39 - Myotonic Muscle Disorders and Periodic Paralysis Syndromes

Abstract

The myotonic muscle disorders are a group of disorders characterized by muscle stiffness, pain, and sometimes intermittent or constant weakness. They can be classified into those with dystrophic changes on muscle biopsy, such as the myotonic dystrophies, resulting in weakness, and those without dystrophic changes, such as myotonia congenita and paramyotonia congenita, where weakness is generally not seen. Myotonia also occurs in several of the periodic paralysis syndromes, both inherited and acquired, as well as on the electromyography (EMG) examination in some metabolic, inflammatory, congenital, and toxic myopathies. The primary periodic paralyses are rare genetic disorders associated with attacks of weakness that may last minutes, hours, or days. The myotonic disorders and periodic paralyses are grouped together as some of them overlap. All are “channelopathies,” associated with mutations of muscle sodium, calcium, potassium, or chloride channels. Genetic linkage and mutational analyses have identified the molecular basis for several of the myotonic muscle disorders and periodic paralysis syndromes, resulting in the classification of these disorders based on a specific ion channel or protein kinase defect. However, in a substantial number of patients, the diagnosis rests on clinical and electrophysiologic findings alone. The classification of these disorders is based on clinical, electrophysiologic, and available molecular findings. The electrophysiologic evaluation is geared toward answering several key questions, including whether myotonia is present or not. A variety of tests can be performed to distinguish among the dystrophic and nondystrophic myotonic muscle disorders, the periodic paralysis syndromes, and other disorders of muscle with accompanying EMG myotonia. In addition to routine nerve conduction studies and needle EMG, muscle cooling, exercise testing, and repetitive nerve stimulation often are very helpful in differentiating among these disorders.