The Effect of Midline Petrous Apex Lesions on Tests of Afferent and Efferent Auditory Function Raymond M. Hurley, Annette Hurley, and Charles I. Berlin 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 effer- ent 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 effer- ent system of the two cases. In addition, three complementary afferent tests of brain stem audi- tory 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 supe- rior olivary complex (SOC) is involved not only in the mediation of the sound evoked efferent reflex assessed in TEOAE suppression, but in the media- tion 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 re- gards to TEOAEs suppression, and ARTs. Specifi- cally, the case with only afferent involvement pro- duced normal TEOAE suppression, a normal MLD, normal ARTs, and abnormal waves III and V of the ABR, whereas the case with both afferent and effer- ent involvement produced abnormal TEOAE sup- pression, a normal MLD, abnormal ARTs, and ab- normal 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 rele- vant test to assay the sound evoked efferent reflex that is mediated by the medial olivocochlear system of the SOC. (Ear & Hearing 2002;23;224–234) Historically, audiological procedures have focused on the assessment of the afferent (ascending) cochle- ar-VIIIth nerve system and have, for the most part, ignored the efferent (descending) auditory system. The efferent innervation to the cochlea consists of the crossed and uncrossed lateral olivocochlear (LOC) system originating from the lateral segments of the superior olivary complex (SOC) and the crossed and uncrossed medial olivocochlear (MOC) system originating from the medial segments of the SOC (see Fig. 1). The crossed segments of the LOC and MOC course with the vestibular nerve just below the IVth ventricle up to the anastomosis of Oort where the efferents enter the cochlea. Collet and colleagues have led studies that assess the efferent (descending) auditory system through contralateral suppression of transient evoked oto- acoustic emissions (TEOAEs) (Collet, Kemp, Vellu- illet, Duclaux, Moulin, & Morgon, 1990; Veuillet, Collet, & Duclaux, 1991; Veuillet, Collet, & Morgon, 1992). Recently, a method has been proposed that clinically assess both ipsilateral as well as contralat- eral effects of the medial efferent auditory system (Berlin, Hood, Hurley & Wen, 1994). This clinical procedure is designed to assess the sound evoked efferent reflex and is based on: 1) the observation that TEOAEs can be suppressed by broad band noise contralaterally, ipsilaterally or bilaterally (Berlin, Hood, Cecola, Jackson, & Szabo, 1993; Ber- lin, Hood, Hurley, Wen, & Kemp, 1995; Berlin, Hood, Wen, Szabo, Cecola, Rigby, & Jackson, 1993; Collet et al., 1990); 2) the outer hair cells (OHCs) are believed to be the generators of otoacoustic emis- sions (Glattke & Kujawa, 1991; Kemp & Chum, 1980; Lonsbury-Martin, Whitehead, & Martin, 1991; Norton & Widen, 1990; Probts, Lonsbury- Martin, & Martin, 1991); and 3) the majority of LOC neurons terminate on the ipsilateral primary neu- rons under the inner hair cells, whereas the major- ity of MOC neurons terminate on the contralateral Department of Communication Sciences & Disorders (R.M.H.), University of South Florida, Tampa, Florida; Department of Communication Sciences & Disorders (A.H.), Southeastern Louisiana University, Hammond, Louisiana; and Kresge Hear- ing Research Laboratory of the South (R.M.H., A.H., C.I.B.), Department of Otorhinolaryngology & Biocommunications, Louisiana State University Health Science Center, New Or- leans, Louisiana. 0196/0202/02/2303-0224/0 • Ear & Hearing • Copyright © 2002 by Lippincott Williams & Wilkins • Printed in the U.S.A. 224