Comparison of Rebound and Applanation Tonometry in Eyes With Focal Corneal Edema.

Objective: To determine the effect of corneal edema on intraocular pressure (IOP) readings and their accuracy and precision when obtained with handheld rebound and applanation tonometers.

Animals and procedures: IOP readings from areas of focal corneal edema were compared to those from clear cornea in 10 eyes of 7 glaucomatous cats in vivo using TONOVET Plus (TVP) and TONOVET (TV01) rebound, and Tono-Pen Vet (TP) applanation tonometers. Four different feline eyes with focal corneal edema were cannulated ex vivo and tonometry readings obtained from clear and edematous cornea at set manometric IOPs from 5 to 70 mmHg.

Results: Rebound tonometry values were 4.0 ± 5.5 (SD) mmHg lower, and TP values were 13.0 ± 5.9 (SD) mmHg higher in regions of edema versus clear cornea in the same eye (p = 0.0075). Relative to manometry, TVP and TV01 accuracy and precision were acceptable for clear cornea readings but were negatively impacted by corneal edema. TP-derived readings grossly underestimated high IOPs relative to manometry in both clear and edematous feline cornea and were significantly less accurate than rebound tonometers in clear cornea (p = 0.03). All tonometers were less precise in edematous cornea; the TP was significantly less precise than the TV01 (p = 0.0419).

Conclusions: Although IOP values obtained with the TVP and TV01 tend to be less accurate and precise in edematous versus clear cornea, rebound tonometry is impacted by corneal edema to a lesser degree than applanation tonometry. In patients with corneal edema, IOP measurements should be obtained from clear corneal regions when possible.