by Steven H. Horowitz, MD, FAAN
Why your doctor might order these tests
Electromyography and nerve conduction studies, commonly known as "EMG," are diagnostic tests that measure the electrical activities of peripheral nerves (outside the spinal cord) and muscles. They are the most important tests for diagnosing many neuromuscular diseases and their severity. Electromyography and nerve conduction studies are important and helpful in diagnosing motor neuron diseases (ALS), spinal root diseases (disc herniations), peripheral neuropathies (diabetes), single nerve damage (carpal tunnel syndrome), neuromuscular transmission disorders (myasthenia gravis), and primary muscle diseases (muscular dystrophies).
What to expect and how to prepare
Both procedures are typically uncomfortable but not very painful, and local or generalized anesthesia is not needed in adults. However, the patient must be able to cooperate for both tests, especially for EMG, when varying degrees of muscle activation/contraction are required. Some patients with anxiety about procedures take their usual medications. Other medications don’t need to be stopped.
You don’t need to stop eating or drinking beforehand. The test is usually done with the patient lying down. Usually you can wear your underwear under a hospital gown that allows the person doing the test to reach your arms and legs easily.
Different patients will have different nerves and muscles studied, depending on what is being tested for. This is because in some nerve diseases all the nerves are affected, often beginning in the feet: They are called polyneuropathies, for example, diabetes. In some nerve conditions only specific nerves are affected, as in carpal tunnel syndrome, which affects the median nerve.
The nitty-gritty details
Nerve conduction studies (NCSs) measure the ability of a peripheral nerve to conduct an electrical impulse from a site of stimulation to a site of recording, measured in meters per second, like miles per hour. So two electrodes will be placed against your skin―one to stimulate a nerve, and the other to record the traveling nerve impulse. The speed of conduction of the electrical impulse along the course of the fastest conducting fibers between the stimulating and recording electrodes is called the nerve conduction velocity (NCV), and the resulting electrical activity is called the action potential. Motor action potentials (MAPs) are what is measured when testing motor nerve fibers, and sensory nerve action potentials (SNAPs) when testing sensory nerve fibers. Only the largest and fastest conducting fibers are evaluated, so NCSs cannot diagnose small-fiber abnormalities and neuropathies.
Electromyography (EMG) usually involves inserting a hair-thin needle into specific muscle to assess the muscle’s electrical activity with and without muscle movement. Specific findings are seen in primary muscle diseases that are different from those seen in nerve diseases. The examiner will ask you to keep the muscle quiet, then contract it slightly, then strongly, to measure the different electrical patterns generated.
These are tests to measure the electrical activities of peripheral nerves and muscles. They are used to diagnose if a patient has a neuromuscular disease, what type it is, and how severe it is. In many neuromuscular diseases, the nerves and muscles generate abnormal electrical activities that can be measured and analyzed with surface electrodes placed on the skin and/or needle electrodes placed in the muscles. Some disorders, such as demyelinating neuropathies and myotonia, can only be diagnosed with these tests. They are frequently used in motor neuron diseases such as amyotrophic lateral sclerosis (ALS); radiculopathies such as nerve root damage from disc herniations; peripheral neuropathies such as diabetic polyneuropathies; mononeuropathies such as carpal tunnel syndrome; and muscle diseases, including myopathies and muscular dystrophies.
Both procedures are somewhat uncomfortable, but local or generalized anesthesia is not necessary. However, patient cooperation is necessary for both, especially EMG, when varying degrees of muscle activation/contraction are required.
Usually a very tiny needle electrode is inserted into the muscle, and the muscle’s electrical activity is assessed as the electrode is inserted while the muscle rests, during minimal activity, and at maximal activity. Electrical activity at insertion or at rest is called “spontaneous activity.” Normal muscle doesn’t have any spontaneous activity, so spontaneous activity is abnormal and often a sign of muscle denervation. Types include fibrillations, positive sharp waves, and, in some disorders, fasciculations.
During minimal muscle activity motor unit potentials (MUPs) are visualized individually or in small groups, and these potentials are measured and characterized. The MUP is generated by the motor unit, which consists of the motor neuron within the spinal cord; the neuron's peripheral extension (the axon), which goes to groups of individual muscle fibers; and the muscle fibers innervated by that particular motor neuron. MUPs are larger in muscles with larger motor units; muscles requiring fine motor control such as the eye or hand muscles may have 10 to 50 muscle fibers per motor unit, whereas postural muscles may have 1,000 to 2,000 muscle fibers per motor unit. With maximal muscle contraction many MUPs are generated that normally "interfere" with one another. Looking at interference patterns helps evaluate the completeness of the contraction.
