Accuracy of intraocular pressure measurements in dogs using two different tonometers and plano therapeutic soft contact lenses

Abstract

Objective : To compare and evaluate the accuracy of intraocular pressure (IOP) measured through a therapeutic contact lens, using applanation (TonoPen XL (R)) and rebound (TonoVet (R)) tonometers in enucleated dog eyes. Animals studied : A total of 30 enucleated eyes from 15 beagle dogs. Procedures To measure accurate IOP, the anterior chamber of each enucleated eye was cannulated with two 26-gauge needles and two polyethylene tubes were connected vertically to an adjustable reservoir bag of normal saline and a pressure transducer. IOP was measured by the TonoPen XL (R) followed by the TonoVet (R) without a contact lens. After a contact lens was applied to the cornea, IOP was re-measured in the same order. Three consecutive IOP measurements were performed using both tonometers. Results : Without the contact lens, the IOP values obtained by both tonometers correlated well according to the regression analysis (TonoVet (R) : gamma(2) = 0.98, TonoPen XL (R) : gamma(2) = 0.97, P < 0.001). The TonoPen XL (R) consistently underestimated values as transducer IOP increased; however, IOP values measured with the TonoPen XL (R) were in close agreement and were less variable than those determined with the TonoVet (R) when a contact lens was applied to the cornea. Bland-Altman analysis was used to determine the lower and upper limits of agreement (TonoVet (R) : - 29.7 and + 21.1 mmHg, TonoPen XL (R) : -3.9 and + 3.6 mmHg) between the two devices. Conclusions : This study suggests that the TonoPen XL (R) is a useful tonometer for dogs wearing therapeutic contact lenses, and importantly, contact lenses would not need to be removed prior to IOP measurement.