Author(s): David Haustein, MD, MBA, Evan Reeves, BS, Connor Swartz, DO
Originally published: September 10, 2015 Last updated: August 16, 2024
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Overview and Description

Definition

The electrodiagnostic evaluation is considered to be an extension of the neurological examination of the peripheral nervous system. At times, it can reveal dysfunction of the nerves and muscles which was not detected in the physical exam.1 Electrodiagnostic studies are commonly used to diagnose entrapment neuropathies, plexopathies, radiculopathies, neuromuscular junction disorders, myopathies, and anterior horn cell disease and can prognosticate functional recovery. It has a role in the localization of the site of the lesion and identifying the pathology.2

Electrodiagnostic testing can help answer the questions of the differential diagnosis considered by the referring physician and the consultant.2 This test should be preceded by verbal or written consent from the patient or their surrogate. The reason for the consultation and the methods employed should be discussed with the patient.3 The typical electrodiagnostic consultation involves a focused neuromuscular history and physical examination, the examination of muscles and nerves utilizing nerve conduction studies (NCS) and needle electromyography (EMG), and the determination of a final diagnosis. According to the American Academy of Neuromuscular and Electrodiagnostic Medicine’s (AANEM) guidelines, the interpretation of NCS without an EMG does not meet its standards and should be the exception in certain situations rather than the standard of practice.4 After the test is performed, the consultant must evaluate the results. Ideally, the entire test should be performed the same day by the same electromyographer for continuity and consistency of the results.3

Hospital Corpsman 2nd Class Melissa Clayton, an electroneurodiagnostic technician at Naval Hospital Pensacola, performs a nerve conduction study on a patient July 18. Clayton recently became the only registered nerve conduction study technologist in the Navy.

Figure 1: Bortz J. Naval Hospital Pensacola sailor strikes a nerve. DVIDS. 2018. Accessed May 12, 2024. https://www.dvidshub.net/image/4572432/naval-hospital-pensacola-sailor-strikes-nerve.

Sensory nerve conduction studies

Sensory nerve action potentials (SNAP) are recorded after the stimulation of a sensory nerve. If the electrical impulse is propagated in a physiologic direction, it is termed an orthodromic study. When the electrical impulse is propagated in the opposite of the physiologic direction, it is termed antidromic.2  

Reporting SNCS should mention whether the test was performed orthodromically or antidromically, whether peak or onset latency was used, and the amplitude (peak to peak or baseline to peak) of the SNAP.6,7 Onset latency reflects the arrival of the impulse in the fastest conducting fibers but can be difficult to measure. Peak latency is easier to record and, therefore, more commonly used.2  

Motor nerve conduction studies

Motor nerve conduction studies measure the compound muscle action (CMAP) potential. The distal latency of the CMAP includes the conduction of the impulse in the nerve, across the neuromuscular junction, and in the muscle fibers. To measure the conduction velocity, it is necessary to stimulate the nerve at two sites, and the distance between the two sites is divided by the difference in latencies. This eliminates the conduction time across the neuromuscular junction and the muscle fiber.2  

Reporting motor nerve conduction studies should include comments on distal latency, amplitude (baseline to negative peak), and conduction velocity along the different nerve segments.6,7 The duration of the negative phase and overall area under the curve are helpful as well.

Late responses

H (Hoffman) reflexes result from the activation of the lowest threshold Ia fibers of a mixed nerve. An afferent volley reaches the motor neurons of the corresponding nerve, and a late response is recorded from the muscle. Clinically the H reflexes can be recorded from the soleus (S1) and the flexor carpi radialis (C6/C7) in adults. Usually, only the H reflex latency is noted. H reflexes may be useful in detecting dysfunction of the sciatic nerve, sacral plexus, or S1 nerve root, as well as assessing large-diameter myelinated nerve function.2,7

F-waves result from the supramaximal antidromic stimulation of a motor nerve. This results in the activation of a subpopulation of anterior horn cells (2-5%) of the corresponding motor nerve and the generation of an orthodromic action potential that is recorded on the distal muscle. This potential has a smaller amplitude and a prolonged latency.2,7

When recording F-waves, indicating the nerve tested, the muscle tested, and the minimal F-wave latency are recommended.6,7 F-wave latency measurement is most useful in detecting multifocal or diffuse demyelinating proximal lesions, particularly in the early stages.7

Blink reflexes are done to evaluate patients with suspected facial nerve, trigeminal nerve, or brainstem lesions and sometimes in the evaluation of patients with polyneuropathy.8

Repetitive nerve stimulation (RNS)

