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Changes in the shape of the anterior and posterior corneal surfaces caused by mydriasis and miosis
Changes in the shape of the anterior and posterior corneal surfaces caused by mydriasis and miosis Detailed analysis Kahori Saitoh, MD, Kenji Yoshida, MD, Yasuhiro Hamatsu, MD, Yutaka Tazawa, MD Purpose: To evaluate changes in the anterior and posterior corneal shape, corneal thickness, and anterior chamber depth (ACD) caused by mydriasis or miosis using scanning-slit corneal topography. Setting: Department of Ophthalmology, Iwate Medical University School of Medicine, Iwate, Morioka, Japan. Methods: Twenty-eight eyes of 28 healthy volunteers with refractive errors of ⫺6.00 to ⫹0.25 diopters were studied. One eye of each subject had instillation of tropicamide–phenylephrine hydrochloride (Mydrin P威) to obtain mydriasis and of pilocarpine hydrochloride 2% (Sanpilo威) to obtain miosis. To assess the corneal shape, the best-fit sphere (BFS), axial power, and tangential power were measured for the anterior and posterior corneal surfaces before and after mydriasis and before and after miosis using scanning-slit corneal topography (Orbscan version 3.0, Orbtek, Inc.). The pupil size, corneal thickness, and ACD were also examined before and after mydriasis and before and after miosis. Results: The mean age of the patients was 31.1 years ⫾ 5.6 (SD) (range 20 to 46 years). The anterior BFS changed from a mean of 8.04 ⫾ 0.3 mm at the time of mydriasis to a mean of 8.00 ⫾ 0.3 mm at the time of miosis. The posterior BFS changed from 6.53 ⫾ 0.3 mm to 6.46 ⫾ 0.3 mm, respectively. Thus, the anterior and posterior cornea became significantly steeper after miosis (P⬍.01). The ACD was significantly more shallow after miosis than after mydriasis. However, there was no significant difference in corneal thickness after mydriasis or miosis. Conclusions: The anterior and posterior corneal shapes changed as a result of mydriasis and miosis, and the refractive power of the cornea significantly increased after miosis. To date, changes in refractive power from changes in pupil size have been attributed to a change in the refractive power of the crystalline lens; however, it is now thought that changes in corneal refractive power also occur. J Cataract Refract Surg 2004; 30:1024–1030 2004 ASCRS and ESCRS
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n the past, corneal shape has been evaluated by analyzing the anterior surface of the cornea, using a photokeratoscope or videokeratoscope in most cases. With the development and popularity of refractive surgery, it has become important to also evaluate the shape of
Accepted for publication October 29, 2003. Reprint requests to Kahori Saitoh, MD, Department of Ophthalmology, Iwate Medical University School of Medicine, 19-1 Uchimaru, Morioka, Iwate 020-8505, Japan. 2004 ASCRS and ESCRS Published by Elsevier Inc.
the posterior surface of the cornea. The use of scanningslit corneal topography has enabled the simultaneous measurement of the anterior and posterior surfaces of the cornea as well as the corneal thickness, anterior chamber depth (ACD), and pupil size. Using scanningslit corneal topography, studies1,2 found changes in the shape of the anterior and posterior surfaces of the cornea before and after keratorefractive surgery such as laser in situ keratomileusis (LASIK). In contrast is the theory that the corneal shape or axial length does not change during near vision and 0886-3350/04/$–see front matter doi:10.1016/j.jcrs.2003.10.040
CORNEAL SURFACE CHANGES INDUCED BY MYDRIASIS AND MIOSIS
that the increase in refractive power is caused by a change in the shape of the crystalline lens.3 A study using the LeGrand eye model4 found no changes in the radius of corneal curvature during accommodation because there were no changes in corneal refractive power between the time of relaxation of accommodation and the time of maximum accommodation. However, a study using a keratometer reports that accommodation affects corneal refractive power during near and far vision.5 Using scanning-slit corneal topography, we examined how the shape of the anterior and posterior surfaces of the cornea, corneal refractive power, pupil size, corneal thickness, and ACD alter after mydriasis and after miosis.
Subjects and Methods The procedures used in this study conformed to the tenets of the Declaration of Helsinki. After they received an explanation of the procedures, all participants provided informed consent. Each subject chose which eye was to be measured. There were no abnormalities in the anterior segment, optic media, or ocular fundus in the tested eyes as observed by slitlamp microscopy and indirect ophthalmoscopy. The best corrected visual acuity was 1.0 or better in all patients, and the mean refractive error was ⫺2.00 diopters (D) ⫾ 1.65 (SD) (range ⫺6.00 to ⫹0.25 D). Individuals with a history of contact lens use were not included. The subjects had no systemic diseases such as diabetes mellitus. Scanning-slit corneal topography (Orbscan version 3.0, Orbtek, Inc.) was used to assess the corneal shape. The measurement was performed by 1 examiner (K.S.) in a dark room (15 lux) at 3 PM and 5 PM. With Orbscan topography, the optical acquisition head scans the eye using vertical light slits that are projected onto the cornea at a 45-degree angle. A total of 40 slits were projected onto the eye. As information on 240 points was contained in each single slit image, 9600 points in 40 images on the entire cornea were measured. Based on data measured at each point, a 3-dimensional shape was constructed from the anterior surface of the cornea to the anterior surface of the lens. The shape of the anterior and posterior surfaces of the cornea, the corneal thickness, and the ACD were analyzed. The values before mydriasis were measured using Orbscan. A parasympatholytic agent of tropicamide–phenylephrine hydrochloride (Mydrin P威) was instilled in the eye. Maximum mydriasis occurred after about 30 minutes, and the measurement during mydriasis was performed again as outlined. After 3 days, the values before miosis in the same eye were measured in a similar manner. Next, a parasympa-
Figure 1. (Saitoh) The point at which the perpendicular line from the tangential line running through the single point A on the anterior or posterior surfaces of the cornea meets the central axis (optic axis) was regarded as B. The distance from A to B was the radius of curvature of this point.
