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Nerve Conduction Study

Original Editor - Mohamed A.Fekri

Top Contributors - Angeliki Chorti and Mohamed Abdelraof Mohamed

Introduction

Definition

Nerve conduction study (NCS) is an electrodiagnostic test that can identify damage to the neural components (or neuromuscular components, if used with other tests) by measuring how fast an electrical signal moves along your nerve until it gets to the muscle and thus, determining how the nerve functions. [1]

NCSs typically measure various components including nerve conduction velocity (NCV), the amplitude of compound motor action potential (CMAP), and sensory nerve action potential (SNAP). Nerve conduction velocity is the most sought after parameter when performing nerve conduction studies and refers to the speed of electrical signal transmission. [2] [3] CMAP is about testing motor nerve fibers from the anterior horn cells to the muscle fiber, while SNAP provides information on the sensory nerve axon and its pathway from the distal receptors in the skin to the dorsal root ganglia. [4]

NCSs are commony combined with electromyography (EMG) for a more in depth evaluation of muscle and nerve function.

[5]

Classification

Main types

NCVs usually fall into 3 types—motor, sensory, and mixed. [4][6] These types vary in terms of technical procedures, and may be performed in sequence in nerve assessment. [6]

Motor NCSs

To determine the motor conduction velocity of a motor nerve, the nerve is stimulated and the response of its target muscles recorded. An active electrode is placed over the muscle body.  A reference electrode is placed distally (ideally over non muscle). The ground electrode is usually placed between the active electrode and the stimulator. The nerve is stimulated near the active electrode and then at a more proximal site.[7] The CMAP generated by the muscle is recorded following the stimulation of the motor nerve.

Terminal latency is a term for the amount of time or delay before the muscle starts depolarizing. This value includes both the amount of time that it takes the nerve to conduct from the point of stimulation to the motor end plate area and the amount of time for the neuromuscular junction transmission to activate the muscle.[8] Once a terminal latency has been recorded, the motor conduction velocity can be determined by stimulation of another, more proximal site along the motor nerve. The computation of motor nerve conduction velocity requires knowing the distance between the two stimulation sites and the difference in the terminal latencies recorded from the more distal and more proximal sites. Dividing the distance by the time gives the nerve conduction velocity over the segment in between the stimuli.[8] Motor studies are orthodromic (measured from proximal to distal).

Sensory NCSs

To determine the sensory conduction velocity of a nerve, the nerve is electrically stimulated, and the response of the nerve picked up at a different location. Sensory studies be orthodromic or antidromic. Orthodromic stimulation occurs if the sensory nerve is stimulated promixally and recorded distally. Antidromic stimulation occurs if the sensory nerve is stimulated distally and recorded proximally. [6]

Mixed NCSs

The motor and sensory component of a mixed nerve are tested at the same time with mixed nerve action potentials (MNAPs). [6] Mixed NCSs typically examine conduction along nerve trunks by stimulating the nerve distally while recording from a more proximal location. [6]

Other types

F-wave NCV studies are another category that evaluates motor nerve conduction in proximal areas e.g. brachial plexus or along the entire length of a nerve. It is usually longer in demyelinating lesions. [9]

H- reflex NCV studies evaluate the reflex arc via sensory and motor circuits e.g. S1 nerve root function.The H-reflex is an electrophysiological response resembling the tendon reflex. To get it, you usually stimulate a peripheral nerve (e.g. the tibial nerve at the knee) and record the muscle response (H-wave). [10]

Indications

NCVs are good at assessing, confirming and identifying the pathophysiology of peripheral nerve disorders. When combined to clinical testing findings, these tests may be used to detect the level of injury, differentiate between sensory and motor involvement, determine prognosis, discern between axon loss or demyelinating damage and assess the condition’s progression. [4]

Common indications for NCV testing alone may include: [11]

  • Focal nerve entrapments e.g. carpal tunnel syndrome [12][13]
  • Demyelinating and Axonal Mononeuropathy [14]
  • Preganglionic lesions (i.e. proximal to the dorsal root ganglion) [15]
  • Polyneuropathies (demyelinating and axonal)

When used with EMG, it can aid in the diagnosis of: [4]

  • Upper motor neuron lesions
  • Lower motor neuron lesions
  • Anterior horn cell disorders
  • Radiculopathies, plexopathies, mononeuropathies
  • Neuromuscular junction disorders, or myopathic conditions.

Technical Considerations

Preparation

Common advice for patients is to keep their skin warm and dry, avoiding the application of creams or lotions. Clothes must be comfortable, to allow easy access to areas to be examined. [16] The examiner should be aware of any relevant safety concerns (see below).

Equipment

NCSs mainly use skin electrodes, usually two (one shock-emitting placed directly over the nerve and one recording over the relevant muscle).

