Cite as: Maida E Opala A. Fundamentals of electromyography for chronic pain physicians. ASRA Pain Medicine News 2023;48. https://doi.org/10.52211/asra050123.010.

Introduction

The complexity of a chronic pain patient’s clinical presentation can create a challenging array of signs and symptoms. When assessing a patient for an underlying peripheral nerve disorder, an electromyography and nerve conduction study (EMG-NCS), can play an invaluable role. EMG-NCSs are utilized as an extension of the clinical neurologic examination, rather than simply a diagnostic assessment. This article will review the basic principles of nerve conduction and electromyography and its clinical utility for the chronic pain physician.

Electrodiagnostic—The Basics

Nerve Conduction Studies (NCS)

NCS are comprised of two components, which independently assess both the motor and sensory proprieties of a peripheral nerve. When referring to the motor function of a peripheral nerve, compound motor action potential (CMAP) is assessed, whereas when assessing the sensory component, it is described by sensory nerve action potential (SNAP).

When performing a NCS, anatomical landmarks are used to identify the course of a peripheral nerve to be stimulated. Once identified, an electrical current is conducted over the course of a peripheral nerve at pre-determined distances. This in turn generates a CMAP, which is an expression of all underlying muscle fiber action potentials, and its function is determined by several objective parameters, including terminal motor latency, motor conduction velocity, and motor amplitude.

A SNAP is recorded at the level of the dorsal root ganglion (DRG) and is generated by stimulating a peripheral sensory nerve recorded from an anatomical cutaneous reference (eg, digit 2 for median sensory study). Sensory conduction velocity (m/s), amplitude (µV), onset, and peak latencies are used to objectively quantify the response from the peripheral sensory nerve to the DRG. These generated potentials are then compared to a set of standardized values to determine if underlying neuropathic pathology is present.

Electromyography (EMG)

When preforming an EMG, a small needle electrode is inserted into the muscle to measure the electrical muscle activity. Volitional activity of muscle is described through analyzing motor unit action potential (MUAP) wave forms via audio-visual recording on the EMG monitor. Assessing the number, rate, and appearance (recruitment pattern) of MUAP aids in the diagnosis of a neurogenic or myopathic disease. Needle examination of muscle at rest is important to determine the presence of spontaneous muscle discharges that are pathologic and consistent with active denervation features (ie, fibrillations, fasciculations, positive sharp waves, etc).

Although testing may result in mild discomfort, it tends to be well-tolerated, yielding a significant diagnostic benefit.

By identifying the location of the pathology in the PNS, a high level of precision may be utilized for prognosticating outcomes and targeting theraputic interventions.

Peripheral Entrapment Neuropathies

Median nerve mononeuropathy (eg, carpal tunnel syndrome (CTS)) as well as ulnar nerve entrapment at the elbow (UNP) are amongst the most common pathologies identified. Differentiating a neurogenic cause of pain from a musculoskeletal etiology at the hand/wrist is possible through electrodiagnostic examination in conjunction with a physical evaluation. Analysis of sensory as well as motor NCS allows one to categorize the severity of mononeuropathy either at the wrist or elbow.

In a retrospective study of 8,501 patients analyzing NCS and symptoms reported via clinical questionnaire, six grades of CTS were described¹. Grade 1 recognized CTS through two very sensitive tests whereas Grade 6 included very severe CTS with reduced CMAP amplitudes. Higher CTS grades were seen with less favorable nerve function with a strong linear relationship between neurophysiologic function and symptoms reported.

By correlating symptoms and electrical mononeuropathy severity, decision making regarding appropriate treatment interventions can be undertaken.

Radiculopathies

EMG can identify the presence of large fiber axonal radiculopathy and its associated severity. Localization of the level of nerve root compromise is possible by examining the myotomal innervation of muscles innervated by an accompanying nerve root. When recording the presence or absence of active denervation and MUAP morphology, one can identify the duration of onset for the focal neuropathy (acute vs chronic). Active denervation can be seen within 7 days in the paraspinal muscles with later spread to proximal muscles at 2-3 weeks and more distal muscles past 4 weeks.² Chronic denervation changes tend to be seen past 2-3 months from intital onset.² In the event of spinal surgery, active denervation may persist indefinitely in the paraspinal muscles.

The yield of the EMG correlates with the number of muscles tested with examination of greater than five muscles resulting up to 80%-95% sensitivity.³

In a prospective study with 183 patients, abnormal electrodiagnostic study (EDX) findings correlated with objective deficits on physical examination in 84% of subjects.⁴ Weakness with abnormal reflexes were the strongest predictors of abnormal EDX (PPD = 86% for abnormal EDX) whereas subjective symptoms (eg, burning, pain) had mild to moderate sensitivity⁶. With the complexity of the diagnostic process for cervical radiculopathy, EDX correlation of the clinical presentation is beneficial to guide therapeutic interventions.

