What is the role of nerve conduction studies (NCSs) in electrodiagnosis?

Updated: Mar 01, 2019
  • Author: Adam B Agranoff, MD; Chief Editor: Stephen Kishner, MD, MHA  more...
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Answer

Answer

Nerve conduction studies (NCSs) are an important part of the complete electrodiagnostic exam. [8] In an NCS, an electrical charge is delivered to a peripheral nerve. That charge is carried down the nerve and generates a muscle contraction. A recording electrode is placed on a muscle innervated by that nerve, and information about the impulse can be recorded, including its latency (the time needed for the impulse to travel from stimulus to recording). The distance traveled and the nerve conduction velocity (NCV) can also be computed.

These measures are a sensitive indicator of nerve damage and look specifically at the integrity of the myelination of the nerve. The needle examination and NCSs are key components of a complete electromyographic exam. [9, 10]

The amplitude of the muscle contraction can be determined and compared to the initial size of the signal, providing information about the number of neurons that are functioning within the nerve. By stimulating the nerve in various places along its course, the practitioner can isolate a specific site of injury. Sensory (first image below) and motor (second image below) nerves may be tested.

Sensory nerve action potential. Sensory nerve action potential.
Motor nerve action potential. Motor nerve action potential.

In a routine carpal tunnel study, for example, conductions of the median nerve above and below the tunnel may be normal, while conduction across the carpal tunnel itself may be impaired. [11, 12, 13] This test isolates the injury to this specific location. Changes in amplitude often signal an axonal injury.

A multitude of entrapments and injuries can be identified in this manner. In the upper extremity, some of these disorders include brachial plexopathies, ulnar entrapments at the cubital and Guyon tunnels, median entrapments at the carpal tunnel, and radial injuries at the spiral groove in the humerus. [14] In the lower extremity, the most common entrapments include the sciatic nerve at the piriformis, the peroneal nerve at the fibular head, and the plantar nerve at the tarsal tunnel. [15]

Peripheral neuropathies also can be identified and monitored objectively. The type of nerve injury (demyelinating, axonal, or mixed) can aid in narrowing the differential diagnosis. [16]  In a study of patients with diabetic peripheral neuropathy, Park and Won reported that NCS variables differ in association with phenotype (normoesthesia, hyperesthesia, or hypoesthesia/anesthesia), as assessed using current perception threshold. The investigators concluded that, while NCSs are an important means of evaluating peripheral nerve abnormalities, their usefulness in the assessment of clinical phenotypes is limited. [17]

With knowledge of neuroanatomy and with measurements from a sample of nerves and muscles from various nerve roots, the clinician can use NCSs to determine where a lesion would need to be located in order to affect those combinations of findings in the extremity that suggest the presence of abnormalities. For example, in a medial cord brachial plexus injury, the ulnar and median nerves are involved along their entire length, with sparing of C5-C6 muscles and the paraspinal muscles. A C7-C8 root injury also affects the median-innervated and ulnar-innervated muscles, but the paraspinals are involved as well. [18, 19]

Remember that a direct relationship exists between nerve diameter and conduction velocity. Larger myelinated fibers (Ia) are the ones measured in NCSs. The smaller fibers (Ib, II, III) are not recorded with standard nerve conduction techniques. Therefore, a small fiber neuropathy may not be detectable by EMG.

Moreover, a prospective study by Fowler et al indicated that NCSs have a higher false-positive rate than does ultrasonography in the diagnosis of carpal tunnel syndrome in asymptomatic patients. Assessing hands with a Carpal Tunnel Syndrome 6 (CTS-6) scale score of 0, the investigators found positive NCS test results in 42.5% of hands, compared with 22.5% in ultrasonographic evaluation. [20]


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