Accuracy and Precision of Rebound and Applanation Tonometers in Eyes with Focal Corneal Edema

Abstract

Purpose : To determine the accuracy and precision of the TONOVET Plus (TVP) and TonoVet (TV01) (ICare, Finland), and the Tono-Pen Vet (TP) (Reichert, USA) in feline eyes with focal corneal edema.

Methods : Intraocular pressure (IOP) measurements were obtained by a single observer in 7 LTBP2-mutant glaucomatous cats (10 eyes) with focal corneal edema in vivo, using the TVP, TV01 and TP. IOP readings obtained from both clear cornea and regions of focal edema in each eye were compared by paired t-test for each tonometer with P<0.05 considered significant. The anterior chambers of 4 eyes of 4 different cats with focal corneal edema were cannulated ex vivo within 5 hrs post-mortem. Manometric IOP was increased in 5 mmHg increments from 5-40 mmHg, then in 10 mmHg increments to 70 mmHg by raising a fluid reservoir. Triplicate IOP values obtained from both normal cornea and regions of focal edema using the TVP, TV01, and TP were compared to manometrically set IOP and to each other via linear regression and one-way ANOVA.

Results : In vivo rebound tonometry values were 4.0 (mean) ± 5.5 (SD) mmHg lower in regions of edema than values from adjacent clear cornea. In contrast, TP readings in vivo were 13.0 (mean) ± 5.9 (SD) mmHg higher in focal regions of edema than in clear cornea (p=0.0075). In ex vivo studies, accuracy and precision were acceptable for TVP and TV01 readings obtained from the clear cornea, but were negatively impacted by corneal edema, which worsened underestimation of IOP. The TP grossly underestimated manometric IOP in cats, whether readings were taken from clear or edematous cornea, and clear cornea TP readings were significantly less accurate than those taken with the TVP and TV01 (p=0.03). All 3 tonometers were less precise when readings were obtained from edematous cornea, and TP readings from edematous cornea were significantly less precise than TV01 (p=0.0419).

Conclusions : Rebound tonometry values are impacted by corneal edema to a lesser extent than applanation tonometry values in feline eyes. However, IOP values obtained from areas of focal edema with the TVP and TV01 tend to be substantially lower and less accurate and precise compared to readings from clear cornea; thus, in cases of corneal edema, IOP measurements should be obtained from clear corneal regions when possible.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.