Electroneurography is a physiologic test that uses electromyelography to objectively measure the difference between potentials generated by the facial musculature on both sides of the face in response to a supramaximal electrical stimulation of the facial nerve. Because all electrodiagnostic testing is performed on the nerve distal to the proposed site of injury, sufficient time is needed for wallerian degeneration to occur, usually 48-72 hours. Testing should begin 3 days from the onset of complete paralysis.
Electrodiagnostic testing measures the facial nerve degeneration indirectly. If a patient does not reach 90% degeneration within the first 3 weeks of the onset of paralysis, some studies suggest that the prognosis is excellent, with over 80%-100% of patients recovering with excellent function.
Patients who reach over 90% degeneration within the first 3 weeks of the onset of paralysis have a much more guarded prognosis, with only 50% having good recovery of facial motion. The rate of degeneration also predicts the prognosis. Patients who have 90% degeneration by 5 days have a worse prognosis than those with 90% degeneration at 14 days.
For more on the workup of Bell palsy, read here.
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Cite this: Selim Benbadis. Fast Five Quiz: How Much Do You Know About Bell Palsy? - Medscape - Sep 29, 2015.