Skeletal Muscle Involvement in Systemic Immunoglobulin Light Chain Amyloidosis: Analysis of Clinical, Electrophysiological and Pathological Features in 11 Patients

ABSTRACT Background To describe the clinical, electrophysiological, and pathological features of systemic light chain (AL) amyloid myopathy. Methods Eleven patients diagnosed with AL amyloid myopathy and who performed muscle biopsies were included. Clinical, electrophysiological, and pathological profiles were retrospectively collected. Results Eight patients reported muscle weakness, muscle strength was generally preserved and five of them exhibited exercise intolerance. Quantitative electromyography was available in eight patients, but only four fulfilled the criteria for myopathic changes. Myopathic MUAPs were also observed in patients with normal muscle strength. Nerve conduction studies showed peripheral neuropathies in seven cases, mostly bilateral carpal tunnel syndromes, but only four manifested with corresponding paresthesia. Degeneration, necrosis, regeneration and inflammatory cells infiltration were mild in muscle biopsies, instead small vessels with thickened walls and luminal stenosis were commonly observed. Amyloid depositions were confirmed in skeletal muscles in 7 cases in intramuscular vessel walls (3/7), endomysium (3/7) and sub‐sarcolemma (3/7), the remaining four patients had amyloid depositions in alternate organs and evidence of myopathy exclusive of other causes. Immunohistochemical analysis showed uneven granular C5b‐9 expression in intramuscular vessel walls (5/7) and sarcolemma (2/7). Vacuoles were found in two cases. Nine patients received chemotherapy, only three of them reported slight improvement of muscle strength. Conclusions In amyloid myopathy, muscle weakness was generally mild and exercise intolerance was a common feature probably attributable to underlying vascular pathology. Muscle biopsies revealed mild inflammation and necrosis, accompanied by uneven granular C5b‐9 deposition in intramuscular vessels, a pattern that may serve as a clue for amyloidosis.