- Email: [email protected]
Correction of Irregular Astigmatism with the Excimer Laser
Correction of Irregular Astigmatism with the Excimer Laser Richard Gibralter, MD,l Stephen L. Trokel, MD2 Background: Correction of irregular astigmatism has not been possible using available keratorefractive technology. Methods: The authors used a topographic map as a guide and created a custom excimer ablation program, designed to create a more regular surface. The program consisted of a combination of phototherapeutic and photo refractive ablation patterns. The amount of tissue to be removed was calculated on the basis of the diameter and steepness of the irregular areas of the corneal surface. Results: A more regular surface, as evidenced by topographic analysis, reduced astigmatism, and improved uncorrected visual acuity, was produced. Conclusion: Using the corneal topographical map as a guide, excimer laser ablation can be used to create a more regular optical surface with improved visual function. Ophthalmology 1994;101:1310-1315
Correction of all types of refractive astigmatism remains a challenge,1.2 but a challenge which is greatest for the patient with irregular astigmatism. These patients often have undergone surgical procedures, such as pterygium removal, penetrating keratoplasty, and cataract extraction, or refractive procedures as myopic keratomileusis, and radial and astigmatic keratotomy. Sometimes the irregular astigmatism follows trauma or a corneal infection. In many of these patients, a spectacle correction produces poor visual acuity, whereas the best visual acuity obtained using contact lenses may be normal. Astigmatism after surgerl is especially disturbing because the patient will be disappointed with the results of an otherwise satisfactory procedure. A patient with a clear cornea after penetrating keratoplasty will not be satisfied with the residual optical disabilities associated with excessive astigmatism even though the cornea is clear. The astigmatism in all these situations may be regular with a reasonable toric Originally received: October 4, 1993. Revision accepted: February 23, 1994. Manhattan Eye, Ear and Throat Hospital. New York. Edward S. Harkness Eye Institute. Columbia·Presbyterian Medical Center, New York. J
2
Dr. Trokel is a consultant to and has a proprietary interest in Visx Corp. Reprint requests to Stephen L. Trokel, MD, Edward S. Harkness Eye Institute, Columbia-Presbyterian Medical Center, 635 W. 165th St. New York. NY 10032.
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surface, even though it is of high degree. On the other hand, the astigmatism may be irregular and asymmetric and not be amenable to treatment with relaxing incisions or with excimer refractive keratectomy using ablation programs designed to remove the to ric elements responsible for regular astigmatism. Studies4 •5 have reported acceptable clinical results using photorefractive keratectomy for the correction of astigmatism. We have developed an approach to patients with irregular astigmatism that has proven capable of producing a more even optical surface on a distorted cornea with improvement in visual function. Photorefractive keratectomy using excimer laser systems operating at 193 nm is under clinical evaluation for correction of myopia, astigmatism, removal of superficial corneal opacities, and treatment of corneal surface disorders. Myopic correction requires6 that the laser flatten the central corneal curvature in a radially symmetric fashion with greater ablation depth in the center compared with the periphery. Removal of opacities requires ablation to a depth sufficient to encompass the pathologic volume and is gently blended toward adjacent normal corneal tissue. Toric corneal surfaces also can be ablated 7 ,8 onto a corneal surface to correct regular astigmatism. However, astigmatism that is not regular or toric cannot be treated by the application of any simple geometric pattern of tissue removal. We have devised a treatment using the excimer laser that addresses this need and has been used to correct
Gibralter and Trokel irregular astigmatism. This approach does not use standard astigmatic ablation patterns, but relies on a treatment plan based on analysis of the topographic image correlated with the biomicroscopic appearance of the cornea. We identify high, steep areas of cornea and ablate them by local, focal treatment using small circular diameter ablations. This technique neutralizes the irregular astigmatism by minimizing the differences between steeper and flatter geographic areas within the optical zone. This selective flattening of the steeper topographic regions is done to create a more regular and spherical anterior corneal surface to improve the optical quality of the corneal surface thereby improving the refractive performance ofthe eye.
