The Scope of the EMG Examination

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Chapter 1 The Scope of the EMG Examination

Electromyography (EMG) is a term that was first coined by Weddell et al in 1943 to describe the clinical application of needle electrode examination of skeletal muscles. Since then, and at least in North America, the nomenclature “EMG” or “clinical EMG” has been used by physicians to refer to the electrophysiologic examination of peripheral nerve and muscle that include the nerve conduction studies (NCS) as well as the needle evaluation of muscles. These terms continue to cause confusion that hinders communication among physicians and healthcare workers. Some physicians refer to the study as EMG/NCS, reserving the name EMG solely to the needle EMG evaluation and adding the term NCS to reflect the nerve conduction studies separately. Others have used the title needle EMG or needle electrode examination to reflect the needle evaluation of muscles, while keeping the term EMG to describe the entire evaluation of nerve and muscle. More recently, a nonspecific term, the “electrodiagnostic (EDX) examination,” has gained popularity to serve as an umbrella covering both the needle EMG and NCS. Other nomenclature used worldwide includes the electrophysiologic examination, which may be confused with the cardiac electrophysiological studies, and the electroneuromyographic (ENMG) examination which is the most accurate description of the study, yet unfortunately not widely used. Finally, physicians performing and interpreting these studies are called electromyographers (EMGers), electrodiagnosticians, or EDX consultants.

Regardless, the designations, EDX, EMG, clinical EMG, or ENMG examinations are best used interchangeably to reflect the entire electrophysiological study of nerve and muscle (NCS and needle EMG), while the terms “needle EMG” or needle electrode examination should be reserved for the specific testing which involves needle electrode evaluation of muscle. This author uses the terms EMG examination and EDX examination interchangeably, and refers to the needle examination of muscle as needle EMG.

The EDX examination comprises a group of tests that are usually complementary to each other and often nec-essary to diagnose or exclude a neuromuscular problem (Table 1-1). These include principally the nerve conduction studies (NCS), that are sensory, motor, or mixed, and the needle EMG, sometimes referred as “conventional” or “routine” needle EMG to distinguish this test from other needle EMG studies including single fiber EMG and quantitative EMG. Also, “concentric” or “monopolar” needle EMG is sometimes utilized to reflect the type of needle electrode used. In addition to the two main components of the EMG examination, three late responses are often incorporated with the NCSs and have become an integral part of the NCSs. These include the F waves also referred to as F responses, the H reflexes also known as H responses, and the blink reflexes. Two specialized tests are often added to the routine EDX study mainly in patients with suspected neuromuscular junction disorders. These include the repetitive nerve stimulations and the single fiber EMG. Finally, a group of specialized studies that require special expertise as well as sophisticated equipment and software, used as a clinical and research tool in the assessment of the microenvironment of the motor unit, include motor unit action potential (MUAP) morphology analysis, MUAP turns and amplitudes analysis, macro EMG, motor unit number estimate (MUNE), and near-nerve recording studies.

Table 1-1 The Spectrum of Clinical Electromyography (Electrodiagnosis)

1. Nerve conduction studies

– Sensory
– Motor
– Mixed
2. Needle electromyography (routine, conventional)

– Concentric
– Monopolar
3. Late responses

– F waves
– H reflexes
– Blink reflexes
4. Specialized electrodiagnostic tests

– Repetitive nerve stimulation
– Single fiber electromyography
– Quantitative electromyography

– Quantitative motor unit action potential morphology analysis
– Turns and amplitude analysis
– Macro electromyography
– Motor unit number estimate (MUNE)

– Near-nerve recording studies

THE REFERRAL PROCESS TO THE EMG LABORATORY

Patients are referred to the EMG laboratory for EDX studies following a clinical assessment by a physician who suspects a disorder of the peripheral nervous system. For example, a patient with intermittent hand paresthesias and positive Phalen’s signs may be referred to the EMG laboratory to evaluate a possible carpal tunnel syndrome. The background and specialty of the referring physician plays a significant role in the planning and execution of the EDX study. In the experience of this author, this usually follows one of these three scenarios:

