Is the test of medial efferent system function a relevant investigation in tinnitus?

The present results are drawn from preliminary observations made in 28 patients suffering from unilateral, permanent, non-pulsatile tinnitus, who were systematically submitted to an investigation of their otoacoustic emissions in both ears. Spontaneous otoemissions (SOAE) were carefully looked for, before recording of transient evoked otoemissions (TEOAE). The effectiveness of the medial olivo-cochlear efferent system (MOC) was also tested. The comparison between TEOAE input/output curves obtained with and without contralateral stimulation allowed a global assessment of the functioning of MOC. Additional information relating to the MOC functioning at the precise frequency of tinnitus and/or around it, was available from the comparison of analogous curves obtained using distortion product otoacoustic emissions (DPOAE) instead of TEOAEs. The results show that: (1) when the amplitude of otoacoustic emissions differs between the two ears, the tinnitus is generally lateralized on the side where amplitude was the lowest, which seems to be related to a greater hearing loss; (2) no general law concerning tinnitus can be drawn from the global testing of MOC functioning; the only result is that the slope of TEOAE input/output function appears as a deciding factor for efficiency of MOC, the least efficiency almost always occurring on the side where the slope is greater, and (3) even in the case of proved global effectiveness of MOC activation, there is, at least in the vicinity and in at least one ear, a part of the cochlea where its functioning is altered. A better comprehension of the mechanisms underlying this symptom will probably come from exhaustive exploration of individual cases rather than a rougher investigation of large samples in which different aetiologies are likely to be mixed.