thomimetic agent of pilocarpine hydrochloride 2% (Sanpilo威) was instilled in the eye 2 times 15 minutes apart to induce miosis. After miosis of less than 3.0 mm in diameter and disappearance of light reflex were confirmed, the measurements were performed during miosis in a similar manner. The following were analyzed to assess the changes induced by mydriasis and miosis.
Pupil Size Numerical values displayed on the instrument were used to examine pupil size.
Best-Fit Sphere The anterior and posterior best-fit sphere (BFS) values for the anterior and posterior surfaces of the cornea were used to examine the changes in corneal shape caused by mydriasis and miosis. The BFS was defined as the radius of curvature (mm) of the sphere constructed based on data input by Orbscan measurements to best fit the anterior or posterior surface of the cornea in the central 10.0 mm diameter zone.
Axial Power The point at which the perpendicular line from the tangential line running through the single point A on the anterior or posterior surface of the cornea meets the central axis (optic axis) was regarded as B (Figure 1). The distance from A to B was the radius of curvature of this point. The anterior or posterior axial power value was obtained by converting the radius of curvature (mm) into a diopter using the physiologic refractive index (1.376). The keratometric axial power was obtained by converting the radius of curvature into a diopter using the corneal conversion refractive index (1.3375). Each value for the anterior and posterior axial powers and keratometric axial power was obtained in the central 5.0 mm zone of the cornea.
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Table 1. Pupil size changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Time
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test
Figure 2. (Saitoh) The radius of the arc (AB) at a single point A on the anterior or posterior surface of the cornea acted as the radius of curvature at this point.
Tangential Power The radius of the arc (AB) at a single point A on the anterior or posterior surface of the cornea acted as the radius of curvature at this point (Figure 2). The anterior or posterior tangential power value was obtained by converting this radius of curvature (mm) into a diopter using the physiologic refractive index (1.376). The keratometric tangential power value was obtained by converting the radius of curvature into a diopter using the corneal conversion refractive index (1.3375). The anterior and posterior tangential powers and keratometric tangential power were obtained within the central 5.0 mm diameter range.
Corneal Thickness Corneal thickness was calculated using the mean value of the points measured within a circular zone of 3.0 mm, 5.0 mm, or 7.0 mm in diameter from the center of each eye. Studies comparing Orbscan with ultrasonic pachymetry found that Orbscan measurements of corneal thickness were greater than ultrasonic pachymetry measurements.6,7 Therefore, the values obtained by multiplying 0.92 and Orbscan measurements are commonly used; this was the method used in this study.
Anterior Chamber Depth For the ACD, the depth from the posterior surface of the cornea to the anterior surface of the lens in the center of the cornea from the Orbscan topography was used.
Level of Significance The Wilcoxon signed rank test was used to evaluate the significance of the results. A P value less than 0.05 was considered statistically significant.
Results Twenty-eight eyes (19 right, 9 left) of 28 normal healthy volunteers (21 men, 7 women) were studied. The mean age was 31.1 ⫾ 5.6 years (range 20 to 46 years). 1026
Pupil Size The mean pupil size in all eyes before mydriasis was 4.6 ⫾ 0.6 mm. The mean pupil size in eyes with mydriasis was significantly larger, 7.8 ⫾ 0.5 mm (P⬍.01). After miosis, the mean pupil size in all eyes decreased significantly, from 4.5 ⫾ 0.8 mm to 2.2 ⫾ 0.4 mm (P⬍.01) (Table 1). Best-Fit Sphere After mydriasis, the mean anterior BFS values increased significantly, from 8.02 ⫾ 0.3 mm to 8.04 ⫾ 0.3 mm (P⬍.05). The mean posterior BFS value also increased significantly, from 6.50 ⫾ 0.3 mm to 6.53 ⫾ 0.3 mm (P⬍.01). Thus, the shape of the anterior and posterior surfaces of the cornea became significantly flatter as a result of mydriasis. After miosis, the anterior BFS value decreased in 17 eyes and remained unchanged or increased in 11 eyes. The mean value in all eyes decreased from 8.02 ⫾ 0.3 mm to 8.00 ⫾ 0.3 mm after miosis; however, the change was not significant. The mean posterior BFS value decreased significantly, from 6.50 ⫾ 0.3 mm to 6.46 ⫾ 0.3 mm after miosis (P⬍.001) (Table 2). Thus, the shape of the posterior surface of the cornea became significantly steeper as a result of miosis. The anterior BFS value was lower during miosis than during mydriasis in 20 of the 28 eyes; in 8 eyes, the value was larger during miosis than mydriasis (Figure 3, A). In all 28 eyes, the mean anterior BFS was 8.04 ⫾ 0.3 mm at the time of mydriasis and 8.00 ⫾ 0.3 mm at miosis; it was significantly lower during miosis than during mydriasis (P⬍.01). The posterior BFS value was lower at the time of miosis than mydriasis in 26 eyes; in 2 eyes, it was larger during miosis than mydriasis (Figure 3, B). The mean value in all eyes was significantly lower during miosis (6.46 ⫾ 0.3 mm) than during mydriasis (6.53 ⫾ 0.3 mm) (P⬍.01). Thus, the corneal shape became significantly steeper for both the
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Table 2. Best-fit sphere changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Mydriasis Time
Anterior
Miosis Posterior
Anterior
Posterior
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
anterior and posterior surfaces during miosis than during mydriasis. Axial Power The mean anterior axial power did not significantly change from before to after mydriasis or miosis. The mean posterior axial power decreased from ⫺6.57 ⫾
Figure 3. (Saitoh) Changes in anterior BFS (A) and posterior BFS (B) caused by mydriasis and miosis (black circles ⫽ eyes in which BFS was smaller in mydriasis than in miosis; diamonds ⫽ eyes in which BFS was larger in miosis than in mydriasis).
