Measurements of IOP made with commonly used tonometers before and after myopic photorefractive keratectomy (PRK) were analyzed to compare the accuracy of these tools.
Study design
This was a retrospective comparative study of 145 eyes of 145 patients in Italy who underwent myopic PRK for refractive errors ranging from −0.50 to −10.25 D. Corneal tomography and IOP measurements with ocular response analyzer (ORA), Corvis ST (CST), rebound tonometry (RT), dynamic contour tonometry (DCT), and Goldmann applanation tonometry (GAT) were performed, in that fixed order, preoperatively and again postoperatively at months 1, 3, and 6. All visits occurred between 2:00 and 4:00 p.m. Correlation analyses were run between changes in IOP and corneal, morphologic, and biomechanical parameters measured after PRK.
Outcomes
After 6 months of follow-up, all devices indicated statistically significant IOP underestimations: −9.6% for RT, −9.7% for ORA, −10.1% for DCT, −11.6% for CST, and −14.1% for GAT. Surprisingly, there were no significant correlations between IOP reduction and variations of corneal morphologic features such as corneal thickness, corneal curvature, or amount of refractive error treated.
Limitations
The IOP measurements with the various instruments were performed in the same order for every patient, with GAT being performed last. This may have overestimated the degree of error of GAT in the post-PRK patient due to increases in aqueous outflow that occur when pressure is applied to the cornea with each subsequent IOP check. The tonopen and pneumatonometer also are common IOP measurement tools but were omitted from this study.
Clinical significance
While GAT has been considered by many to be the gold standard for measuring IOP, this study demonstrates that GAT may be more prone to underestimating IOP than other tonometers in the setting of prior myopic PRK. It may be wise to consider alternative tonometry tools in post-myopic PRK patients, or to at least keep in mind that since all tonometry instruments underestimate IOP to some degree in this patient population, IOP targets should be set accordingly.