Bridget T. Carey
OVERVIEW
In the practice of pain medicine, nerve conduction studies and electromyography provide useful information through the diagnosis of peripheral neurologic injury. By precisely identifying and localizing spinal root and peripheral nerve injury, these studies provide an important role in confirming the anatomical site active in pain generation. This knowledge increases the success of focal procedural and other interventions. It should be noted that these tools should be used to optimize patient care or help guide surgical and interventional practice.
DEFINITIONS
Electromyography (EMG) and nerve conduction studies (NCS) are electrodiagnostic tests performed to evaluate the function of the peripheral nervous system. These tests are usually but not invariably performed together and interpreted in conjunction. Often the single term “EMG” is used to mean an examination comprising both NCS and EMG studies, though this is technically an imprecise usage. Both examinations evaluate and measure aspects of the electrophysiological properties inherent in peripheral nerve and muscle tissue. Through assessment of these neurophysiological parameters, these studies provide accurate diagnostic information to the clinician regarding the integrity and function of the spinal nerve roots, nerve plexi, peripheral nerves, neuromuscular junction, and muscle.
INDICATIONS
EMG/NCS are performed in patients for the following reasons:
Symptoms consistent with possible peripheral nerve injury, either in isolation or combination, include the following:
CONTRAINDICATIONS
NCS and EMG are relatively safe tests, with few potential associated complications or side effects. Some patients find the study uncomfortable; however, when performed correctly, both NCS and needle EMG examination are well tolerated by most patients.
There are no absolute contraindications to NCS/EMG; however, the following relative contraindications are considered:
The examination is generally modified to exclude muscles at noncompressible sites to eliminate the risk of hematoma.
Although not absolutely contraindicated, examination of the paraspinal musculature is generally avoided in patients on anticoagulation due to potential adverse effects of possible hematoma formation adjacent to spinal structures.
RELEVANT ANATOMY
Figure 12-1. Dermatome map.
Figure 12-2. UE nerve schematic drawing.
Figure 12-3. LE nerve schematic drawing.
TECHNICAL ASPECTS AND CONSIDERATIONS
NCS/EMGs are most frequently performed in the outpatient office setting. NCS may be performed by a trained technician or physician; however, the EMG portion is always performed by a physician. Both examinations are performed in “real time,” with findings elicited during the examination influencing the extent and specific subsequent course of the study. Study design and the interpretation of all information derived from these tests must be performed by a physician with training in electrodiagnostic medicine and a solid understanding of clinical neurophysiology (Figure 12-4).
Figure 12-4. EMG/NCS machine. A typical EMG/NCS system which has settings for both nerve conduction studies and electromyography.
NERVE CONDUCTION STUDIES
Nerve conduction studies (NCS) involve the delivery of an electrical stimulus externally over the skin along the course of a peripheral nerve. This stimulus activates the selected underlying nerve; that is, itdepolarizes the nerve causing an action potential. The response evoked by stimulation of the nerve is then measured at a site distant to the stimulation by a recording electrode placed on the surface of the skin. Motor nerves, sensory nerves, and reflex pathways involving both can be studied in this method, with slight variations in technique (Figures 12-5 and 12-6).
Figure 12-5. NCS stimulator. The bipolar prongs are held against the skin overlying the nerve to provide an electrical potential that induces depolarization over the stimulated nerve segment.
Figure 12-6. Surface and needle electrodes. Recording and ground electrodes for NCS, and single-use disposable needle electrodes for EMG.
Motor NCS are performed by stimulating over the selected nerve, and recording the compound muscle action potential (CMAP) with a surface electrode placed over a muscle that the selected nerve is known to innervate. Standard procedures have been established for the commonly (and less commonly) studied nerves that identify specific sites for stimulation and recording.
In general for all motor NCS:
For example, a recording electrode for the median nerve is placed over the abductor pollicis brevis (APB) muscle over the thenar eminence, and the nerve is first stimulated at the wrist (distal site) and then at the antecubital fossa (proximal site) (Figures 12-7 and 12-8).