0.27 D before mydriasis to ⫺6.48 ⫾ 0.28 D after mydriasis; thus, the minus power decreased. This change was statistically significant (P⬍.01). The change in posterior axial power caused by miosis showed that the mean minus power significantly increased (P⬍.05), from ⫺6.57 ⫾ 0.27 D before miosis to ⫺6.62 ⫾ 0.27 D after miosis (Table 3). The mean keratometric axial power did not significantly change after mydriasis or miosis. The mean anterior axial power was 47.86 ⫾ 1.93 D during mydriasis and 48.09 ⫾ 1.74 D during miosis. The plus power significantly increased (P⬍.05) during miosis. The mean posterior axial power was ⫺6.48 ⫾ 0.28 D during mydriasis and ⫺6.62 ⫾ 0.27 D during miosis. Thus, the minus power significantly increased during miosis (P⬍.01). The mean keratometric axial power was 43.19 ⫾ 1.81 D at the time of mydriasis and 43.54 ⫾ 1.82 D at the time of miosis. Thus, the plus power increased significantly at the time of miosis (P⬍.01). Tangential Power The mean anterior tangential power did not change significantly from before mydriasis or miosis to after mydriasis or miosis. The mean posterior tangential power did not change significantly from before mydriasis to after mydriasis. For miosis, the mean minus power significantly decreased (P⬍.05), from ⫺6.10 ⫾ 0.29 D before miosis to ⫺6.06 ⫾ 0.29 D after miosis. The mean keratometric tangential power decreased from 42.25 ⫾ 2.39 D before mydriasis to 41.50 ⫾ 2.15 D after mydriasis, with a significant plus power decrease (P⬍.01). However, the mean posterior tangential power did not change significantly from before miosis to after miosis (Table 4). The mean anterior tangential power was 47.42 ⫾ 1.87 D at the time of mydriasis and 47.65 ⫾ 1.73 D
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Table 3. Axial power changes caused by mydriasis and miosis. Mean ⫾ SD (D) Anterior Time
Mydriasis
Posterior Miosis
Mydriasis
Keratometric Miosis
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
Table 4. Tangential power changes caused by mydriasis and miosis. Mean ⫾ SD (D) Anterior Time
Mydriasis
Posterior
Miosis
Mydriasis
Miosis
Keratometric Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
Table 5. Corneal thickness changes caused by mydriasis and miosis. Mean ⫾ SD (m) Mydriasis Time
3 mm zone
5 mm zone
Miosis 7 mm zone
3 mm zone
5 mm zone
7 mm zone
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
at miosis; thus, the plus power increased significantly during miosis compared with mydriasis (P⬍.05). The mean posterior tangential power was ⫺6.11 ⫾ 0.27 D at the time of mydriasis and ⫺6.06 ⫾ 0.29 D at the time of miosis, showing a significant decrease (P⬍.05) in minus power at the time of miosis. The mean keratometric tangential power was 41.50 ⫾ 2.15 D during mydriasis and 42.94 ⫾ 2.52 D during miosis, with a significant plus power increase during miosis (P⬍.01). Corneal Thickness The mean corneal thickness increased significantly after mydriasis and miosis at the 3.0 mm, 5.0 mm, and 7.0 mm central zones. However, there was no significant difference between mydriasis and miosis in any zone (Table 5). 1028
Table 6. Anterior chamber depth changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Time
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test
Anterior Chamber Depth The mean ACD increased significantly, from 2.98 ⫾ 0.29 mm before mydriasis to 3.11 ⫾ 0.25 mm after mydriasis (P⬍.01). The ACD became significantly shallower, changing from a mean of 2.99 ⫾ 3.00 mm before miosis to 2.82 ⫾ 0.32 mm after miosis (P⬍.01) (Table 6). The ACD in all subjects was significantly deeper in mydriasis than in miosis.