1. The referring physician is also the EDX consultant (electromyographer). In other words, the patient is examined first by the EDX consultant (usually a neurologist or physiatrist) who performs and interprets the EDX study. The advantage of this situation is that the neurological examination is often thorough and the differential diagnosis is limited. Hence, the selection of NCSs and the choice of sampled muscles on needle EMG are well guided by the neurological findings. Though this situation is ideal, it is not common or practical in a busy EMG laboratory. Also with this approach, the electromyographer may encounter one or two pitfalls. The first is that he/she may perform a very limited and suboptimal study and become excessively biased by the clinical information, resulting in a significant number of diagnostic errors. The second hazard is that the EDX consultants may change the interpretation of similar findings among different studies to suit and support the clinical diagnosis. For example, a diabetic patient with denervation of quadriceps, iliacus, thigh adductors, and lumbar paraspinal muscles may be diagnosed in the EMG laboratory as consistent with lumbar radiculopathy or diabetic amyotrophy depending on the temporal course of the symptoms, pain characteristics, status of diabetic control, or findings on imaging of the spine.
2. The referring physician is well versed with the anatomy and disorders of the peripheral nervous system and the EDX examination. The physician is often a neurologist or physiatrist, but occasionally a neurosurgeon or an orthopedist. In this situation, the referral information includes brief, yet focused, clinical information, and a limited differential diagnosis. In these situations, the EDX consultant performs an EDX study on the symptomatic limb(s) to confirm or exclude the suspected diagnosis or, sometimes, makes an alternative diagnosis which may have not been considered by the referring physician.
3. The referring physician is not well versed with disorders of the peripheral nervous system. Often, the referral working diagnoses in these patients are vague, nonspecific, or extensive. Since the EDX study has limitations related to patient discomfort, expense, and time constraints, a directed neurological history and a brief neurological examination is often mandatory before planning and executing the EDX study. Unfortunately, contacting the referring physician to extract more specific information is often not fruitful.

THE EMG LABORATORY PROCEDURES

Testing an Adult

Patients referred to the EMG laboratory should have a referral form completed by the referring physician with relevant clinical information and preferably a pertinent neurological differential diagnosis (Figure 1-1). Referring physicians should also describe the EDX study to their adult patients, particularly in regard to the discomfort associated with it, without creating unnecessary heightened anxiety. If unclear about the technical details of the procedure, they should encourage their patients to contact the EMG laboratory prior to the test date, to get a verbal or written description of the procedure. Such written descriptions should be widely available in all referring physicians’ offices (Table 1-2).

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Figure 1-1 A sample of the referring request for an EMG examination.

(Adapted from Katirji B. The clinical electromyography examination. An overview. Neurol Clin N Am 2002;20:291–303.)

Table 1-2 A Sample of a Descriptive Explanation of the EMG Examination to be Given to Patients before Undergoing Testing

WHAT YOU SHOULD KNOW ABOUT YOUR EMG TESTING
The EMG (ElectroMyoGraphy) Examination
The EMG examination is a diagnostic examination of nerve and muscle function. Your doctor has arranged this test to assist in establishing a diagnosis and plan treatment. The EMG examination includes (1) nerve conduction studies and (2) muscle testing.
Nerve conduction studies are performed by placing discs on the skin over nerves and muscles and recording the responses to electrical stimulation of the nerves. The nerves are stimulated with mild electrical impulses that give an unusual and surprising sensation (much like the sensation in the fingers experienced when you hit you elbow on a desk).
The muscle testing involves direct recording of muscle activity at rest and during contraction by inserting small needles into various muscles. A pinprick sensation is experienced when the needle is inserted and sometimes a mild, dull ache is noted while the needle is in place. No electrical shocks are given. The needle picks up the electrical activity generated by normally by the muscle. This electrical activity is displayed on a screen and over a loudspeaker so that the physician can see and hear it.
The EMG examination is safe, well tolerated, and involves only minor discomfort. It takes about one and a half hours to complete the study. However, it is not unusual for more time to be required.

