Contralateral suppression of transiently evoked otoacoustic emissions and tinnitus

The present paper reports individual data obtained in three different patients who consulted for unilateral tinnitus in the Department of Otolaryngology. After pure tone and high-frequency audiometry, Audioscan audiometry was recorded, and tinnitus measurement which comprised a determination of pitch and loudness matches. Spontaneous, transient evoked and 2f1-f2 distortion product otoacoustic emissions at 65 dB SPL stimulus intensity were then determined. The functioning of the medial olivocochlear system (MOC) was also tested from a comparison between OAE input/output curves obtained in the presence and absence of 30 dB SL contralateral stimulation by a broadband noise: MOC global effectiveness was assessed through transient evoked emissions while those concerning distortion product emissions allowed a precise testing at the tinnitus frequency itself. The examples here displayed illustrate the diversity of results that can be found in such investigations, thereby preventing a general law to be established from the global testing of MOC functioning. On the contrary, the local testing at the precise frequency of tinnitus revealed the existence of an alteration of MOC functioning in at least one ear as shown by either a weak, null or inverse effect of contralateral stimulation. A better understanding of the sensorineural forms of tinnitus should come from the extensive investigation of MOC efficiency along the basilar membrane when routine clinical investigations lead one to suspect an involvement of this system, due to discordant damaging between inner and outer hair cells. Such studies would allow one to test the validity of hypotheses which invoke MOC-related mechanisms as an essential link for the emergence and persistence of tinnitus.