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Discussion With the recent popularity of refractive surgery, the evaluation of the posterior corneal shape, not just the anterior corneal shape, has become important. An analysis of corneal shape using conventional methods can only assess the anterior corneal shape. With Orbscan topography, the posterior surface of the cornea can also be evaluated. Previous studies using Orbscan were mainly of refractive surgery such as LASIK, and they showed that the shape of the cornea changed from before LASIK to after LASIK.1,2 This change occurs because in addition to the change in the anterior corneal shape by the excimer laser, there is an anterior shift of the posterior corneal surface caused by a reduction in strength resulting from corneal thinning. It has been reported that the shape of the cornea changes during near vision and far vision and that the refractive power of the cornea may increase during near vision when measured by keratometry.3 We therefore examined how changes in pupil size affect the shape of the anterior and posterior surfaces of the cornea using Orbscan topography. Accommodation involves contraction of the ciliary muscle and relaxation of the zonules, resulting in thickening of the lens. In the present study, we instilled a parasympathomimetic agent into the subject’s eye to make the ciliary muscle contract. This simulated what occurs during accommodation. We also instilled a sympathomimetic agent to relax the ciliary muscle, simulating what occurs during nonaccommodation. The BFS value was used to examine the corneal shape. The BFS is expressed by the spherical radius of curvature that best fits the shape of the anterior or posterior surface of the cornea. The greater the value, the flatter the surface of the cornea. The smaller the value, the steeper the surface of the cornea. The mean anterior and posterior BFS significantly flattens after mydriasis and becomes significantly steeper after miosis. Thus, the shape of the cornea changed significantly as a result of mydriasis and miosis. We believe the reason the corneal shape was steeper during miosis than during mydriasis was the result of a contractive force acting on the scleral spur where the ciliary body is attached. As a result, we believe the contractive force acting on the peripheral cornea connecting to the scleral spur steepened the cornea. The axial power and tangential power were used to examine the changes in the refractive power of the
cornea caused by mydriasis or miosis and to compare the anterior, posterior, and keratometric corneal powers between mydriasis and miosis. We had previously analyzed axial power and tangential power in the central 3.0 mm zone and central 5.0 mm zone. There were significant changes in the central 5.0 mm zone between mydriasis and miosis, but not in the 3.0 mm zone. Thus, we used the former data. In the anterior corneal surface, the axial and tangential powers increased by a plus power of 0.23 D during miosis. Regarding the posterior corneal surface during miosis, the axial power increased by a minus power of 0.14 D while the tangential power increased by a plus power of 0.50 D. In a study using a keratometer, Piers´cionek and coauthors5 suggest that corneal power might change during accommodation. Regarding keratometric corneal power during miosis, the axial power increased by a plus power of 0.35 D and the tangential power increased by a plus power of 1.44 D. The refractive power of the cornea increased more during miosis than with mydriasis. The mean ACD in our subjects was 2.98 mm before mydriasis and miosis, smaller than the value of 3.23 mm reported for measurements using the Orbscan.8 The ACD became significantly deeper during mydriasis and significantly more shallow during miosis. This was attributed to an increase in lens thickness during miosis. Corneal thickness is thought to be greater at the periphery than at the central area. Our results show that the cornea was thicker at the central 5.0 mm zone than at the central 3.0 mm zone and that it was thickest at the central 7.0 mm zone. There have been many reports on the relationship between age and refraction and central corneal thickness, which is known to change as a result of various factors. Although it has been reported that age, sex, and refraction are not correlated with corneal thickness,9 it has also been reported that the cornea thins with age10 and as the degree of myopia increases.11 There is also diurnal variation in corneal thickness. In general, the cornea is thickest upon waking in the morning and becomes thin toward evening, but it has also been reported that it is thinnest at 3 PM.12 Thus, corneal thickness changes depend on various factors. The central corneal thickness in all eyes increased on both mydriasis and miosis. The reason might be the 30-minute eyelid closure after the instillation of the mydriatic or miotic drops before Orbscan measurements. However, no significant changes in the central
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corneal thickness were found between mydriasis and miosis. Our study also led to a theory of pseudoaccommodation in pseudophakic eyes. Some argue that the mechanism of pseudoaccommodation occurs when the IOL shifts anteriorly during near vision, creating shortsightedness and enabling near vision.13 Although none of our eyes was pseudophakic, the effect of changes in corneal shape during near vision may be 1 cause of pseudoaccommodation in pseudophakia. This requires further investigation. In conclusion, the shape and refractive power of the cornea significantly changed as a result of mydriasis and miosis. This suggests that changes in lens thickness and corneal shape are involved in the change in refractive power during near vision.
