Electromyography (EMG) and nerve conduction studies (NCSs) are valuable diagnostic tools that help neurologists locate and determine the causes of diseases that affect muscles and peripheral nerves. In EMG, a small needle is inserted into a muscle in order to measure electrical activity. In nerve conduction studies, electrodes are placed on the skin overlying a nerve, and other recording electrodes are attached at a different point over the same nerve. A small shock is applied, and the electrical impulse is recorded.
While EMG and NCSs are different tests, they're often used together because the information gained from each test is complementary — that is, the two tests together tend to be more informative than either used alone, except in specific situations.
Understanding NCS Results
The electrical signal sent along the axon of a nerve is called an action potential. In nerve conduction studies, these action potentials are artificially generated by electrical stimulation in order to assess how the axon responds.
There are two main portions to a nerve conduction study: sensory and motor. Recording from a sensory nerve gives a sensory nerve action potential (SNAP), and recording from a muscle yields a compound muscle action potential (CMAP).
Other terms you may encounter in an EMG or NCS report include the following:
- Amplitude- The electrical signal is represented as a wave, and the amplitude is its height.
- Conduction Velocity (CV)- The conduction velocity describes the speed at which the electrical impulse travels along the nerve.
- Duration- This describes the width of an electrical wave.
- Conduction Block- The diminution of signal across an anatomical region such as the wrist. This suggests nerve entrapment, as in carpal tunnel syndrome.
- F reflex-The F waves is a kind of electrical echo, in which the impulse travels up to the spine and then back down along the same fiber. It thereby gives a sense of the conduction along the entire length of a motor nerve.
- H reflex- The H wave is the electrical equivalent of a reflex in the leg. An impulse travels to the spinal cord via a sensory nerve, then travels back along a motor nerve.
These measures give information about both motor and sensory components of the peripheral nervous system. They also suggest whether the axon or the myelin sheath of nerves is more damaged by a neuropathy. Myelin helps action potentials travel faster, and so in problems of myelin (myelinopathies), conduction velocity is more decreased. In problems with the axon (axonopathies), fibers that are intact can conduct signals at normal speeds, but there are fewer fibers, which leads to a weaker signal and decreased amplitude.
Understanding EMG Results
When an EMG is performed, electrical activity from muscle fibers is measured and demonstrated as waves on a screen and static-like noises played on a speaker. The technician both listens to these sounds and watches the monitor in order to detect abnormalities.
When a nerve stimulates a muscle to contract, the result is a brief burst of electrical activity called a motor unit action potential (MUP). In diseases of peripheral nerves, muscles sometimes start having spontaneous activity on their own. This can be detected by EMG as fibrillations and positive sharp waves on the monitor. Sometimes the abnormality causes visible muscle twitches called fasciculations.
If a nerve has been injured and then regrows, the nerve tends to branch out to include a wider area. This causes abnormally large MUPS.
In contrast, MUPS are abnormally small or brief, and suggest the presence of a disease of a muscle (a myopathy).
Doctors interpreting EMG results may also mention the term "recruitment pattern." As a muscle is contracted, nerve fibers signal more and more bits of muscle (called motor units) to join in and help. In a neuropathic disorder, the amplitude of different motor units is strong, but there are fewer of them because the nerve is unable to connect to as many units. In myopathies, the number of motor units is normal, but the amplitude is smaller.
The pattern of electrical discharges from the muscle can give additional information as to the cause of the problem, and may even help determine how long a problem has been present.
The interpretation of EMG and NCSs is not always straightforward and may not always lead to just one possible diagnosis — but the tests can reduce the number of diagnostic possibilities.
Alport AR, Sander HW, Clinical Approach to Peripheral Neuropathy: Anatomic Localization and Diagnostic Testing. Continuum; Volume 18, No 1, February 2012
Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002