Nerve Conduction Studies (NCSs)
NCSs measure how well peripheral nerves transmit electrical impulses from a site of stimulation to a site of recording, and are measured in meters per second. Most laboratories use surface electrodes to stimulate and record, but if more precision is required, needle electrodes can be inserted close to a nerve or even within it (microneurography). The speed of conduction of the electrical impulse along the course of the fastest conducting fibers between the stimulating and recording electrodes is called the nerve conduction velocity (NCV), and the resulting electrical activity is called the action potential.
Motor action potentials (MAPs) are recorded when testing motor nerve fibers, and sensory nerve action potentials (SNAPs) when testing sensory nerve fibers. Only the largest and fastest conducting and middle-sized nerve fibers are evaluated, so this technique will not uncover small-fiber abnormalities; therefore, results will be normal in small-fiber neuropathies.
Some nerve conduction procedures also test conductivity of the nerves from the site of stimulation up to the spinal cord and back again; these are called “late responses.” They measure nerve conduction through the proximal segments of the extremity, the mixtures of nerves just outside the spinal canal called plexi, and the nerve roots as they leave or enter the spinal cord and spinal canal, so these procedures are used to diagnose and track neuropathies that affect the nerves or roots inside the torso.
Interpreting Test Results
Starting from the motor neuron in the spinal cord and moving outwards:
1. Motor neuron disease. The NCSs are usually normal until late in the disease. The most important study is the EMG, which shows abnormal spontaneous activity with needle insertion and at rest; enlarged and complex MUPs when measured individually; and reduced interference patterns with maximal muscle activity due to loss of motor units. These can occur in only a few muscles to start, but they also frequently occur in muscles that are still strong and thus subclinically involved. As these diseases progress, more muscles become abnormal and more severely so.
2. Nerve root disorders. Here the NCSs are usually normal, although late responses can be delayed in affected areas. Similar but milder-type EMG abnormalities to the motor neuron diseases can be seen in areas specific to the affected root; elsewhere the findings are normal.
3. Peripheral neuropathies. Because these are general disorders that affect many or all peripheral nerves, there are widespread abnormalities. Most start distally in the feet and then progress to involve the more proximal lower extremities and the upper extremities. NCSs will demonstrate reduced NCVs, often both sensory and motor, as in diabetes, early in the disease process; the sensory conductions may be more affected than the motor in these early stages. The greatest slowing in NCVs in the early stages is distally. As nerve damage increases, the NCVs will slow (particularly in the demyelinating neuropathies), and the MAP and SNAP amplitudes will decrease (especially in neuropathies preferentially affecting axons). Neuropathies affecting motor nerves will have EMG changes somewhat like motor neuron diseases, but usually milder. Peripheral neuropathies that only affect small nerve fibers will have normal NCSs and EMG findings if they are “pure,” but some patients have lesser degrees of damage to large fibers as well (“mixed neuropathy”) and thus can have mild abnormalities on EMG/NCSs.
4. Mononeuropathies. These are types of peripheral nerve damage that affect only one or a few nerves. They can be caused by internal or external injuries or compression of individual peripheral nerves, such as the median nerve at the carpal tunnel (carpal tunnel syndrome), the ulnar nerve at or just below the ulnar groove at the elbow, and the peroneal nerve as it crosses the fibula on the outside of the knee. Here, the NCVs usually show slowing in conduction across the damaged nerve segment, but if the nerve is completely transected there will be no visible nerve activity below the affected area at all. 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.
5. Disorders of neuromuscular transmission. Two diseases―myasthenia gravis and Lambert-Eaton myasthenic syndrome―involve disturbances in the transmission of electrical activity across a gap (neuromuscular junction) between motor nerve endings and muscle fibers. To study them, additional short trains of nerve stimulations and needle EMG (single fiber) are used.
6. Muscle diseases (myopathies or dystrophies). In these disorders, which can be inherited; caused by infection; or metabolic, hormonal, or immune, the disease only affects the muscles, so the NCSs are normal. The EMG shows abnormalities in the muscle electrical activities that are most often widespread and usually correlate with the degree of muscle weakness. The EMG findings are different from those seen in nerve disorders.