Does the order of intraocular pressure measurement affect tonometry results?

Purpose: To determine whether the order of intraocular pressure (IOP) measurement affects readings, regardless of which eye is measured first. Methods: Intraocular pressure was measured in 31 and 41 dogs using applanation and rebound tonometry, respectively. Initially, IOP was measured in the first (randomly chosen) eye (reading A), followed by measurement in the fellow eye (reading B), and a third (repeated) measurement in the first eye (reading C). After 15 minutes, measurements were repeated in reverse order (readings D - F). Results: Applanation tonometry revealed significant differences between readings A & B (15.6 ± 2.3 and 14.8 ± 2.7 mm Hg, respectively, p =.02), A & C (15.6 ± 2.3 and 14.5 ± 2.4 mm Hg, respectively, p =.002), D & E (14.5 ± 2.3 and 13.7 ± 2.1 mm Hg, respectively, p =.02), D & F (14.5 ± 2.3 and 13.9 ± 1.9 mm Hg, respectively, p =.05), and A & E (15.6 ± 2.3 and 13.7 ± 2.1 mm Hg, respectively, p =.001). Rebound tonometry yielded similar results, with additional differences between B & C (19.1 ± 3.0 and 18.2 ± 2.4 mm Hg, respectively, p =.002) and E & F (18.7 ± 3.3 and 18.2 ± 3.3 mm Hg, respectively, p =.02). Conclusions: Intraocular pressure measured in the first eye, whether right or left, is higher than in the fellow eye. Repeated tonometry in the same visit could result in a significant IOP decrease, though the magnitude may not be clinically appreciable.