeneration of the Spinal Cord
Vitamin B-12 deficiency results in an acquired severe condition known as subacute combined degeneration of the spinal cord. Since the cervical, or upper spinal cord degenerates, the neuropathy appears first in the hands, followed by numbness of both feet, as occurred in the 38-year-old woman reported by Dr. Kshitij Mankad in "The American Journal of Medicine." Patients often become unsteady and have difficulty with balance and gait. Vitamin B-12 injections can cure this illness, if they are started within a few months of symptom onset.