Megavitamin-B6 syndrome has been reported in doses as low as 24 mg/day.Symptoms may also be dependent on the form of vitamin B6 taken in supplements. It has been proposed that vitamin B6 in supplements should be in pyridoxal or pyridoxal phosphate form rather than pyridoxine as these are thought to reduce the likelihood of toxicity. A tissue culture study, however, showed that all B6 vitamers that could be converted into active coenzymes (pyridoxal, pyridoxine and pyridoxamine) were neurotoxic at similar concentrations. It has been shown, in vivo, that supplementing with pyridoxal or pyridoxal phosphate increases pyridoxine concentrations in humans, meaning there are metabolic pathways from each vitamer of B6 to the all other forms. Consuming high amounts of vitamin B6 from food has not been reported to cause adverse effects.Early diagnosis and cessation of vitamin B6 supplementation can reduce the morbidity of the syndrome. Cause
While vitamin B6 is water-soluble, it has a half-life of 25–33 days and accumulates in the body where it is stored in muscle, plasma, the liver, red blood cells and bound to proteins in tissues. Potential mechanisms
The common supplemental form of vitamin B6, pyridoxine, is similar to pyridine which can be neurotoxic. Pyridoxine has limited transport across the blood–brain barrier explaining why the central nervous system is spared. Cell bodies of motor fibers are located within the spinal cord that is also restricted by the blood-brain barrier explaining why motor impairment is rare.
Megavitamin-B6 syndrome is a collection of symptoms that can result from chronic supplementation, or acute overdose, of vitamin B6. While it is also known as hypervitaminosis B6, vitamin B6 toxicity and vitamin B6 excess, megavitamin-b6 syndrome is the name used in the ICD-10. Signs and symptoms
The predominant symptom is peripheral sensory neuropathy that is experienced as numbness, pins-and-needles and burning sensations (paresthesia) in a patients limbs on both sides of their body. Patients may experience unsteadiness of gait, incoordination (ataxia), involuntary muscle movements (choreoathetosis) the sensation of an electric zap in their bodies (Lhermittes sign), a heightened sensitivity to sense stimuli including photosensitivity (hyperesthesia), impaired skin sensation (hypoesthesia), numbness around the mouth, and gastrointestinal symptoms such as nausea and heartburn. The ability to sense vibrations and to sense ones position are diminished to a greater degree than pain or temperature. Skin lesions have also been reported. Megavitamin-B6 syndrome may also contribute to burning mouth syndrome. Potential psychiatric symptoms range from anxiety, depression, agitation, and cognitive deficits to psychosis.Symptom severity appears to be dose-dependent (higher doses cause more severe symptoms) and the duration of supplementation with vitamin B6 before onset of systems appears to be inversely proportional to the amount taken daily (the smaller the daily dosage, the longer it will take for symptoms to develop). It is also possible that some individuals are more susceptible to the toxic effects of vitamin B6 than others.