Electroneurography

Electroneurography (synonyms: electroneurography (ENG); ENG diagnostics) is a diagnostic procedure used to measure the nerve conduction velocity (NLG) of motor and sensory nerve tracts of peripheral nerves (nerve tracts of nerve cells responsible for muscle movement and skin sensitivity). This is an electrophysiological measurement method using surface or needle electrodes. The method is used to determine the location of nerve lesions (injuries to nerve pathways caused, for example, by bruising) and to characterize neuropathies (nerve diseases). Electroneurography is primarily part of the diagnostic process in neurology (the study of the nervous system) and is an important element of routine examination. The following article provides an overview of how the examination is performed and its theoretical background.

Indications (areas of application)

The procedure

The primary goal of electroneurography is to measure the so-called nerve conduction velocity. This is a physiological value that provides information about the state of peripheral axons and their myelin sheaths (nerve conduits and their nerve sheaths on arms and legs). To record this value, electrical conduction through electrodes is necessary. The measurement is made at a point on the extremities from which the nerve under examination is easily accessible (i.e., very close to the surface of the skin). The following nerves are accessible to electroneurography and are usually examined:

  • Radial nerve – the so-called radial nerve belongs to the brachial plexus (brachial plexus) and can be examined on the upper arm, forearm, and hand (extensor indicis muscle)
  • Nervus medianus – the median nerve also belongs to the brachial plexus and can also be found on the upper arm, forearm and hand (M abducis pollicis brevis)
  • Ulnar nerve – the so-called ulnar nerve also belongs to the brachial plexus and is located next to the upper arm, forearm and hand (M. abductor digiti minimi) especially in the area of the elbow close under the skin surface
  • Nervus ischiadicus – the so-called sciatic nerve or sitting leg nerve belongs to the lumbosacral plexus (lumbar-cruciate plexus) and can be found on the upper thigh
  • Tibial nerve – the tibial nerve is a main branch of the sciatic nerve and is located in the area of the lower leg and foot (abductor hallucis muscle) close enough under the skin surface for measurement
  • Common peroneal nerve – the common fibular nerve is also a main branch of the sciatic nerve and divides into the superficial and profundal peroneal nerve in the course; the measurement is made in the lower leg area as well as on the foot (extensor digitorum brevis muscle)
  • Sural nerve – this nerve is purely sensitive and accessible for examination on the lower leg and foot.

Nerve conduction velocity is not measured directly, but calculated. For this purpose, the nerve trunk to be examined is stimulated by an electrical stimulus at one of the easily accessible points (duration: approx. 0.1-1 second; frequency: approx. 0.1-1.0/second). The action potentials (electrical excitation waves of the nerve) are derived taking into account the time and amplitude magnitude at the corresponding muscle (time interval from the stimulation of the nerve to the arrival of the excitation at the muscle and strength of the excitation reaching the muscle). When the muscle excitation is derived (muscle action potential), the time of transmission of the excitation to the muscle is also measured. To determine the pure nerve conduction velocity, the nerve must be stimulated at two points and the time spans subtracted from each other. There are two different nerve conduction velocities, sensitive NLG (conduction velocity of a sensitive nerve pathway) and motor NLG (conduction velocity of a motor nerve pathway). The motor NLG is determined as described above, i.e., the nerve is stimulated proximally (e.g..e.g. on the forearm) and the excitation is derived distally (e.g. on the hand). The direction of excitation is orthodromic, that is, in the physiological direction along the limb away from the trunk. In the sensitive NLG, the excitation is both orthodromic and antidromic (the excitation is reversed from distal (hand) to proximal (forearm). Sensory NLG is a more sensitive measurement parameter than motor NLG. The entire measurement consists of several steps:

  1. Acquisition of spontaneous activity – After the electrodes are attached, the excitation is measured at rest (without stimulation). Unusual excitations, called fibrillations and fasciculations, as well as positive sharp waves or pseudomyotonic discharges (pathological excitations) indicate a fresh lesion of the nerve trunk.
  2. Acquisition of muscle action potentials – by stimulation as described above.

Nerve conduction velocity is expressed in meters/second and is approximately 45-65 meters/second in healthy adults. A pathological (pathological) finding is the slowing of NLG due to primary damage to the nerve sheath and a reduction in amplitude magnitude due to primary damage to the axon. The following terms are used to classify traumatic (injury-related) nerve lesions:

  • Neurapraxia – blockage of excitation conduction while maintaining continuity of the nerve fiber (axon and nerve sheath), e.g., when the nerve is compressed (crushed).
  • Axonotmesis – blockage of excitation conduction with destruction of the axon but preserved continuity of the myelin sheath (nerve sheath).
  • Neurotmesis – complete severance of the nerve.

Another examination option of electroneurography is the electrodiagnostic reflex examination of the orbicularis oculi reflex (blink reflex). Here, the so-called supraorbital nerve is stimulated at its exit point and the muscle action potentials of the orbicularis oculi muscle (eye sphincter) are derived. This examination is used, for example, in facial nerve palsy (paralysis of the facial motor nerve).