Electrodiagnostic Testing

Electrodiagnostic testing has been advocated as an important element in diagnostic evaluation of facial paralysis. It is used to determine the extent of facial nerve injury and to provide prognostic information for the use of treatment planning. The electrical tests that we have found the most helpful include electroneuronography (ENOG), volitional motor unit potentials on electromyelogram (EMG), spontaneous EMG responses, and Hilger nerve stimulation minimum nerve excitability test (NET). ENOG is used to document the extent of facial nerve degeneration in comparison with the contralateral side. It should not be performed until approximately 3 to 4 days after the development of a complete unilateral paralysis. ENOG provides valuable information up to 21 days after injury. Degeneration to less than 10% function compared with the contralateral side is considered significant and is considered to be the threshold for possible surgical exploration.2 Sources of error do occur in ENOG, including electrode placement, skin impedance, masseter muscle artifact, equipment variability, and lack of standardization. Consequently, ENOG results alone should not be considered sufficient in selecting patients for surgery.

In NET, a 2-3 j mA difference between sides of stimulation threshold is considered significant. Hilger NET testing is used to collaborate findings on ENOG as well as a possible substitute for ENOG in good prognostic situations. Hilger stimulation is usually quicker, easier to perform, and more economical to the patient than is ENOG testing. The test, however, introduces subjectivity in that it relies on visual detection of response.

EMG responses reflect postsynaptic membrane potentials that may be either initiated at the neuromuscular junction voluntarily or spontaneously across the membrane potential. The presence of voluntary EMG facial motoring unit potentials early on after an acute facial paralysis was noted by Granger3 to be associated with ultimate recovery from a facial paralysis. Motor unit potentials in four or five muscle groups during the first 3 days after onset of an acute facial paralysis was associated with satisfactory outcome in more than 90% of the patients. Thus, the presence of EMG voluntary facial motor unit potentials despite clinically absent motion has important prognostic implications. Spontaneous EMG activity has prognostic implications at 6 months after a major intratemporal facial nerve injury. Fibrillation potentials in the muscle are considered evidence of continuing complete denervation. Polyphasic potentials are considered evidence of regeneration, as well as a good prognostic finding. Reevaluation with EMG at the 6-month point has been selected because this provides adequate time for an intratemporal facial nerve injury to regenerate at the expected 1-mm/day growth rate to the facial periphery.

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