Electrophysiology of Myopathy Approach to the Patient with Myopathy in the EMG Laboratory

20 Electrophysiology of Myopathy Approach to the Patient With Myopathy in the EMG Laboratory Nithi S. Anand and David Chad Summary The myopathic disorders represent a heterogeneous group of diseases with a variety of causes. Although electrodiagnostic testing rarely allows an entirely specific diagnosis to be made, such testing can be extremely helpful in first confirming the presence of myopathy and therefore helping to appropriately categorize it. Standard motor nerve conduction studies generally do not demon- strate substantial abnormalities, except occasional reductions in compound motor potential amplitude in severe cases or where predominantly distal disease is present. Needle EMG remains the most important part of neurophysiological examination. In most myopathic conditions, sponta- neous activity is increased, although it is most prominently increased in inflammatory or necrotizing myopathic processes. The presence of myotonic discharges can be very helpful in limiting the dif- ferential diagnosis. Complex repetitive discharges, which are present in myopathic conditions, are nonspecific. Motor unit potentials in myopathy are generally of low-amplitude and short duration, except in very chronic conditions, in which the motor unit potentials can actually become of long duration and high amplitude. Finally, it is critical to remember that an entirely normal EMG does not exclude the presence of myopathy. Key Words: Dermatomyositis; fibrillation potential; myopathy; myositis; myotonic discharge; myotonic dystrophy; polymyositis. 1. INTRODUCTION The term myopathy is used to denote diseases of the striated muscle. Diseases of muscle are common in neurological practice, and EMG has an important role in their diagnosis and management. We suggest that most patient encounters in the EMG laboratory start with a brief history. This is performed to ascertain the mode of onset, progression, pattern of muscle involvement, and to inquire about relevant medical history, including medications (e.g., statin drugs, coumadin). The history is followed by a focused neurological examination to identify the weak muscle groups and confirm the pattern of muscle involvement. In general, the pattern of muscle involvement in myopathy is proximal more than distal, although there are exceptions to this general rule. In this context, it is useful to have in mind a classification of muscle diseases (Table 1). It is worth remembering that lesions in other parts of the nervous system, such as watershed cerebral infarctions, motor neuron diseases, neu- romuscular junction (NMJ) disorders, and motor neuropathies might mimic the clinical picture of myopathy. It is also useful to remember key clinical findings of common From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ 325 326 Anand and Chad Table 1 Classification of Myopathies Muscular dystrophies Duchenne muscular dystrophy Becker muscular dystrophy Emery–Dreifuss muscular dystrophy Limb girdle muscular dystrophy Facioscapulohumeral muscular dystrophy Oculopharyngeal muscular dystrophy Myotonic muscular dystrophy Proximal myotonic myopathy Congenital Centronuclear myopathy Congenital fiber type disproportion Nemaline rod myopathy Central core myopathy Metabolic Glycolytic pathway myopathy (acid maltase deficiency, McArdle’s, and Taruis) Lipid storage myopathy (carnitine palmitoyltransferase deficiency and carnitine deficiency) Mitochondrial myopathy (chronic progressive external ophthalmoplegia, Kearn–Sayre syndrome, mitochondrial encephalopathy with lactic acidosis, and myoclonus epilepsy with ragged red fibers) Ion channelopathies Periodic paralyses (hypokalemic and hyperkalemic) Myotonia congenita Malignant hyperthermia Inflammatory Polymyositis Dermatomyositis Inclusion body myositis Connective-tissue disorders Infections Toxic Chloroquine Colchicine Statins ICU related Endocrine Corticosteroid excess Hypothyroid Hyperthyroid myopathies because these distinguishing features provide important clues to the final diagnosis (Table 2). Although EMG plays an important role in the diagnosis of myopathic disorders, it should always be considered as an extension of the clinical examination. The final diagnosis of myopathy rests not only on the EMG findings but also on patient’s history, the clinical examination, laboratory tests, such as creatine kinase, and muscle biopsy. Before delving into details of EMG testing, it is helpful to review the anatomy and physiology of normal and myopathic muscle. Electrophysiology of Myopathy 327 Table 2 Clinical Patterns in Myopathy Myopathies with ocular palsies (ptosis and ophthalmoparesis) Myotonic dystrophy (ptosis only) Facioscapulohumeral muscular dystrophy (FSHD) (ptosis only) Mitochondrial