Factors Affecting Nerve Conduction Study

There are some factors you should know about when you diagnose patients with potential nerve damage. These technical and environmental issues, potentially resulting in false results and false diagnosis, should be readily recognised and addressed. [17]

Physiological Factors

Temperature
  • Temperature is the most important of all the physiologic factors.
  • Normal physiologic range of limb temperature (approximately 21–34°C).
  • Physiologically, cooler temperatures result in the delayed opening of sodium channel sand and prolong the depolarisation (prolonged distal latency), resulting in slowed conduction velocities for the nerve being studied.
  • In addition, longer channel opening time results in a larger influx of sodium. Subsequently, each nerve fiber depolarisation is larger and longer resulting in a higher amplitude and longer duration for both compound muscle action potentials (CMAPs) and sensory nerve action potentials (SNAPs)
  • For motor and sensory conduction velocities, conduction velocity slows between 1.5 and 2.5 m/s for every 1°C drop in temperature, and distal latency prolongs by approximately 0.2 milliseconds per degree.
  • There may be significant variation in limb temperature among individuals, moreover, there is a marked variation of temperature for a given nerve, with a trend toward cooler temperatures as the nerve travels distally and superficially within the respective limb. Furthermore, skin surface temperature typically is 1 to 2°C warmer in a warm limb compared to the near nerve temperature.
Maintenance of Temperature
  • Distal limb temperatures should be routinely recorded and monitored in all patients and ideally maintained between 32 and 34°C.
  • Limbs can be heated with heating lamps, warming packs, or hydrocollators.
  • Remember there may be a heating delay between when the skin and the underlying nerve reach the desired temperature.
  • For profoundly cool limbs, it may require 20 to 40 minutes for the underlying nerve temperature to equilibrate.
Gender
  • Variation in compound action potentials (CAPs) has been observed between men and women probable due to differences in digit circumference, with negative linear correlation found between CAP amplitude and circumference for people of the same sex. [18]
Age
  • Conduction velocities decrease slightly with age in adults, most likely as a consequence of the normal loss of motor and sensory neurons that occur with aging.
  • This is more prominent for individuals older than 60 years, in whom conduction velocity decreases by approximately 0.5 to 4.0 m/s/decade. The effect is slightly more pronounced for sensory than for motor fibers.
  • Age also affects CMAP and SNAP amplitudes. SNAP amplitudes are known to decrease substantially with advanced age.
Height
  • Taller individuals commonly have slower conduction velocities than shorter individuals. [19]
  • Normal conduction velocities are slower in the lower extremities, where the limbs are longer than in the upper extremities. [20]

Non-physiological Factors

  1. Electrode impedance mismatch and 60 Hz interference
  2. Stimulus artifact
  3. Cathode position: reversing stimulator polarity
  4. Supramaximal stimulation
  5. Co-stimulation of adjacent nerves
  6. Electrode placement for motor studies
  7. Distance between recording electrodes and nerve
  8. Distance between active and reference recording electrodes
  9. Limb position and distance measurements

Safety Considerations

NCVs are noninvasive electrophysiological techniques and are considered low-risk procedures. [1] Nevertheless, some safety issues should be taken into account when using these tests. You can find contraindications as well as areas of caution for the application of NCSs and its combination with needle EMG below.

  • Avoid areas covered by a splint or cast, open wound or active skin infections, surface burns, or recent skin grafts; it may be unsafe to apply electrodes in these areas or there may be a risk of further damage or infection
  • Avoid application in severe bleeding disorders or platenet count less than 50.000/ μL; an hematoma may be formed with needle insertion
  • External cardiac pacing wires or external defibrilators are also cited as another contraindication when applied directly or near the device since NCVs may interact with these external devices [21]
  • Implanted electrical devices e.g. pacemakers, cardiac defibrillators, neurostimulators have sensing and stimulation functions that may be affected by NCVs. Although some evidence implies no significant clinical impact, [22] caution should still be taken.
  • When used with EMG application, some avoid areas affected with lympedema for risk of infection. Care should also be taken when the patient receives anticoagulation or antiplatenet therapy when using needle EMG. Avoid deep paraspinal muscles and prefer to use smaller needles. Testing of diaphragm, serratus anterior, supraspinatus, rhomboids, cervical or thoracic paraspinal muscles should be done cautiously to avoid risk of pneumothorax
  • Needle EMG may also be safely applied in patients with prosthetic joints, [23] and although some pose theoretical concerns in cases of external fixation treatments, there is no clear contraindication to using NCS/EMG to date.
  • No known contraindications have been reported for performing NCS and needle EMG in pregnancy, with no relative reports in recent neurophysiology studies.

[4][24]

Resources

Orthobullets provides a summary of techniques, indications and procedures

Guidelines by AANEM for the application of NCSs

Daube JR: Nerve conduction studies, in MJ Aminoff (ed): Electrodiagnosis in Clinical Neurology. New York, Churchill Livingstone, 1980, pp 229–264.