Figure 12-7. Median nerve conduction studies, distal stimulation. Recording electrodes are placed over the abductor pollicis brevis muscle, and the nerve is stimulated at the wrist (distal stimulation site).
Figure 12-8. Median motor nerve conduction studies, proximal stimulation. Recording electrodes are placed over the abductor pollicis brevis muscle, and the nerve is stimulated at the antecubital fossa (proximal stimulation site).
For the peroneal nerve, a recording electrode is placed over the extensor digitorum brevis (EDB) muscle in the foot. The nerve is first stimulated at the ankle (distal site), then below the neck of the fibula, then again above the neck of the fibula (Figure 12-9).
Figure 12-9. Peroneal motor nerve conduction studies, distal stimulation. Recording electrodes are placed over the extensor digitorum brevis muscle, and the nerve is stimulated at the anterior ankle (distal stimulation site).
Several electrophysiological parameters are recorded, measured, and analyzed, including:
Results are then analyzed in comparison with normative data for the specific nerve and electrophysiological parameter studied (Figure 12-10 and Table 12-1).
Figure 12-10. Median motor tracing. Motor nerve conduction study of the median nerve. The top line displays the compound muscle action potential (CMAP) obtained by distal stimulation, and the bottom line displays the CMAP obtained by proximal stimulation. Numerical data pertaining to the CMAP responses is displayed on the right of the screen.
TABLE 12-1. Motor Nerve Conduction Studies: Electrophysiological Parameters and Normal Values of Frequently Evaluated Nerves
Sensory NCS are performed by stimulating the sensory nerve endings in a specific nerve distribution, and measuring the sensory nerve action potential (SNAP) this produces.
As with motor NCS, standard procedures have been established for the commonly (and less commonly) studied nerves that identify specific sites for stimulation and recording.
For example, for a median sensory study, we stimulate the skin of the (palmar) index finger, and record over the median nerve at the wrist (Figure 12-11).
Figure 12-11. Median sensory nerve conduction study. Recording electrodes are placed over the course of the median nerve at the wrist, and ring electrodes are used to stimulate the median sensory nerves in the index finger.
The electrophysiological parameters recorded, measured, and analyzed for sensory NCS are:
Figure 12-12. Median sensory tracing. Sensory nerve conduction study of the median nerve. The median sensory nerve action potential (SNAP) is displayed on the top 2 lines, and in this case the bottom 2 lines display the ulnar SNAP. Numerical data pertaining to the SNAP responses is displayed on the right.
TABLE 12-2. Sensory Nerve Conduction Studies: Electrophysiological Parameters and Normal Values of Frequently Evaluated Nerves
ELECTROMYOGRAPHY
A needle electrode is placed percutaneously into a specific target muscle. The needle electrode then records the physiologic electrical activity of the muscle, which is amplified and displayed on an oscilloscope screen. The electrical signal from the muscle is seen visually as a waveform, and is also translated into sound, which is heard on a speaker in “real time.”
Figure 12-13. Needle EMG of tibialis anterior muscle. A recording needle electrode is introduced percutaneously into the tibialis anterior muscle. A surface ground electrode is applied over the skin.
Figure 12-14. Needle EMG extensor carpi radialis muscle. A recording needle electrode is introduced percutaneously into the extensor carpi radialis muscle. A surface ground electrode is applied over the skin.
Muscle fibers conform to identifiable electrophysiologic patterns in health and in disease. When the nerve supply to a muscle is impaired, the muscle will respond to the injury in characteristic ways, which can be recognized and measured (Figure 12-15 and Table 12-3).
Figure 12-15. EMG. Motor unit potentials are recorded by the needle and displayed on the screen for analysis.
