A difference of 3.5 mamp or more indicates an unfavorable prognosis. Amplitudes are a 24-48 hour delayed representation of actual events occurring at site of lesion. Topographic Diagnosis - Topographic testing to determine the anatomical level of a peripheral lesion is possible because of the mixed function of the nerve and the branching pattern within the temporal bone. Stapedius nerve Impedance audiometry can record the presence or absence of stapedius muscle contraction to sound stimuli 70 to 100 dB above hearing threshold. Diagnostic Studies Audiometry Pure tone audiometry records cochlear nerve function.
Stapedial reflex is part of topographic testing.
Speech discrimination, tone decay, auditory evoked potentials are used to screen for retrocochlear lesions, e.g., tumors of the cerebellopontine angle. High resolution scans needed for base of skull lesions. MRI is best for soft tissue assessment and tumors of the facial nerve. When immediate repair in contaminated or extensive wounds is not feasible, proximal and distal stumps should be tagged. The transected ends lose response to electrical stimulation within 72 hours.