References 1. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 106:406–409; discussion by RK Maloney, 409– 410 2. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology 2001; 108:317– 320 3. Brown N. The change in shape and internal form of the lens of the eye on accommodation. Exp Eye Res 1973; 15: 441–459 4. LeGrand Y. Form and Space Vision [translated by M Millodot, GG Health]. Bloomington, IN, Indiana University Press, 1967
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5. Piers´cionek BK, Popiołek-Masajada A, Kasprzak H. Corneal shape change during accommodation. Eye 2001; 15: 766–769 6. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness measurements with the Orbscan Topography System and ultrasonic pachymetry. J Cataract Refract Surg 1997; 23:1345–1350 7. Gonza´lez-Me´ijome JM, Cervin˜o A, Yebra-Pimentel E, Parafita MA. Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry. J Cataract Refract Surg 2003; 29: 125–132 8. Auffarth GU, Tetz MR, Biazid Y, Vo¨lcker HE. Measuring anterior chamber depth with the Orbscan Topography System. J Cataract Refract Surg 1997; 23:1351– 1355 9. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures. A review and meta-analysis approach. Surv Ophthalmol 2000; 44: 367–408 10. Chang S-W, Tsai I-L, Hu FR. The cornea in young myopic adults. Br J Ophthalmol 2001; 85:916–920 11. Cho P, Lam C. Factors affecting the central corneal thickness of Hong Kong-Chinese. Curr Eye Res 1999; 18:368–374 12. Fujita S. Diurnal variation in human corneal thickness. Jpn J Ophthalmol 1980; 24:444–456 13. Lesiewska-Junk H, Kałuz˙ny J. Intraocular lens movement and accommodation in eyes of young patients. J Cataract Refract Surg 2000; 26:562–565 From the Department of Ophthalmology, Iwate Medical University School of Medicine, Morioka, Japan. Presented at the 107th Annual Meeting of the Japanese Ophthalmological Society, Fukuoka, Japan, April 2003. None of the authors has a financial or proprietary interest in any material or method mentioned.
J CATARACT REFRACT SURG—VOL 30, MAY 2004
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n the past, corneal shape has been evaluated by analyzing the anterior surface of the cornea, using a photokeratoscope or videokeratoscope in most cases. With the development and popularity of refractive surgery, it has become important to also evaluate the shape of
Accepted for publication October 29, 2003. Reprint requests to Kahori Saitoh, MD, Department of Ophthalmology, Iwate Medical University School of Medicine, 19-1 Uchimaru, Morioka, Iwate 020-8505, Japan. 2004 ASCRS and ESCRS Published by Elsevier Inc.
the posterior surface of the cornea. The use of scanningslit corneal topography has enabled the simultaneous measurement of the anterior and posterior surfaces of the cornea as well as the corneal thickness, anterior chamber depth (ACD), and pupil size. Using scanningslit corneal topography, studies1,2 found changes in the shape of the anterior and posterior surfaces of the cornea before and after keratorefractive surgery such as laser in situ keratomileusis (LASIK). In contrast is the theory that the corneal shape or axial length does not change during near vision and 0886-3350/04/$–see front matter doi:10.1016/j.jcrs.2003.10.040
CORNEAL SURFACE CHANGES INDUCED BY MYDRIASIS AND MIOSIS
that the increase in refractive power is caused by a change in the shape of the crystalline lens.3 A study using the LeGrand eye model4 found no changes in the radius of corneal curvature during accommodation because there were no changes in corneal refractive power between the time of relaxation of accommodation and the time of maximum accommodation. However, a study using a keratometer reports that accommodation affects corneal refractive power during near and far vision.5 Using scanning-slit corneal topography, we examined how the shape of the anterior and posterior surfaces of the cornea, corneal refractive power, pupil size, corneal thickness, and ACD alter after mydriasis and after miosis.
Subjects and Methods The procedures used in this study conformed to the tenets of the Declaration of Helsinki. After they received an explanation of the procedures, all participants provided informed consent. Each subject chose which eye was to be measured. There were no abnormalities in the anterior segment, optic media, or ocular fundus in the tested eyes as observed by slitlamp microscopy and indirect ophthalmoscopy. The best corrected visual acuity was 1.0 or better in all patients, and the mean refractive error was ⫺2.00 diopters (D) ⫾ 1.65 (SD) (range ⫺6.00 to ⫹0.25 D). Individuals with a history of contact lens use were not included. The subjects had no systemic diseases such as diabetes mellitus. Scanning-slit corneal topography (Orbscan version 3.0, Orbtek, Inc.) was used to assess the corneal shape. The measurement was performed by 1 examiner (K.S.) in a dark room (15 lux) at 3 PM and 5 PM. With Orbscan topography, the optical acquisition head scans the eye using vertical light slits that are projected onto the cornea at a 45-degree angle. A total of 40 slits were projected onto the eye. As information on 240 points was contained in each single slit image, 9600 points in 40 images on the entire cornea were measured. Based on data measured at each point, a 3-dimensional shape was constructed from the anterior surface of the cornea to the anterior surface of the lens. The shape of the anterior and posterior surfaces of the cornea, the corneal thickness, and the ACD were analyzed. The values before mydriasis were measured using Orbscan. A parasympatholytic agent of tropicamide–phenylephrine hydrochloride (Mydrin P威) was instilled in the eye. Maximum mydriasis occurred after about 30 minutes, and the measurement during mydriasis was performed again as outlined. After 3 days, the values before miosis in the same eye were measured in a similar manner. Next, a parasympa-
Figure 1. (Saitoh) The point at which the perpendicular line from the tangential line running through the single point A on the anterior or posterior surfaces of the cornea meets the central axis (optic axis) was regarded as B. The distance from A to B was the radius of curvature of this point.
thomimetic agent of pilocarpine hydrochloride 2% (Sanpilo威) was instilled in the eye 2 times 15 minutes apart to induce miosis. After miosis of less than 3.0 mm in diameter and disappearance of light reflex were confirmed, the measurements were performed during miosis in a similar manner. The following were analyzed to assess the changes induced by mydriasis and miosis.
Pupil Size Numerical values displayed on the instrument were used to examine pupil size.
