Overview and Description
The Electrodiagnostic evaluation is considered to be an extension of the neurological examination of the peripheral nervous system. At times it can reveal some dysfunction of the nerves and muscles, which was not detected in the physical exam.1 It is commonly used to diagnose entrapment neuropathies, plexopathies, radiculopathies, neuromuscular junction disorders, myopathies, anterior horn cell disease and can prognosticate functional recovery. It has a role in localization of the site of the lesion, and identifying the pathology.2
Electrodiagnostic testing is utilized to help answer the questions of the differential diagnosis considered by the referring physician, and the consultant2. This test should be preceded by a verbal or written consent from the patient or a surrogate if the patient is unable to give consent. In emergent situations when neither is possible, the consultant can proceed without consent. 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 (NCSs) and needle electromyography (EMG), and the determination of a final diagnosis. According the AANEM’s 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 practice4 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
Nerve conduction (NCS):
When a nerve is electrically stimulated the action potential can be recorded through surface electrodes and is termed compound nerve action potential (CNAP). CNAPs measure the potentials of large myelinated fibers.2,5 The nerve, the side tested, the site of stimulation and recording, the distances between stimulation and recording site, and the temperature should be documented in the report.6
Sensory nerve conduction (SNCS):
Sensory nerve action potentials (SNAP) are recorded after the stimulation of a sensory nerve. If it was propagated in a physiologic direction it is termed orthodromic. Antidromic impulses are recorded an unphysiologic direction.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 (MNCS):
MNCS 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 MNCS should include comment on distal latency, amplitude (baseline to negative peak) and conduction velocity along the different nerve segments. 6,7 Duration of the negative phase and overall area under the curve are helpful as well.
H reflexes (Hoffman) result from the activation of the lowest threshold Ia fibers of a mixed nerve. An afferent voley 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 are most useful in detection of S1 radiculopathies. 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 small amplitude (100-200µV) and a prolonged latency. 2,7
When recording F-waves, indication of the nerve tested, 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 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 (MG), 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, sensory nerve conduction studies, and electromyography to rule out a neuropathic or a 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
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 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 are waves seen in the absence of needle movement or voluntary muscle contraction. 2 Fibrillation potentials(fibs), positive sharp waves (PSW), complex repetitive discharges, 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 comment on size, amplitude, duration and phases of the MUAPs, motor unit recruitment, and the interference pattern. 6,7
Relevance to Clinical Practice
Electrodiagnostic testing can differentiate between demyelination and axonal loss in a nerve.
- In demyelinating lesions the compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) are large distal to the lesion, and demonstrate slowing, potential conduction block and temporal dispersion when stimulated proximal to the lesion. In pure demyelinating lesions there should not be findings of axonal loss on EMG.
- In axonal loss the CMAP and SNAP amplitudes are reduced with stimulation of the nerve proximal and distal to the lesion as compared with the contralateral side with relatively preserved latencies and conduction velocities. EMG of the muscles supplied by the affected nerve show decreased recruitment followed by the appearance of denervation potentials. In the chronic stages reinervation follows with enlargement of the motor unit size, increased number of phases and duration. 2,5
- Electrodiagnosis is useful in 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.
- Identifying whether a neuropathy is predominantly sensory or motor, length dependent, and primarily axonal or demyelinating process 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. They disappear with treatment. 9
- In ongoing denervation fibs, PSWs, decreased recruitment and polyphasic motor units are present. In chronic denervation fibs and 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 extremities10
Measurement and prediction of outcome:
- NCS can be an indicator of the severity of a lesion. The larger amplitude of the nerve action potential distal to the lesion the better prognosis for recovery. 7
- EMG can be a measure of the severity of a lesion. Abnormal spontaneous activity indicates axon loss. The absence of motor unit action potentials in the muscles supplied by a nerve indicates no action potentials traversing the lesion due to either axon loss or conduction block.
