Needle EMG muscle identification: A systematic approach to needle EMG examination.

The proper performance of needle electromyography (EMG) requires that the examiner obtain a brief but comprehensive history, perform a directed examination and generate a short differential diagnosis as part of the initial patient encounter. Equally as important is to set reasonable expectations for this study's performance as electronic media do not necessarily portray all of the nuances of an electrodiagnostic study. In addition to these preliminary steps, this minimonograph discusses equipment used in EMG evaluations, EMG examination techniques, muscles commonly sampled, pain reduction techniques, and special considerations that may require study modification such as anticoagulation, lymphedema, obesity and supervening infection. Clinicians performing these studies will maximize useful data collection while minimizing patient discomfort if all of these recommendations are followed.

Introduction

A broad knowledge base is required for performing an electrodiagnostic (EDX) medicine consultation. It includes understanding of anatomy, normal physiology, pathophysiology, EDX medicine techniques, basic principles of electricity, as well as signal processing and analysis. A successful examination requires patient rapport and cooperation. A successful needle examination requires attention to a number of specific issues in a series of steps:

Clinical evaluation

Patient preparation

Muscle selection

Muscle localization

Muscle examination

Special Considerations

Clinical evaluation

The referring provider’s clinical history and physical examination should be reviewed before beginning the EDX medicine consultation. While not obligatory, it is recommended that verbal informed consent be obtained and documented prior to the performance of any EDX procedure. It is strongly recommended that the examiner establish the salient features of the patient’s history along with the performance of a directed examination in order to verify or amend the initial clinical impression. A successful EDX consultation will address the questions posed by the referring provider as well as any new diagnostic possibilities that arise during one’s own abbreviated history and physical. Additional historical information may be obtained during the needle examination. This serves an additional purpose of distracting the patient from the discomfort of this portion of the examination.

It is important to include clinically-weak muscles during the needle examination. For example, if an L5 radiculopathy is suspected and the extensor hallucis longus is the only weak muscle, it should be included in the study. You should also review the results of any previous studies to help design selection and order of the muscles to test. One should plan the study to answer the questions as efficiently as possible with the least degree of patient discomfort.

Preparing the patient for the study

Most patients will have received information about the needle examination prior to the study and may have a few questions. Nonetheless, it is still helpful to explain that this examination differs from the nerve conduction studies in that no external electrical stimuli are applied. The patient should also be informed that nothing is inserted through the needle or removed from the needle as it will only be used to record muscle activity. One of the authors typically explains needle electromyography (EMG) as a “muscle microphone.” You should explain that the needle will be inserted into a number of muscles and that there will be some discomfort, which is unavoidable but generally well-tolerated. You should also explain that needles are discarded after each use. Do not use statements such as, “A fine wire electrode will be placed …” Many will be caught unaware seconds later when they feel a needle stick, and the patient will lose confidence in you. The patient will appreciate knowing approximately how long the study will take and how many muscles will be examined. Prepare the skin over each muscle with alcohol or other appropriate agent before needle insertion. Although this has not been shown to reduce infection, many needle EMG videos depict this behavior leading patients to expect that this will be performed. Before each needle insertion, you should inform the patient of the approximate location and alert them to an imminent “stick or poke.”

Selection of muscles for examination

The groups of muscles to be tested are initially selected on the basis of the clinical hypotheses (e.g., proximal muscles for myopathy, single limb for radiculopathy, widespread for motor neuron disease, etc.) The individual muscles selected for examination should be superficial, easily palpated, and readily identified. They should be located away from major vessels, nerve trunks, and viscera. Select muscles that are less uncomfortable for the patient. For example, the thenar and small foot muscles are often more uncomfortable, and they should only be tested when the information is not available from other muscles. Exceptions to this general guideline would be an evaluation for ulnar neuropathies and median neuropathy at the wrist. The first dorsal interosseous pedis is useful in polyneuropathy examinations and will be described later in this monograph. Since the appearance of motor unit action potentials (MUAPs) can vary greatly between different muscles, the muscles selected should be familiar to the examiner, both in how to test the muscle and the range of normal findings.

