Organ-specific autoantibodies with muscle weakness.

The motor unit includes the anterior horn cell, the motor axon and the muscle fibers it innervates, and the neuromuscular junction. Diseases of the motor unit usually present with weakness. Diagnosis of motor unit disorders involves the history, physical examination, electrophysiologic studies of nerve and muscle, and blood testing for creatine kinase, genetic disorders, and autoantibodies. Antibody testing is often useful for the identification of specific immune-mediated motor unit disorders. Identification of these disorders is important because they are often treatable. Antibodies with disease specificity include those directed against autoantigens with and without organ specificity. Several autoantibodies to non-organ-specific antigens are associated with subgroups of immune myopathies. Organ-specific autoantibodies in motor unit disorders with weakness occur in myasthenia gravis, especially with thymoma, a myopathy associated with Waldenstrom's macroglobulinemia, Lambert-Eaton myasthenic syndrome, and multifocal motor neuropathy.