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Nikhil R Menon, Jenelle Mallios, Steven A Kane, Steven E Brooks; Factors affecting IOP measurement with the Icare tonometer and Tonopen. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2681. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the effect of corneal location, probe angle, actual intraocular pressure (IOP), corneal edema, corneal epithelium, and soft contact lens, on Icare tonometer and Tonopen measurements.
IOP was controlled in 5 porcine eyes using a large-bore needle placed within the anterior chamber and connected to an adjustable-height water reservoir. IOP was measured at two corneal locations, central or paracentral, using each device over a range of IOPs ranging from 5 to 40 mmHg. Measurements were taken centrally with the Icare using probe incident angles from 0 to 15 degrees. IOP was also measured across a -1.00D or +1.00D soft hydrogel contact lens, before and after the induction of corneal edema by transiently raising the IOP to 72 mmHg, as well as before and after removal of the corneal epithelium.
Central and paracentral measurements were not significantly different from each other using either the Icare (mean±SD 8.2±0.4 v. 8.6±0.8, p=0.39) or Tonopen (mean±SD 11.4±0.8 v. 11.2±0.4, p=0.66). Icare probe angulation did not significantly affect measured IOP (mean±SD 6.0±0.0 v. 5.6±0.4, p=0.37), but resulted in instrument error when the angle exceeded 15 degrees. A +1.00D soft CL caused a small but significant increase in measured IOP by Icare (mean±SD 9.5±0.5 v. 6.5±0.5, p=0.02), but a -1.00D lens did not (mean±SD 6.0±0.0 v. 6.5±0.5, p=0.42). No significant differences were found with contact lenses using Tonopen. Both Icare and Tonopen tended to underestimate the true pressure; however, accuracy and inter-device agreement improved with increasing IOP. Neither corneal edema nor absence of the corneal epithelium significantly affected measurements with either device (p>0.05 in all cases).
In a controlled experimental model system, IOP measured with Tonopen or Icare was not significantly influenced by corneal location, edema, absence of epithelium or, in the case of the Icare, by small deviations in probe angle up to 15 degrees, a particularly important feature in children and uncooperative adults. Measurements may be significantly affected by the presence of a plus power soft contact lens. Both devices tended to underestimate actual IOP, with the Icare tending toward lower values than the Tonopen. These differences decreased as actual IOP increased.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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