Materials and Methods We use an argon-fluoride excimer laser system (Visx TwentyTwenty, Visx, Inc, Sunnyvale, CA) operating at 193 nm, a fluence of 160 mJ/cm 2 and 5 Hz. Myopic corrections are achieved by flattening the cornea with a pattern oflaser ablation determined by a computer-controlled iris diaphragm. Laser correction of astigmatism creates a toric flattening with either sequential cylindrical then myopic ablations, or a single toric ablation. Both of these patterns are oriented to flatten the steep corneal meridian. Neither of these patterns would appear to be useful for the patients with astigmatism as typified by those we report here with highly irregular, postoperative astigmatism (Figs 1 and 2). In one patient, a refractive problem developed after myopic keratomileusis; in the other patient the problem developed after penetrating keratoplasty for keratoconus. To create a regular optical surface, we customized an ablation program using circular elements of varying diameters to flatten identified high areas and a standard myopic ablation to achieve central flattening and eliminate residual myopia. The laser was calibrated with a test 4-diopter (0) myopic ablation in a polymethylmethacrylate block to verify beam fluence and laser system performance. The two patients described here were both mechanically and optically intolerant of their contact lenses. Both were aware of the fact that they were participating in an investigative protocol monitored by the Federal Food and Orug Administration and the Investigational Review Board of the Manhattan Eye, Ear and Throat Hospital. An informed consent was obtained, and both subjects were told of the risks, benefits, and alternative treatments to use of the excimer laser. The examination and refraction were performed by the same investigator (RG) and the corneal computer-assisted topographic analyses (Corneal Modeling System, New York, NY) were performed by the same technician before and after the surgical laser treatments. Treatment was planned by determining elevated areas and their dimensions on the topographic maps. The details of surgical planning are described below, which aimed at creating a more uniform optical surface with a desired average keratometry.
Irregular Astigmatism
Technique The patients received topical proparacaine hydrochloride and 5 mg oral diazepam. They then were positioned under the operating microscope of the excimer laser. The contralateral eye was covered with a light-tight barrier shield during the laser treatment. The patient was instructed to fix on a blinking red light in the delivery system of the laser. A Merocel sponge (Merocel Corp, Mystic, CT) soaked in 4% lidocaine was placed over the cornea for 45 seconds to facilitate epithelial removal. We used a 57 Beaver blade to gently stroke the surface of the cornea with short tangential motions to remove the epithelium over the distorted area to be treated. A mixture of dextran and methylcellulose (Tears Naturale II) was used to moisten a Merocel spear which is used to coat uniformly the stromal surface of the corneal surface to be treated. The computer was programmed to perform circular cylindrical (phototherapeutic keratectomy [PTK]) and refractive (photorefractive keratectomy [PRK]) ablations. The placement of the ablations was based on defining high and steep areas on the corneal surface by analyzing computer-assisted pretreatment topography studies. The 4-mm laser treatments were placed over the steeper corneal zones and then residual myopia was corrected treating directly over the entrance pupil as in photorefractive keratectomy for myopia. After treatment, the patients received tobramycin sulfate-dexamethasone sodium phosphate ointment (Tobradex), and the eye was pressure patched. In addition, the patients were given oxycodone (Percocet) immediately after the treatment and additional analgesia for postoperative pain. The patients were examined daily until complete epithelialization occurred. They were then treated with FML (0.1 % fluorometholone) three times daily for 1 month, followed by once daily for 4 weeks.
Patients and Treatments The first patient was a 24-year-old man who had myopic keratomileusis performed in the left eye in June 1991. This was followed by a second procedure in November 1991 in which partial removal of the surface lamellae was undertaken to eliminate ectopic corneal epithelial interface inclusions. After these procedures, the patient was left with a residual myopic astigmatism. Visual acuity in the nonoperated right eye was 20/200 which was corrected to 20/16 with a -8.50-0 lens. Visual acuity in the operated left eye was 20/200- which improved to 20/25 with a spectacle correction of -2.00 -3.50 X 180. Results of examination of the left eye showed no signs of inflammation, and no conjunctival injection was present. Results of slit-lamp biomicroscopic examination showed the outline of the myopic keratomileusis. Areas of epithelial inclusion and intrastromal haze were apparent at the stromal interface at the 6-0'clock position with a separate area visible at the lO-o'clock position. Centered in the haze at the 6-0'clock position was a 0.5-mm transparent zone which was thought to be either an inclusion cyst or an area of stromal loss in the cap filled from above by an
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Gibralter and T roke!
Irregular Astigmatism Table 2. Treatment Plan for Patient 2 Based on Analysis of Computerized Keratography·
Table 1. Treatment Plan for Patient 1 Based on Analysis of Computerized Keratography·
Zone A B
C D PTK
Ablation Type
Diameter (mm)
PTK PTK PTK PRK
4 4 4 6
Depth (ILm)
ZO
20 20 20
Refractive Power (diopters)
Zone
0 0 0 -1.50
A B
C D E
= phototherapeutic program; PRK = photorefractive program.