Special Instructions
At the time of your EMG appointment, your skin should be clean and without lotions, oils, or creams. No special preparation is required. You can take all your medications as prescribed by your doctor. Please notify the technologist if you are taking a blood thinner (warfarin or Coumadin), are on medication for myasthenia gravis (Mestinon), or have a pacemaker or stimulator. There are no after effects and you may return to your usual activities upon leaving the EMG laboratory.

Test Results
The results of the EMG examination are sent to your doctor, who, in turn, will explain them to you and plan the appropriate treatment.

Reprinted with permission from Katirji B. The clinical electromyography examination. An overview. Neurol Clin N Am 2002;20:291–303.

Upon arrival at the EMG laboratory for testing, the patient should be informed in detail of the procedures planned based on the referral information and clinical manifestations. Reading a written description is useful, but a verbal description of the procedure by the EDX technologist, the electromyographer, or both is usually more comforting and reassuring to the patient.

The practice of electrodiagnosis is widely regarded as a practice of medicine. The electromyographer must have a good fund of knowledge of the anatomy, physiology, and disorders of the peripheral nervous system, and be familiar with the techniques that are necessary for performing the EDX study. Although a formal training in clinical EMG is necessary, the skills of the electromyograper are usually based on the number and type of patients studied. In practice, the electromyographer functions similar to a radiologist, by providing diagnostic studies directed by the patient clinical symptoms and working diagnosis. Hence, the EMG study should be as independent as possible, by providing an objective physiological assessment of the neuromuscular system.

NCSs and repetitive nerve stimulations (RNSs) may be performed by the electromyographer, EDX technologist, or both. Well-trained, preferably certified, EDX technologists should work under close supervision of the electromyographer. All NCSs and RNSs should be viewed by the physician before proceeding with the needle EMG. Additional NCS may be added pending the needle EMG findings. For example, the superficial peroneal sensory NCS should be added to the routine NCS of the lower extremity, if the needle EMG examination reveals denervation in L5 innervated muscles, to confirm the location of the lesion to the intraspinal canal as seen with an L5 radiculopathy.

Needle EMG is performed by the electromyographer. The data are obtained live and are not easily stored or reviewed. A concentric or monopolar needle electrode with the smallest diameter possible should be utilized, to reduce the pain and discomfort associated with needle insertion. The patient should be comforted throughout the procedure; if requested, a pause should be granted in the midst of the study.

Testing a Child

The EMG examination often creates extreme anxiety in young children and their parents. If possible, children should be accompanied by at least one nonanxious parent throughout the study for comfort purposes. The parent may hold the child’s hand or sit next to the child. Occasionally, both parents may not withstand observing the test performed on their child, and in these situations, they are better kept away during the active component testing.

Most teenagers tolerate the test well. High current nerve stimulations that are excessively supramaximal should be avoided to reduce the pain and discomfort. In an extremely anxious child (and occasionally adult), the needle EMG should focus on muscles with the highest likelihood of abnormality, since only few muscles may be ultimately sampled. For example, sampling the vastus lateralis and deltoid may be the only possible muscles examined in an anxious child with possible proximal myopathy.

Sedation of young children, particularly those between the ages of 2 and 10 years, is advocated but still debated. Sedation has the advantage of allowing the performance of NCSs and repetitive nerve stimulation without any concern about movement artifacts. However, sedation has the disadvantage of rendering the needle EMG more difficult, if the child does not activate enough MUAPs needed for accurate analysis. In these situations, the evaluation of both MUAP morphology and recruitment may be suboptimal. Sedation of young children is not without risks and should be done under physician and nurse supervision and constant monitoring of vital signs and oxygenation. The use of chloral hydrate in the past was not always successful and occasionally resulted in deep and prolonged sedation. This drug has been replaced by agents that induce rapid hypnosis without excitation such as midazolam hydrochloride (Versed®) or propofol (Diprivan®).