References

  1. ↑ 1.0 1.1 Tavee J. Nerve conduction studies: Basic concepts. Handb Clin Neurol. 2019;160:217-224.
  2. ↑ Joynt RL. Correlation studies of velocity, amplitude, and duration in median nerves. Arch Phys Med Rehabil. 1989 Jun;70(6):477-81.
  3. ↑ Walsh ME, Sloane LB, Fischer KE, Austad SN, Richardson A, Van Remmen H. Use of Nerve Conduction Velocity to Assess Peripheral Nerve Health in Aging Mice. J Gerontol A Biol Sci Med Sci. 2015 Nov;70(11):1312-9.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Ramani PK, Lui F, Arya K. Nerve Conduction Studies and Electromyography. [Updated 2025 Feb 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK611987/
  5. ↑ Dr. Simon Freilich. Nerve conduction study and EMG demonstration. Available from: https://youtube/1vQIEXUZ30k
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities, and value in localization. Neurol Clin. 2002 May;20(2):305-38, v.
  7. ↑ Weiss L, Weiss J, Pobre T. Oxford American Handbook of Physical Medicine & Rehabilitation. Oxford University Press, USA. 2010.
  8. ↑ 8.0 8.1 Reeves A, Swenson R. Disorders of the nervous system. Online: Dartmouth Medical School; 2008.
  9. ↑ Chung T, Prasad K, Lloyd TE. Peripheral neuropathy: clinical and electrophysiological considerations. Neuroimaging Clin N Am. 2014 Feb;24(1):49-65.
  10. ↑ Jin X, Zhu Y, Lu FZ, Wu XD, Zhu DQ, Weber R, Dunn B, Jiang JY. H-reflex to S1-root stimulation improves utility for diagnosing S1 radiculopathy. Clin Neurophysiol. 2010 Aug;121(8):1329-35.
  11. ↑ Weber GA. Nerve conduction studies and their clinical applications. Clin Podiatr Med Surg. 1990 Jan;7(1):151-78.
  12. ↑ Buchthal F, Rosenfalck A, Trojaborg W. Electrophysiological findings in entrapment of the median nerve at wrist and elbow. J Neurol Neurosurg Psychiatry 1974; 37: 340–60.
  13. ↑ Buchthal F, Rosenfalck A. Sensory conduction from digit to palm and from palm to wrist in the carpal tunnel syndrome. J Neurol Neurosurg Psychiatry. 1971 Jun;34(3):243-52.
  14. ↑ Raynor EM, Ross MH, Shefner JM, Preston DC. Differentiation between axonal and demyelinating neuropathies: identical segments recorded from proximal and distal muscles. Muscle Nerve. 1995 Apr;18(4):402-8.
  15. ↑ Chen ZY, Xu JG, Shen LY, Gu YD. Phrenic nerve conduction study in patients with traumatic brachial plexus palsy. Muscle Nerve. 2001 Oct;24(10):1388-90.
  16. ↑ Novello BJ, Pobre T. Electrodiagnostic Evaluation of Peripheral Neuropathy. [Updated 2023 Jan 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK563169/
  17. ↑ Stetson, D.S., Albers, J.W., Silverstein, B.A. and Wolfe, R.A. (1992), Effects of age, sex, and anthropometric factors on nerve conduction measures. Muscle Nerve, 15: 1095-1104.
  18. ↑ Bolton CF, Carter KM. Human sensory nerve compound action potential amplitude: variation with sex and finger circumference. J Neurol Neurosurg Psychiatry. 1980 Oct;43(10):925-8.
  19. ↑ Campbell WW Jr, Ward LC, Swift TR. Nerve conduction velocity varies inversely with height. Muscle Nerve. 1981 Nov-Dec;4(6):520-3.
  20. ↑ Nobue A, Kunimasa Y, Tsuneishi H, Sano K, Oda H, Ishikawa M. Limb-Specific Features and Asymmetry of Nerve Conduction Velocity and Nerve Trunk Size in Human. Front Physiol. 2020 Dec 3;11:609006.
  21. ↑ Plaut T, Weiss L. Electrodiagnostic Evaluation of Critical Illness Neuropathy. [Updated 2022 Sep 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562270/
  22. ↑ Schoeck AP, Mellion ML, Gilchrist JM, Christian FV. Safety of nerve conduction studies in patients with implanted cardiac devices. Muscle Nerve. 2007 Apr;35(4):521-4.
  23. ↑ American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM). Risks of Electrodiagnostic Medicine. July 2014; 1–8. Available from: https://www.aanem.org/docs/default-source/documents/aanem/practice/risksinedx.pdf?sfvrsn=a112b935_0 [accessed 22/6/25]
  24. ↑ Gechev A, Kane NM, Koltzenburg M, Rao DG, van der Star R. Potential risks of iatrogenic complications of nerve conduction studies (NCS) and electromyography (EMG). Clin Neurophysiol Pract. 2016 Oct 13;1:62-66.
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