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The Use of EMG/NCS in Neuromuscular Disorders
By Lanny Y. Xue, M.D., Ph.D., Division of Neurology/Epileptology
Electrodiagnostic studies play a key role in the evaluation of patients with a wide variety of neuromuscular disorders such as radiculopathy, brachial and lumbosacral plexopathy, neuropathy, neuromuscular junction disorders, myopathy, and amyotrophic lateral sclerosis. Needle EMG (electromyography) and nerve conduction study (NCS) form the core of the electrodiagnostic study. According to the American Academy of Neurology (AAN), “Needle EMG (NEMG), in combination with nerve conduction studies, is the gold standard methodology for assessing the neurophysiologic characteristics of neuromuscular diseases.”
EMG and nerve conduction studies are best considered an extension of the clinical examination. Indeed, these studies cannot be properly planned, performed, or interpreted without knowing the patient’s symptoms and findings on the clinical examination. Numerous nerves and literally hundreds of muscles can be studied. To study them all would be neither desirable for the patient. In every case, the study must be individually planned, based on the clinical differential diagnosis, and then modified as the study progresses and further information is gained.
Nerve Conduction Studies (NCS)
After the history is taken and a directed physical examination is performed, every study begins with the nerve conduction study (NCS). The findings on the NCSs are used in the planning and interpretation of the needle examination which follows. Motor, sensory or mixed nerve studies can be performed, stimulating the nerve with the recording electrodes placed over a distal muscle, a cutaneous sensory nerve, or the entire mixed nerve, respectively. The findings of motor, sensory, and mixed nerve studies often complement one another and yield different types of information associated with distinct patterns of abnormalities, depending on the underlying pathology.
Electromyography (EMG)
After nerve conduction studies are performed, the electrophysiologic evaluation moves on to needle electromyography (EMG), which is the study of motor unit potentials. Most commonly, a concentric needle electrode is passed through muscle fibers, where it detects both intracellular and extracellular potentials.
Like the nerve conduction studies, each needle EMG study must be individualized based on the clinical findings and differential diagnosis and modified as the test proceeds and more data are obtained. For each study, a balance must be reached between the need to study a sufficient number of muscles to reach or exclude a diagnosis and the limits of the patient’s ability to tolerate the examination. When the examination is performed skillfully most patients tolerate it well, with only minor discomfort.
Clinical-Electrophysiologic Correlations
Carpal Tunnel Syndrome (CTS)
Median nerve entrapment at the wrist is the most common of all entrapment neuropathies. In nearly all patients, the usual site of compression occurs in the carpal tunnel and results in a constellation of symptoms and signs known as the carpal tunnel syndrome (CTS). Although CTS usually is bilateral, both clinically and electrically, the dominant hand generally is more severely affected. Patients complain of wrist and arm pain associated with paresthesia in the hand.
Carpal tunnel syndrome (CTS) is very common. In the U.S., approximately 5% of the general population will suffer from the effects of carpal tunnel syndrome. Caucasians have the highest risk of being diagnosed with CTS compared with other races. Women suffer more from CTS than men with a ratio of 3:1 between the ages of 45–60 years of age.
It is important to emphasize that CTS is a clinical diagnosis. Although the reported causes of CTS are numerous, most cases are idiopathic. The pathophysiology of CTS typically is demyelination.
The EMG/NCS evaluation of a patient suspected of having CTS is directed toward the following: (a) demonstrating focal slowing or conduction block of median nerve fibers across the carpal tunnel; (b) excluding median neuropathy in the region of the elbow; (c) excluding brachial plexopathy; and (d) excluding cervical radiculopathy.
There is a group of patients with clinical symptoms and signs of CTS in whom these routine studies are normal (approximately 10%-25% of CTS patients). In such patients, the electrodiagnosis of CTS will be missed unless further testing is performed using more sensitive nerve conduction studies. Those studies usually involve a comparison of the median nerve to another nerve in the hand. The diagnostic yield increases from approximately 75% using routine motor and sensory studies to approximately 95% using these more sensitive techniques.
Radiculopathy
The clinical hallmark of radiculopathy includes pain and paresthesias radiating in the distribution of a nerve root, often associated with sensory loss and paraspinal muscle spasm. Motor dysfunction also may be present. There are a vast number of causes of radiculopathy. The most common are structural lesions, including herniated discs, bony impingement from spondylosis, and mass lesions such as epidural abscesses and metastatic tumors to the spine. Radiculopathy caused by degenerative bone and disc disease most often affects the cervical (C3-C8) and lumbosacral (L3-S1) segments.
In patients with radiculopathy, nerve conduction studies typically are normal, and the electrodiagnosis is established on needle EMG. The main reason to perform nerve conduction is to exclude other conditions that may mimic radiculopathy, especially entrapment neuropathy and plexopathy.
The electromyographic study may be normal in some patients with radiculopathy if the lesion is acute, the radiculopathy is purely demyelinating, or the sensory nerve root is predominantly affected.
Brachial and Lumbosacral Plexopathy
In cases of suspected brachial or lumbosacral plexopathy, nerve conduction studies and EMG often are used to localize the lesion accurately, to assess its severity, and to exclude radiculopathies and various mononeuropathies that can mimic plexopathy clinically. The electrophysiologic evaluation of plexopathy is demanding for the electromyographer. Detailed knowledge of the anatomy of the nerve roots, plexus, and peripheral nerves is required. Extensive bilateral studies, with emphasis on the sensory conduction studies and needle EMG, frequently are needed to localize the lesion. The etiology of plexopathy includes traumatic injury, tumor, hematoma, inflammation, infarction, diabetes, radiation, and childbirth.
