Electroneuronography

Electroneuronography

Electroneuronography (ENoG) is a non-invasive neurological test introduced by Esslen and Fisch in 1979, designed to assess the integrity and conductivity of peripheral nerves. It involves applying brief electrical stimulation to a nerve point beneath the skin while simultaneously recording the electrical activity (compound action potentials) at another point along the nerve's path in the body.

The response is displayed on a cathode ray tube (CRT) or computer monitor. Electrodes taped to the skin are used for both stimulation and recording, with electrically conducting gel or paste sometimes applied to enhance signal quality. Alternatively, the recording electrodes can capture the muscle's electrical activity innervated by the nerve, making electroneuronography closely related to electromyography in such cases.

Typically, electroneuronography tests focus on nerves in the limbs (arms and legs), but one common application is assessing the facial nerve, particularly in cases of facial muscle weakness (Bell's palsy). Conducted by an audiologist, the test compares the function of the facial nerve on both sides of the face. The stimulation electrode is positioned at the stylomastoid foramen, while the recording electrode is near the nasolabial fold. ENoG is considered the sole objective measure of facial nerve integrity.

Audiologists have conducted electroneuronography tests since their inception, replacing acoustic reflex measurements. The procedure involves calculating the difference between compound muscle action potentials generated near the nose in response to electrical stimulation near the ear. Variability in recordings can stem from electrode placement, stimulating current, and skin resistance.

Although Esslen and Fisch initially placed electrodes on the nasolabial fold, alternative positions like the nasal ala have been explored. While the nasal alae demonstrated superior waveform characteristics and required less stimulation, no significant difference was observed in other aspects compared to the nasolabial fold.

Patients may experience discomfort during nerve stimulation, but most find the procedure tolerable given its potential therapeutic benefits. Typically, measurements are taken first on the unaffected side of the face and then on the affected side. Bipolar stimulation is applied at the stylomastoid foramen, with recording electrodes near the nose. A ground electrode is positioned on the forehead, away from the facial nerve. Audiologists aim to optimize results by minimizing stimulation while maximizing efficiency. The extent of nerve damage is quantified by comparing nerve conduction on the affected side to the healthy side.

Various alternative procedures exist for assessing facial nerve integrity, including electromyography, acoustic reflex testing, MRI, CT scanning, transcranial magnetic stimulation, blink reflex tests, and maximal/minimal stimulation tests. However, electroneuronography remains the preferred objective test due to its reliability, often conducted before other methods.