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QUESTION:
How can the use of vitamin B12 (cyanocobalamin) supplements prevent or treat neuropathy?
ANSWER:
Vitamin B12 is a well known water soluble vitamin necessary for a number of metabolic reactions and prevention of medical complications, most commonly hematopoietic disorders and spinal cord related neuropathies.1-4 Unfortunately, approximately 1.5% to 15% of the general population are known to be deficient of vitamin B12, thereby putting them at increased risk for these medical complications.
As it relates to neuropathy, the most common complication related to vitamin B12 deficiency is a neuroanatomical abnormality commonly seen in the spinal cord. The below diagram represents a normal cross section of a spinal cord (on top) and a cross section representing the areas affected during deficiencies in vitamin B12. As a quick review, the spinal cord is primarily a bunch of nerve fibers and cell bodies that are transmitting nerve impulse from the periphery to the brain (cerebral cortex) and from the cerebral cortex to the peripheral parts of the body.5 The white areas in the cross section represent nerve fibers that are myelinated. The gray areas (mainly the dorsal and anterior horns) are unmyelinated nerve fibers. As previously stated, the nerve fibers are transmitting information up and down the spinal cord. Each area of the spinal cord generally represents a certain type of nerve fibers and information being communicated with the brain. As it relates to nerve fibers that are commonly affected by vitamin B12 deficiency, there are two general areas or "tracks" of nerve fibers being affected. Those two areas represented on the images below are the dorsal columns of the spinal cord and the lateral corticospinal tract.3
The dorsal columns are further broken down into the fasciculus cuneatus (representing nerve fibers traveling the upper body and extremities) and fasciculus gracilis (representing nerve fibers traveling up the spinal cord from the lower body and extremities).5 The nerve fibers in these two areas of the dorsal column are carrying sensory information (pressure, vibration, touch, proprioception) to the brain for interpretation.5 As noted in the bottom figure, this area is affected by vitamin B12 deficiency and more specifically these nerve fibers basically lose their myelination.3 A demyelination of these fibers will significantly impact the delivery of sensory information to the brain. As such, patients with neuropathy associated with vitamin B12 will commonly present with reductions in the ability to perceive feelings of pressure, vibration, touch, as well as position of the body part in space (i.e., proprioception).
How does vitamin B12 deficiency result in a demyelination of these nerve fibers?
In
addition to its use in a number of other cellular reactions or
functions, vitamin B12 is also involved in the metabolism of odd-chain
fatty acids (methylmalonyl CoA directly and propionyl CoA indirectly).3 Normally odd-chain fatty acids are converted to propionyl CoA which is then converted to methylmalonyl CoA. The methylmalonyl CoA then gets metabolized to succinyl CoA by the enzyme methylmalonyl CoA mutase and vitamin B12.3 When vitamin B12 is deficient, there is an accumulation of both propionyl CoA and methylmalonyl CoA.
The accumulation of propionyl CoA replaces the acetyl CoA in neuronal
membranes, thereby resulting in a demyelination of the nerve fibers.3
The replacement of acetyl CoA within the neuron could also impact
acetylcholine synthesis since it is derived from acetyl CoA and choline.3 A reduction in acetylcholine could cause dementias thus mimicking neurologic symptoms similar to that of Alzheimer's disease.
Therefore, vitamin B12 deficiencies can result in peripheral neuropathies, sensorimotor dysfunction, and even dementia. Given that these complications are also common to other disease states, the inclusion of vitamin B12 deficiency on the differential diagnosis would be prudent to consider especially given its ease of reversibility in most cases.
TAKE HOME POINTS:
Approximately 1.5% to 15% of the general population are known to be deficient of vitamin B12 thereby putting them at increased risk for these medical complications including neuropathy.
Vitamin B12 deficiency results in a demyelination of ascending nerve fibers located in the dorsal columns and descending nerve fibers in the lateral corticospinal tracts of the spinal cord.
In addition, vitamin B12 deficiency could also affect the synthesis of acetylcholine thereby resulting in dementia.
This demyelination of various nerves within the spinal cord and possible impact on acetylcholine synthesis is the reason that patients with vitamin B12 deficiency present with signs and symptoms of peripheral neuropathy, sensorimotor dysfunction, and/or dementia.
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