Electrodiagnostic testing is the core diagnostic modality for patients with a suspected myopathy. It consists of nerve conduction studies (NCS) and electromyography (EMG). Despite recent advances in molecular genetics and significant improvement in imaging quality, it is still a pertinent part of the diagnostic process in most patients. Electrodiagnostic studies are considered an extension of the physical examination and are most useful in the workup of a patient with a suspected myopathy.
NCS usually precedes needle EMG and provides valuable information about the function of sensory and motor nerve fibers. NCS is normal in the majority of patients with disorders of muscle and assists in excluding disease mimickers. In some cases, specialized tests, such as repetitive nerve stimulation, can be employed to evaluate disorders of the neuromuscular junction, as another cause of pure motor weakness.
The performance of EMG for evaluation of myopathy involves the placement of a needle recording electrode inside the muscle and analysis of electrical potentials at rest and with muscle activation. The selection of muscles for electrodiagnostic examination depends on the clinical scenario and technical limitations. Testing clinically weak muscles increases the yield of the test. Most myopathies affect proximal muscles; therefore, limb-girdle and paraspinal muscles are usually tested. In certain myopathies, distal muscles are preferentially involved (myofibrillar myopathies, distal muscular dystrophies). In such cases, this can lead not only to abnormal EMG but also abnormal motor nerve conductions due to muscle atrophy.
Electrodiagnostic studies not only allow confirmation of myopathy diagnosis and assist in identifying etiology, but they also can be used for the selection of a suitable site for muscle biopsy or to direct further genetic testing.
Skeletal muscle consists of multiple muscle fascicules, and each fascicule is composed of multiple muscle fibers. A muscle fiber is a multinucleated cell containing myofibrils, structures responsible for muscle contraction, and an enfolding plasma membrane (sarcolemma). There are also three layers of associated connective tissues: endomysium surrounding each muscle fiber, perimysium between different muscle fascicules, and epimysium (the outermost layer) covering all fascicules. The epimysium is connected to tendon or aponeurosis that attaches muscle to the bone or occasionally to the skin or connective tissue.
At rest, the normal muscle membrane has only resting potential; therefore, resting muscle EMG is “electrically silent.” Only when the needle electrode is close to the neuromuscular junction can small normal endplate potentials be recorded.
When a motor neuron is activated, the action potential travels along the nerve and arrives at the neuromuscular junction, where after a series of electrochemical processes, depolarization of the muscle fiber occurs. The action potential then propagates along the sarcolemma by shifts in sodium and potassium ion-driven currents, generating a depolarizing electrical field along with adjacent membrane segments. Thus, activation of each motor neuron generates a motor unit action potential, consisting of the summation of individual muscle fiber action potentials within a single motor unit. Assessment of motor unit action potential parameters is an essential part of needle EMG evaluation.
Electrodiagnostic studies must be considered in a patient presenting with a weakness to confirm the presence of muscle disease and rule out alternative diagnoses and myopathy mimickers. Additionally, this study can reduce differential diagnoses and point towards specific types or groups of myopathies. The test may also help identify suitable sites for biopsy, as it can detect affected muscle that is not clinically weak and identify muscles to be avoided for biopsy due to end-stage denervation changes.
Electrodiagnostic testing is important in investigating patients with suspected myopathy, excepting clearly hereditary myopathies with positive family history, where proceeding directly to genetic testing is reasonable and preferable.
In general, electrical stimuli applied during routine NCS are safe. However, in patients with external pacers, microcurrents can trigger dangerous arrhythmias, and therefore nerve conduction studies should not be performed until external pacer wires are removed. Implantable pacemakers and defibrillators, as well as central lines, have been shown to be safe. Reasonable caution should be taken with proximal stimulation near these internal devices.
There are no absolute contraindications for needle EMG. Anticoagulation poses potential risks and represents a relative contraindication. Safely measures should be taken to avoid hemorrhagic complications.
Modern electrodiagnostic equipment combines a hardware unit with a stimulator, amplifier, control panel, and a computer containing special software for signal processing and storage. Various quantitative analysis tools were introduced in recent years that enhance and complement the visual analysis of recorded signals. Quantitative EMG is used extensively for research purposes and in some clinical situations. Equipment components that contact the patient include surface electrodes and needle electrodes.
The American Association of Electrodiagnostic Medicine set up requirements for the qualifications of an electrodiagnostic physician. An appropriately trained physician must perform all needle EMGs. Nerve conduction studies can be performed by a trained assistant under that physician’s supervision.