* The treatment plan is shown in Figure 3; focal ablations are based on
PTK
topographic map in Figure 1.
ingrowth of corneal epithelium. The intraocular pressure was 14 mmHg, and the central cornea thickness measured 0.498 mm with ultrasonic pachymetry (DGH-2000, DGH Technology, Frazer, PA). Keratometry of the left cornea showed mild distortion of the mires, and readings were estimated at 40.50/42.50 axis 30 0 • The anterior chamber was deep with no iritis or dilation of iris vessels, the crystalline lens was clear, and results of the retinal examination were normal. The treatment plan for this patient (Fig 3; Table I) was based on biomicroscopic observation and the topographic analysis in Figure I which showed steep areas on the corneal surface. In patient 1, to flatten local steep areas and create a more uniform surface, three confluent 4-mm circular zones in the inferior portion of the myopic keratomileusis lenticle were ablated. Each of these zones were ablated to a depth of 20 ILm, and their edges were barely touching. All 4-mm treatment zones were programmed with an edge transition of 0.35 mm. The final ablation zone was a fourth ablation of 6 mm in diameter with a refractive power of -1.50 0 (20 ILm depth) which was centered over the entrance pupil. The focal excimer laser ablations (Fig 3; Table I) were based on flattening the high or steep zones which involved the lower portion of the myopic keratomileusis lenticle and are seen in the preoperative corneal map (Fig 1). Patient 2 was a 40-year-old woman whose right eye with keratoconus had been treated 23 years previously with a penetrating keratoplasty. Over the last several years, the patient became increasingly intolerant of contact lens use and was unable to wear spectacles because of the significant anisometropia. Visual acuity in the left eye was
=
Ablation Type
Diameter (mm)
PTK PTK PTK PTK PRK
4 4 4 4 6
phototherapeutic program; PRK
Depth (ILm)
10
20 20 20 53 =
Refractive Power (diopters)
0 0 0 0 -4.00
photorefractive program.
* The treatment plan is shown in Figure 4; focal ablations are based on topographical map in Figure 2.
20/25 without optical correction after penetrating keratoplasty followed by relaxing incisions for postsurgical astigmatism. Visual acuity in the right eye was counting fingers at 3 feet but improved to 20/25 with a myopic astigmatic lens of -6.25 -3.25 X 45. Ultrasonic pachymetry in this right eye measured 0.582 mm, and keratometry readings were 51.5/50.5 axis 140 0 with irregular mires. The applanation intraocular pressure was 16 mmHg. Corneal topography (Fig 2) showed that the inferonasal portion of the graft was steep when compared with the superotemporal region. In addition, a relatively steep area was located within the superotemporal region at the 10-0'clock position. Based on the topographic analysis and the slit-lamp appearance, the excimer treatment plan was designed as five separate treatment zones (Fig 4; Table 2). Zone I was a 4-mm PTK treatment diameter to a depth of 10 ILm . Zones 2, 3, and 4 were also 4-mm in diameter PTK treatments and each was to a depth of 20 ILm. Some blending of zones 2,3, and 4 was achieved by moving the eye from treatment zone 2, 3, and then to 4 as the treatment proceeded. A central 6-mm-diameter myopic PRK was performed over the entrance pupil for a - 3.81-0 correction at the corneal plane which required a 53-lLm treatment depth. The patient was intentionally undercorrected because of concern related to the contribution of the focal treatments to the overall corneal flattening. In both patients, the peripheral therapeutic treatment zones were intentionally placed so that they overlapped the central 6-mm treatment zone by I mm. This was done
( Top left, Figure 1. Topographic map of patient 1. The optical distortion resulted after multiple keratorefractive procedures. Top right, Figure 2. Topographic map of patient 2. The local steepening associated with the corneal graft and irregular astigmatism is visible inferiorly. Center left, Figure 3. Treatment plan for patient 1 (see Table 1). Center right, Figure 4. Treatment plan for patient 2 (see Table 2). Bottom left, Figure 5. Posttreatment topographic corneal map of patient 1. Bottom right, Figure 6. Posttreatment topographic corneal map of patient 2.
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Volume 101, Number 7, July 1994
to avoid creating a local discontinuity at the intersection of the PRK and the PTK ablation zone.
Results At 1 month after treatment, patient 1 had a visual acuity without correction of 20/50 and a corrected visual acuity of 20/25 with +0.50 -1.50 X 135. At 3 months, visual acuity without correction had improved to 20/30 without spectacle correction and 20/25 with a -0.25 lens. The refraction stabilized at 6 months with an uncorrected visual acuity of 20/50 and a refraction of -0.75 -0.50 X 50. The final topographic map can be reviewed in Figure 5. Visual acuity in patient 2 was 20/40 without spectacle correction which improved to 20/20 with a -1.50 -2.25 X 65 corrective lens 1 month after the PTK procedure. At 3 months after surgery, the patient's uncorrected visual acuity was 20/40-, correcting to 20/25 with a -2.50 -2.25 X 75 corrective lens. At 6 months, the uncorrected visual acuity stabilized at 20/50, improving to 20/25 with -2.75 -2.50 X 75. The improvement in the corneal surface is reflected in the map shown in Figure 6.
