MTHFR and mitochondrial neurogastrointestinal encephalomyopathy: Demyelinating polyneuropathy can also be prevalent in homocysteine remethylation disorders—the adult form of cobalamin C disease (CblC) and MTHFR (methylenetetrahydrofolate reductase) deficiency causing gait-ataxia and psychiatric symptoms, often together with a cognitive decline (which are better responsive to treatment than early onset forms [41]), as well as in MNGIE (mitochondrial neurogastrointestinal encephalomyopathy) [42].