Progressive Proximal Muscle Weakness Due to a 51 CAG Repeat Expansion in Exon 1 of the Androgen Receptor Gene: A Case Report of Kennedy Disease

Rare disease Background:Kennedy disease, also known as spinal and bulbar muscular atrophy (SBMA), is a rare and incurable X-linked neuromuscular disorder mainly affecting men aged 30 to 60 years.Polymyositis can present similarly, but can be excluded by measuring muscle enzymes, performing muscle imaging, and electromyography.This report describes the case of a 52-year-old man with a 10-year history of progressive limb weakness due to Kennedy disease, established by genetic testing. Case Report:A 52-year-old man presented with a 10-year history of gradually progressive proximal limb weakness and persistently elevated creatine kinase levels ranging from 808-2300 U/L (normal 39-308 U/L).One year prior to this admission, the limb weakness had worsened, but initial electromyography, neuroimaging, and muscle biopsy showed no specific abnormalities.Despite a trial of immunosuppressive therapy due to suspected polymyositis, there was no clinical improvement.Neurological examination later revealed gynecomastia, proximal muscle atrophy, and bilateral tongue atrophy with tremor.Electromyography showed chronic neurogenic changes and reduced sensory nerve action potentials.Repeat expansion analysis identified a hemizygous pathogenic CAG repeat expansion in exon 1 of the androgen receptor gene using a short-read next-generation sequencing-based repeat detection algorithm (ExpansionHunter), with an estimated repeat number of 51 (range 50-53).At 6-month follow-up, the patient demonstrated mild progression of motor symptoms but remained functionally stable. Conclusions:This report presents a rare case of Kennedy disease, initially diagnosed as polymyositis, and highlights the importance of follow-up with genetic testing when neurological and electromyography investigations are not typical for polymyositis.Early identification of Kennedy disease helps avoid unnecessary immunosuppressive treatments.