TMD fails to provide accurate estimates of ICP mostly because the acoustic impedance and its changes due to the acoustic reflex are dominantly determined by the structures and functional properties of the middle ear, and only marginally influenced by changes in ICP. A measurable acoustic phenomenon that originates in the inner ear would, at least in theory, allow for more precise assessment of the pressure of the peri- and endo-lymph, and consequently, of ICP. Otoacoustic emission (OAE), which is a sound generated by subtle oscillations of the endo- and perilymph caused by contractions of the outer hairy cells of the inner ear in response to a loud sound, seems to offer such a possibility. The sound is transmitted to the stapes, and further through the ossicles, to the tympanic membrane from which it can be detected with a sensitive microphone inserted into the ear canal. OAE is used in clinical practice to test for hearing deficits in babies and children who are too young to cooperate. The equipment can be made portable, and is relatively easy to use. Two approaches are commonly utilized that increase the unfavorable signal-to-noise ratio and facilitate extraction of the OAE waveform: transient evoked otoacoustic emission (TEOAE) and distortion product otoacoustic emission (DPOAE). In a recent US patent issued to Meyerson and colleagues 1 thought the use of both the TEOAE and DPOAE for measurement of ICP. TEOAE is used first to determine the optimum OAE response frequency, after which the pair of pure tones is deployed in a DPOAE paradigm such that the cubic distortion product frequency equals the optimum response frequency while the ratio of frequencies f2/f1 is set to 5:4, and of intensities I2/I1 to 6:5. The inventors also proposed formulae that relate ICP to the intensity or phase of the measured OAE signal, and described how the other physiological signals or behaviors that are known to affect ICP such as small oscil-lations of ICP with each heart beat, respiration, or posture changes, can be used to confirm the validity of the obtained measurements (e.g. the absence of modulation of the measured OAE phase with respiration may indicate occlusion of the cochlear aqueduct, in which case OAE cannot provide any information about ICP). There is little data up to date about the clinical utility or accuracy of otoa-coustic emission as a measure of ICP. A pilot study of Frank and colleagues that evaluated different modalities of OAE in 12 healthy volunteers and 5 patients with implanted ventricular catheters for direct ICP monitoring revealed that increased ICP or conditions known to increase ICP (e.g. posture changes, abdomen compression, coughing) were associated with notable decreases (between -2.1 and -7.9SPL) in intensity of the evoked OAE. All results were however reported only as group averages, and no attempt was made to derive a quantitative one-to-one relation between the OAE intensity and ICP.
1 Meyerson, S.C., Avan, P.A., Buki, B.: US20036589189 (2003).