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Objectives: Evoked tympanic membrane displacement (TMD) measurements, quantified by Vm, record small volume changes in the ear canal following stimulation of the acoustic reflex. Vm shows a correlation with intracranial pressure (ICP) and has been proposed as an option to non-invasively measure ICP. The spontaneous pulsing of the tympanic membrane, driven by the cardiovascular pulse, may contaminate the recordings and contribute to high measurement variability in some subjects. This study hypothesised that the larger the spontaneous vascular pulse, the larger the variability in Vm.

OBJECTIVE Evoked tympanic membrane displacement (TMD) is a non-invasive technique for assessing intracranial pressure (ICP). The aim of this study was to define reference intervals (RIs) in the healthy population. APPROACH Measurements were made in 154 healthy adults. Results were quantified by V m, which is the most frequently described TMD measurement. Distributions were determined for sitting and supine posture. Differences between right and left ears were explored using a Wilcoxon signed-rank test. Postural changes were used to assess pressure transfer between the cerebral spinal fluid (CSF) and the perilymph. MAIN RESULTS The range in which 95% of scores fall is -283 to 722 nl (M = 132 nl) left sitting, -232 to 623 nl (M = 97 nl) right sitting, -543 to 717 nl (M = 37 nl) left supine and -584 to 504 nl (M = -15 nl) right supine. No significant difference was seen between the left and right ears in the sitting position; a significant difference was seen in the supine position. A significant effect of posture was seen for both the left and right ears. Postural changes indicated pressure transfer between the CSF and perilymph more often in the right ear (75.3%) than the left (61.9%). Pressure transfer could not be assumed in either ear for 13.4% of participants. SIGNIFICANCE We present the largest dataset of evoked TMD in healthy individuals and the first set of RIs for V m. A patient cohort with both invasive ICP and evoked TMD measurements is needed to validate the technique for clinical use.