Noninvasive intracranial pressure assessment using otoacoustic emissions: An application in glaucoma

Abstract

The theory that glaucoma patients have a lower intracranial pressure (ICP) than healthy subjects is a controversial one. The aim of this study was to assess ICP noninvasively by determining the relationship between distortion product otoacoustic emission (DPOAE) phase and body position and to compare this relationship between patients with primary open angle glaucoma (POAG), patients with normal tension glaucoma (NTG), and controls. The relationship was also calibrated using published data regarding invasive measurements of ICP versus body position. DPOAEs were measured in 30 controls and 32 glaucoma patients (17 POAG, 15 NTG) at the following body positions (assuming 90° as upright): 45, 30, 20, 10, 0 (supine), -10, and -20°. DPOAE phase had a clear, nonlinear relationship with body position. The mean DPOAE phase shifts between the two most extreme body positions (45 to -20°) were 73.6, 80.7, and 66.3° for healthy, POAG, and NTG, respectively (P = 0.73), and the groups showed the same, nonlinear behaviour. This indicates that there is no evidence that glaucoma patients have a reduced ICP. When calibrated with invasive data, ICP and DPOAE phase were linearly related over an ICP of 3 mmHg. This suggests that, more broadly, DPOAEs could be used in the future to monitor changes in ICP in a clinical setting and to measure dynamic changes in ICP such as diurnal fluctuations or changes induced by certain medications.

Intraocular and intracranial pressure in glaucoma patients taking acetazolamide

“…There was a significant difference in ICP (the AAZ effect on ICP) between the groups (P = 0.035). Given the conversion factor mentioned above, the observed AAZ effect on ICP from baseline to the final 2 measurements of the 2 hour measurement period post AAZ ingestion would equate to approximately a 4 mmHg decrease in ICP for patients compared to controls [6]. Despite the apparent trend that AAZ had a greater effect on ICP than IOP, the AAZ induced…”

“…The methods of this study were similar to a previous study from our lab [6]. In short, in order to be eligible to participate in this study, all subjects had to meet the following inclusion criteria: 50 to 70 years of age and detectable DPOAEs in at least one ear.…”

“…Due to the subject specific, unknown offset of the DPOAE phase (the quantity provided by the device), data were presented relative to the calculated baseline value. Next, the resulting DPOAE phase shift was converted into an ICP change from baseline using a conversion factor of 4 degrees/mmHg [6]. From the time series of ICP measurements (each 15 minutes during two hours, with the AAZ intake in patients and the sip of water in controls at t = 0), we determined a trend by using a linear regression for each subject.…”

“…One possible theory is that glaucoma patients have a reduction in intracranial pressure (ICP) [2][3][4]. But this has not been confirmed in some more recent studies [5][6][7].…”

“…Although DPOAEs are not yet able to give absolute pressure readings, they can show changes in ICP, due to changing body positions or the use of drugs like AAZ. We recently calibrated the relationship between DPOAE change and ICP change by comparing the body-position dependency of DPOAEs with that of ICP [5,6].…”