Propofol (Diprivan®) is the most popular intravenous sedative-hypnotic anesthetic widely used in the United States since 1989 because of its rapid onset of action and recovery. Plasma levels decline quickly as a result of high metabolic clearance and prompt distribution to the tissues. These properties account for propofol’s rapid onset and short duration of action. Clinically, maintenance of adequate sedation requires a constant infusion of propofol. Discontinuation of propofol anesthesia usually results in a rapid decrease in plasma concentrations and prompt awakening. Although the terminal elimination half-life of the drug is 1 to 3 days, the sedative effects typically dissipate within 5 to 10 minutes after the infusion is discontinued. Longer anesthesia cases or sedation in the intensive care unit may produce higher plasma concentrations and thus prolong awakening time. With the advent of propofol, the use of sedation of young children undergoing EMG testing has become more feasible since the time to awakening after a one- to two-hour infusion is extremely short. Also, it is possible to titrate the dose to allow partial awakening that is good enough to assess MUAP morphology and recruitment.

In infants, using a pediatric stimulator is recommended since the distance between the cathode and anode is smaller. Shock artifacts that occasionally obliterate the response partially or completely or prevent accurate measurement of amplitude or latency are common with distal stimulations at the wrists and ankles. This is due to the very short distance between the cathodes and recording electrodes with distal stimulations.

Testing in the Intensive Care Unit

EDX testing of critically ill patients with suspected neuromuscular disorders in the intensive care unit (ICU) is often difficult and may be frustrating because of several limitations (Table 1-3). Particular attention should be given to the patient’s skin temperature since peripheral vasoconstriction is common and may lower skin temperature. A core temperature of greater than 36°C or skin temperature greater than 32°C should be aimed for, since lower temperatures result in slowing of distal latencies and conduction velocities. Excessive tissue edema may be associated with low amplitude sensory or motor responses or interferes with supramaximal stimulations and giving a false impression of axonal loss. The edema may be generalized (as with hypoalbunimemia) or limited to the legs (such as with congestive heart failure), hands (such as with extravasation of fluids from intravenous lines), or neck (such as following tracheostomy or central line placement). Many ICU patients have a bleeding diasthesis or are on anticoagulation that prevents extensive needle EMG testing. Excessive sweating, skin breakdown, central lines, pacemakers, monitoring devices, or communicable diseases also influence the type of procedure, the particular site, and extremity tested.

Table 1-3 Limitations of Electrodiagnostic Testing in the Intensive Care Unit

Limitation Result
Nerve conduction studies
Cool extremities Delayed distal latencies and conduction velocities
Edema Low amplitude or unevoked sensory and motor responses
Excessive sweating Artifacts and inadequate or unevoked responses
Skin breakdown Inability to stimulate or record
Central line Inability to stimulate near the line in fear of cardiac stimulation
Pacemaker Inability to stimulate near the wires or pacer in fear of cardiac stimulation
Anterior neck swelling Inability to percutaneously achieve supramaximal simulation of the phrenic nerves
Needle EMG
Bleeding diasthesis Inability to complete a thorough needle EMG
Coma or deep sedation Inability to accurately assess MUAP morphology or recruitment
Agitation Inability to accurately assess the insertional and spontaneous activities
Intubation/ventilation Inability to turn the patient to needle test the paraspinal muscles

In spite of these limitations, EDX testing, including needle EMG, NCS, and repetitive nerve stimulation, may be performed safely in the ICU, and often provide significant assistance in neuromuscular diagnosis and prognosis. Reviewing the history, physical examination, and medication history as well as discussing the queries and testing plan with the ICU team may prove beneficial to avoid possible pitfalls. Except in rare situations, the EDX tests done in the ICU are often less extensive than the studies done in the EMG laboratory, often with less NCS and needle EMG sampling. However, enough details are usually obtained to diagnose or exclude certain neuromuscular disorders that may be encountered in the ICU. In acute situations, serial studies are often necessary for final diagnosis and prognosis.