In conjunction with MRI studies and clinical examination, EMG testing plays an invaluable role in confirming anatomical nerve root compromise when planning for interventional approaches. EMG-NCS also enables the evaluation of a mononeuropathy with superimposed large fiber radiculopathy in a single limb, also known as “double crush syndrome” (ie, median nerve and C6 radiculopathy) that can be missed without EDX evaluation.

However, as with any diagnostic test EMG-NCS does have its limitations. The inability of the test to identify a purely sensory radiculopathy can create frustrations for the patient and clinician alike.

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The role of EMG-NCS is to serve as a functional assessment used as an extension of the physical examination and not in isolation.

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Plexopathies

Persistent weakness with sensory disturbance in a limb can be a resultant cause of brachial or lumbosacral plexopathies. Etiologies vary, including trauma, autoimmune, radiation, iatrogenic, thoracic outlet syndrome, and malignancy. EDX plays a major role in localizing the lesion to the plexus, especially in patients not responding to therapies aimed at spinal segments. EDX consists of standard limb testing protocols, frequently evaluating median, ulnar, and radial CMAPs and standard SNAPs. Additional testing of medial and lateral antebrachial cutaneous nerves is common practice in suspected brachial injuries with comparing results of NCS to the contralateral, unaffected side. Since the SNAP response originates in the DRG, reduced or absent responses, frequently in a patchy non-dermatomal distribution, are suggestive of a potential post-ganglionic lesion. Isolated abnormal CMAP, with a preserved SNAP recording, is more likely suggestive of a pre-ganglionic pathology.

Common mimickers of radiculopathy, such as IT band syndrome, greater trochanteric bursitis, intraarticular hip pathology, rotator cuff tendinopathy, elbow epicondylitis, and De Quervain tenosynovitis, may create diagnostic confusion in a pain patient. As a result, clinical correlation in conjunction with the use of an EDX may help to delineate underlying pathologies in a complex pain patient.

Polyneuropathies

EDX remains a feasible approach to diagnosing polyneuropathy (PNS), a frequent cause of paresthesia’s of the limbs. EMG-NCS can identify large fiber pathology frequently presenting with abnormal reflexes, proprioceptive and vibration sense disturbance, and variable degrees of symmetric distal weakness. Small fiber polyneuropathy resulting in only painful dysesthesias cannot be identified on EDX as the testing is aimed at evaluating large nerve fibers. Distinguishing between acquired or hereditary disease, axonal vs demyelinating nature, is of great importance to determine etiology of disease and to seek appropriate therapy.

PNS presenting with amplitude loss on sensory and/or motor NCS are categorized as axonal PNS. Whereas slowing of conduction velocities and prolongation of terminal and peak latencies are some features of demyelinating disease. In a retrospective study of 328 patients, abnormal yield of sural or superficial peroneal sensory response was confirmatory of a large fiber axonal polyneuropathy in most patients (68%).⁵

The most common etiology of PNS worldwide remains Type II diabetes mellitus followed by alcohol induced peripheral neuropathy.^6,7 In certain cases, establishing an etiology is not possible (eg, cryptogenic polyneuropathy).⁸

Superimposed large fiber radiculopathy of an existing PNS is not infrequent. EDX testing in a patient with low back pain and PNS can be of benefit to pain physicians. By evaluating for more than one causative etiology in a patient with dysesthesias, weakness, or proprioceptive difficulties, the risk of omitting additional pathology contributing to patient presentation is reduced.

Myopathies

Generalized muscle aches and discomfort with associated fatigue are some of the symptoms of myopathy. EDX testing of muscles in patients with proximal muscle myalgias can aid in diagnosing intrinsic muscle disease (acquired or hereditary) that may present with generalized aching.

Conclusion

EMG-NCS is an invaluable tool for clinicians when differentiating complex pain presentations and delineating approaches to care. Its role is to serve as a functional assessment used as an extension of the physical examination and not in isolation. Given its high sensitivity and specificity to axonal loss and demyelinating disease respectively, its utility can play a profound role in patient care. Although not without its limitations, when a patient may partially meet clinical criteria, an unclear differential diagnosis is contemplated or when planning for surgical intervention based on the degree of neuropathic pathology, an EMG-NCS can play an invaluable role to help guide clinical decision making.

Eugene Maida, MG BCh, BAO, FRCPC, is an assistant clinical professor in the departments of Medicine and Anesthesia at McMaster University in Hamilton Ontario, Canada.

Adrian Opala, MD is an assistant clinical professor the Department of Medicine at McMaster University in Hamilton Ontario, Canada.

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References

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