Intraocular Pressure and Photorefractive Keratectomy: A... : Cornea (original) (raw)
Glaucoma is the second leading cause of blindness in the United States. Risk factors for glaucoma include age, race, elevated intraocular pressure (IOP), myopia, family history, 1 and vascular factors. 2 Excimer laser photorefractive keratectomy (PRK) is a safe, effective, and popular procedure for correcting myopia. 3,4 Recently, IOP reduction has been reported in eyes, that have undergone PRK. 5–7 This procedure reduces both central corneal thickness (CCT) and the curvature of the cornea. Noninvasive studies have found a relationship between the corneal thickness and Goldmann applanation tonometer (GAT) readings. 8–11 When the cornea is thicker, the GAT reading is higher. When the cornea is thinner, the GAT reading is lower. Invasive manometric studies of applanation tonometry in vivo have shown that corneal thickness can have a major influence on the accuracy of GAT. 12–14 Also, corneal curvature has been reported to influence tonometer accuracy. 15–17 Lower corneal curvature produces lower GAT IOP measurements and higher curvature produces higher GAT IOP measurements.
This lowering of IOP may delay the diagnosis of future glaucoma patients and underestimate the actual IOP during the follow-up phase of glaucoma patients, resulting in deterioration of the visual function. This study was designed as a prospective case series to compare four different kinds of measurements of three different instruments and to find the most valid method for the evaluation of the IOP after PRK.
MATERIALS AND METHODS
In this prospective study, we collected data from 149 consecutive eyes of 97 patients; 54 were women and 43 were men. Patients were aged 31.16 ± 8.24 ranging 20–54 years. All PRK surgeries were done by one surgeon (H.J.G.) between November 1995 and June 1997. No Institutional Review Board approval was required for this study.
Preoperative measurements were taken as baseline values. Both pre-and postoperative measurements were taken between 3 and 6 PM by a single ophthalmic technician. Preoperative data included visual acuities, refraction, and IOP measurements from the central part of the cornea with GAT, noncontact tonometer, and Tono-Pen XL (Mentor, Norwell, MA, U.S.A.). IOP was also measured from the temporal peripheral part of the cornea 1.5–2.0 mm from the limbus with Tono-Pen. CCT measurements with ultrasonic pachymetry and keratometric readings were also performed. The same measurements were done after surgery.
Data from patients with glaucoma or with known familiar history of glaucoma, patients receiving IOP lowering medications before or after PRK, patients with follow-up of <12 months, and steroid responders under local corticosteroid treatment with postsurgical IOP increases >7 mmHg were excluded.
The mean preoperative spherical equivalent error was −6.07 ± 3.58 diopters (D) (range, −1.00 to −17 D), the mean preoperative CCT was 537.59 ± 6 μm (range, 514–552 μm). The mean preoperative keratometry was 43.95 ± 1.43 D.
Refractive surgery was performed with Schwind II keratome 193-nm ArF excimer Laser (Schwind Eye Tech-Solutions, Kleinostheim, Germany). The ablation zone was 6 mm in diameter with a single zone in myopia ≤6 D and multiple zones in cases of higher myopia. Postoperatively, all patients received 0.1% fluorometholone, four drops a day for the first month, gradually tapering for the next 2–3 months depending on the healing response of the patient.
The results were evaluated statistically with a Student paired t test and linear regression analysis. Differences were considered significant when p < 0.05. We were aware in the data analysis that two eyes from the same person were analyzed. Our method of calculating the data compared preoperative versus postoperative values in ten individual eyes.
RESULTS
Twelve-month postoperative measurements were compared to baseline values. Twelve-month data were used because we found that after 12 months, the healing process was completed and the IOP and refraction had stabilized. Results at 18 months were not completed in all 149 eyes.
The mean preoperative and postoperative refractions, CCT and keratometry, are reported in Table 1. Refraction, CCT, and keratometric measures were all reduced significantly (p < 0.05 for each).