Discussion The choice of the depth of treatment for the localized steep zones was based on an approximation relating the thickness of a solid spherical element to its diameter for each diopter of power. Depth in microns per diopter
=
1/3 diameter2 (7)
This equation approximates the depth of each dioptric elevation at a given diameter. That is, a 3-mm diameter elevation will be 3 ILm per diopter, a 4-mm diameter will be approximately 5 ILm thick per diopter, and a 5-mm diameter will be approximately 8 ILm thick for each diopter of power visible in the keratographic map. The thickness varies linearly with the power but with the square of the diameter. Depth in microns
=
1/3 diameter2 X Diopters(7)
This means that approximately 4 D of refractive power will represent 20 ILm of thickness in a 4-mm diameter on the corneal surface. While this relation relates to spherical volumes, it also serves as a guide to the thickness of local irregular areas of corneal steepness. It can be used to calculate depths of zones of relative corneal steepening which contribute to the asymmetric astigmatism in the optical zone of these patients. The limits of this analysis are based on the smoothing functions of the topographic software which tend to average changes over several millimeters to present a more readily interpretable image. The degree of flattening desired and the location of the treatment zones were based on analysis of the preoperative corneal topographic analysis, resulting in the treatment plans detailed in Tables 1 and 2. Both patients
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had postoperative topographic analysis (Figs 5 and 6) which showed that the treatment produced a more spherical and regular corneal surface as well as the central flattening which was desired to correct the residual myopia in addition to the correction of the irregular astigmatism. There has been contention over whether to treat the refractive state or to alter the treatment plan by identifying local abnormalities of the corneal surface. In these two patients, the extensive distortion and asymmetry of the corneal surface suggested that a standard spherocylindric ablation would not be effective in improving the refractive state of the eye. The extensive corneal unevenness implied that focal treatment and flattening of the localized steep zones might improve the regularity of the corneal surface and the visual function of the patient. The hypothesis can be entertained that a refraction may not provide the best way to alter the corneal surface even though it provides an adequate level of vision. The computer-assisted corneal modeling system helped to identify the steeper regions within the optical zone. It was also unclear to what extent treating these local steep zones would add to the central myopic PRK treatment. In patient 1, there was a pretreatment myopia of - 2.00 D. Only -1.50 D were programmed for treatment to prevent overcorrection. Similarly, the residual myopic correction of patient 2, who had a myopic refraction preoperatively of -6.25, was intentionally undercorrected to only -4.00 s to prevent a hyperopic shift. This resulted in a significant undercorrection of - 2. 7 5 D. It remains unclear as to the amount of the myopic correction that should be performed when it is combined with focal treatments for correction of asymmetric astigmatism. Refinements of the relatively crude planning process that we used to devise the treatment can readily be imagined. Most essential are improvements in analyzing and interpreting corneal topography to provide accurate determination of the depth and size of required treatmellts to create a uniform optical surface. In principle, the information provided by topographic analysis could be used to generate treatment algorithms which could be incorporated into the laser controls. Another possibility is a molded gel with ablation characteristics identical to corneal tissue which would provide a smoothing surface that could use simple unmodified ablation algorithms. The final therapeutic goal remains to convert a highly irregular corneal surface to a predetermined regular and symmetric surface curvature. Excimer laser photoablation as a treatment of both symmetric and asymmetric postoperative astigmatism will continue to improve. The future holds promise as the crude attempts reported here to correct focal topographic abnormalities in the optical zone of the cornea were sufficiently successful to encourage ways to link analysis of the corneal surface to treatment by excimer ablation. Improvements in analysis and treatment, especially those which join topographical analysis with control of the pattern of excimer laser photoablation may solve the difficult problem of irregular astigmatism.
Gibralter and T rokel . Irregular Astigmatism
References I. Nordan LT. Quantifiable astigmatism correction: concepts and suggestions. J Cataract Refract Surg 1986;12:507-18. 2. Lindstrom RL. The surgical correction of astigmatism: a clinician's perspective. Refract Corneal Surg 1990;6:441-54. 3. Hall GW, Campion M, Sorenson CM, Monthofer S. Reduction of corneal astigmatism at cataract surgery. J Cataract Refract Surg 1991;17:407-14. 4. Seiler T, Bende T, Wollensak J, Trokel S. Excimer laser keratectomy for correction of astigmatism. Am J Ophthalmol 1988;105:117-24.
