Electrodiagnosis

Original Editor - Innocent Abugu

Top Contributors - Innocent Abugu and Lucinda hampton  

Introduction

Nerve conduction study
Electrodiagnostic testing is the core diagnostic modality for patients with a suspected myopathy.
  • It consists of nerve conduction studies (NCS) and electromyography (EMG).
  • Electrodiagnostic studies are considered an extension of the physical examination and are most useful in the workup of a patient with a suspected myopathy[1]
  • Less commonly used electrodiagnostic testing includes: somatosensory evoked potentials (SEP); single fibre EMG (SFEMG); repetitive stimulation. [2]

Electrodiagnostic testings used to evaluate conditions such as:

In physiotherapy practice, electrodiagnosis can be performed by a clinical electrophysiologic certified specialist.

Electromyography

Surface Electromyography
Electromyography (EMG) involves the evaluation and recording of muscle activity. The instrument for electromyography is the electromyograph. The electromyograph detects electrical potential generated by a muscle and produces a record of the muscle activity called electromyogram.

EMG is used for identifying neuromuscular diseases and disorders of motor control. EMG is usually performed with nerve conduction study. The electrical activity of a muscle can be measured using a surface electrode (surface EMG), for a large region of muscle, and a needle EMG for a lesser region.

The surface EMG, while it gives a good indication of which muscle groups are active in for example tremor or dystonia, gives little information as to the fine structure of those muscles.[3] It entails placing the electrodes on the skin overlying a muscle to determine the electrical activity of the muscle.

Needle EMG entails inserting a needle electrode into a muscle, recording and amplifying the electrical signals generated from resting or contracting muscle fibers, and interpreting the signals to determine the function of the muscle fibers and motor units.[4]

Needle Electromyography

Muscles are usually tested at rest and minimal muscle contraction.[2] It is imperative that the patient is told what to expect before the test. The test should be discontinued if the patient becomes uncomfortable or requests that it should be stopped.

Limitations

  1. Adipose tissue can affect the recordings of a surface EMG.
  2. Surface EMG can measure only superficial muscles.
  3. Needle EMG involves voluntary activation of muscles, so can be less effective in unresponsive/uncooperative patients, paralyzed patients, children and infants.
[5]

Nerve Conduction Studies

Nerve conduction study
Nerve conduction study also known as nerve conduction velocity test is used to measure the speed of the electrical activity of a nerve. Nerve conduction studies can test sensory or motor nerve fibers and can determine both the speed of conduction as well as the amplitude of the electrical signal evoked following stimulation of a nerve.[6] The sensory nerve action potential (SNAP) provides information on the sensory nerve axon and its pathway from the distal receptors in the skin to the dorsal root ganglia, while the compound muscle action potential (CMAP) is an assessment of the motor nerve fibers from their origins in the anterior horn cells to their termination along muscle fibers.[7]

Motor Conduction Studies

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.[2] 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 depolarising. 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.[6] 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.[6] Motor studies are orthodromic (measured from proximal to distal). 

Sensory Conduction Studies

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.

[8]

Somatosensory Evoked Potentials

Somatosensory evoked potentials (SEPs) assess the function of somatosensory pathways by stimulation of sensory nerves. SEPs may be recorded by stimulation of mixed or pure sensory nerves in the upper and lower extremities, in dermatomal areas of the skin, and from some cranial nerves with sensory function. [9]

Single Fiber Electromyography

Single fiber electromyography (SFEMG) is a highly selective diagnostic test that involves assessment of individual muscle fiber action potentials (MFAPs). Single fiber electromyography (SFEMG) is the most sensitive electrophysiological test for myasthenia gravis and other neuromuscular junction pathology. It is also useful in the assessment of motor unit morphology in some neuromuscular diseases. [10] 

Repetitive Nerve Stimulation

Repetitive nerve stimulation (RNS) involves the repeated transcutaneous electrical stimulation of all the motor fibers within a peripheral nerve, which generates successive impulses.[11] Repetitive nerve stimulation is used to diagnose myasthenia gravis and other disorders of the neuromuscular junction (NMJ).

References

  1. Paganoni S, Amato A. Electrodiagnostic evaluation of myopathies. Physical Medicine and Rehabilitation Clinics. 2013 Feb 1;24(1):193-207.Available from:https://www.statpearls.com/ArticleLibrary/viewarticle/115846 (last accessed 7.11.2020)
  2. 2.0 2.1 2.2 2.3 Weiss L, Weiss J, Pobre T. Oxford American handbook of physical medicine & rehabilitation. Oxford University Press, USA; 2010 Mar 15.
  3. Whittaker RG. The fundamentals of electromyography. Practical neurology. 2012 Jun 1;12(3):187-94.
  4. Rubin DI. Needle electromyography: basic concepts and patterns of abnormalities. Neurologic clinics. 2012 May 1;30(2):429-56.
  5. Dr. Simon Freilich. Nerve conduction study and EMG demonstration. Available from: https://youtu.be/1vQIEXUZ30k
  6. 6.0 6.1 6.2 3. Reeves A, Swenson R. Disorders of the nervous system. Online: Dartmouth Medical School; 2008.
  7. Tavee J. Nerve conduction studies: Basic concepts. InHandbook of clinical neurology 2019 Jan 1 (Vol. 160, pp. 217-224). Elsevier.
  8. Mohammad Sadique. Neurology#2 Nerve Conduction Velocity Studies (NVC). Available from: https://youtu.be/3FW1ZcGMW8I
  9. Waldman SD. Pain review. Elsevier Health Sciences; 2009 Feb 23.
  10. Lagueny A. Single-fibre electromyography. Revue medicale de Liege. 2004;59:141.
  11. Gooch CL, Ashizawa T. Neuromuscular Junction Diseases. Neurology Secrets E-Book. 2010 Apr 30:83.