This is performed to assess the neuromuscular junction (NMJ). NMJ disorders are classified into post synaptic disorders (e.g., myasthenia gravis), presynaptic disorders (e.g., Eaton Lambert Syndrome), and combined pre and post-synaptic disorders (e.g., aminoglycoside induced myasthenic syndrome).9 It is first necessary to perform routine motor and sensory nerve conduction studies and electromyography to rule out other neuropathic or myopathic disorders.7

Supramaximal stimulation is used in RNS with the recording electrodes firmly attached in a belly tendon fashion; similarly, the stimulating electrodes should be firmly attached to minimize movement. The limb should be immobilized and warm.9

Indication of the nerve stimulated, the side, the muscle used for recording, if the muscle was at rest or if the test was done after exercise, the duration of the exercise, and the time interval after the exercise is necessary. The rate of stimulation and the number of stimuli used should be mentioned.6

EMG testing

Theoretically, the electrophysiologic signal resulting from the voluntary firing of a MU is the summation of the firing of all the muscle fibers (MF) within the motor unit (MUAP). Generally, MUAPs have a simple triphasic configuration. The increasing phases or turns (complexity) is a sensitive, but nonspecific, marker of abnormality. The recruitment pattern reflects the graded activation of MUAPs on voluntary contraction, while the interference pattern reflects full activation.7

Insertional activity results from the movement of the needle in the muscle. Spontaneous activity is waves seen in the absence of needle movement or voluntary muscle contraction.2 Fibrillation potentials (fibs), positive sharp waves (PSW), complex repetitive discharges (CRDs), and myotonic discharges are abnormal discharges from a single muscle fiber. Fasciculations, myokymic discharges, cramps, and neuromyotonic discharges emerge from the motor unit.

Reporting should include the muscles tested, the side, the presence or absence of abnormal insertional activity, or spontaneous activity at rest. Reporting muscle activity on volition should include comments on size, amplitude, duration, and phases of the MUAPs, motor unit recruitment, and the interference pattern.6,7

Relevance to Clinical Practice

Pathophysiology

Electrodiagnostic testing can differentiate between demyelination and axonal loss in a nerve.

  • In demyelinating lesions, the CMAP and SNAP demonstrate normal parameters distal to the lesion and demonstrate slowing, potential conduction block, and temporal dispersion when stimulated proximal to the lesion. In purely demyelinating lesions, there should not be findings of axonal loss on EMG.
  • In axonal loss, after Wallerian degeneration has occurred, the CMAP and SNAP amplitudes are reduced with stimulation of the nerve proximal and distal to the lesion as compared with the contralateral side. The latencies and conduction velocities are relatively preserved. EMG of the muscles supplied by the affected nerve shows decreased recruitment followed temporally by the appearance of spontaneous activity. In the chronic stages, reinnervation follows with enlargement of the motor unit size, increased number of phases, and duration.2,5

Figure 2: Electromyogram.png. (2024, January 9). Wikimedia Commons. Retrieved 13:17, May 16, 2024 from https://commons.wikimedia.org/w/index.php?title=File:Electromyogram.png&oldid=840036794

Localization

  • Electrodiagnosis is useful in the localization of the site of the nerve lesion, whether it involves the root, plexus, or peripheral nerve. The presence of a normal SNAP in an area of sensory loss is indicative of a lesion proximal to the dorsal root ganglion or in a central sensory pathway. Focal conduction slowing or block on nerve conduction studies can localize a site of entrapment. The distribution of the abnormalities in the muscles tested can also clarify the site of the lesion.2,7
  • EMG can be helpful in identifying neuromuscular junction disorders, myopathic and neuropathic disorders through the MUAP size, configuration, and stability.

Assessment

  • Identifying whether a neuropathy is predominantly sensory or motor, generalized or length-dependent, and primarily axonal or demyelinating can also guide the classification and workup.7
  • EMG can predict the activity of some myopathic disorders. The presence of fibs and PSW in association with short-duration MUAPs indicates active disease in inflammatory myopathy. These findings may resolve with treatment.9
  • In ongoing denervation, fibs, PSWs, decreased recruitment, and polyphasic motor units are present. In chronic denervation fibs, PSWs are minimal and small in amplitude, with large-sized polyphasic MUAPS.
  • EDX study is commonly indicated for evaluation in children with suspected polyneuropathy, mononeuropathy, and various focal neurological symptoms in one or more extremities.10

Measurement and prediction of outcome

  • NCS can assist with prognostication: the larger the amplitude of the nerve action potential distal to the lesion, the better the prognosis for recovery.7

EMG can measure the severity of a lesion. Abnormal spontaneous activity indicates axonal loss. The absence of motor unit action potentials in the muscles supplied by a nerve indicates no action potentials can traverse the lesion due to either axon loss or complete conduction block.