Best-Fit Sphere The anterior and posterior best-fit sphere (BFS) values for the anterior and posterior surfaces of the cornea were used to examine the changes in corneal shape caused by mydriasis and miosis. The BFS was defined as the radius of curvature (mm) of the sphere constructed based on data input by Orbscan measurements to best fit the anterior or posterior surface of the cornea in the central 10.0 mm diameter zone.
Axial Power The point at which the perpendicular line from the tangential line running through the single point A on the anterior or posterior surface of the cornea meets the central axis (optic axis) was regarded as B (Figure 1). The distance from A to B was the radius of curvature of this point. The anterior or posterior axial power value was obtained by converting the radius of curvature (mm) into a diopter using the physiologic refractive index (1.376). The keratometric axial power was obtained by converting the radius of curvature into a diopter using the corneal conversion refractive index (1.3375). Each value for the anterior and posterior axial powers and keratometric axial power was obtained in the central 5.0 mm zone of the cornea.
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Table 1. Pupil size changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Time
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test
Figure 2. (Saitoh) The radius of the arc (AB) at a single point A on the anterior or posterior surface of the cornea acted as the radius of curvature at this point.
Tangential Power The radius of the arc (AB) at a single point A on the anterior or posterior surface of the cornea acted as the radius of curvature at this point (Figure 2). The anterior or posterior tangential power value was obtained by converting this radius of curvature (mm) into a diopter using the physiologic refractive index (1.376). The keratometric tangential power value was obtained by converting the radius of curvature into a diopter using the corneal conversion refractive index (1.3375). The anterior and posterior tangential powers and keratometric tangential power were obtained within the central 5.0 mm diameter range.
Corneal Thickness Corneal thickness was calculated using the mean value of the points measured within a circular zone of 3.0 mm, 5.0 mm, or 7.0 mm in diameter from the center of each eye. Studies comparing Orbscan with ultrasonic pachymetry found that Orbscan measurements of corneal thickness were greater than ultrasonic pachymetry measurements.6,7 Therefore, the values obtained by multiplying 0.92 and Orbscan measurements are commonly used; this was the method used in this study.
Anterior Chamber Depth For the ACD, the depth from the posterior surface of the cornea to the anterior surface of the lens in the center of the cornea from the Orbscan topography was used.
Level of Significance The Wilcoxon signed rank test was used to evaluate the significance of the results. A P value less than 0.05 was considered statistically significant.
Results Twenty-eight eyes (19 right, 9 left) of 28 normal healthy volunteers (21 men, 7 women) were studied. The mean age was 31.1 ⫾ 5.6 years (range 20 to 46 years). 1026
Pupil Size The mean pupil size in all eyes before mydriasis was 4.6 ⫾ 0.6 mm. The mean pupil size in eyes with mydriasis was significantly larger, 7.8 ⫾ 0.5 mm (P⬍.01). After miosis, the mean pupil size in all eyes decreased significantly, from 4.5 ⫾ 0.8 mm to 2.2 ⫾ 0.4 mm (P⬍.01) (Table 1). Best-Fit Sphere After mydriasis, the mean anterior BFS values increased significantly, from 8.02 ⫾ 0.3 mm to 8.04 ⫾ 0.3 mm (P⬍.05). The mean posterior BFS value also increased significantly, from 6.50 ⫾ 0.3 mm to 6.53 ⫾ 0.3 mm (P⬍.01). Thus, the shape of the anterior and posterior surfaces of the cornea became significantly flatter as a result of mydriasis. After miosis, the anterior BFS value decreased in 17 eyes and remained unchanged or increased in 11 eyes. The mean value in all eyes decreased from 8.02 ⫾ 0.3 mm to 8.00 ⫾ 0.3 mm after miosis; however, the change was not significant. The mean posterior BFS value decreased significantly, from 6.50 ⫾ 0.3 mm to 6.46 ⫾ 0.3 mm after miosis (P⬍.001) (Table 2). Thus, the shape of the posterior surface of the cornea became significantly steeper as a result of miosis. The anterior BFS value was lower during miosis than during mydriasis in 20 of the 28 eyes; in 8 eyes, the value was larger during miosis than mydriasis (Figure 3, A). In all 28 eyes, the mean anterior BFS was 8.04 ⫾ 0.3 mm at the time of mydriasis and 8.00 ⫾ 0.3 mm at miosis; it was significantly lower during miosis than during mydriasis (P⬍.01). The posterior BFS value was lower at the time of miosis than mydriasis in 26 eyes; in 2 eyes, it was larger during miosis than mydriasis (Figure 3, B). The mean value in all eyes was significantly lower during miosis (6.46 ⫾ 0.3 mm) than during mydriasis (6.53 ⫾ 0.3 mm) (P⬍.01). Thus, the corneal shape became significantly steeper for both the
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Table 2. Best-fit sphere changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Mydriasis Time
Anterior
Miosis Posterior
Anterior
Posterior
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
anterior and posterior surfaces during miosis than during mydriasis. Axial Power The mean anterior axial power did not significantly change from before to after mydriasis or miosis. The mean posterior axial power decreased from ⫺6.57 ⫾
Figure 3. (Saitoh) Changes in anterior BFS (A) and posterior BFS (B) caused by mydriasis and miosis (black circles ⫽ eyes in which BFS was smaller in mydriasis than in miosis; diamonds ⫽ eyes in which BFS was larger in miosis than in mydriasis).
