The electroneurography or electroneurography (ENG) is a method of electric diagnostics in neurology for the determination of the functional state of a peripheral nerve . Among other things, the nerve conduction velocities and their distribution, the amplitude and the refractory period are recorded.
The basic principle is to stimulate a nerve (mostly on the extremities ) with a short (0.1–2.0 ms) electrical impulse ( square wave voltage ). This leads to depolarization of the nerve, which is transmitted in both directions via the nerve fiber ( axon ). The induced voltage change can then be measured along the nerve, depending on the issue. In routine medical diagnostics, valuable information about the functional state of nerves and the location of a disorder can be obtained. However, it is always the sum of the response from a large number of nerve fibers. According to the agreement, the nerve conduction velocity of the rapidly conducting fibers is therefore specified.
The nerve conduction velocities of motor nerve fibers can be easily determined by stimulating the nerve in two different locations and measuring the stimulus responses in the muscle. The difference between the conduction times ( latencies ), for example, is determined in ms and the difference between the stimulus locations, for example, in mm. Knowing the distance and time difference enables the nerve conduction velocity to be calculated by calculating the distance / time quotient.
The amplitude of the stimulus response gives a rough indication of the number of nerve fibers that are forwarding, but also depends, among other things, on the position and shape of the lead electrode.
By deriving the F-wave , statements about the nerve conduction up to the soma of the nerve cell , which is located in the anterior horn of the spinal cord for arm and leg nerves, can also be made for motor nerves . The F-wave is a late response potential that fluctuates in its expression and has a low amplitude and occurs after the stimulation.
An A-wave can indicate a lesion of the nerve on the sensed portion of the nerve.
After a depolarization of the nerve membrane, the initial state must first be restored through active processes until the membrane can be stimulated again. This time is called the refractory period . Determining the refractory period provides important information about the functional state of the nerve.
Equipment requirements are a correspondingly controllable stimulator, a measuring system for the voltage with memory and an evaluation unit for measuring the recorded potentials. Corresponding devices are available in many neurological departments and practices.
The person being examined usually perceives the electrical stimulation of the nerve as uncomfortable, and sometimes also as painful. However, the sensitivity is quite different from person to person.
In principle, electroneurography is suitable for investigating the function of nerves that run sufficiently close to the surface to be able to both electrically stimulate them and derive a response potential. This is mainly the case for nerves in the extremities .
It is possible to differentiate between damage to the myelin sheath (the isolation of a single nerve fiber) and damage to the axons (the nerve fibers themselves). Destruction of the myelin sheath leads to a reduction in nerve conduction velocity by impairing the saltatory conduction of excitation. In contrast, the loss of the axons leads to a reduction in the amplitude of the stimulus response. In many diseases, however, both phenomena occur with an emphasis on one aspect.
In the case of hereditary or inflammatory demyelinating nerve diseases ( HMSN-I or Guillain-Barré syndrome and variants) there is damage to the myelin sheaths and consequently a reduction in nerve conduction velocities. In the case of polyneuropathy caused by diabetes mellitus , mainly the myelin sheaths are damaged, and in the case of alcohol-toxic causes mainly the axons.
The electrical serial stimulation of motor nerves makes it possible to investigate disturbances in the transmission of excitation from nerves to skeletal muscles. In myasthenia , for example, there is a decrease in the stimulus response in the course of 5–10 (3 / s) equally strong stimuli, which is referred to as decrement .
Electromyography is often useful for further assessment of the motor nerves (which control muscles) . In order to further assess the function of sensitive nerves, evoked potentials must be recorded for the sections near the body and for transmission in the spinal cord and brain .
- Bastian Conrad, Christian Bischoff, Reiner Benecke: The EMG book . Thieme, Stuttgart / New York 2005, ISBN 3-13-110341-8 .
- Peter Vogel: Course book Clinical Neurophysiology . Thieme, Stuttgart / New York 2001, ISBN 3-13-128111-1 .