Cutting Edge/Unique Concepts/Emerging Issues

  • There is an increasing role for ultrasound examination of the nerves and muscles in the electrodiagnostic laboratory. Ultrasound can provide structural information about the nerve, such as visualization of the nerve in both long and short axes, measurements of cross-sectional area, echogenicity, flattening ratios, mobility, vascularity, and bowing/notching  In addition, certain scenarios may be encountered whereby the additional imaging information makes the diagnosis clear, such as when localization cannot be determined by electrodiagnostic means, severe neuropathies with absent responses and in cases in which the electrodiagnostic studies were normal but the clinical suspicion is still strong.11 Ultrasound can guide needle placement in electromyography, thus improving safety and accuracy. It may also help localize nerves that are challenging to find when performing NCS.
  • EMG probably contributes to asymptomatic hemorrhage in approximately 1% of patients, but clinically significant bleeding has only been reported a few times. Therapeutic anticoagulation does not significantly increase this risk. With standard procedures, there have been no reports of patients developing cardiac arrhythmia from nerve conduction studies. No special precautions are necessary in patients with implantable cardiac devices or intravenous lines.12
  • MR neurography is a technique that provides noninvasive visualization of nerve pathology and injury at the level of the fascicles, allowing the identification of both focal and non-focal neuropathies.13
  • There is an increasing role for conventional and quantitative MRIs in the diagnosis of inherited muscle disease, quantification of disease burden, and monitoring of disease progression.14
  • Motor unit number estimation (MUNE) is a technique that can be used to determine the approximate number of motor units in a nerve. It can measure motor unit loss, change in the motor unit size, and collateral reinnervation. It can be used to study amyotrophic lateral sclerosis, poliomyelitis, spinal muscular atrophy, aging, and neuropathies in the research setting.15
  • Motor unit number index (MUNIX) is a new technique correlating with the number of motor units in a muscle.  It is comparable to MUNE methods but is less time-consuming and requires less electrical stimulation. It can be used to monitor the progression of neuromuscular disease (e.g., ALS and demyelinating polyneuropathies).  It can detect disease in the affected muscles before denervation is apparent in the affected muscle.16
  • Electrical impedance myography (EIM) is a new, non-invasive technique for the evaluation of neuromuscular disease that relies upon the application and measurement of high-frequency, low-intensity electrical current. It can detect pathologic changes in muscles (e.g. myocyte atrophy, reinnervation, edema, deposition of connective tissue and fat), and grade the severity of neuromuscular diseases.17

Gaps in Knowledge/Evidence Base

  • In electrodiagnostic studies, axonotmesis and neurotmesis cannot usually be distinguished, as the primary difference between the two is the supporting structure integrity.2
  • Patients with small fiber neuropathies may show normal electrodiagnostic studies as they do not involve the larger fibers, which are studied in electrodiagnostic testing.7
  • EMG of the MUAP can measure only physiologic changes that affect the electrical signal of the muscle fiber and is unable to detect some pathologic changes (e.g., abnormal accumulation of storage products like glycogen).7
  • Many findings in electrodiagnostic testing are sensitive indicators of abnormality but are not specific to a disease. An example is that fibrillation potentials and positive sharp waves are present in both neuropathic and myopathic disorders and sometimes even in neuromuscular disorders.7
  • Despite the stable and perhaps increasing demand for pediatric EDX testing, the availability and the approaches to perform the test vary from center to center.10
  • Neuromuscular ultrasound, an emerging diagnostic subspecialty field, has become an important extension of the electrodiagnostic examination. However, there are no formal guidelines on how to report NMUS results appropriately.18
  • Ultrasound elastography is a technique that measures the elastic properties of tissues and has been explored as a noninvasive way to evaluate changes in nerve tissue composition. Its role in the evaluation of peripheral nerve disorders is yet to be clearly defined, although it appears to be promising as an adjunct to other diagnostic studies and as a potential measure of treatment response.19

