The effect of midline petrous apex lesions on tests of afferent and efferent auditory function.

Objectives: Historically, audiological procedures have focused on the assessment of the afferent (ascending) cochlear-VIIIth nerve system and have, for the most part, ignored the efferent (descending) auditory system. We report afferent and efferent auditory test results for two cases with a cholesterol cyst of the right petrous apex; one lesion involves the afferent segment of the auditory system, and the second lesion involves both the afferent and efferent segments of the auditory system. These "natural experiments" provide a unique opportunity to study the effect of a space-occupying lesion of the petrous apex on afferent and efferent function of the auditory system.

Design: Transient evoked otoacoustic emission (TEOAE) suppression studies were performed to assess the effect of the cholesterol cyst on the efferent system of the two cases. In addition, three complementary afferent tests of brain stem auditory function were administered: 1) acoustic reflex thresholds (ARTs); 2) masking level difference (MLD); and 3) auditory brain stem response (ABR). These tests are complementary because the superior olivary complex (SOC) is involved not only in the mediation of the sound evoked efferent reflex assessed in TEOAE suppression, but in the mediation of the ARTs, the MLD, and the ABR.

Results: The two cases with midline petrous apex lesions, one not involving the VIII-cochlear efferent auditory system, differed from each other with regards to TEOAEs suppression, and ARTs. Specifically, the case with only afferent involvement produced normal TEOAE suppression, a normal MLD, normal ARTs, and abnormal waves III and V of the ABR, whereas the case with both afferent and efferent involvement produced abnormal TEOAE suppression, a normal MLD, abnormal ARTs, and abnormal waves III and V of the ABR.

Conclusions: These cases illustrate that although several auditory tests can be mediated within the same or adjacent anatomical structures, i.e., the SOC, they may not be equally affected by the same lesion due to different physiology. Further, the TEOAE suppression paradigm is a clinically relevant test to assay the sound evoked efferent reflex that is mediated by the medial olivocochlear system of the SOC.