Testing of the respiratory system in the ICU is another important part of the application of EDX testing that has not been used frequently. Its major role is to investigate the cause of respiratory insufficiency or failure to wean off mechanical ventilation by testing components of the peripheral nervous system involved in ventilation, including the diaphragm and phrenic nerve. Phrenic motor NCS by surface stimulation, recording from the skin over the diaphragm, may be performed in the ICU setting, but are not uncommonly limited by neck swelling, central lines, and pacemaker wires. Diaphragmatic needle EMG examination of the diaphragm may be performed, but is cumbersome in the ICU and patients may not be alert enough to cooperate with testing.

EMG LABORATORY REPORT

When completed, the EDX consultant should explain the findings in brief to the patient, bearing in mind that the electromyographer is often not the referring or treating physician. Discussion of a serious illness, such as amyotrophic lateral sclerosis, may be best left to the referring physician. Suggestions for clinical management should not be discussed with the patient (except in general terms if necessary) unless the referring physician has requested a formal neuromuscular consultation.

The results of the EDX study should be conveyed promptly to the referring physician(s). An EMG laboratory report is the best way to transmit the results of the EDX assessment to the referring physician. Occasionally, the EDX consultant should contact the referring physician if the EMG findings reflect a grave disease or if a planned surgery needs to proceed or be cancelled due to these findings.

Generating a concise and understandable EMG laboratory report is an important function of the electromyographer. The EDX report should be typed (not hand written) since it constitutes an integral part of the patient’s medical records. The report should contain all the pertinent data acquired during the study, despite that some referring physicians are only interested in the final conclusion (Figure 1-2). In addition to the demographic data (patient name, age, birth date, sex, hospital number, date of study, and referring physician), the EMG laboratory report should include the following:

1. Reason for referral to the EMG laboratory. This should include a brief and pertinent clinical summary, the temporal course of the illness (with date of onset if applicable), and the complicating factors which may influence the EDX findings. These factors include diabetes mellitus, local swelling, limb deformity, history of poliomyelitis, or previous lumbar or cervical spinal surgery. An example of a statement outlining a brief history and the reason for referral is the following: “Acute right wrist drop noted after recent abdominal surgery on 6/4/2004. The patient has had diabetes mellitus for 3 years and a remote history of anterior cervical diskectomy. Evaluate for right radial neuropathy and brachial plexopathy.”
2. Nerve conduction studies. This segment of the report should always be part of the EMG laboratory report, and is particularly directed to physicians who are well versed with the EDX examination. Recording and revealing limb(s) temperature is extremely useful, since many of the NCS parameters are greatly affected by cool limbs. Since F wave and H reflex latencies are length-dependent, the patient’s height should be also shown on the report. The tabulated NCS form should be detailed but not overcrowded with unnecessary data. Nerves stimulated, stimulation sites, and recording points are extremely important. Amplitudes (distal and proximal), latencies, conduction velocities, and F wave latencies should be noted. Normal laboratory values should also be shown if possible (Figure 1-2A).
3. Needle EMG. This should list all the muscles examined with their detailed findings. The insertional activity (increased, decreased, myotonic discharges, etc.), spontaneous activity (fibrillation potentials, fasciculation potentials, complex repetitive discharges, etc.), MUAP activation (normal, fair, poor) and recruitment (normal, decreased, early), and MUAP morphology (amplitude, duration, percentage polyphasia) should be all reported (Figure 1-2B). If an advanced EMG study is done (quantitative MUAP analysis, MUNE, single fiber EMG, etc.), the data should be outlined in detail.
4. Summary. It is a good practice to recap the pertinent aspects of the EDX study in one or two paragraphs. All the abnormalities and relevant negatives should be highlighted. This summary sets the stage for formulating a meaningful impression.
5. Impression (or conclusion). This is the most important component of the EMG report since it represents the final link between the electromyographer and referring physician. The impression should be brief, yet clear and disclose as much information as possible. The EDX examination often makes an anatomic or physiologic diagnosis but not a final clinical diagnosis. For example, the EDX study often diagnoses a median mononeuropathy at the wrist but not a carpal tunnel syndrome, or a necrotizing myopathy but not polymyositis. In these situations, the electromyographer may report that the finding are “consistent with or compatible with” the appropriate suspected clinical syndrome. When a peripheral nerve lesion is detected, the site of pathology with its severity, chronicity, and pathophysiology, should be delineated. The EMGer may also provide a brief list of differential diagnoses that are based on the EDX findings. For example, if myotonia is detected on needle EMG, a list of the common inherited myotonic disorders and the drug-induced myotonias may be useful to the referring physician. Rarely, such as with the Lambert-Eaton myasthenic syndrome, the EDX examination may be diagnostic of a specific disorder.