Anatomical localization of the muscle

A needle EMG examination is inextricably linked to human anatomy. A thorough knowledge of musculoskeletal anatomy is essential to the successful practice of EDX medicine. Most importantly, the practitioner must always be confident of which muscle is being examined. Achieving that certainty is easily accomplished when the EDX medicine consultant is confident of needle placement through a detailed knowledge of the pertinent anatomy (see Appendix A) (Geiringer, 1999, Leis and Schenk, 2012). Knowledge of anatomy is preferable to fixed distances for identifying the optimal point for needle insertion. Estimates of where to insert the needle based upon fixed distances from an anatomical landmark quickly fail in practice. A fixed distance will mean one thing in an infant, another in an obese adult, and quite another in a tall adult. Apparent muscle locations vary with limb and joint position as well as with associated edema and pathological processes that result in atrophy or hypertrophy. It is only through a detailed understanding of the three dimensional relationships that do not vary among patients that allows a practitioner to develop confidence in needle electrode placement. If sufficiently superficial, the muscle to be tested should be palpated during intermittent contraction to localize its borders with the examiner’s thumb and index finger before needle insertion in order to define the optimal insertion site. The location of end-plate regions should also be taken into account so that they may be avoided.

Performing the EDX medicine consultation

An EDX medicine consultation includes a number of distinct skills that are described in detail below:

Needle EMG techniques

Data collection/EMG activity analysis

Pain minimization

Needle EMG techniques

The ability to record normal and abnormal electrical activity from muscle is operator dependent. Needle EMG requires a number of skills and knowledge (Appendix E). Needle placement and data recording are absolutely necessary in order to obtain accurate and reliable waveforms. This critical step is generally underemphasized. A few simple guidelines allow this crucial aspect of needle EMG examinations to be performed correctly and efficiently. The following discussion outlines some of the considerations.

Needle electrodes

There are a variety of needle electrode lengths and types. Needle electrode selection depends on a number of patient and examiner considerations (Appendices B and C). Needle electrodes must be sterile. Disposable, standard electrodes are available at a reasonable cost and should be used for each patient. While more expensive needle electrodes, such as single fiber needle EMG electrodes, may be sterilized and reused, this is not recommended for those employed in routine practice. Such electrodes are typically sharp and undistorted. Rarely, they may not be sharp and will resist insertion. If an electrode penetrates the skin with difficulty, passing it through a sterile cotton ball or sponge may identify snags from bent tips. To determine if a batch of electrodes are not well made, they should be examined under a low power microscope. Needles must be straight. A needle that has been bent should not be straightened for continued use since a small break in the insulation may cause a short circuit and result in needle EMG signal distortion.

The recording surface must be the correct size and shape, as well as absolutely clean. Disposable, sterile needles from the manufacturer may rarely be left with a very thin, poorly conducting film on the surface. This film increases the impedance and may cause a low-voltage, irregular, positive waveform (popping noise). This must be recognized since it may be mistaken for end-plate noise, positive sharp waves, or fibrillation potentials. The film may be dispersed within a few seconds in the muscle. If not, the needle should be replaced. The shaft must be stable in the hub to prevent it from breaking off in a patient. The connections to the cable must be intact. A poor connection can result in intermittent 60 Hz or irregular interference. Electrical impedance should be checked if a break or short is suspected (correct impedance at 60 Hz is 5–20 kΩ).

There is a debate as to whether concentric needles or monopolar needles should be used for the needle EMG examination. While there are some differences, they are relatively minor. Nonetheless, it is important that the examiner use the same type of needle electrode that was used in obtaining the normal values used in his or her laboratory. The authors prefer concentric needles because they do not require a surface reference, the signal is crisper, and the examination may be conducted more rapidly.

Needle insertion

The muscle to be tested should be palpated during intermittent contraction to localize its borders with the examiner’s thumb and index finger. It is helpful to make the skin taut at the site of insertion, particularly where the skin is loose. The taut skin is best pulled a short distance distally over the muscle to reduce bleeding (when released, the skin will pull back over the needle site in the muscle). The needle electrode should be held firmly in the fingers like a pen and inserted smoothly and quickly through the skin into the subcutaneous tissue or superficial layers of the muscle at approximately a 45 degrees angle. This minimizes the force necessary to achieve penetration, and it also may distract the patient prior to skin puncture. Rest the hand holding the needle on the skin in order to make needle movement comfortable and precise. Your opposite hand is located on the boundaries of the muscle for assistance in localization during needle movement. A small flick of the examiner’s index finger over the intended insertion site may assist in reducing the patient’s perceived discomfort (Boon et al., 2008).