5. McDonnell PJ, Moreira H, Clapham TN, et al. Photorefractive keratectomy for astigmatism. Initial clinical results. Arch Ophthalmol 1991;109:1370-3. 6. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: A technique for laser refractive surgery. J Cataract Refract Surg 1988;14:46-52. 7. Clapham T, D'Arcy J, Bechtel L, Giockier H, Munnerlyn C. Analysis of an adjustable slit design for correcting astigmatism. SPIE Proc 1991;1423:2-7. 8. McDonnell PJ, Moreira H, Garbus J, et al. Photo refractive keratectomy to create toric ablations for correction of astigmatism. Arch Ophthalmol 1991;109:710-13.
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Correction of all types of refractive astigmatism remains a challenge,1.2 but a challenge which is greatest for the patient with irregular astigmatism. These patients often have undergone surgical procedures, such as pterygium removal, penetrating keratoplasty, and cataract extraction, or refractive procedures as myopic keratomileusis, and radial and astigmatic keratotomy. Sometimes the irregular astigmatism follows trauma or a corneal infection. In many of these patients, a spectacle correction produces poor visual acuity, whereas the best visual acuity obtained using contact lenses may be normal. Astigmatism after surgerl is especially disturbing because the patient will be disappointed with the results of an otherwise satisfactory procedure. A patient with a clear cornea after penetrating keratoplasty will not be satisfied with the residual optical disabilities associated with excessive astigmatism even though the cornea is clear. The astigmatism in all these situations may be regular with a reasonable toric Originally received: October 4, 1993. Revision accepted: February 23, 1994. Manhattan Eye, Ear and Throat Hospital. New York. Edward S. Harkness Eye Institute. Columbia·Presbyterian Medical Center, New York. J
2
Dr. Trokel is a consultant to and has a proprietary interest in Visx Corp. Reprint requests to Stephen L. Trokel, MD, Edward S. Harkness Eye Institute, Columbia-Presbyterian Medical Center, 635 W. 165th St. New York. NY 10032.
1310
surface, even though it is of high degree. On the other hand, the astigmatism may be irregular and asymmetric and not be amenable to treatment with relaxing incisions or with excimer refractive keratectomy using ablation programs designed to remove the to ric elements responsible for regular astigmatism. Studies4 •5 have reported acceptable clinical results using photorefractive keratectomy for the correction of astigmatism. We have developed an approach to patients with irregular astigmatism that has proven capable of producing a more even optical surface on a distorted cornea with improvement in visual function. Photorefractive keratectomy using excimer laser systems operating at 193 nm is under clinical evaluation for correction of myopia, astigmatism, removal of superficial corneal opacities, and treatment of corneal surface disorders. Myopic correction requires6 that the laser flatten the central corneal curvature in a radially symmetric fashion with greater ablation depth in the center compared with the periphery. Removal of opacities requires ablation to a depth sufficient to encompass the pathologic volume and is gently blended toward adjacent normal corneal tissue. Toric corneal surfaces also can be ablated 7 ,8 onto a corneal surface to correct regular astigmatism. However, astigmatism that is not regular or toric cannot be treated by the application of any simple geometric pattern of tissue removal. We have devised a treatment using the excimer laser that addresses this need and has been used to correct
Gibralter and Trokel irregular astigmatism. This approach does not use standard astigmatic ablation patterns, but relies on a treatment plan based on analysis of the topographic image correlated with the biomicroscopic appearance of the cornea. We identify high, steep areas of cornea and ablate them by local, focal treatment using small circular diameter ablations. This technique neutralizes the irregular astigmatism by minimizing the differences between steeper and flatter geographic areas within the optical zone. This selective flattening of the steeper topographic regions is done to create a more regular and spherical anterior corneal surface to improve the optical quality of the corneal surface thereby improving the refractive performance ofthe eye.
Materials and Methods We use an argon-fluoride excimer laser system (Visx TwentyTwenty, Visx, Inc, Sunnyvale, CA) operating at 193 nm, a fluence of 160 mJ/cm 2 and 5 Hz. Myopic corrections are achieved by flattening the cornea with a pattern oflaser ablation determined by a computer-controlled iris diaphragm. Laser correction of astigmatism creates a toric flattening with either sequential cylindrical then myopic ablations, or a single toric ablation. Both of these patterns are oriented to flatten the steep corneal meridian. Neither of these patterns would appear to be useful for the patients with astigmatism as typified by those we report here with highly irregular, postoperative astigmatism (Figs 1 and 2). In one patient, a refractive problem developed after myopic keratomileusis; in the other patient the problem developed after penetrating keratoplasty for keratoconus. To create a regular optical surface, we customized an ablation program using circular elements of varying diameters to flatten identified high areas and a standard myopic ablation to achieve central flattening and eliminate residual myopia. The laser was calibrated with a test 4-diopter (0) myopic ablation in a polymethylmethacrylate block to verify beam fluence and laser system performance. The two patients described here were both mechanically and optically intolerant of their contact lenses. Both were aware of the fact that they were participating in an investigative protocol monitored by the Federal Food and Orug Administration and the Investigational Review Board of the Manhattan Eye, Ear and Throat Hospital. An informed consent was obtained, and both subjects were told of the risks, benefits, and alternative treatments to use of the excimer laser. The examination and refraction were performed by the same investigator (RG) and the corneal computer-assisted topographic analyses (Corneal Modeling System, New York, NY) were performed by the same technician before and after the surgical laser treatments. Treatment was planned by determining elevated areas and their dimensions on the topographic maps. The details of surgical planning are described below, which aimed at creating a more uniform optical surface with a desired average keratometry.