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Figure 1-2 Sample of an EMG report. A: Nerve conduction studies. B: Needle EMG.

If the EDX study is normal, the impression should also state that the study did not find evidence of the specific disorder(s) for which the patient was referred. If the EDX examination was limited or incomplete, such as due to poor patient tolerance, this should be explicitly explained in the impression. In situations where multiple EDX findings are detected, they should preferably be listed relevant to their individual relation to the suspected diagnosis, followed by the likely incidental or asymptomatic findings. If a repeat EMG study is needed, the report’s impression should state the proposed time frame for such a study.

The electromyographer should be as objective as possible and not rely fully on the clinical information in making a diagnosis that is not substantiated by the EDX findings. For example, the EDX of a patient with a remote elbow fracture and suspected tardy ulnar palsy may show an axon-loss ulnar mononeuropathy without focal slowing or conduction block but with denervation of the ulnar innervated muscles in the forearm. The electromyographer should report that the ulnar mononeuropathy is localized at or above the elbow and refrain from localizing the lesion to the elbow. Apart from prognostication in patients with nerve injuries, the EDX report should not include treatment or management recommendations. In situations where the electromyographer is the treating physician or is asked to provide a neuromuscular consultation, a detailed neurological history, examination, diagnosis, management, and prognosis should be included in a distinct neurological consultation report.

SUGGESTED READINGS

Aminoff MJ. Electromyography in clinical practice, 3rd ed. New York: Churchill-Livingstone, 1998.

Dumitru D. Electrodiagnostic medicine, 2nd ed. Philadelphia: Hanley and Belfus, 2001.

Katirji B. Clinical electromyography. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, editors. Neurology in clinical practice. 4th ed. Boston: Butterworth-Heinemann; 2004:491-520.

Katirji B, editor. Clinical electromyography. Neurology clinics. Philadelphia: WB Saunders, 2002.

Katirji B. The clinical electromyography examination. An overview. Neurol Clin N Am. 2002;20:291-303.

Kimura J. Electrodiagnosis in diseases of nerve and muscle: principles and practice, 3rd ed. New York: Oxford University Press, 2001.

Preston DC, Shapiro BE. Electromyography and neuromuscular disorders. Philadelphia: Elsevier/Butterworth-Heinemann, 2005.

Shapiro BE, Katirji B, Preston DC. Clinical electromyography. In: Katirji B, Kaminski HJ, Preston DC, Ruff RL, Shapiro EB, editors. Neuromuscular disorders in clinical practice. Boston: Butterworth-Heinemann; 2002:80-140.

Smith SJM. Electrodiagnosis. In: Birch R, Bonney G, Wynn Parry CB, editors. Surgical disorders of the peripheral nerves. Edinburgh: Churchill-Livingstone; 1998:467-490.

Wilbourn AJ, Ferrante MA. Clinical electromyography. In: Joynt RJ, Griggs RC, editors. Baker’s clinical neurology on CD-ROM. Philadelphia: WB Saunders, 2000.

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