Intraocular pressure measurement using ICare rebound tonometer in different positions of eye and different locations on cornea

Intraocular pressure (IOP) is one of the most crucial aspects for diagnosis and treatment plan among patients with glaucoma. Although the gold standard for IOP measurement is Goldmann applanation tonometer (GAT)([1]), it must be mounted to a slit lamp biomicroscope. However, rebound tonometer has become popular due to its ease of operation and portable design, does not require topical anesthesia, and results do not differ significantly from those of GAT([2]). The purpose of this cross-sectional study is to investigate the difference in IOP measurement with iCare IC200 in different angles of the eye and different corneal locations. All participants underwent IOP measurement by GAT twice. Then, IOP was measured with iCare by a single physician. IOP was measured in a straight manner in the upright patient position; then participants were asked to look at fixation targets, which located in four different points. IOP was measured in upgaze, downgaze, medial gaze, and lateral gaze. Then, IOP was measured at 2mm from limbus in superior, inferior, nasal, and temporal cornea. All methods were measured twice, and the mean was used for calculation. The physician who measured IOP by iCare was masked from GAT results. A total of 168 eyes were tested with a mean age of 62.15 +/- 12.34 years. Mean IOP measured by GAT and iCare at the central cornea was 15.53 +/- 5.57 and 14.78 +/- 6.14 mmHg, respectively. The standardized mean difference (SMD) between iCare and GAT was 0.13 (-0.09-0.34), which is insignificant. The average IOP was 0.6, 0.47, 0.91, and 0.44 mmHg lower than the primary position in upgaze, downgaze, medial gaze, and lateral gaze 15degrees angulated positions respectively (p<.01). IOPs at 2mm from limbus in the inferior, nasal, and temporal cornea were 0.5, 0.69, and 0.57 mmHg lower than IOP measured at the central cornea (p=<.01). IOP measurements with iCare in different angles of eye were statistically significantly lower than in the primary position. Similarly, IOPs at different locations on cornea were lower than at the central cornea. However, the difference in IOP measurements with iCare in different angles of the eye and different corneal locations was in the trivial range and might be clinically insignificant.