TABLE 12-3. Needle Electromyography: Electrophysiological Parameters and Normal Values of Frequently Evaluated Upper Extremity Muscles
In the design of an electromyography (EMG) study, muscles are selected for study based on their peripheral innervation, from spinal root through terminal peripheral nerve branch. In this way, the location of a nerve injury can be “mapped” by determining which muscles reflect denervation injury and which muscles are spared (Tables 12-4 and 12-5).
TABLE 12-4. Example: Lower Extremity Emg Study
TABLE 12-5. Example: Upper Extremity Emg Study
Additional muscles may then be selected as indicated, based on the above “screening” of roots and peripheral nerves.
For example, if electrophysiological evidence of denervation is observed solely in the first dorsal interosseous muscle, on the basis of this finding alone, nerve injury could localize to either of 2 sites:
To determine which of these 2 possibilities is correct, subsequent examination could then be performed of
CLINICAL BENEFITS
NCS/EMG are studies of “function” as opposed to “structure” and therefore complement imaging studies, which evaluate structure only. For example, an MRI can show that a nerve is compressed, but cannot provide information about the functional integrity of the compressed nerve, whereas NCS/EMG can.
Case Example
Patient with symptoms of neck pain radiating into the medial aspect of right arm, with episodic paresthesias affecting right fourth and fifth fingers. Cervical spine MRI reveals multilevel degenerative disease with resulting various degrees of impingement of C6, C7, and C8 exiting nerve roots.
Scenario #1
This patient has active right C8 radiculopathy, which likely accounts for both pain and paresthesias. There is a high likelihood that this patient would benefit from a focal procedural intervention targeted at the right C8 root.
Scenario #2
This patient likely has neck pain from C6 radiculopathy and/or mechanical causes, and hand paresthesias from ulnar neuropathy. A focal procedural intervention targeted at the right C8 root is far less likely to result in satisfactory outcome for this patient.
Neurologists consider electrodiagnostic studies, including NCS and EMG, to be an extension of the neurologic examination. On the basis of peripheral neuroanatomy, the localization of a nerve injury to a specific spinal root, plexus, or peripheral nerve can at times be determined clinically through careful neurologic examination alone. Some injuries may involve a clear dermatomal/myotomal pattern, as opposed to a peripheral nerve distribution, or vice versa. Frequently, however, clear localizing signs and/or symptoms are either not identifiable, or may be confounded by additional factors. In some cases, dermatomal/myotomal patterns closely approximate the territory subserved by a peripheral nerve. In other cases, more than one diagnosis may be present. In these cases, NCS/EMG plays an essential role in the diagnosis and localization of neuropathic injury. It is important to recognize that in small percentage of patients, in an appropriately performed test, subtle lesions may not be identified on the EMG/NCV and in these settings the physicians’ clinical acumen should prevail.
Suggested Reading
Donofrio PD, Albers JW. Polyneuropathy: classification by nerve conduction studies and electromyography. Muscle Nerve. 1990;(13):889-903.
Gruis KL, Little AA, Zebarah VA, Albers JW. Survey of electrodiagnostic laboratories regarding hemorrhagic complications from needle electromyography. Muscle Nerve. 2006;(34):356-358.
Guarantors of Brain. Aids to the Examination of the Peripheral Nervous System. Saunders Elsevier Limited; 2000.
Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 3rd ed. Oxford University Press; 2001.
Lynch SL, Boon AJ, Smith J, et al. Complications of needle electromyography: hematoma risk and correlation with anticoagulation and antiplatelet therapy. Muscle Nerve. 2008;(38):1225-1230.
Oh SJ. Clinical Electromyography Nerve Conduction Studies. 2nd ed. William & Wilkins; 1993.
Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders, Clinical-Electrophysiologic Correlations. Boston, MA: Butterworth-Heinemann; 1998.
Wilbourn AJ, Aminoff MJ. The electrodiagnostic examination in patients with radiculopathies. Muscle Nerve. 1998;(21):1612-1631.