Mean preoperative and postoperative refractions
Mean preoperative and postoperative IOP measurements are reported in Table 2. Each method of pressure measurement reported a significant reduction in IOP (p < 0.05); however, GAT reported a more prominent reduction in pressure than the peripheral Tono-Pen measurement (p < 0.0001). Goldman indicated a reduction of 10.52 ± 13.64%, whereas Tono-Pen indicated a drop of only 1.47 ± 7.61% (Fig. 1).
Mean preoperative and postoperative IOP measurements
Cell bar chart. Error bars, ±1 SE. There was a significant difference between the reduction in IOP as measured by GAT and Tono-Pen (p < 0.0001).
We found a significant correlation between baseline myopia and postoperative reduction of IOP with GAT (r2 = 0.503, p < 0.0001) (Fig. 2). We found a statistically significant correlation between percent change in each of the IOP measurement methods and the percent change in CCT and keratometry. The results were less prominent in the peripheral Tono-Pen measured group (Table 3).
Percent reduction in IOP
There is a significant correlation between the baseline myopia and the reduction in IOP as measured by GAT.
There was a 4% reduction in CCT without a change in GAT. However, there is a 1:1 reduction in GAT for CCT drops >4%. In other words, a 24% drop in CCT will produce a 20% drop in GAT.
Correlating the reduction in refraction with the change in IOP measurements, there were statistically significant results in each of the three central corneal measurements of IOP (p < 0.05) but not in the peripheral measurements by Tono-Pen (p = 0.63) (Table 3).
Preoperative values of IOP taken by each of the measurement techniques correlated with high significance (p < 0.0001), with the postoperative IOP values taken by Tono-pen in the periphery of the cornea (Fig. 3).
The best correlation of a postoperative IOP measurements with preoperative GAT was the peripheral Tono-Pen measurement.
DISCUSSION
Glaucoma is still regarded as one of the leading causes of blindness world wide. IOP is considered the most important risk factor in primary open angle glaucoma. 1 After PRK for myopia, there is a measured reduction of IOP. 5–7,18–20 This may delay the diagnosis of glaucoma and underestimate the IOP during the follow-up of glaucoma patients, which may result in deterioration of visual function.
In our study, we found a correlation between treated myopia and the lowering of IOP, as was reported in other studies (Fig. 2). 4–6 This reflects an error introduced into the process of measurements by changes in the corneal thickness, corneal curvature, and corneal structure.
This study was limited in several ways. The measurements in this study were performed in accordance with clinical protocols and, therefore, are not randomized. Furthermore, the noncontact tonometry was performed first, without topical anesthesia. The other measurements were performed after instillation of a topical anesthetic. There was a potential for the anesthetic to slightly alter IOP in the contact measurements.
We decided to compare the preoperative results with postoperative results at 12 months and not earlier, considering the fact that the healing process and use of topical steroids may affect ocular rigidity and may have some influence on the parameters obtained. Munger et al. 18 found evidence in their data that confirmed this process of nonstability during the first postoperative 6 months.
The GAT is a variable force applanation tonometer based on the Imbert-Fick Law that states that the pressure within a sphere (p) is roughly equal to the external force (f) needed to flatten a portion of the sphere divided by the area (a) of the sphere, which is flattened. p = f/a. This law applies to surfaces that are perfectly spherical, dry, flexible elastic, and infinitely thin. Because the cornea does not satisfy all these requirements this formula was modified to p = f / a + M − N, with (M) representing the attractive force of surface tension of the tears and (N) representing the modulus of elasticity of the cornea. Goldmann designed his applanation tonometer with a tip diameter of 3.06 mm because, at that diameter, the opposing forces of corneal elasticity and surface tension tend to cancel each other out and the force of 1 g applied to the tonometer head area corresponds to an IOP of 1 mmHg. The thickness of the cornea has been shown to influence the pressure measurement. A thin cornea results in falsely low readings and a thick cornea results in falsely high readings. 8–14 The IOP is also influenced by corneal curvature. 17 Our results, as others, 5–7,18 show lowering in IOP measured by GAT after PRK. We believe that these readings underestimate the IOP and are influenced by thinning of the central cornea where the measurement is taken and by changes in the central corneal curvature (Table 3). There was a 4% reduction in CCT without a change in GAT. However, for CCT drops >4%, there is a 1:1 reduction in GAT. In other words, a 24% drop in CCT will produce a 20% drop in GAT.
