BACKGROUND
Late-onset multiple acyl-CoA dehydrogenase deficiency can mimic inflammatory myopathy. We report a probable ETFDH-related case to illustrate diagnostic reconsideration after corticosteroid nonresponse when biopsy and genetic findings do not establish a definitive etiology.
CASE REPORT
A 60-year-old man presented with proximal weakness for more than 1 year and elevated muscle enzymes and aminotransferases. No sertraline or other selective serotonin reuptake inhibitor exposure was reported. Corticosteroid treatment produced no objective strength improvement. Biopsy showed marked lipid accumulation. Sanger sequencing confirmed ETFDH c.1227A>C (p.Leu409Phe), classified as pathogenic, and c.1732C>T (p.Arg578Trp), classified as a variant of uncertain significance. Metabolic testing and functional validation were not performed. Strength and aminotransferases improved during combined riboflavin and coenzyme Q10 treatment, but an intercurrent brainstem infarction confounded recovery. At 1 month, creatine kinase and lactate dehydrogenase were not rechecked.
CONCLUSIONS
The findings supported probable, not definitive, ETFDH-related late-onset disease. Lipid-storage pathology should prompt reconsideration of a treatable metabolic myopathy while acquired alternatives remain under consideration. Improvement during combined treatment was supportive but nonspecific and did not verify the diagnosis or isolate a riboflavin effect.

Keywords: Genetic Testing; Muscle Weakness; Peripheral Nervous System Diseases; Riboflavin