Refining non-invasive techniques to measure intracranial pressure: comparing evoked and spontaneous tympanic membrane...

Tympanic membrane displacements (TMDs) are used to non-invasively gauge inner-ear fluid pressure. Inner-ear fluid pressure equalizes with intracranial pressure (ICP) via the cochlear aqueduct and therefore TMDs can indirectly evaluate ICP. We studied the relationship between two TMD modalities, evoked and spontaneous. Evoked TMD is a reflex response to an auditory stimulus and the established stapes-footplate mechanism explains how evoked TMDs change with ICP. Spontaneous TMD refers to a pulsatile TMD waveform expressed in the form of pulse amplitudes (TMD-PAs), the origins of which are poorly understood. We investigated whether both modalities respond similarly to an ICP change, suggesting a common mechanism.

ICP was manipulated in 20 healthy volunteers by a postural change from sitting (lower ICP) to supine (higher ICP). Differences between paired sitting and supine TMD results generated ΔEvoked and ΔSpontaneous values.

Evoked TMDs became more inward on lying supine while spontaneous TMDs became more outward. There was no evidence of a correlation between ΔEvoked and ΔSpontaneous (Right ears: r  =  -0.38, p  =  0.10, 95% CI  -0.75 to 0.21; Left ears: r  =  0.34, p  =  0.16, 95% CI  -0.17 to 0.75).

This suggests the stapes-footplate mechanism is not the primary mechanism explaining how spontaneous TMDs respond to changing ICP.