During needle insertion and the study of insertional activity, the study is best served if the patient is not asked to do anything more than relax. There are many pertinent reasons to avoid relying on patient input for your localization of a muscle. No voluntary contractions should be required to confirm needle placement, for the following reasons:

Some patients are not capable of activating one or any muscle (e.g., with nerve palsy, hemiparesis, coma, upper motor neuron disorders, or non-organic weakness, etc.).

Some muscles are not palpable from the surface in any patient, (e.g., tibialis posterior).

Voluntary contractions can be misleading. As an example, if the needle is mistakenly placed in the flexor carpi radialis rather than the targeted pronator teres, testing localization with forearm pronation will not reveal the error, as both muscles subserve this function.

Patients tend to become less comfortable with needle EMG as time passes. The sequence of palpation, contraction, repalpation, needle insertion, and recontraction takes time, which extends the length of the examination.

The patient’s confidence in you might waiver if you spend as much time searching for each muscle as examining it. Diagnostic ultrasound may be useful for precise placement of the needle electrode into the muscle, especially in patients with a large body mass index. Other uses of ultrasound are summarized later in this monograph.

Above all, you must be completely confident that you are examining the muscle that you intended to study.

Needle movement

The muscle is examined by moving the needle along a straight line into the muscle in short steps (0.5–1 mm). Large movements are more painful and end-plate areas may not be recognized. The needle should not be released between movements. The pace of needle movement should not be rushed. A brief pause (1 s or more) between each site is needed to listen and watch for slower onset abnormal activity. The needle is advanced in 5 to 30 such steps depending on muscle diameter. After traversing the diameter, the needle is withdrawn from the muscle, but not from the skin, and then reinserted at a different angle in the same location; 2–4 such passes through the muscle are made until an adequate number of sites within the muscle have been examined. Most texts describe the standard needle EMG examination as requiring 5 sites in each of 4 quadrants be evaluated so that 20 total sites have been sampled per muscle. However, 3 sites in 2–4 quadrants may be sufficient. Significant amounts of spontaneous activity are usually observed with the first few sampling sites.

Data collection/EMG activity analysis

The muscle should be examined at multiple sites both at rest and during contraction using the methods previously described. Either resting or contracting muscle may be tested initially. Resting muscle is preferred first since it is sometimes more difficult to obtain full relaxation than a contraction. However, if a muscle is already contracting at the desired level on insertion, it should be tested in that position. Do not intermittently relax and contract a muscle at one site. That leads to more local muscle injury, bleeding, and subsequent pain.

Resting muscle

The resting muscle is tested for spontaneous activity at a gain of 50 µV/cm. When the needle is well within the muscle, it should be left undisturbed for a number of seconds to listen for fasciculations. It is not always easy to obtain muscle relaxation. In tense patients or during a painful examination, relaxation can be enhanced by:

Carefully positioning the patient at the beginning to provide the best relaxation and save time overall

Adequately supporting the limb and, at times, passively manipulating the limb

Contraction of an antagonist

Distraction with conversation

Reassurance

Changing needles

Once you have the needle under the skin, a more gentle movement of the needle can be used to pierce the superficial fascia. As the fascia is approached, listen for the rumbling of “distant” motor units. The muscle might not be relaxed enough to proceed, and pushing the needle into a moderately or strongly contracting muscle may be unnecessarily painful. Give specific directions about how to relax the muscle. For example, “Roll that thigh out toward me.” Medical terms such as “dorsiflex your ankle” should be avoided. A similar outcome can be obtained by saying, “Toes up towards your nose.” It is not useful to simply request that the patient relax, especially at increasingly higher volumes on your part. The typical responses are, “I thought I was relaxed,” or, “I’m as relaxed as I can be with your sticking that needle in me.”

