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Electrophysiological Testing - EMG/NCV

Diagnostic Study - Description & Definition

Background

Electrophysiological studies include nerve conduction velocity (NCV) testing and needle electromyography (EMG). These procedures are used as part of the diagnostic evaluation of the peripheral nervous system and the localization of abnormalities along peripheral nerves or motor neurons.1,2 NCV and EMG studies are particularly useful for diagnosing radiculopathy and entrapment neuropathy. 

Historical Overview

In 1771, Luigi Galvani demonstrated that electrical stimulation of muscle produced contraction. In 1849, Emil du Bois-Reymond discovered that it also was possible to record electrical activity during a voluntary muscle contraction. This electrical response was not quantified until the development of improved recording equipment including the cathode ray oscilloscope by Gasser and Erlanger in 1922 and the amplifier and oscillograph by Matthews in 1928. The first modern EMG machine was built by Jasper in 1942. 

In 1929, Adrian and Bronk developed a method to record a single motor unit action potential (MUAP) in human subjects by connecting concentric and bipolar needle electrodes to an amplifier and a loud speaker. In 1938, Denny-Brown and Pennybacker recorded and described fasciculations, or action potentials of single contracting or spontaneously firing motor units. They also described fibrillations by separating fasciculations from the action potentials of single, dennervated muscle fibers. Subsequently, Larrabee, Hodes and German measured the combined potential of individual muscle fibers from the surface of the muscle after stimulating the supplying nerve. This became known as a compound muscle action potential (CMAP). In 1957, Lambert and Eaton differentiated pre-synaptic and post-synaptic neuromuscular transmission disorders after describing the electrophysiological features of a newly recognized disease they called the myasthenic syndrome.

Description

For NCV studies, the function of peripheral nerves is evaluated by electrically stimulating the nerve and recording the response at the muscle or the nerve.1,2 When compared with normative data, the responses can yield information about demyelination and axonal loss. NCVs include motor, sensory and mixed nerve testing. The results from NCV studies can be affected by a number of variables, including technique used, skin temperature (positively correlated) and age (inversely correlated).1,2

A motor NCV test is performed by applying a single supramaximal stimulus on at least two proximal points along the nerve. The resulting compound muscle action potential (CMAP) is recorded by a surface electrode on the innervated muscle. NCV is calculated as the distance (ie, length between the stimulating and recording electrodes) divided by latency (ie, time from the onset of the stimulus to the onset of the response; NCV Figure). In patients with carpal tunnel syndrome, for example, distal latency is prolonged because the carpal canal is located distally to the most distal site of median nerve stimulation.3

For EMG studies, muscle function is evaluated by inserting a needle electrode into a muscle and recording the intrinsic electrical activity of muscle fibers1,2 (EMG Figure). There are three components to the needle EMG: observation at rest, motor unit potential (MUP) after minimal voluntary contraction and the recruitment pattern of MUPs after maximal contraction. The amplitude, duration, shape and rate of MUPs are indicators of motor unit function.2 Normally, MUP amplitude and duration are relatively constant. Abnormal activities include fibrillations from single muscle fibers and fasciculations from groups of muscle fibers. In patients with carpal tunnel syndrome, focal demyelination after prolonged entrapment can produce fibrillation, positive sharp waves (PSWs) and reduced recruitment on EMG.2

Diagnoses

Carpal tunnel syndrome

Tests should include motor, sensory and mixed NCV studies for the median and ulnar nerves of the affected arm. The most sensitive test is sensory NCV across the palm-to-wrist segment. Other sensitive tests include the finger-to-wrist NCV and distal motor latency. A CTS diagnosis can be confirmed with 85–91% sensitivity and 95% specificity.4 To document any secondary axonal degeneration, a needle EMG in the abductor pollicis brevis is recommended.2

Cubital tunnel syndrome

Tests should include motor, sensory and mixed NCV studies for the median and ulnar nerves of the affected arm. The sensory and mixed NCVs for the finger-to-wrist and wrist-to-elbow segments are frequently abnormal. An inching approach on the ulnar motor nerve, from 6 cm below the medial epicondyle to ≥4 cm proximal to it, helps localize the area of compression. The sensitivity for identifying the site of compression ranges from 83% to 96% of patients.5

