Acoustic middle ear reflexes in laboratory animals using clinical equipment: technical considerations

Acoustic middle ear muscle reflexes (AMR) were examined in nonanesthetized rabbits using clinical oto-impedance devices to determine the feasibility of utilizing clinically available instruments in studies of laboratory animals, particularly with regard to parameters such as probe tone frequency and intensity. The Karolinska Sjukhuset clinical impedance instrument and the Madsen Electronics ZO174 standard clinical impedance meter were used. The results of this study show that reliable AMRs and tympanograms can be obtained from rabbits with both clinical instruments. Probe tone frequencies in the range of 800-1,200 Hz were most effective for eliciting tympanograms and the AMR in rabbits. The probe tone level was also found to be an important factor, with the most reliable sound pressure level being around 70 dB SPL, and below that of the reflex threshold. Both clinical instruments recorded AMR threshold elevations in animals with 7th-nerve and cochlear lesions. AMR decay was observed at 8 and 12 kHz, but no decay was seen in normal-hearing animals up to 4 kHz, suggesting that this frequency may be suitable for detecting and measuring pathological decay in the rabbit. The overall results suggest that standard oto-impedance instruments with a probe tone frequency of 1,000 Hz may be used effectively in experimental studies of several aspects of the AMR in laboratory animals, such as rabbits.