Polyneuropathy
Polyneuropathy literally means dysfunction or disease of many or all peripheral nerves. Because peripheral nerves can react in only a limited number of ways to disease, polyneuropathies of many different origins may present with similar symptoms and signs. The underlying pathology of the vast majority of polyneuropathies is axonal degeneration, usually affecting both motor and sensory fibers. Axonal polyneuropathies include nearly all diabetic, toxic, metabolic, drug-induced, nutritional, connective tissue, and endocrine-associated polyneuropathies. For any patient with a polyneuropathy, the presence of demyelination has special diagnostic significance.
Nerve conduction studies and EMG play key roles in the evaluation of patients with suspected polyneuropathy. Because polyneuropathy has hundreds of potential causes, the first step in its evaluation is to reduce the differential diagnosis to a smaller more manageable number of possibilities. This usually can be accomplished by acquiring several critical pieces of information from history, physical examination, and electrophysiologic studies.
Electrophysiologic studies can be used (1) to confirm the presence of a polyneuropathy, (2) to assess its severity and pattern, (3) to determine whether motor, sensory, or a combination of fibers are involved, and, most importantly, (4) to assess whether the underlying pathophysiology is axonal loss or demyelination. The information obtained from electrophysiologic testing, in conjunction with key pieces of clinical information, usually allows the differential diagnosis to be narrowed considerably so that further laboratory testing can be more appropriately applied and a final diagnosis reached.
Mononeuropathies
Median neuropathy at the wrist (carpal tunnel syndrome, CTS) has been described above. Other commonly encountered mononeuropathies include proximal median neuropathy, ulnar neuropathy at the elbow, ulnar neuropathy at the wrist, radial neuropathy, peroneal EMG/NCS in Neuromuscular Disorders, Continued from page 5
neuropathy at the fibular head, femoral neuropathy, tarsal tunnel syndrome, and facial and trigeminal neuropathy. EMG and NCS play an important role in the diagnosis of mononeuropathy, localization of entrapment, and assessment of extent and severity.
Neuromuscular Junction Disorders
Disorders affecting the neuromuscular junction (NMJ) are generally pure motor syndromes that usually preferentially affect proximal, bulbar, or extraocular muscles. These disorders include myasthenia gravis, Lambert-Eaton myasthenic syndrome, and botulism. They are confused occasionally with myopathies. With knowledge of normal NMJ physiology, most of the abnormalities affecting the NMJ can be differentiated using a combination of nerve conduction studies, repetitive stimulation, exercise testing, and needle EMG.
Myopathy
Myopathies present as pure motor syndromes without any disturbance of sensory or autonomic function. In most myopathies, symptoms tend to be bilateral and affect proximal muscles preferentially.
EMG can be useful in directing the site for a muscle biopsy in a patient with a myopathy. The EMG examination has the advantage that multiple muscles and sites can be sampled easily and often can suggest a suitable muscle to biopsy.
Amyotrophic Lateral Sclerosis (ALS)
ALS is one of the most common neuromuscular diseases worldwide, and people of all races and ethnic backgrounds are affected. One or two out of 100,000 people develop ALS each year. ALS most commonly strikes people between 40 and 60 years of age, but younger and older people can also develop the disease. Men are affected slightly more often than women.
ALS is a degenerative, progressive disorder that affects both the upper motor neuron and lower motor neuron. ALS is remarkably specific for the motor system. Most often, ALS is a regional disease that usually starts in one body segment and then progresses to adjacent myotomes. Most cases begin with insidious weakness in either a distal upper or a lower extremity.
Differential diagnosis for ALS includes cervical/lumbar stenosis, demyelinating motor neuropathy, inclusion body myositis, benign fasciculation syndrome, and myotonic disorders.
Because the prognosis in ALS is uniformly poor, it is essential that the correct diagnosis be reached. EMG and nerve conduction studies are most often used to confirm the diagnosis of ALS. More importantly, however, they are used to help exclude other conditions, some potentially treatable, which may mimic ALS.
Nowhere else is the clinical-electrophysiologic correlation more important than in ALS. Electrodiagnostic studies, by themselves, cannot make a diagnosis of ALS. Rather, ALS remains a clinical diagnosis supported by electrodiagnostic findings. The electromyographer must appreciate that other disorders may display similar electrodiagnostic findings to ALS (e.g., coexistent cervical and lumbar radiculopathy) and that it is the combination of clinical and electrodiagnostic findings that allows a final diagnosis to be reached.
Summary
EMG and NCS serve as a quantitative extension of the neurologic examination. NCSs localize compressive lesions, determine the extent of sensory and motor nerve contribution to neuropathy, assess NMJ integrity, and distinguish between demyelination and axonal denervation. EMG distinguishes traumatic injury and degeneration due to motor neuronopathies, radiculopathies, or peripheral neuropathy.
Needle EMG evaluation provides information about chronicity of disease and plays a critical role in diagnosis of myopathies. Specialized NCS and EMG techniques are the most sensitive measures of neuromuscular junction injury. Coupling a patient’s history, clinical examination, and electrodiagnostic studies frequently yields an informative diagnosis, allowing for effective therapeutic intervention.
EMG and NCS studies are performed by a number of New England Neurological Associates neurologists in various NENA office locations, including: Lanny Y. Xue, M.D., Ph.D., in Newburyport; Andreas P. Schoeck, M.D., in Lawrence; Jennifer A. Grillo, M.D., in Lawrence; Richard S. Finkelman, M.D., in Haverhill; Vladan Milosavljevic, M.D., in Lowell; Min Zhu, M.D., Ph.D., in Lowell; Antonio Silva Sayago, M.D., in Lawrence.
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