Irregular Astigmatism
Technique The patients received topical proparacaine hydrochloride and 5 mg oral diazepam. They then were positioned under the operating microscope of the excimer laser. The contralateral eye was covered with a light-tight barrier shield during the laser treatment. The patient was instructed to fix on a blinking red light in the delivery system of the laser. A Merocel sponge (Merocel Corp, Mystic, CT) soaked in 4% lidocaine was placed over the cornea for 45 seconds to facilitate epithelial removal. We used a 57 Beaver blade to gently stroke the surface of the cornea with short tangential motions to remove the epithelium over the distorted area to be treated. A mixture of dextran and methylcellulose (Tears Naturale II) was used to moisten a Merocel spear which is used to coat uniformly the stromal surface of the corneal surface to be treated. The computer was programmed to perform circular cylindrical (phototherapeutic keratectomy [PTK]) and refractive (photorefractive keratectomy [PRK]) ablations. The placement of the ablations was based on defining high and steep areas on the corneal surface by analyzing computer-assisted pretreatment topography studies. The 4-mm laser treatments were placed over the steeper corneal zones and then residual myopia was corrected treating directly over the entrance pupil as in photorefractive keratectomy for myopia. After treatment, the patients received tobramycin sulfate-dexamethasone sodium phosphate ointment (Tobradex), and the eye was pressure patched. In addition, the patients were given oxycodone (Percocet) immediately after the treatment and additional analgesia for postoperative pain. The patients were examined daily until complete epithelialization occurred. They were then treated with FML (0.1 % fluorometholone) three times daily for 1 month, followed by once daily for 4 weeks.
Patients and Treatments The first patient was a 24-year-old man who had myopic keratomileusis performed in the left eye in June 1991. This was followed by a second procedure in November 1991 in which partial removal of the surface lamellae was undertaken to eliminate ectopic corneal epithelial interface inclusions. After these procedures, the patient was left with a residual myopic astigmatism. Visual acuity in the nonoperated right eye was 20/200 which was corrected to 20/16 with a -8.50-0 lens. Visual acuity in the operated left eye was 20/200- which improved to 20/25 with a spectacle correction of -2.00 -3.50 X 180. Results of examination of the left eye showed no signs of inflammation, and no conjunctival injection was present. Results of slit-lamp biomicroscopic examination showed the outline of the myopic keratomileusis. Areas of epithelial inclusion and intrastromal haze were apparent at the stromal interface at the 6-0'clock position with a separate area visible at the lO-o'clock position. Centered in the haze at the 6-0'clock position was a 0.5-mm transparent zone which was thought to be either an inclusion cyst or an area of stromal loss in the cap filled from above by an
1311
Ophthalmology
Volume 101, Number 7, July 1994
ill • '
I
I
ill •
I
•
I
,ID
,«J
,41
.. 'lao
· .III
1312
Gibralter and T roke!
Irregular Astigmatism Table 2. Treatment Plan for Patient 2 Based on Analysis of Computerized Keratography·
Table 1. Treatment Plan for Patient 1 Based on Analysis of Computerized Keratography·
Zone A B
C D PTK
Ablation Type
Diameter (mm)
PTK PTK PTK PRK
4 4 4 6
Depth (ILm)
ZO
20 20 20
Refractive Power (diopters)
Zone
0 0 0 -1.50
A B
C D E
= phototherapeutic program; PRK = photorefractive program.
* The treatment plan is shown in Figure 3; focal ablations are based on
PTK
topographic map in Figure 1.
