Reversible upper limb muscle weakness with selective loss of thick filaments

Selective loss of thick myofilaments was described in critically ill patients who developed acute quadriplegic myopathy (AQM) after treatment with high doses of corticosteroids or nondepolarizing neuromuscular blocking agents (NMBAs).1 Myosin filament loss was also observed in the childhood type of dermatomyositis1 and in patients with congenital myopathies1,2⇓ and HIV-related myopathy.3 A healthy 58-year-old man with no medical history developed an insidious, progressive proximal muscle weakness of upper limbs. He never received steroids, NMBAs, or any other medication. Family history was negative for neuromuscular disorders. Routine laboratory studies and antinuclear antibodies, C-reactive protein, and extractable nuclear antigens tests were normal. On two determinations, serum creatine kinase level was elevated twofold to fourfold. Nerve conduction studies were normal. EMG performed at the upper and lower limbs showed low-amplitude and short-duration motor unit potentials only in deltoid, biceps, and triceps brachialis muscles; spontaneous activity was absent. Cervical spinal MRI did not show any spinal cord or radicular compression. When the patient came to our attention, 8 months after the onset of symptoms, he lifted his arms over the …