0.27 D before mydriasis to ⫺6.48 ⫾ 0.28 D after mydriasis; thus, the minus power decreased. This change was statistically significant (P⬍.01). The change in posterior axial power caused by miosis showed that the mean minus power significantly increased (P⬍.05), from ⫺6.57 ⫾ 0.27 D before miosis to ⫺6.62 ⫾ 0.27 D after miosis (Table 3). The mean keratometric axial power did not significantly change after mydriasis or miosis. The mean anterior axial power was 47.86 ⫾ 1.93 D during mydriasis and 48.09 ⫾ 1.74 D during miosis. The plus power significantly increased (P⬍.05) during miosis. The mean posterior axial power was ⫺6.48 ⫾ 0.28 D during mydriasis and ⫺6.62 ⫾ 0.27 D during miosis. Thus, the minus power significantly increased during miosis (P⬍.01). The mean keratometric axial power was 43.19 ⫾ 1.81 D at the time of mydriasis and 43.54 ⫾ 1.82 D at the time of miosis. Thus, the plus power increased significantly at the time of miosis (P⬍.01). Tangential Power The mean anterior tangential power did not change significantly from before mydriasis or miosis to after mydriasis or miosis. The mean posterior tangential power did not change significantly from before mydriasis to after mydriasis. For miosis, the mean minus power significantly decreased (P⬍.05), from ⫺6.10 ⫾ 0.29 D before miosis to ⫺6.06 ⫾ 0.29 D after miosis. The mean keratometric tangential power decreased from 42.25 ⫾ 2.39 D before mydriasis to 41.50 ⫾ 2.15 D after mydriasis, with a significant plus power decrease (P⬍.01). However, the mean posterior tangential power did not change significantly from before miosis to after miosis (Table 4). The mean anterior tangential power was 47.42 ⫾ 1.87 D at the time of mydriasis and 47.65 ⫾ 1.73 D
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Table 3. Axial power changes caused by mydriasis and miosis. Mean ⫾ SD (D) Anterior Time
Mydriasis
Posterior Miosis
Mydriasis
Keratometric Miosis
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
Table 4. Tangential power changes caused by mydriasis and miosis. Mean ⫾ SD (D) Anterior Time
Mydriasis
Posterior
Miosis
Mydriasis
Miosis
Keratometric Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
Table 5. Corneal thickness changes caused by mydriasis and miosis. Mean ⫾ SD (m) Mydriasis Time
3 mm zone
5 mm zone
Miosis 7 mm zone
3 mm zone
5 mm zone
7 mm zone
*P⬍.01, Wilcoxon signed rank test **P⬍.05, Wilcoxon signed rank test
at miosis; thus, the plus power increased significantly during miosis compared with mydriasis (P⬍.05). The mean posterior tangential power was ⫺6.11 ⫾ 0.27 D at the time of mydriasis and ⫺6.06 ⫾ 0.29 D at the time of miosis, showing a significant decrease (P⬍.05) in minus power at the time of miosis. The mean keratometric tangential power was 41.50 ⫾ 2.15 D during mydriasis and 42.94 ⫾ 2.52 D during miosis, with a significant plus power increase during miosis (P⬍.01). Corneal Thickness The mean corneal thickness increased significantly after mydriasis and miosis at the 3.0 mm, 5.0 mm, and 7.0 mm central zones. However, there was no significant difference between mydriasis and miosis in any zone (Table 5). 1028
Table 6. Anterior chamber depth changes caused by mydriasis and miosis. Mean ⫾ SD (mm) Time
Mydriasis
Miosis
*P⬍.01, Wilcoxon signed rank test
Anterior Chamber Depth The mean ACD increased significantly, from 2.98 ⫾ 0.29 mm before mydriasis to 3.11 ⫾ 0.25 mm after mydriasis (P⬍.01). The ACD became significantly shallower, changing from a mean of 2.99 ⫾ 3.00 mm before miosis to 2.82 ⫾ 0.32 mm after miosis (P⬍.01) (Table 6). The ACD in all subjects was significantly deeper in mydriasis than in miosis.