Tonic pressure should be kept on the needle hub while studying insertional activity. If you do not, particularly with concentric needle electrodes, there will be a tendency for the electrode to “bounce” back out of the muscle at the same distance that you just moved it inward. Insertions should be smooth, firm, and with small amplitude forward movements of 0.5 mm or so. There is no advantage in using hard jabbing motions as they may cause pain.

Contracting muscle

The contracting muscle is examined using the same needle methods as for resting muscle. The contracting muscle is best examined with the muscle held at a level of contraction that activates a few motor units (low-to-moderate effort). Selective activation of the muscle of interest and adjacent muscles is needed to determine needle position when examining deep muscles, muscles that are difficult to palpate, or small muscles. Steps in testing a contracting muscle include:

Withdrawing the needle to a subcutaneous position before asking for muscle contraction.

Positioning the limb and muscle and initiating contraction before moving the needle into the muscle. Advance the needle until you encounter MUAPs with a rapid rise time and a sharp, clicking sound.

Proper limb positioning such that the activity of synergistic and adjacent muscles is limited.

Asking the patient to perform a movement that only requires activation of the muscle being examined.

Palpating the contracting muscle in order to help guide the needle movement.

Other considerations

Small muscles are best tested with an oblique needle course through the muscle to lengthen the needle’s path. Deep muscles and obese patients require a needle of adequate length. If the needle were to break off, it would likely do so at its hub, which is its weakest point. If a needle were to be inserted to a depth greater than its length and it broke, it would be difficult to remove. Some muscles, such as the deep paraspinal muscles, may be difficult to reach without a long needle, even in average-sized patients. Needles of up to 120 mm length should be available and should be used in such circumstances.

Pain control/minimization

Most patients are able to tolerate the discomfort of the needle examination without difficulty, but a few need special approaches. A review article on safety and pain in EDX studies summarizes the salient features of needle EMG associated pain (Boon et al., 2008).

Pain minimization requires attention to all interactions with the patient, in particular the techniques of the needle examination itself. Approaches that can be helpful in all patients are described in Appendix C.

Overview of adjunctive ultrasound

Ultrasound has been evolving as an adjunctive electrodiagnostic methodology based on its ability to assess normal and pathological anatomy (Boon et al., 2012a, Boon et al., 2012b). A complete discussion of neuromuscular ultrasound is beyond the scope of this discuss save for a brief description of using this technique for needle EMG guidance. However, it can be summarized as allowing the examiner to sample muscles in patients with atypical surface anatomy, significant body mass index, deep muscles, denervated muscles and muscles often not routinely examined such as the diaphragm (Boon et al., 2008). Ultrasound was also demonstrated to improve sampling accuracy in a cadaveric model, especially in less experienced examiners (Boon et al., 2011). In summary, ultrasound guidance should be considered in situations wherein the examiner has a concern about accurate needle placement.

Special considerations

A number of special issues presented by a few patients must be considered before initiating the needle examination. As the article by London (2017) addresses many of these items, only a brief summary will be listed in Appendix D and includes:

Anticoagulants and bleeding disorders

Infection

Cardiac valvular disease

Obesity

Skin conditions

After the study

Before leaving the room, check to be sure that all puncture sites are dry and that no bruising is evident. If bleeding is still present, 1–2 min of firm pressure applied by either the EDX examiner or the patient will usually stop it. An ice pack is useful to minimize additional bleeding if a small hematoma has formed. Ensure that the patient can get dressed unassisted, or be sure they have help. Some patients ask about persisting discomfort after the examination. They can be advised that their muscles may ache for a few hours, but this will usually disappear overnight. If necessary, mild analgesics such as non-steroidal anti-inflammatory agents may be used, (e.g., acetaminophen).

	Concentric Needles	Monopolar Needles
Recording area	Smaller (stable)	Larger (variable)
Recording properties	Stable	Variable; may polarize
Background	Less noise (better common mode rejection)	Noise from surrounding muscles
Reference electrodes	Needle shaft	Separate surface electrode
Motor unit potentials	Smaller	Larger
Motor unit quantitation	More reliable	Less reliable
Discomfort	No difference, if disposable	Less than non-disposable
Cost	More expensive	Less expensive