Cutting Edge/Unique Concepts/Emerging Issues
- There is an increasing role for ultrasound examination of the nerves and muscles as a complementary technique to the electrodiagnostic examination. It can guide needle placement in electromyography thus improving safety and accuracy. It may also help localize nerves that are challenging to find to perform 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. 11
- MR Neurography is a technique which is able to provide non invasive visualization of nerve pathology and injury to level of the fascicles earlier and with higher sensitivity than NCS allowing identification of both focal and non -focal neuropathies. 12
- There is an increasing role for conventional and quantitative MRIs in diagnosis of inherited muscle disease, quantification of disease burden, and monitoring of disease progression 13
- 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 is been used to study amyotrophic lateral sclerosis, poliomyelitis, spinal muscular atrophy, aging and a neuropathies in the research setting. 14
- 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. 15
- Electrical impedance myography 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. 16
Gaps in Knowledge/Evidence Base
- In electrodiagnostic studies axonotemeis and neurotemesis 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 for a disease. An example is that fibrillation potentials and positive sharp 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 with the approaches to perform the test varies 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 appropriately report NMUS results. 17
- 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. 18
- Podnar, S., Critical reappraisal of referrals to electromyography and nerve conduction studies. Eur J Neurol, 2005. 12(2): p. 150-5.
- Robinson, L.R., Role of neurophysiologic evaluation in diagnosis. J Am Acad Orthop Surg, 2000. 8(3): p. 190-9.
- Mackin, G.A., et al., Guidelines for ethical behavior relating to clinical practice issues in electrodiagnostic medicine. Muscle Nerve, 2005. 31(3): p. 400-5.
- Aanem, Proper Performance and Interpretation of Electrodiagnostic Studies. [Corrected]. Muscle Nerve, 2015. 51(3): p. 468-71.
- Preston, D.C. and B.E. Shapiro, Electromyography and neuromuscular disorders : clinical-electrophysiologic correlations. 1998, Boston: Butterworth-Heinemann. xiii, 581 p.
- Jablecki, C.K., et al., Reporting the results of needle EMG and nerve conduction studies: an educational report. Muscle Nerve, 2005. 32(5): p. 682-5.
- Barboi, A.C. and P.E. Barkhaus, Electrodiagnostic testing in neuromuscular disorders. Neurol Clin, 2004. 22(3): p. 619-41, vi.
- Katirji, B., The clinical electromyography examination. An overview. Neurol Clin, 2002. 20(2): p. 291-303, v.
- Oh, S.J., Principles of clinical electromyography : case studies. 1998, Baltimore: Williams & Wilkins. xi, 604 p.
- Kang, P.B., et al., Utility and practice of electrodiagnostic testing in the pediatric population: An AANEM consensus statement. Muscle Nerve, 2020. 61(2): p. 143-155.
- London, Z.N., Safety and pain in electrodiagnostic studies. Muscle Nerve, 2017. 55(2): p. 149-159.
- Kollmer, J., M. Bendszus, and M. Pham, MR Neurography: Diagnostic Imaging in the PNS. Clin Neuroradiol, 2015. 25 Suppl 2: p. 283-9.
- Fischer, D., U. Bonati, and M.P. Wattjes, Recent developments in muscle imaging of neuromuscular disorders. Curr Opin Neurol, 2016. 29(5): p. 614-20.
- Gooch, C.L., et al., Motor unit number estimation: a technology and literature review. Muscle Nerve, 2014. 50(6): p. 884-93.
- Fatehi, F., et al., The utility of motor unit number index: A systematic review. Neurophysiol Clin, 2018. 48(5): p. 251-259.
- Rutkove, S.B., Electrical impedance myography: Background, current state, and future directions. Muscle Nerve, 2009. 40(6): p. 936-46.
- Hobson-Webb, L.D. and A.J. Boon, Reporting the results of diagnostic neuromuscular ultrasound: an educational report. Muscle Nerve, 2013. 47(4): p. 608-10.
- Wee, T.C. and N.G. Simon, Ultrasound elastography for the evaluation of peripheral nerves: A systematic review. Muscle Nerve, 2019. 60(5): p. 501-512.
Original Version of the Topic
Eathar A. Saad, MD, Abir Naguib, MD. The electrodiagnostic consultation and report. 9/10/2015
Eathar A. Saad, MD
Ipsen, Research Grant paid to institution, PI for Pediatric lower limb spasticity study
Julio Vazquez-Galliano, MD
Nothing to Disclose
Yuxi Chen, MD
Ipsen, Research Grant paid to institution, PI for Pediatric lower limb spasticity study