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Electroneuronography

 
 
 

Electroneuronography (ENoG) is a neurological non-invasive test that was first described by Esslen and Fisch in 1979 and is used to examine the integrity and conductivity of a peripheral nerve. It consists of a brief electrical stimulation of the nerve in one point underneath the skin, and at the same time recording the electrical activity (compound action potentials) at another point of the nerve's trajectory in the body. The response is displayed in a cathode ray tube (CRT) or through the video monitor of a computer. The stimulation as well as the recording are carried out by disc electrodes taped to the skin, and the technician may use electrically conducting gel or paste to bolster the signals being input and output. Alternatively, the recording electrodes may also be used to pick up the electrical activity of a muscle innervated by that nerve. In such instances electroneuronography is closely related to electromyography.

Usually, nerves in the limbs (arms and legs) are tested in this way, but one of the most common applications of electroneuronography is the test of the facial nerve, such as in cases of muscle weakness in one side of the face (Bell's palsy). It is performed by an audiologist, who carries out tests to compare the two sides of the face. The stimulation electrode is located at the stylomastoid foramen and the recording electrode is located near the nasolabial fold. The ENoG test is the only objective measure of facial nerve integrity.

Electroneuronography tests are performed by audiologists, and have been since their invention in the late 1970s, when they replaced acoustic reflex measurements. Typically, the system calculates the difference between compound muscle action potentials generated near the nose (nasolabial fold) in response to supramaximal electrical stimulation near the ear (stylomastoid foramen). Thus, the electrical stimulus travels along the facial nerve, allowing it to be specifically pinpointed. Increasing sensitivity and specificity of the recordings has been a constant goal, and it is believed that variability arises from the location and pressure of the electrodes, the stimulating current, and skin resistance. Esslen and Fisch placed the electrodes on the nasolabial fold, and this has become the standard, but May and Hughes experimented with electrodes placed on the nasal ala, citing better waveforms. The two positions were compared with respect to supramaximal threshold, waveform shape/amplitude, and repeatability. With regard to the supramaximal threshold, the nasal alae demonstrated a superior biphasic waveform while requiring less input stimulation to yield adequate results. In all other categories, however, there was no statistical difference between taking measurements at the nasolabial fold compared to the nasal alae.

It is common for a general feeling of discomfort to accompany the electrical stimulation of the nerve, but nearly all patients prefer to undergo the procedure in order to effect a treatment for their condition. Measurements are generally taken on the normal, unaffected side of the face first, and then on the abnormal side. Bipolar stimulation is generated at the stylomastoid foramen, while the recording electrodes are attached at the terminal ends of the nerve near the nose. A ground electrode is placed in the center of the patient’s forehead, sufficiently far from the facial nerve as to not give an output reading. A variety of stimulation locations may also be employed, to get the best possible results. Audiologists aim to get the most efficient readings possible by optimizing results with a minimal input stimulus. The amount of damage is calculated as a ratio of how much nerve conduction has been retained by the affected side compared to the healthy value. Massive amounts of clinical experience may be required to accurately interpret the data received from testing, and misreading the results may put the patient at serious risk of developing further damage or creating a problem in otherwise healthy facial nerves.

Several alternative procedures exist for testing facial nerve integrity. Electromyography, Acoustic reflex testing (formerly the gold standard), MRI, CT scanning, transcranial magnetic stimulation, blink reflex tests, and maximal/minimal stimulation tests may also be used to asses the viability of the nerves. Currently, however, electroneuronography serves as the only objective test compared to these options, and the test is preferentially performed before the others.

 
 
     

 

 

 

         

 

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