Metformin-Associated Functional Vitamin B12 Deficiency Presenting as Subacute Combined Degeneration in a 57-Year-Old Man With Diabetes Mellitus

Unusual clinical course Background:Functional vitamin B12 deficiency, also known as metformin-associated cobalamin deficiency, can occur in patients receiving metformin due to impaired gastrointestinal absorption of vitamin B12.This condition can coexist with diabetic neuropathy and may result in impaired myelin formation in the spinal cord and peripheral nerves, leading to subacute combined degeneration of the spinal cord.This report describes a patient with diabetes mellitus and metformin-associated functional vitamin B12 deficiency who presented with subacute combined degeneration of the spinal cord and gastrointestinal symptoms. Case Report:A 57-year-old man with a 1-year history of metformin therapy for type 2 diabetes mellitus presented with anorexia, vomiting, weight loss, and gait ataxia.Neurological examination revealed impaired superficial sensation at the T10 to L1 level and bilaterally diminished patellar reflexes.Despite a normal serum vitamin B12 level, further metabolic evaluation revealed a substantially elevated homocysteine level.Electromyography demonstrated peripheral neuropathy in the right upper limb, involving both motor and sensory axons.Spinal magnetic resonance imaging showed characteristic T2 hyperintensity of the dorsal columns.Based on these findings, a definitive diagnosis of subacute combined degeneration was made.The patient was promptly treated with high-dose intramuscular vitamin B12 supplementation and supportive care, resulting in clinically significant improvement in subsequent weeks. Conclusion:This case of metformin-associated cobalamin deficiency with subacute combined degeneration of the spinal cord highlights the importance of monitoring vitamin B12 status and neurological symptoms in patients with metformin-treated diabetes.