Insidious peripheral neuropathy occurring under treatment in infantile MTHFR deficiency.

5,10-Methylenetetrahydrofolate reductase (MTHFR) deficiency was diagnosed in a 1-month-old baby with signs of cerebral distress. Under a classic treatment using methionine supplementation, methyl donor (betaine) folinic acid, vitamin B(6) and vitamin B(12), the neuromotor development was satisfactory. At 15 years of age, however, despite no clear modification of the biochemical markers in body fluids, she developed a clinically overt peripheral axonal neuropathy. Only partial clinical improvement was obtained after reinforcement of betaine doses. Surveillance of the peripheral nerve is indicated in MTHFR deficiency, including in the infantile form with a good therapeutic compliance.