Cervical radiculopathy

The most important test is the EMG.2,6 With nerve root compression, sensory NCVs are normal because the dorsal root ganglion (sensory somata) is distal to the site of compression. Similarly, motor NCVs will not reflect demyelination because the area of injury is proximal to the tested nerve sites. EMG will detect degenerative changes in muscles innervated by the specific nerve root. Because the radicular pattern of muscle involvement leads to a specific root level, the paraspinal muscles also must be evaluated. If the paraspinals are involved, the diagnosis of radiculopathy can be confirmed. However, a negative EMG does not rule out radiculopathy, especially if only the sensory part of the nerve is affected or there is no motor axonal loss. The sensitivity of NCV/EMG in radiculopathy is about 80–85%.2

Colles’ fracture

For Colles’ fracture, NCV/EMG is only required when the swelling from the fracture is causing clinical signs and symptoms of carpal tunnel syndrome.  In this situation, electrophysiological studies can be used to verify the secondary diagnosis of carpal tunnel syndrome and its severity.

Rheumatoid arthritis

For rheumatoid arthritis (RA), NCV/EMG is only required when the flexor tenosynovitis from the RA is causing clinical signs and symptoms of carpal tunnel syndrome.  In this situation the electrophysiological studies can be used to verify the secondary diagnosis of carpal tunnel syndrome and its severity.

 

Normal Study Findings - Images (For abnormal findings images, click on Diagnoses below)
Normal Nerve Conduction Values
Set up for Carpal Tunnel Syndrome electrophysiological testing.
Electrophysiological testing findings consistent with Carpal Tunnel Syndrome.
Normal Study Findings - Video
Diagnoses Where These Studies May Be Used In Work-Up (with abnormal findings images)
Comments and Pearls
  • NCV and EMG testing does not supplant a careful patient history and physical examination; all are complementary components of a diagnostic evaluation.7
  • While the diagnoses of carpal tunnel syndrome, cubital tunnel syndrome and cervical radiculopathy can be established by taking a complete history and performing a physical examination, it is appropriate to investigate the diagnosis further with EMG/NCV with the following scenarios: (1) when considering surgical treatment, (2) if there is any doubt about the diagnosis, or (3) when there are overlapping neurological difficulties such those observed with double crush syndrome.
  • NCV/EMG and clinical examination are operator dependent; test results and diagnostic decision-making process are influenced by the experience, skill and expertise of the clinician performing the evaluations.7
  • Other factors include the clinical setting and culture, specializations and sub-specializations, insurance and medico-legal issues, personality and patient coopertion.7
References
  1. Hilburn JW. General principles and use of electrodiagnostic studies in carpal and cubital tunnel syndromes. With special, attention to pitfalls and interpretation. Hand Clin 1996;12:205-21.
  2. Oh SJ. Principles of Clinical Electromyography: Case Studies. Philadelphia: Lippincott Williams & Wilkins; 1998.
  3. Lee DH, Claussen GC, Oh S. Clinical nerve conduction and needle electromyography studies. J Am Acad Orthop Surg 2004;12:276-87.
  4. Jablecki CK, Andary MT, Floeter MK, et al. Practice parameter: Electrodiagnostic studies in carpal tunnel syndrome. Report of the American Association of Electrodiagnostic Medicine, American Academy of Neurology, and the American Academy of Physical Medicine and Rehabilitation. Neurology 2002;58:1589-92.
  5. Kincaid JC. AAEE minimonograph #31: the electrodiagnosis of ulnar neuropathy at the elbow. Muscle Nerve 1988;11:1005-15.
  6. Wilbourn AJ, Aminoff MJ. AAEE minimonograph #32: the electrophysiologic examination in patients with radiculopathies. Muscle Nerve 1988;11:1099-114.
  7. Fuller G. How to get the most out of nerve conduction studies and electromyography. J Neurol Neurosurg Psychiatry 2005;76 Suppl 2:ii41-6.
  8. Werner RA and Andary M. Electrodiagnostic Evaluation of Carpal Tunnel syndrome. Muscle Nerve 44: 597-607, 2011.
  9. Bland JDP. A Neourophysiological Grading Scale for Carpal Tunnel Syndrome.  Muscle Nerve 23: 1280-1283, 2000.
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