Conclusions: Our findings for the first time demonstrate in vivo a significant redox imbalance in Cbl-C defect that may contribute to the disease progression and indicating that antioxidant drugs should be used in combination with standard therapy. Case report: Bilateral visual loss occurred within five days in a 23-year old women with methylmalonic acidemia (MMA). The disease was metabolically well controlled by strict diet and carnitine supplementation since early childhood and the visual loss was not associated with metabolic decompensation. Ophtalmologic exams showed the typical signs of optic neuropathy, and common etiologies of this pathology were ruled out. Moderate enhancement of both optic nerves was present on MRI. Treatment attempts with high dose corticosteroids, and coenzyme Q10 combined with vitamin E were ineffective. Subacute partial bilateral neurosensory hearing loss occurred three months later, further complicating patient care.