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Discussion With the recent popularity of refractive surgery, the evaluation of the posterior corneal shape, not just the anterior corneal shape, has become important. An analysis of corneal shape using conventional methods can only assess the anterior corneal shape. With Orbscan topography, the posterior surface of the cornea can also be evaluated. Previous studies using Orbscan were mainly of refractive surgery such as LASIK, and they showed that the shape of the cornea changed from before LASIK to after LASIK.1,2 This change occurs because in addition to the change in the anterior corneal shape by the excimer laser, there is an anterior shift of the posterior corneal surface caused by a reduction in strength resulting from corneal thinning. It has been reported that the shape of the cornea changes during near vision and far vision and that the refractive power of the cornea may increase during near vision when measured by keratometry.3 We therefore examined how changes in pupil size affect the shape of the anterior and posterior surfaces of the cornea using Orbscan topography. Accommodation involves contraction of the ciliary muscle and relaxation of the zonules, resulting in thickening of the lens. In the present study, we instilled a parasympathomimetic agent into the subject’s eye to make the ciliary muscle contract. This simulated what occurs during accommodation. We also instilled a sympathomimetic agent to relax the ciliary muscle, simulating what occurs during nonaccommodation. The BFS value was used to examine the corneal shape. The BFS is expressed by the spherical radius of curvature that best fits the shape of the anterior or posterior surface of the cornea. The greater the value, the flatter the surface of the cornea. The smaller the value, the steeper the surface of the cornea. The mean anterior and posterior BFS significantly flattens after mydriasis and becomes significantly steeper after miosis. Thus, the shape of the cornea changed significantly as a result of mydriasis and miosis. We believe the reason the corneal shape was steeper during miosis than during mydriasis was the result of a contractive force acting on the scleral spur where the ciliary body is attached. As a result, we believe the contractive force acting on the peripheral cornea connecting to the scleral spur steepened the cornea. The axial power and tangential power were used to examine the changes in the refractive power of the
cornea caused by mydriasis or miosis and to compare the anterior, posterior, and keratometric corneal powers between mydriasis and miosis. We had previously analyzed axial power and tangential power in the central 3.0 mm zone and central 5.0 mm zone. There were significant changes in the central 5.0 mm zone between mydriasis and miosis, but not in the 3.0 mm zone. Thus, we used the former data. In the anterior corneal surface, the axial and tangential powers increased by a plus power of 0.23 D during miosis. Regarding the posterior corneal surface during miosis, the axial power increased by a minus power of 0.14 D while the tangential power increased by a plus power of 0.50 D. In a study using a keratometer, Piers´cionek and coauthors5 suggest that corneal power might change during accommodation. Regarding keratometric corneal power during miosis, the axial power increased by a plus power of 0.35 D and the tangential power increased by a plus power of 1.44 D. The refractive power of the cornea increased more during miosis than with mydriasis. The mean ACD in our subjects was 2.98 mm before mydriasis and miosis, smaller than the value of 3.23 mm reported for measurements using the Orbscan.8 The ACD became significantly deeper during mydriasis and significantly more shallow during miosis. This was attributed to an increase in lens thickness during miosis. Corneal thickness is thought to be greater at the periphery than at the central area. Our results show that the cornea was thicker at the central 5.0 mm zone than at the central 3.0 mm zone and that it was thickest at the central 7.0 mm zone. There have been many reports on the relationship between age and refraction and central corneal thickness, which is known to change as a result of various factors. Although it has been reported that age, sex, and refraction are not correlated with corneal thickness,9 it has also been reported that the cornea thins with age10 and as the degree of myopia increases.11 There is also diurnal variation in corneal thickness. In general, the cornea is thickest upon waking in the morning and becomes thin toward evening, but it has also been reported that it is thinnest at 3 PM.12 Thus, corneal thickness changes depend on various factors. The central corneal thickness in all eyes increased on both mydriasis and miosis. The reason might be the 30-minute eyelid closure after the instillation of the mydriatic or miotic drops before Orbscan measurements. However, no significant changes in the central
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corneal thickness were found between mydriasis and miosis. Our study also led to a theory of pseudoaccommodation in pseudophakic eyes. Some argue that the mechanism of pseudoaccommodation occurs when the IOL shifts anteriorly during near vision, creating shortsightedness and enabling near vision.13 Although none of our eyes was pseudophakic, the effect of changes in corneal shape during near vision may be 1 cause of pseudoaccommodation in pseudophakia. This requires further investigation. In conclusion, the shape and refractive power of the cornea significantly changed as a result of mydriasis and miosis. This suggests that changes in lens thickness and corneal shape are involved in the change in refractive power during near vision.
References 1. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 106:406–409; discussion by RK Maloney, 409– 410 2. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology 2001; 108:317– 320 3. Brown N. The change in shape and internal form of the lens of the eye on accommodation. Exp Eye Res 1973; 15: 441–459 4. LeGrand Y. Form and Space Vision [translated by M Millodot, GG Health]. Bloomington, IN, Indiana University Press, 1967
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5. Piers´cionek BK, Popiołek-Masajada A, Kasprzak H. Corneal shape change during accommodation. Eye 2001; 15: 766–769 6. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness measurements with the Orbscan Topography System and ultrasonic pachymetry. J Cataract Refract Surg 1997; 23:1345–1350 7. Gonza´lez-Me´ijome JM, Cervin˜o A, Yebra-Pimentel E, Parafita MA. Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry. J Cataract Refract Surg 2003; 29: 125–132 8. Auffarth GU, Tetz MR, Biazid Y, Vo¨lcker HE. Measuring anterior chamber depth with the Orbscan Topography System. J Cataract Refract Surg 1997; 23:1351– 1355 9. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures. A review and meta-analysis approach. Surv Ophthalmol 2000; 44: 367–408 10. Chang S-W, Tsai I-L, Hu FR. The cornea in young myopic adults. Br J Ophthalmol 2001; 85:916–920 11. Cho P, Lam C. Factors affecting the central corneal thickness of Hong Kong-Chinese. Curr Eye Res 1999; 18:368–374 12. Fujita S. Diurnal variation in human corneal thickness. Jpn J Ophthalmol 1980; 24:444–456 13. Lesiewska-Junk H, Kałuz˙ny J. Intraocular lens movement and accommodation in eyes of young patients. J Cataract Refract Surg 2000; 26:562–565 From the Department of Ophthalmology, Iwate Medical University School of Medicine, Morioka, Japan. Presented at the 107th Annual Meeting of the Japanese Ophthalmological Society, Fukuoka, Japan, April 2003. None of the authors has a financial or proprietary interest in any material or method mentioned.
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