New applanation tonometer for myopic patients after laser refractive surgery

Abstract

this study assesses the agreement between intraocular pressure (iop) measurements taken with the Goldmann applanation tonometer (GAt) and a new experimental applanation tonometer with a convexly shaped apex (CT) after laser myopic refractive surgery. Two different CT radii (CT1 and CT2) were designed with a finite element analyser, and a prospective double masked study on 102 eyes from 102 patients was carried out. A Bland-Altman plot and intra-class correlation coefficient (ICC) were calculated to assess the agreement between GAT measurements and the measurements of both CT1 and CT2 before and after myopic laser assisted in situ keratomileusis (LASiK; n = 73) and photorefractive keratectomy (pRK; n = 29). We evaluated a subset of two subgroups (n = 36 each) for intra and interobserver (iA/ie) error. from the whole cohort, the best iop agreement was observed between GAtpre and CT1post surgery: 16.09 ± 2.92 vs 16.42 ± 2.87 (p < 0.001); ICC = 0.675 (95% CI: 0.554-0.768). In the analysis of LASiK vs PRK, GATpre and CT1post showed the highest agreement, although LASIK measurements were more accurate than pRK, as the icc = 0.718 (95% CI: 0.594-0.812) and ICC = 0.578 (95% CI: 0.182-0.795) respectively. Excellent agreement was observed for IA/IE, and there was an icc > 0.8 (95% CI) in all cases. CT1 proved more accurate in the LASIK subgroup. In conclusion, our new version of GAt could be used with post-surgery LASiK patients as a more accurate measurement device compared to the current reference tonometer. The Goldmann applanation tonometer (GAT; Haag-Streit, Switzerland) is the current reference tonometer for measuring intraocular pressure (IOP) in daily clinical practice due to its acceptably accurate measurements, reproducibility and reliability 1-4. However, GAT readings are influenced by corneal central thickness (CCT) and corneal biomechanics (CB), which vary widely among normal individuals 1,5,6. In myopic laser refractive surgery (LRS) patients, variations induced in both CCT and CB lead to differences in GAT readings. This change in IOP has been inconsistent in previous studies regardless of the type of surgery: Mardelli et al. 7 found a significant reduction of 1 ± 2.8 mmHg in IOP measurements after ablating 23 μm in photorefractive keratectomy (PRK) patients; while Duch et al. 8 found an underestimation of about 2.9 mmHg per 70 μm ablation in CCT after laser assisted in situ keratomileusis (LASIK). These IOP underestimations affect the reliability of GAT as gold standard after LASIK and PRK procedures 9-11. LASIK is one of the most frequent types of eye surgery and a total of 40 million people had this operation worldwide between 1991 and 2016 12. Moreover, the prevalence of glaucoma is 2 to 4 times higher among the myopic population 13. Thus, it is very important to obtain an accurate and real estimation of IOP in order not to miss ocular hypertension after myopic LRS. The aim of our study is to describe a new modified GAT as well as its translational application and clinical outcomes by evaluating IOP meas...

Reliability of Intraocular Pressure Measurement by Goldmann Applanation Tonometry After Refractive Surgery: A Review [Response To Letter]

“…They complained that we forgot to mention the very interesting paper on the convex tonometer they published in 2020. 2 …”

“…1 They complained that we forgot to mention the very interesting paper on the convex tonometer they published in 2020. 2 Unfortunately, we could not include it in our review because, as they stated in the letter, their paper was published in April 2020, and our article was submitted in the same period, so we could not be aware of their publication.However we are very glad to know that they described a device that is able to overcome the problem of measuring the IOP after CRS, because the problems raised by the CRS in several eye measurements, including IOP measurements and IOL power calculation, have been one of our main interests in the last years. [3][4][5][6] Concerning the paper by McCafferty et al 7 we agree, it was our mistake.…”

“…1 They complained that we forgot to mention the very interesting paper on the convex tonometer they published in 2020. 2 Unfortunately, we could not include it in our review because, as they stated in the letter, their paper was published in April 2020, and our article was submitted in the same period, so we could not be aware of their publication.…”

Reliability of Intraocular Pressure Measurement by Goldmann Applanation Tonometry After Refractive Surgery: A Review of Different Correction Formulas [Letter]

“…We have developed a new applanation tonometer that does not underestimate IOP after surgery: the Convex Tonometer (CT). 2 We studied n=102 myopic laser refractive surgery patients (29 PRK, 73 LASIK) and CT had a strong correlation in the LASIK subgroup with GAT before surgery. Such outcome correlated as well with our finite element analysis, in which we found a lower resistance from the center of the cornea that encompassed a wider amount of contact between the operated cornea and GAT.…”