Evaluation of intraocular pressure according to corneal thickness before and after excimer laser corneal ablation for myopia

Abstract

Intraocular pressure is affected by corneal thickness and biomechanics. Following ablative corneal refractive surgery, corneal structural changes occur. The purpose of the study is to determine the relationship between the mean central corneal thickness (CCT) and the change in intraocular pressure measurements following various corneal ablation techniques, using different measurement methods. Two hundred myopic eyes undergoing laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) were enrolled into a prospective, non-randomized study. Corneal parameters examined included full ocular examination, measurement of CCT, corneal topography, corneal curvature and ocular refractivity. Intraocular pressure measurements were obtained using three different instruments-non-contact tonometer, Goldmann applanation tonometer and TonoPen XL (TonoPen-Central and TonoPen-Peripheral). All measurements were performed pre-operatively and 4 months post-operatively. Post-operative intraocular pressure was significantly lower than pre-operative values, with all instruments (p value < 0.001, Student's t-test). The post-operative intraocular pressure decrease was smallest using the Tonopen-XL compared to the Goldmann applanation tonometer and non-contact tonometer (p value < 0.001, ANOVA). Intraocular pressure readings are significantly reduced following corneal ablation surgery. We determined in our myopic patient cohort that the TonoPen XL intraocular pressure measurement method is the least affected following PRK and LASIK as compared to other techniques.

Comparison of three methods of tonometry in normal subjects: Goldmann applanation tonometer, non-contact airpuff tonometer, and Tono-Pen XL

“…Insufficient fluorescein use causes false-low readings, and excessive fluorescein use causes false-high readings. The presence of corneal epithelial defects and previous corneal surgeries also complicate measurements 11. Finally, there is a contamination risk, so the tool tip must be cleaned after each use 12…”

<p>A Review on Different Tonometers for Intraocular Pressure Measurement After Photorefractive Keratectomy or Small Incision Lenticule Extraction</p>

“…31 A significant reduction in keratometry of 5.56 D was also reported (P<0.05). 12 Another study by Zare demonstrated a significant reduction in corneal resistance factor and corneal hysteresis in post PRK eyes when compared with their preoperative baseline. 37 The study also compared the effect of PRK with or without adjunctive mitomycin C on postoperative IOP readings and corneal biomechanical properties, which were measured by Goldmann applanation tonometry and ocular response analyzer.…”

“…11,12,31,36,37,39 Study by Hamed Azzam has demonstrated a significant reduction of mean CCT by 75.5μm in post PRK eyes, when compared with the mean CCT before operation. 12 Munger showed that the measured change in CCT is found to be dependent on the amount of spherical correction during PRK in hyperopic eyes, but not on the cylindrical correction. 31 Six months after PRK, every diopter of applied spherical correction will result in 5.0± 0.9μm of corneal thinning.…”

“…A total of 15 studies have demonstrated a significant reduction in the measured IOP by Goldmann applanation tonometer in post PRK eyes when compared with the baseline before operation. 9,12,13,15,16,20,24,[29][30][31][32]35,38,39,50 Aristeidou demonstrated a fall in measured IOP value by Goldmann applanation tonometer before and after PRK. The preoperative measurement of IOP was 12.9± 2.8 mmHg, which was then reduced to 11.4 ±2.9mmHg after one month, 11.1 ±2.3 mmHg after three months, 11.1 ±2.5 mmHg after 6 months and 11.0 ±2.8 after 12 months.…”

Author's reply

“…As they mentioned, there are limitations associated with intraocular pressure (IOP) measurements after refractive surgery. [ 1 2 ] Therefore, no acceptable definition is available for steroid-induced ocular hypertension after refractive surgery, and various authors have employed different criteria to ascertain this. For instance, Busool et al classified patients as steroid responders if they had an IOP elevation of at least 25% while on topical steroid treatment (minimum of 28 mmHg) followed by an IOP drop of at least 25% when steroid treatment was discontinued.…”