myopathies Oculopharyngeal muscular dystrophy Grave’s diseasea Myasthenia gravisa Myopathies with facial weakness Myotonic dystrophy FSHD Congenital myopathies Congenital facial diplegia (Mobius syndrome)a Myopathies with dysarthria/dysphagia/dysphonia Myotonic dystrophy Inflammatory myopathies Oculopharyngeal muscular dystrophy Thyroid myopathya Myopathies with hypertrophied muscles Duchenne’s and Becker’s dystrophy Limb–girdle dystrophy Hypothyroid myopathy Myotonia congenita Cysticercosisa Malignant hyperpyrexiaa Myopathies with muscle pain and tenderness Polymyositis and dermatomyositis Myopathies with connective-tissue disease Polymyalgia rheumaticaa Acute rhabdomyolysisa Infection (influenza, trichinosis) Myopathies with notable distal weakness Myotonic dystrophy (mainly type 1 but also type 2) Inclusion body myositis Distal myopathies Scapuloperoneal syndromes Miscellaneous patterns “Popeye arms” and inverted axillary folds in FSHD Skin rash and Gottron’s papules in dermatomyositis Lymphadenopathy in malignancy, infection, and sarcoidosisa Episodic hypoglycemia/encephalopathy in carnitine deficiencya aConditions not primarily diseases of the muscle. 2. NORMAL MUSCLE ANATOMY, HISTOLOGY, PHYSIOLOGY, AND THE ORIGIN OF THE NORMAL ELECTRICAL ACTIVITY OF MUSCLE 2.1. Anatomy/Histology Muscle fibers in adults are approx 50 µm in size. They are polygonal in shape and are bundled into fascicles (Fig. 1). Each fascicle contains approx 20 to 60 muscle fibers, and each 328 Anand and Chad Fig. 1. Schematic diagram of the connective tissue sheaths of muscle. Muscle fibers are bundled into fascicles bordered by perimysial connective tissue. From DeGirolami and Smith, 1982 with permission. muscle fiber consists of 50 to 100 myofibrils (Fig. 2). The myofibrils are divided into sarcom- eres, which are the smallest contractile units. The sarcomere is that portion of the myofibril that extends from Z band to Z band (Fig. 3). The sarcomere consists of two filaments, the thick (myosin) filaments alternating with the thin (actin) filaments. The four major contractile proteins that are present in a myofilament are actin, myosin, troponin, and tropomyosin. The T tubules (Fig. 2) are located near the junction of the A and I bands and transmit the ini- tial depolarization at the motor end plate. They are opposed on each side by the terminal cisterns of the sarcoplasmic reticulum. The terminal cisterns act as the storage space for calcium ions. This sarcoplasmic reticulum-T tubule-sarcoplasmic reticulum complex is called the “triad” and is responsible for converting electrical signals (membrane action potentials) to chemical signals (calcium release). In a normal muscle, the muscle fibers are organized into functional groups called motor units. The term “motor unit,” first used by Liddell and Sherrington in 1929, refers to a single lower motor neuron and the muscle fibers it innervates. One motor neuron innervates multi- ple muscle fibers, but each muscle fiber is innervated by only one motor neuron. The average number of muscle fibers in a motor unit determines the innervation ratio of the muscle. This ratio varies greatly from one muscle to another and is related to the degree of dexterity required of that muscle. For instance, the innervation ratio of the external ocular muscles is approx 10 muscle fibers to 1 motor neuron, that of intrinsic hand muscles is approx 100 muscle fibers to 1 motor neuron, and that of gastrocnemius is approx 2000 muscle fibers to 1 motor neuron. The muscle fibers belonging to a motor unit are typically distributed widely, reaching over as many as 100 fascicles. They are randomly distributed over a circular or oval Electrophysiology of Myopathy 329 Fig. 2. Structure of a single muscle fiber cut both horizontally and longitudinally. Individual myofibrils are surrounded and separated by sarcoplasmic reticulum. T tubules are continuous with extracellular fluid and interdigitate with sarcoplasmic reticulum. Note the regular association of T tubule with sarcoplasmic reticulum to form membranous triads. Note the location of myonuclei at the periphery of the muscle fiber and the presence of many mitochondria. From Westmoreland et al., 1994 with permission. region that can reach approx 20 to 30% of the muscle’s cross sectional area, with an average diameter of 5 to 10 mm. In general, there is a mosaic pattern of as many as 20 to 50 overlap- ping motor units in the same muscle, although a slightly higher density of muscle fibers belonging to the same motor unit is observed at the center of the motor unit. 2.2. Physiology Muscle contraction occurs as a result of a series of steps. The process starts with an action potential traveling along the motor nerve axon and reaching the axon terminal

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