Correspondence to: Professor Kerry R Mills Department of Clinical Neurophysiology, King’s College Hospital, Denmark Hill, London SE5 9RS, UK; kerry.mills1kingsch.nhs.uk
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.
Electromyography (EMG), the recording of electrical activity in muscle, should be regarded as an extension of the clinical examination. It can distinguish myopathic from neurogenic muscle wasting and weakness. It can detect abnormalities such as chronic denervation or fasciculations in clinically normal muscle. It can, by determining the distribution of neurogenic abnormalities, differentiate focal nerve, plexus, or radicular pathology; and it can provide supportive evidence of the pathophysiology of peripheral neuropathy, either axonal degeneration or demyelination. EMG is an obligatory investigation in motor neurone disease to demonstrate the widespread denervation and fasciculation required for secure diagnosis.
EMG METHODOLOGY
Recordings are made with a disposable concentric needle electrode inserted into the muscle. A fine wire in the axis of the needle is insulated from the shaft, the end of the needle being cut at an acute angle. The area of the recording surface determines the volume of muscle that the needle can “see”. Conventional EMG needles record from a hemisphere of radius of about 1 mm. Within this volume there are some 100 muscle fibres. The many hundreds of muscle fibres belonging to one motor unit are distributed widely throughout the cross section of the muscle and, therefore, within the pick-up region of the needle there may be just 4–6 fibres of a single motor unit. Analysis of the waveforms and firing rates of single motor or multiple motor units can give diagnostic information.
Electromyographers are skilled at interpreting both the appearance of muscle activity and the sound of the activity transmitted through a loud speaker. Normal resting muscle is silent. Patients often have difficulty completely relaxing a muscle. The motor unit activity associated with incomplete relaxation is distinguished from abnormal spontaneous activity by its rhythmicity. Motor units when first recruited or on the point of being de-recruited fire regularly at 6–10 spikes …
Electromyography (EMG) is the clinical study of the electrical activity of motor units
motor units
In biology, a motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle.
and their muscle fibers, individually and collectively. EMG typically evaluates electrical activity with the muscle at rest and during periods of voluntary muscle contraction [1].
EMG measures the electrical activity of the muscle during rest, slight contraction and forceful contraction. Muscle tissue does not normally produce electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen on the oscilloscope, but after that, no signal should be present.
Motor neurons transmit electrical signals that cause muscles to contract. An EMG uses tiny devices called electrodes to translate these signals into graphs, sounds or numerical values that are then interpreted by a specialist.
Surface ElectroMyoGraphy (SEMG) is a non-invasive technique for measuring muscle electrical activity that occurs during muscle contraction and relaxation cycles. “Electromyography is unique in revealing what a muscle actually does at any moment during movement and postures.
Your doctor may order an electromyography, or EMG, to see how well your nerves are working. If your EMG is normal and you continue to have symptoms of neuropathy, your doctor may order a skin biopsy to look at nerves that are too small to be tested with an EMG.
EMG is most often used when a person has symptoms of weakness, pain, or abnormal sensation. It can help tell the difference between muscle weakness caused by the injury of a nerve attached to a muscle, and weakness due to nervous system disorders, such as muscle diseases.
Abnormal EMG results can show up in two ways. First, the muscle may show electrical activity at rest. On the other hand, the muscle may show abnormal electrical activity during contraction. This shows up as an abnormal action potential pattern with changes in the size or shape of the wave.
Pain is the most common complication of EMG2 causing some level of discomfort in all patients either from the nerve conduction portion or the needle examination. Most surveys find pain more common in the needle portion. The study is typically well tolerated but for some patients it is nearly unbearable.
EMG will show damage in muscles innervated by damaged motor axons. EMG will not detect injuries to purely sensory nerves, and NCS may not detect subtle partial injuries, injuries to nerve branches that are not recorded from, or injuries distal to the site of study.
If they are both available, EMG should be the first choice. They may be performed together when diagnosis is challenging. CT may especially be preferred for bone-related pathological conditions, whereas MRI may be preferred for soft tissue-related pathological conditions.
EMG changes are first seen in the muscles closest to the site of nerve injury, underscoring the importance of examination of the paraspinous muscles. A disadvantage of EMG is the delay in the appearance of reliable abnormalities until 7 to 10 days after a root injury.
How painful is an EMG test? You may feel some pain or discomfort when your provider inserts the needle into your skin and muscles. But most people can complete the test without issues. After the test, the muscles they tested may feel tender for a few days.
Introduction. With steady improvement of recording apparatus, nerve conduction studies (NCSs) have become a simple and reliable test of peripheral nerve function. ...
Electrical Stimulation of the Nerve. Cathode and Anode. ...
Recording of Muscle and Nerve Potentials. Surface and Needle Electrodes. ...
Nerve conduction studies (NCS) assess peripheral motor and sensory functions by recording the evoked response to stimulation of peripheral nerves. Motor NCS require stimulation of a peripheral nerve while recording from a muscle innervated by that nerve.
Conventional nerve conduction studies deal primarily with measurements of the distal nerve segments in an extremity. More recent techniques are applicable to less accessible anatomical regions, as illustrated by elicitation of the blink reflex, F wave, and H reflex, and the use of the inching technique.
Sensory NCSs are performed with the two-pronged stimulator and pair of recording electrodes over the nerve. These studies may be performed antidromically (stimulating proximally and recording adjacent to the sensory receptor) or orthodromically (stimulating distally and recording proximally).
Introduction: My name is Kimberely Baumbach CPA, I am a gorgeous, bright, charming, encouraging, zealous, lively, good person who loves writing and wants to share my knowledge and understanding with you.
We notice you're using an ad blocker
Without advertising income, we can't keep making this site awesome for you.
