Electromyogram

Original Editor - Ahmet Kocyigit

Top Contributors - Ahmet Kocyigit, Nehal Shah, Angeliki Chorti and Kim Jackson

Introduction

Electrodiagnosis is a technique which uses electrical means to understand bioelectric signals that emanate from nerves and muscles of our body and is helps diagnose neuromuscular disorders. Various electrodiagnostic tests consist of Nerve Conduction Studies, Electromyography, Late responses, Repetitive Nerve Stimulation Studies, Somatosensory Evoked Potentials etc.

Various electrodiagnostic protocols are employed while testing a patient and these are usually a combination of the above-mentioned tests. It is imperative to perform these studies as per the protocols and guidelines that are being published from time to time

Electromyography

Electromyography is a process in which the electrical signals of the muscles are captured via an electrode. Electrical signals from the muscles represent anatomical and physiological properties of the muscle, produced during muscle contraction in a normal muscle and even at rest in an abnormal muscle and are controlled by the nervous system. EMG tests can provide data about the impulses from the nerves responsible for contraction and the reactions of the muscle fibres to the said impulses. Depending on the device used, the resulting raw data can be exported as a graph called an electromyograph, therefore in some cases giving the name electromyography to the original test.

Electromyography can be used

• For Research purpose
• For Clinical diagnostic procedure

Electromyography (EMG) is one of the many electrodiagnostic tests conducted to study the electrical activity within a muscle that helps in understanding the pathology of a neuromuscular disorder. EMG gives easy access to understand physiological processes related to muscle movement, force generation by a muscle and many such muscle functions. Hence EMG provides many important information regarding muscle state and function. As a result of this EMG studies can be abused easily.Electromyography involves testing a muscle using various types of electrodes depending upon the protocol to be employed for that particular condition.

Uses of EMG

It is helps diagnose the exact location, extent, and severity of a nerve lesion, status and level of nerve regeneration. It is also helpful in diagnosing various inflammatory and non-inflammatory nerve pathology like myopathies and dystrophies etc. Hence it can be described as both a neurological and a musculoskeletal test that is targeted to the peripheral nervous system pathway.

Clinical EMG studies are used to diagnose various conditions like

• Entrapment Syndromes - Carpal Tunnel Syndrome, Thoracic Outlet Syndrome, Tarsal Tunnel Syndrome etc
• Radiculopathy
• Neuropathies
• Myopathies
• Site, extent, severity of Peripheral nerve injuries
• Anterior Horn Cell lesions

Techniques of performing EMG studies

EMG studies can be done by using

• Surface Electrodes
• Needle Electrodes

Surface EMG (sEMG)

Surface electrodes are noninvasive electrodes which are placed over the muscles to record myoelectric signals. Surface measurements of muscle activity are generally reserved for research purposes. Using an adhesive electrode on the skin over the targeted area enables an easier test. However, a singular superficial electrode measurement picks up signals from multiple muscle fibres and all the tissue in between, compromising signal integrity thus making it non-viable for diagnostic uses.

Needle EMG

This technique involves inserting a needle electrode into the muscle which needs to be tested. The needle can be relocated to a different site in the same muscle or a different muscle as needed. Due to the proximity of the needle to the muscle surface, this is a more accurate and reliable method used for clinical diagnostic purposes. This needs rigorous training and certification before one can start performing this technique. Needle EMG is the preferred method for diagnostic purposes due to being more targeted and reliable than a surface electrode. Although the process is considered safe, the potential risks of pain, bleeding, infection, and pneumothorax remain as a result of the needle being used.

Motor Unit and Motor Unit Action Potential

A motor unit comprises of a motor neuron, its axon and all the muscle fibres innervated by that axon (Burke and Edgerton, 1975) A motor unit is an anatomical and functional unit of the neuromuscular system. Electrical activity generated within a motor unit during a muscle contraction can be recorded and analyzed using surface or needle electrodes. This type of electrical activity generated within a muscle and recorded by electrodes is called Motor Unit Action Potential. These motor units are then analyzed for shape, duration, amplitude and frequency.

Stages of performing clinical EMG studies

Clinical EMG studies are usually performed using needle electrodes and can be performed and analyzed in three stages:

• Spontaneous Activity - Analyzing EMG activity while the muscle is at rest. Normally a relaxed muscle does not display any activity on the screen. But in a diseased muscle, electrical activities are produced even at rest called Spontaneous Activity and it is an abnormal finding. Different types of Spontaneous Activity are detected in different conditions and depending upon thepresence of a particular activity, a diagnosis is confirmed. Spontaneous Activity consists of
  • Fibrillation Potentials
  • Positive Sharp Waves
  • Complex Repetitive Discharges
  • Fasciculation Potentials
  • Neuromyotonia
  • Myokymia
  • Cramps
• Voluntary Activity - Gentle contraction of a muscle is performed. This shows Motor Unit Action Potentials which are analyzed for shape, amplitude, duration and frequency
• Interference Pattern - The patient is instructed to perform a strong isometric contraction which produces a dense pattern of overlapping MUAPs called Interference Pattern.

Nerve Conduction Studies

The speed at which an impulse propagates across a peripheral nerve is called Nerve Conduction Velocity (NCV). This test is used to evaluate the nerve in clinical scenarios . Nerve Conduction study (NCS) can be performed on any peripheral nerve by stimulating it giving an electrical stimulation over two sites - preferably superficial enough to be stimulated. The analysis is made for latency ( time elapsed between nerve stimulation and contraction of a muscle innervated), amplitude ( determines the physiologically intact axons in a nerve) and NCV value. Data gathered from an NCS can be used to determine the type and extent of damage to a nerve.

EMG and NCV studies go hand in hand to make the conclusion regarding the physiological status of the neuromuscular system and confirm a diagnosis. Interpretation of these studies has to be correlated with the thorough clinical assessment to make a final diagnosis. It is imperative to have formulated differential diagnosis with a thorough evaluation and other laboratory investigations before performing EMG NCV tests.

EMG in Rehabilitation

Electromyography has also found uses within certain fields of rehabilitation, biofeedback therapy being one of them. Conditioning of biological action is a proven concept. This approach has been successfully used with visual and auditory feedback in the past, and converting the outputs of an electromyograph into similar feedback had varying degrees of success in coordinating muscle movement for the muscles of the pelvic floor. The same principle also showed promise for patients with recent knee surgeries although to a lesser extent. Alternative uses for the surface variance of EMG have also been tested to find mixed results, one of them being the inspiratory muscles.

EMG in Research

Electrophysiological properties of the human body are still a subject of vigorous study due to the intricacies and complexity of the nervous system as a whole. EMG has been proven to be an invaluable tool in collecting data and helped build some of the current concepts of the musculoskeletal system in the literature. As research progresses, combined use of EMG with other types of electrodiagnostic tools resulted in a vast array of studies to discover and evaluate new approaches for rehabilitation such as motor imagery and sensorial feedback. Studies aiming to implement EMG in more specific areas such as activities of daily living have also been prevalent, especially with the progress of technological adaptations of EMG.

Technological Research and Development

Thanks to the multidisciplinary research on the subject, EMG has not remained only to be a clinical testing device. From gait analysis to wheelchairs using a human-machine interface with EMG sensors this technology proved to be an exciting prospect. This potential also paved the way for sensor technology to become more accessible and less costly.

While it is impossible to refuse the fact that the problems of objectivity undoubtedly increased proportionally with the ease of access to these devices , it also allowed for many new areas and approaches to come to life in the field of rehabilitation, much like the 3d printing technology.