References

  1. Podnar S. Critical reappraisal of referrals to electromyography and nerve conduction studies. European Journal of Neurology. 2005;12(2):150-155. doi:10.1111/j.1468-1331.2004.00979.x 
  2. Kincaid JC. Neurophysiologic Studies in the Evaluation of Polyneuropathy. Continuum (Minneap Minn). 2017;23(5, Peripheral Nerve and Motor Neuron Disorders):1263-1275. doi:10.1212/CON.0000000000000521
  3. American Association of Neuromuscular & Electrodiagnostic Medicine Ethics Committee. Guidelines for ethical behavior relating to clinical practice issues in neuromuscular and Electrodiagnostic Medicine. Muscle Nerve. 2022;65(4):391-399. doi:10.1002/mus.27501
  4. American Association of Neuromuscular & Electrodiagnostic Medicine. Proper performance and interpretation of Electrodiagnostic Studies. Muscle Nerve. 2020;61(5):567-569. doi:10.1002/mus.26835
  5. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic-Ultrasound Correlations. Elsevier science publishers; 2021.
  6. Jablecki CK, Busis NA, Brandstater MA, Krivickas LS, Miller RG, Robinton JE. Reporting the results of needle EMG and nerve conduction studies: An educational report. Muscle Nerve. 2005;32(5):682-685. doi:10.1002/mus.20422
  7. Barboi AC, Barkhaus PE. Electrodiagnostic testing in neuromuscular disorders. Neurologic Clinics. 2004;22(3):619-641. doi:10.1016/j.ncl.2004.03.007
  8. Preston DC, Shapiro BE. Blink Reflex. In: Electromyography and Neuromuscular Disorders. Elsevier; 2021:52-56.
  9. Oh SJ. In: Principles of Clinical Electromyography: Case Studies. Lippincott Williams & Wilkins; 1998:604.
  10. Kang PB, McMillan HJ, Kuntz NL, et al. Utility and practice of electrodiagnostic testing in the pediatric population: An aanem consensus statement. Muscle Nerve. 2020;61(2):143-155. doi:10.1002/mus.26752
  11. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders : Clinical-Electrophysiologic-Ultrasound Correlations. Fourth edition. Elsevier, Inc.; 2021. Accessed August 6, 2024. https://search.ebscohost.com/login.aspx?direct=true&AuthType=sso&db=cat09285a&AN=osc.1150900721&site=eds-live&scope=site
  12. London ZN. Safety and pain in electrodiagnostic studies. Muscle Nerve. 2017;55(2):149-159. doi:10.1002/mus.25421
  13. Martín-Noguerol T, Montesinos P, Hassankhani A, Bencardino DA, Barousse R, Luna A. Technical update on MR neurography. Seminars in Musculoskeletal Radiology. 2022;26(02):093-104. doi:10.1055/s-0042-1742753
  14. Fischer D, Bonati U, Wattjes MP. Recent developments in muscle imaging of neuromuscular disorders. Current Opinion in Neurology. 2016;29(5):614-620. doi:10.1097/wco.0000000000000364
  15. Gooch CL, Doherty TJ, Chan KM, et al. Motor unit number estimation: A rechnology and literature review. Muscle Nerve. 2014;50(6):884-893. doi:10.1002/mus.24442
  16. Fatehi F, Grapperon A-M, Fathi D, Delmont E, Attarian S. The utility of Motor Unit Number index: A systematic review. Neurophysiologie Clinique. 2018;48(5):251-259. doi:10.1016/j.neucli.2018.09.001
  17. Sanchez B, Martinsen OG, Freeborn TJ, Furse CM. Electrical impedance myography: A critical review and outlook. Clinical Neurophysiology. 2021;132(2):338-344. doi:10.1016/j.clinph.2020.11.014
  18. Hobson‐Webb LD, Boon AJ. Reporting the results of diagnostic neuromuscular ultrasound: An educational report. Muscle Nerve. 2013;47(4):608-610. doi:10.1002/mus.23742
  19. Wee TC, Simon NG. Ultrasound elastography for the evaluation of peripheral nerves: A systematic review. Muscle Nerve. 2019;60(5):501-512. doi:10.1002/mus.26624
  20. File:Electromyogram.png. (2024, January 9). Wikimedia Commons. Retrieved 13:17, May 16, 2024 from https://commons.wikimedia.org/w/index.php?title=File:Electromyogram.png&oldid=840036794
  21. Bortz J. Naval Hospital Pensacola sailor strikes a nerve. DVIDS. 2018. Accessed May 12, 2024. https://www.dvidshub.net/image/4572432/naval-hospital-pensacola-sailor-strikes-nerve.

Original Version of the Topic

Eathar A. Saad, MD, Abir Naguib, MD. The electrodiagnostic consultation and report. 9/10/2015

Previous Revision(s) of the Topic

Eathar A. Saad, MD, Julio Vazquez-Galliano, MD, Yuxi Chen, MD. Basic Electrodiagnostics for the Referring Physician. 9/28/2020

Author Disclosure

David Haustein, MD, MBA
PM Consulting, Consulting Income, Owner
University of Missouri, Employment, Associate Dean
BMJ Best Practice, Payment for authorship, Payment for authorship

Evan Reeves, BS
Nothing to Disclose

Connor Swartz, DO
Nothing to Disclose