No special preparation is necessary for electrodiagnostic studies. Patients should avoid applying lotions, creams, or oils to the skin a few days before or at least on the day of the procedure.
Electrodiagnostic studies for evaluating myopathies include nerve conduction studies (NCS) and needle EMG (EMG).
Routine Nerve conduction studies include sensory and motor studies, analyzing distal latencies, amplitudes, and conduction velocities for tested nerves. Sensory nerve action potentials (SNAPs) are usually normal in disorders of muscles unless there is a superimposed sensory polyneuropathy or disorder affecting muscle and nerve simultaneously (critical illness neuropathy/myopathy, amyloidosis). Compound muscle action potentials (CMAPs) are normal in proximal myopathies but can be abnormal in distal myopathies when significant muscle atrophy is present.
Electromyography can be performed using a needle electrode or surface electrode.
EMG analysis includes assessing spontaneous activity, motor unit potential configuration, and a pattern of recruitment.
There is no spontaneous activity in normal muscle, with the exclusion of potentials that could be recorded if the needle is positioned near a neuromuscular junction (end-plate noise and end-plate spikes). The most common abnormal spontaneous potentials seen in myopathic disorders include fibrillations, positive sharp waves, complex repetitive discharges, and myotonic discharges. The presence of these discharges provides additional clues to the diagnosis.
Configuration of MUAPs and Motor Unit Recruitment
Electrodiagnostic studies are safe and generally well tolerated by patients. Iatrogenic side effects are very rare. In nerve conduction studies, transdermal electrical stimulation is used, producing a theoretical risk for electrical complications. Needle EMG carries the potential complications of any needle insertion, including infection, hemorrhage, tissue injury, and pneumothorax.
Electrodiagnostic studies are an integral part of the core investigations in patients with a suspected myopathy. These studies can help establish a diagnosis of myopathy; in some cases, point towards etiology, and guide the selection of muscle when a biopsy is considered.
Abnormal Spontaneous Activity
Topographical Distribution of Affected Muscles in Different Myopathies
The most typical pattern for most myopathies is predominantly proximal muscle involvement. The comprehensive electrodiagnostic evaluation should also include neck extensor muscles, distal muscles, and facial muscles to assess less typical patterns.
In some myopathies, electrodiagnostic testing can be normal. For example, in steroid myopathy cases, the most common drug-induced myopathy, NCS, and EMG are usually normal. Steroid myopathy affects predominantly type IIb fibers, while EMG assesses predominantly type 1 muscle fibers. As steroids are commonly used to treat dermatomyositis and polymyositis, worsening of weakness can be due to exacerbation or undertreatment of inflammatory myopathy or due to the development of steroid myopathy. Lack of abundant spontaneous activity facilitates this distinction and can point towards steroid myopathy.
EMG also has a low yield in disorders that alter mechanical but not electrical properties of the muscle, as seen in some metabolic and congenital myopathies.
Electrodiagnostic testing is a valuable technique to evaluate a patient with a suspected myopathy. The standard test usually includes nerve conduction studies and needle EMG examination. The care of patients with disorders of muscle often involves different specialties, including neurologists, rheumatologists, orthopedic surgeons, primary care providers, physical and occupational therapists, speech pathologists, cardiologists, and others.
The traditional approach to diagnosis has evolved in the era of breakthroughs in molecular genetics and neuroimaging and requires a tailored approach to each patient where an electrodiagnostic study is required. Enhancement of knowledge about electrodiagnostic studies and their role in diagnosing neuromuscular disorders among different healthcare providers will improve the appropriateness of referrals and yield of the tests. [Level 3] Proper selection of patients and the use of appropriate techniques can guide further diagnostic investigations, including genetic testing and muscle biopsy, as well as treatment strategies.
Overall, electrodiagnostic testing is safe and well-tolerated by most patients. However, sometimes it can cause sufficient patient discomfort leading to aborted study or inconclusive results. It has been shown that a lack of information or incorrect information about the test is associated with higher anticipated pain and test failure in some patients. Studies also showed that needle EMG is less painful than expected and patients who underwent electrodiagnostic studies are willing to repeat the test if necessary. Anticoagulation is only a relative contraindication for needle EMG, usually requiring careful planning and approach, but studies have shown the procedure to be safe.
Improving communications, expanding knowledge of principles, indications, and limitations of the electrodiagnostic evaluation and its role in the patient workup, and further management help make timely clinical decisions and improve treatment outcomes.
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