ingrowth of corneal epithelium. The intraocular pressure was 14 mmHg, and the central cornea thickness measured 0.498 mm with ultrasonic pachymetry (DGH-2000, DGH Technology, Frazer, PA). Keratometry of the left cornea showed mild distortion of the mires, and readings were estimated at 40.50/42.50 axis 30 0 • The anterior chamber was deep with no iritis or dilation of iris vessels, the crystalline lens was clear, and results of the retinal examination were normal. The treatment plan for this patient (Fig 3; Table I) was based on biomicroscopic observation and the topographic analysis in Figure I which showed steep areas on the corneal surface. In patient 1, to flatten local steep areas and create a more uniform surface, three confluent 4-mm circular zones in the inferior portion of the myopic keratomileusis lenticle were ablated. Each of these zones were ablated to a depth of 20 ILm, and their edges were barely touching. All 4-mm treatment zones were programmed with an edge transition of 0.35 mm. The final ablation zone was a fourth ablation of 6 mm in diameter with a refractive power of -1.50 0 (20 ILm depth) which was centered over the entrance pupil. The focal excimer laser ablations (Fig 3; Table I) were based on flattening the high or steep zones which involved the lower portion of the myopic keratomileusis lenticle and are seen in the preoperative corneal map (Fig 1). Patient 2 was a 40-year-old woman whose right eye with keratoconus had been treated 23 years previously with a penetrating keratoplasty. Over the last several years, the patient became increasingly intolerant of contact lens use and was unable to wear spectacles because of the significant anisometropia. Visual acuity in the left eye was
=
Ablation Type
Diameter (mm)
PTK PTK PTK PTK PRK
4 4 4 4 6
phototherapeutic program; PRK
Depth (ILm)
10
20 20 20 53 =
Refractive Power (diopters)
0 0 0 0 -4.00
photorefractive program.
* The treatment plan is shown in Figure 4; focal ablations are based on topographical map in Figure 2.
20/25 without optical correction after penetrating keratoplasty followed by relaxing incisions for postsurgical astigmatism. Visual acuity in the right eye was counting fingers at 3 feet but improved to 20/25 with a myopic astigmatic lens of -6.25 -3.25 X 45. Ultrasonic pachymetry in this right eye measured 0.582 mm, and keratometry readings were 51.5/50.5 axis 140 0 with irregular mires. The applanation intraocular pressure was 16 mmHg. Corneal topography (Fig 2) showed that the inferonasal portion of the graft was steep when compared with the superotemporal region. In addition, a relatively steep area was located within the superotemporal region at the 10-0'clock position. Based on the topographic analysis and the slit-lamp appearance, the excimer treatment plan was designed as five separate treatment zones (Fig 4; Table 2). Zone I was a 4-mm PTK treatment diameter to a depth of 10 ILm . Zones 2, 3, and 4 were also 4-mm in diameter PTK treatments and each was to a depth of 20 ILm. Some blending of zones 2,3, and 4 was achieved by moving the eye from treatment zone 2, 3, and then to 4 as the treatment proceeded. A central 6-mm-diameter myopic PRK was performed over the entrance pupil for a - 3.81-0 correction at the corneal plane which required a 53-lLm treatment depth. The patient was intentionally undercorrected because of concern related to the contribution of the focal treatments to the overall corneal flattening. In both patients, the peripheral therapeutic treatment zones were intentionally placed so that they overlapped the central 6-mm treatment zone by I mm. This was done
( Top left, Figure 1. Topographic map of patient 1. The optical distortion resulted after multiple keratorefractive procedures. Top right, Figure 2. Topographic map of patient 2. The local steepening associated with the corneal graft and irregular astigmatism is visible inferiorly. Center left, Figure 3. Treatment plan for patient 1 (see Table 1). Center right, Figure 4. Treatment plan for patient 2 (see Table 2). Bottom left, Figure 5. Posttreatment topographic corneal map of patient 1. Bottom right, Figure 6. Posttreatment topographic corneal map of patient 2.
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Volume 101, Number 7, July 1994
to avoid creating a local discontinuity at the intersection of the PRK and the PTK ablation zone.
Results At 1 month after treatment, patient 1 had a visual acuity without correction of 20/50 and a corrected visual acuity of 20/25 with +0.50 -1.50 X 135. At 3 months, visual acuity without correction had improved to 20/30 without spectacle correction and 20/25 with a -0.25 lens. The refraction stabilized at 6 months with an uncorrected visual acuity of 20/50 and a refraction of -0.75 -0.50 X 50. The final topographic map can be reviewed in Figure 5. Visual acuity in patient 2 was 20/40 without spectacle correction which improved to 20/20 with a -1.50 -2.25 X 65 corrective lens 1 month after the PTK procedure. At 3 months after surgery, the patient's uncorrected visual acuity was 20/40-, correcting to 20/25 with a -2.50 -2.25 X 75 corrective lens. At 6 months, the uncorrected visual acuity stabilized at 20/50, improving to 20/25 with -2.75 -2.50 X 75. The improvement in the corneal surface is reflected in the map shown in Figure 6.
