Rapidly progressive myopathy: unveiling light chain amyloidosis as an initial manifestation of multiple myeloma: a case report and literature review.

We present the case of a 79-year-old man with rapidly progressive myopathy as the initial manifestation of light chain amyloidosis associated with multiple myeloma. The patient experienced progressive lower limb weakness resulting in difficulty climbing stairs. Ancillary tests revealed slightly elevated serum creatine kinase levels. The electromyogram revealed a diffuse myogenic pattern while muscle MRI indicated fatty replacement of the quadriceps muscles. Muscle biopsy revealed the presence of amyloid deposits in the vessel walls. An elevated level of lambda (246 mg/L) light chain was detected. The bone marrow aspiration results were consistent with the diagnosis of multiple myeloma. In conclusion, even if amyloid myopathy is a rare condition, routine screening for amyloid deposits in muscle biopsy is crucial and should be performed systematically. In the present case, it enabled a rapid diagnosis and the beginning of treatment.