Immunoglobulin light chain (AL) amyloidosis is a systemic disease characterized by the deposition of abnormally folded monoclonal light chains in different organs. It causes a myopathy in around 2% of cases, usually manifesting with symmetrical proximal muscle weakness, axial weakness, and dysphagia.

A 56-year-old male with no significant medical comorbidities presented with a 1-year history of progressive, painless and asymmetric bilateral shoulder weakness and atrophy. The weakness started on the right side and after 6 months spread to the left. There was no diplopia, dysphagia, facial, neck or lower limb weakness, or sensory symptoms. Examination was notable for bilateral scapular winging, and moderate-to-severe asymmetric weakness and atrophy of shoulder girdle and upper arm muscles, right worse than left. Biceps and brachioradialis reflexes were mildly reduced bilaterally. Otherwise, strength, reflexes and sensory exam were normal. EMG demonstrated a proximal myopathy with fibrillation potentials.  CK was mildly elevated at 439 U/L (N < 308). Monoclonal gammopathy screen showed a small IgA lambda monoclonal gammopathy with markedly elevated lambda free light chain at 20.7 mg/dl (N: 0.57-2.63) with an abnormal Kappa/lambda ratio of 0.0787 (N: 0.26-1.65).

A right triceps muscle biopsy demonstrated interstitial amyloidosis involving the skeletal muscle. Mass-spectrometry–based proteomics identified an AL lambda amyloidosis peptide profile. Fat aspirate and bone marrow biopsy were also positive for amyloid deposits. The patient was referred to hematology who started a combination of daratumumab, bortezomib, cyclophosphamide and dexamethasone.

Our case represents a novel presentation of AL amyloidosis as an asymmetric scapulohumeral myopathy and highlights that amyloid myopathy, regardless of the weakness pattern, should be included in the differential diagnosis of any chronic myopathy and that serum immunofixation and free light chains should always be part of a myopathy workup.