Electoneurography (ENG)

introduction

Electronurography (CLOSELY, electroneurography) is a diagnostic method in neurology, with the help of which the ability of nerves is determined to transmit electrical impulses and thereby be able to excite a muscle, for example. This technique makes it possible to stimulate nerves and derive their electrical activity superficially, so that more precise statements can be made with the result about the neurological basis of a patient's complaints.

This measurement determines the nerve conduction velocity (NLG) and thus the length of time it takes from the excitation of the nerve to the response, for example in the form of a muscle twitch of a muscle that is supplied by the nerve. In order for the normal conduction of a nerve to be guaranteed, both the nerve itself (Axon) as well as the covering around the nerve (Myelin sheath) be intact.

Indication / areas of application

In the clinic, electroneurography is used to Check the functional state of nerves. This can be necessary for a wide variety of diseases.

• Accidents of all kinds, for example cuts
• Entrapment for example Carpal tunnel syndrome
• Damage to Nerve fibers (Axons)
• to Alcohol abuse (Polyneuropathy)
• Damage to the surrounding shell (Myelin), for example in diabetes (Diabetic neuropathy),
• Damage to the transmission between nerves and muscles, for example Myasthenia gravis.

In addition, electroneurography can be used to distinguish whether the patient's symptoms are on a Nerve damage or one Muscle damage is based. Ultimately, electroneurography serves to Nerve damage (Degenerations) to be precisely classified in order to follow the healing process after nerve damage.

Anatomy / physiology of the nerve

A single nerve consists of a large number of nerve fibers. these can motorized (for movement), sensory (for the feeling) or autonomous (involuntary activities such as digestion). Most of the nerves in our body are made up of these three types of nerves. The nerves that are important for electroneurography are, however mostly purely motor or purely sensory.

In general, one differentiates nerves according to the size or the diameter of the nerve fiber and whether the nerve also insulates (myelinated) is. Generally one can say that nerves with larger diameter conduct electrical impulses faster and nerves with insulation also the electric current lead faster. This leads to one in both cases faster response of the area supplied by the nerves, for example if you burn your finger painfully on the stove (sensory) and then pull your hand away (motor).

examination

Various parameters are recorded in electroneurography. A general distinction is made between the investigation and motor and sensory nerves. It can just nerves let examine their electrical Changes in potential can be detected on the surface by electrodes is because deeper needle electrodes are almost never used for electroneurography.

1. Electronurography in motor nerves
Often, electroneurography is performed on motor nerves. The motor nerves include those that run from the brain to muscles and for the Control the movements of the body are responsible. When examining a motor nerve, the nerve is passed through a Skin electrode irritatedwhereupon it discharges (depolarized) and this electrical voltage difference spreads in both directions of the nerve.

Are the nerve and the supplied muscle intactso it comes to one Contraction of the muscle. This time span is only a few milliseconds and is measured from the time difference between the voltage between the first and the second electrode and using a nominal scale compared with values ​​from healthy volunteers. In addition to the elapsed time from the stimulation of the nerve to the contraction of the muscle (nerve conduction speed), electroneurography often uses the Strength of muscle contraction as well as the Strength of the electrical potential, which arrives at the muscle, measured.

2. Electronurography on sensitive nerves
Sensitive nerves, on the other hand, conduct stimuli from the Skin to the brain so that we can, for example, notice when an object is too hot and we can burn ourselves. This perception in the skin happens through Sensory cellsthat are linked to nerves, which in turn transmit the signal to the brain. To examine the function of sensitive nerves becomes a certain skin area by a superficial skin electrode stimulates and irritated.

Due to the irritation of the skin, a electrical impulse along the nerve found, which transmits this sensation to the brain. That's why you can go along the nerve a voltage change with a second electrode determine and also the Nerve conduction velocity as well as the strength of the signal to calculate.

Side effects / complications

Electoneurography has almost no side effects. That's why it is also considered low risk routine procedurewhich is carried out very frequently every day. Some patients experience the electrical irritation to the nerves as uncomfortable or slightly painful.