The noncontact tonometer is a constant force applanation tonometer that deforms the corneal apex by means of a jet of air. 21 The force of the air jet, which is generated by a solenoid-activated piston, increases linearly over time. An optoelectronic applanation monitoring system estimates the flattening of the central cornea and gives reading of the IOP. The noncontact tonometer is accurate when the IOP is near normal, but its accuracy is diminished with higher pressures and in eyes with poor fixation. 21,22 With noncontact tonometer, we found lowering of the IOP readings after PRK similar to that found with GAT. Catterjee et al. 5 used noncontact tonometer in their study and obtained similar results.
Tono-Pen is a portable variable force applanation tonometer that uses the principle of the MacKay-Marg tonometer. Its tip has a strain gauge that is activated when it touches the cornea. The built-in microprocessor logic circuit senses a trough force and registers that until an acceptable measurement is achieved. Four to ten acceptable measurements are averaged to give a final IOP that is displayed as a digital readout. There is good correlation between pressure measurements taken with Tono-Pen compared with manometric readings in human autopsy eyes. 23 In a study by Kao et al., 24 Tono-Pen was found to be accurate when compared with GAT. Also, multiple noncentral corneal readings provided useful approximation of IOP in eye bank eyes. 25 Our central measurements with Tono-Pen pre-and post-PRK were similar to the measurements taken by GAT and noncontact tonometer, but measurements taken over the temporal periphery of the cornea 1.5–2 mm from the corneoscleral limbus show only a minimal reduction in IOP (Fig. 1). These measurements were taken over a thick cornea where Bowman's layer still exists. Levy et al. 20 recently reported their findings concluding that the Tono-Pen measurements appeared to be less affected than the GAT measurements after PRK.
Abbasoglu et al. 19 found reliable measurements of IOP in the periphery of the cornea using a GAT and a pneumotonometer (which is also an applanation tonometer that uses the principle of the MacKay-Marg tonometer), although their follow-up was only 3 months and the IOP was measured under local cortisone treatment during the corneal healing process. They recorded the measurements in adduction, a variant that can elevate IOP. 25–27 Thus, peripheral measurements were shown to be more reliable after PRK. 19
In our study, we avoided lateral gaze, considering the reports in the literature that IOP measurements may be affected by eccentric gaze during readings. 25,26 All procedures for IOP determination were performed in primary straight gaze.
In summary, we found significant reduction in IOP after PRK when measurements were taken over the central ablated thin and more flat cornea. Contributing factors to IOP changes include corneal curvature, thickness, and absence of the Bowman's layer. We detected a correlation between IOP readings and CCT and between IOP readings and keratometric values. The influence of the absence of Bowman's layer after PRK is not yet clear. The same measurements performed pre-and post-laser in situ keratomileusis and compared to our results may quantify the contribution of the Bowman's layer to changes in IOP readings after refractive surgery. We found that peripheral measurements by Tono-Pen are the most accurate and correlate best to the preoperative measurements (Fig. 1, Tables 2 and 3). We conclude that follow-up of IOP in PRK-operated patients is more accurate by Tono-Pen peripheral corneal measurements.
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Keywords:
Excimer laser; Photorefractive keratectomy; Intraocular pressure; Tenometry
© 2001 Lippincott Williams & Wilkins, Inc.