Electrodiagnosis

Original Editor - Innocent Abugu

Top Contributors - Innocent Abugu, Angeliki Chorti, Jasrah Javed, Lucinda hampton and Tolulope Adeniji  

Introduction

Electrodiagnosis is a technique which uses electrical means to understand bioelectric signals that emanate from nerves and muscles of our body. It is an integral part of diagnosis and rehabilitation in neuromuscular disorders. Clinicians use electrodiagnosis when confirming a diagnosis, or establishing prognosis for a particular disorder.

Electricity for nerve and muscle electrophysiologic studies was applied for the first time in the 19th Century when Erb described the galvanic-faradic test. Lord Adrian then demonstrated the clinical application of strength-duration curves, and they became widely used in the assessment of peripheral nerve injuries during the 1939-45 War.

Electrodiagnostic studies are considered an extension of the physical examination and are most useful in the workup of:

• Peripheral neuropathy
• Entrapment neuropathy (including carpal tunnel syndrome, ulnar neuropathy at the elbow, peroneal (aka fibular) neuropathy at the fibular head, tarsal tunnel syndrome),
• Plexopathy
• Radiculopathy
• Spinal stenosis
• Neuromuscular junction disorders (myasthenia gravis, Lambert–Eaton syndrome),
• Myopathies disorders of the anterior horn cell

In some countries e.g. USA, electrodiagnosis is performed by a clinical electrophysiologic certified specialist.

Electrodiagnostic testing

There are various electrodiagnostic techniques for testing e.g. Nerve Conduction Studies, Electromyography (EMG), Late responses, Repetitive Nerve Stimulation Studies, Somatosensory Evoked Potentials (SEP), single fibre EMG (SFEMG) Below you can find all relevant information.

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 analysed for shape, duration, amplitude and frequency.

Electromyography

Surface Electromyography by Paulstewart77 CC BY-SA 4.0

Electromyography (EMG) involves the evaluation and recording of muscle activity. The electrical activity is recorded and converted in visual or auditory information through 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 smaller muscle.

Muscles are usually tested at rest and minimal muscle contraction. Testing may involve various types of electrodes depending upon the protocol to be employed for that particular condition. It is imperative that the patient is told what to expect before performing the test. The test should be discontinued if the patient becomes uncomfortable or requests that it should be stopped.

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

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.

Nerve Conduction Studies

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. 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.

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. 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. 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. 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.

Somatosensory Evoked Potentials (SEP)

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.

Single Fiber Electromyography (SFEMG)

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.  

Repetitive Nerve Stimulation (RNS)

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

Electrodiagnosis in Older Adults

Ageing is characterized by physiological functions declines and it may be accompanied by changes such as decline in activities of daily living, muscle function including postural control and muscle activation etc. These changes may impact on older adult functional ability and independent living. Electromyography(EMG) is one of the electrodiagnostic tools that can provide information about age related changes in neuromuscular adjustments and help to identify factors that may contribute to risk of falls in older adults. Kindly refer to this article to learn more about age-related neuromuscular adjustments assessed by EMG.