The nerve conduction velocity is usually derived by means of devices applied to the skin Adhesive electrodes. Attaching these electrodes is painless. However, a allergic reaction on the adhesive arise, especially with patients having Patch allergy.

However, permanent damage cannot occur with correct tension. However, particular caution applies to patients with Pacemakerchern. Here it should be carefully weighed whether the examination is urgently necessary or also through another investigation method could be used.

Pain

In electroneurography, nerves are stimulated by small current surges in order to be able to measure the transmission of the electrical excitation and, as a result, to be able to assess the functionality of the corresponding nerve. The current surges are usually delivered by electrodes attached to the skin. This is not painful. Be rare small needles pricked the skin to measure the electrical currents. In this case, pain occurs which is similar to the pain when blood is drawn. The electrical impulses are, depending on the intensity, called by some patients unpleasant, but perceived much less often as painful.

Accordingly, severe pain is not to be expected during electroneurography. In the supply area of ​​the irritated nerves it can under certain circumstances briefly Tingling or numbness come, but they quickly disappear again.

Values ​​/ amplitude

In electroneurography, the nerve conduction speed of the nerve to be examined is determined. Since the nerves also mediate muscle function, it comes through the electrical impulses to a muscular response that becomes visible as a contraction. Depending on how strong the muscle contraction is, a will be shown when the examination is recorded higher or lower stimulus amplitude. The stronger the muscular response, the higher the amplitude. Hence the amplitude is a Measure of the transmission of stimuli of the nerve to the muscle. If there are many functional nerve fibers in the nerve to be examined, the amplitude is large. If the nerve fibers are restricted in their function or even destroyed, this is reflected in a lower muscular stimulus amplitude. Under certain circumstances, the muscle contraction can also fail completely, so that only small or no amplitudes are shown during the recording.

In addition, the amplitude in electroneurography is also dependent on the lead electrodes used, in particular on their shape and position.

Evaluation / standard values

The evaluation of the results of the electroneurography is carried out as follows: The Stimulus and derivation electrode be at the beginning of the investigation applied to the skin at a certain distance. Then the electrical impulse is emitted to the stimulation electrode and the time it takes for the nerve to conduct the impulse to the recording electrode is determined. With the help of the previously determined distance between the electrodes and the determined conduction time, the Calculated the conduction velocity of the nerve become. This varies slightly depending on the nerve, as the conduction velocity depends on various factors, such as the Thickness of the nerve, depends on tissue temperature and myelination of the nerve (Myelin surrounds the nerve as a kind of insulating layer). The measurement results are usually in the thousandths of a second range, so that the nerves of the arm can have conduction speeds of > 45m / s are normal and the normal values ​​for the lower leg nerves > 40m / s lie. A reduced conduction speed can therefore indicate a nerve conduction disorder, for example on a Polyneuropathy as part of a Diabetes mellitus or other diseases, such as carpal tunnel syndrome.

Please also read our page Diagnosis of polyneuropathy.

Carpal tunnel syndrome

The so-called Carpal tunnel syndrome there is a bottleneck for the Median nerve on the flexor side of the wrist. The structures are under the Flexor retinaculum, a connective tissue plate, pinched.

Possible causes are, for example, a very one-sided overload of the wrist or a inflammation in this area. This causes the tissue to swell and put pressure on the nerves. Due to the compression of the nerve it can on the one hand become Pain, on the other hand too Numbness or tingling sensations come by hand in the corresponding supply area. As the damage progresses, motor function losses can also occur over time.

In order to assess the damage to the nerve from the compression, a Electronurography of the median nerve help. The electrodes are placed according to its course Forearm and wrist glued and the Line speed of the nerve can be determined. If the nerve conducts significantly more slowly than normal, pressure damage can be assumed. A new electroneurography after therapy for the carpal tunnel syndrome can finally provide information about the success of the therapy, i.e. whether the median nerve has sufficiently recovered from its compression.