Middle-ear influence on otoacoustic emissions. II: Contributions of posture and intracranial pressure

Although it seems likely that body till or surgically provoked variations i n intracranial pressure (ICP) can result in variations of intralabyrinthine pressure, the channels for pressure transmission remain controversial and the reasons why evoked otoacoustic emissions (EOAEs) exhibit attendant modi fications are unclear. The theoretical framework implemented in the compani on paper [Avan et al, part I, 2000] provides sensitive and non-invasive mea ns to identify the middle-ear mechanism(s) entailed in EOAE changes. It was thus applied to analyze the influence of posture on EOAE phases and magnit udes as a function of frequency, in a series of experiments involving body tilt from sitting to supine (0 degrees or -30 degrees). Controlled ICP vari ations were surgically carried out in a series of hydrocephalic patients an d the resulting EOAE changes were compared to posture data and model predic tions. In all cases, the EOAE changes closely resembled those due to an inc rease in the stiffness of the stapes' annular ligament, in keeping with the assumption that ICP gets transmitted to intralabyrinthine spaces and modif ies the hydrostatic load on the stapes, thereby influencing EOAE features. A small additional contribution of middle-ear pressure to EOAE changes was identified in addition to the main stapes component. Dynamical EOAE measure ments showed that sudden ICP changes were transmitted to the inner ear with in 8-30 s. The high sensitivity of EOAE phases below 2 kHz to ICP changes, together with the absence of any significant confounding middle-ear effect, favors EOAEs for a reliable non-invasive monitoring of ICP and intralabyri nthine pressures. (C) 2000 Elsevier Science B.V. All rights reserved.