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Acute changes in atmospheric pressure seem to induce changes in intraocular pressure (IOP) measurements for both Pascal dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT) — albeit, in different directions.1 Research published in the Journal of Glaucoma shows that the difference between IOP measurements from the 2 tonometers increases with atmospheric pressure changes.1
The researchers initiated this study in the hopes of understanding the difference in the IOP measured by DCT and GAT at different atmospheric pressures, on the basis of the premise that atmospheric pressure significantly impacts the IOP as measured by GAT but might not with DCT.1
They found that an acute increase in ambient atmospheric pressure seems to initially decrease, and then increase, IOP measured by GAT. The IOP measured by DCT, however, seems to slightly and progressively lower with an acute increase in ambient atmospheric pressure, bringing the difference between the tonometers toward a smaller value.1
The investigators looked at the IOP of both eyes of 22 subjects measured with both devices at 4 different atmospheric pressure levels. They started at 1 Queretaro atmospheric pressure (QATM), then increased to 1.1 QATM, 1.2 QATM, and 1.25 QATM (equivalent to sea level), with measurements taken 5 minutes after reaching each level.1
The mean IOP as measured by GAT at 1 QATM was 12.23 mm Hg (range, 8 mm Hg to 20 mm Hg), and mean IOP by DCT was 16.36 mm Hg (range, 12.1 mm Hg to 25.3 mm Hg).1 The mean difference at this elevation between GAT and DCT was 4.14 mm Hg (range, −0.1 mm Hg to 7.5 mm Hg; P <.001). At 1.1QATM, mean GAT IOP was 11.05 mm Hg (± 2.68 mm Hg) and mean DCT IOP was 15.60 mm Hg (± 3.02 mm Hg), for a mean difference between instruments of 4.56 mm Hg (± 1.81 mm Hg; P <.001). At 1.2 QATM, mean GAT IOP was 11.14 mm Hg (± 2.53 mm Hg), and mean DCT IOP was 15.39 mm Hg (± 2.91 mm Hg).1 The difference between instruments was 4.25 mm Hg (± 2.12 mm Hg; P <.001).1 At 1.25 QATM, the mean GAT IOP was 12.39 mm Hg (± 3.11 mm Hg), and mean DCT IOP was 14.91 mm Hg ± 2.73 mm Hg.1 The difference between instruments after 5 minutes of adaptation was 2.53 mm Hg (±1.62 mm Hg; P <.001).1
Analysis shows that the difference between GAT and DCT increases by 1 mm Hg per 673 meter of increase of altitude above sea level.1
The investigators suggest a fundamental difference in how the instruments establish IOP may explain some of this. The GAT approach creates a static measurement of the force required to flatten a fixed area of the cornea. This approach is dependent on corneal properties, such as central corneal thickness (CCT), which can influence the result. Clinicians can avoid accounting for factors such as corneal hysteresis and CCT by using DCT, which is essentially unaffected by corneal characteristics.1,2
Instead, DCT’s contour-matched tonometer tip has a concave surface that allows the cornea to assume the shape that it naturally assumes when pressure on both sides of the cornea is equal and distortion of the cornea is minimal.2 It calibrates itself to atmospheric pressure (sets to 0) when it is turned on and measures gauge pressure (IOP relative to atmospheric pressure).1
“As is evident from this study, increasing atmospheric pressure from 0.803 ATM (1 QATM) to 1 ATM (1.25 QATM) reduces the difference between DCT and GAT tonometry to levels less clinically significant,” the investigators explained.1 “Using GAT at even 0.963 ATM (1.2 QATM, equivalent to an altitude of 315 meters above sea level) causes a significant and increasing disparity with DCT. In other words, there is a real danger of underestimating gauge IOP when using GAT at ever-higher altitudes.”1
“Relying on GAT measurements for multicenter clinical trials and for the serial follow-up of glaucoma patients across practices located at different cities and altitudes might prove an additional bias that needs to be considered,” the researchers advise.1
References
1. Albis-Donado O, Rodríguez-Camacho B, Bhartiya S, et al. Effects of acute atmospheric pressure changes on dynamic contour tonometry and goldmann applanation tonometry in normal individuals: a pilot study. J Glaucoma. 2020;29(9):756-760. doi: 10.1097/IJG.0000000000001592.
2. Kaufmann C, Bachmann L, Thiel M. Comparison of dynamic contour tonometry with goldmann applanation tonometry. Invest Ophthalmol Vis Sci. 2004;45(9):3118-3121. doi: 10.1167/iovs.04-0018.