Discussion The choice of the depth of treatment for the localized steep zones was based on an approximation relating the thickness of a solid spherical element to its diameter for each diopter of power. Depth in microns per diopter
=
1/3 diameter2 (7)
This equation approximates the depth of each dioptric elevation at a given diameter. That is, a 3-mm diameter elevation will be 3 ILm per diopter, a 4-mm diameter will be approximately 5 ILm thick per diopter, and a 5-mm diameter will be approximately 8 ILm thick for each diopter of power visible in the keratographic map. The thickness varies linearly with the power but with the square of the diameter. Depth in microns
=
1/3 diameter2 X Diopters(7)
This means that approximately 4 D of refractive power will represent 20 ILm of thickness in a 4-mm diameter on the corneal surface. While this relation relates to spherical volumes, it also serves as a guide to the thickness of local irregular areas of corneal steepness. It can be used to calculate depths of zones of relative corneal steepening which contribute to the asymmetric astigmatism in the optical zone of these patients. The limits of this analysis are based on the smoothing functions of the topographic software which tend to average changes over several millimeters to present a more readily interpretable image. The degree of flattening desired and the location of the treatment zones were based on analysis of the preoperative corneal topographic analysis, resulting in the treatment plans detailed in Tables 1 and 2. Both patients
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had postoperative topographic analysis (Figs 5 and 6) which showed that the treatment produced a more spherical and regular corneal surface as well as the central flattening which was desired to correct the residual myopia in addition to the correction of the irregular astigmatism. There has been contention over whether to treat the refractive state or to alter the treatment plan by identifying local abnormalities of the corneal surface. In these two patients, the extensive distortion and asymmetry of the corneal surface suggested that a standard spherocylindric ablation would not be effective in improving the refractive state of the eye. The extensive corneal unevenness implied that focal treatment and flattening of the localized steep zones might improve the regularity of the corneal surface and the visual function of the patient. The hypothesis can be entertained that a refraction may not provide the best way to alter the corneal surface even though it provides an adequate level of vision. The computer-assisted corneal modeling system helped to identify the steeper regions within the optical zone. It was also unclear to what extent treating these local steep zones would add to the central myopic PRK treatment. In patient 1, there was a pretreatment myopia of - 2.00 D. Only -1.50 D were programmed for treatment to prevent overcorrection. Similarly, the residual myopic correction of patient 2, who had a myopic refraction preoperatively of -6.25, was intentionally undercorrected to only -4.00 s to prevent a hyperopic shift. This resulted in a significant undercorrection of - 2. 7 5 D. It remains unclear as to the amount of the myopic correction that should be performed when it is combined with focal treatments for correction of asymmetric astigmatism. Refinements of the relatively crude planning process that we used to devise the treatment can readily be imagined. Most essential are improvements in analyzing and interpreting corneal topography to provide accurate determination of the depth and size of required treatmellts to create a uniform optical surface. In principle, the information provided by topographic analysis could be used to generate treatment algorithms which could be incorporated into the laser controls. Another possibility is a molded gel with ablation characteristics identical to corneal tissue which would provide a smoothing surface that could use simple unmodified ablation algorithms. The final therapeutic goal remains to convert a highly irregular corneal surface to a predetermined regular and symmetric surface curvature. Excimer laser photoablation as a treatment of both symmetric and asymmetric postoperative astigmatism will continue to improve. The future holds promise as the crude attempts reported here to correct focal topographic abnormalities in the optical zone of the cornea were sufficiently successful to encourage ways to link analysis of the corneal surface to treatment by excimer ablation. Improvements in analysis and treatment, especially those which join topographical analysis with control of the pattern of excimer laser photoablation may solve the difficult problem of irregular astigmatism.
Gibralter and T rokel . Irregular Astigmatism
References I. Nordan LT. Quantifiable astigmatism correction: concepts and suggestions. J Cataract Refract Surg 1986;12:507-18. 2. Lindstrom RL. The surgical correction of astigmatism: a clinician's perspective. Refract Corneal Surg 1990;6:441-54. 3. Hall GW, Campion M, Sorenson CM, Monthofer S. Reduction of corneal astigmatism at cataract surgery. J Cataract Refract Surg 1991;17:407-14. 4. Seiler T, Bende T, Wollensak J, Trokel S. Excimer laser keratectomy for correction of astigmatism. Am J Ophthalmol 1988;105:117-24.
5. McDonnell PJ, Moreira H, Clapham TN, et al. Photorefractive keratectomy for astigmatism. Initial clinical results. Arch Ophthalmol 1991;109:1370-3. 6. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: A technique for laser refractive surgery. J Cataract Refract Surg 1988;14:46-52. 7. Clapham T, D'Arcy J, Bechtel L, Giockier H, Munnerlyn C. Analysis of an adjustable slit design for correcting astigmatism. SPIE Proc 1991;1423:2-7. 8. McDonnell PJ, Moreira H, Garbus J, et al. Photo refractive keratectomy to create toric ablations for correction of astigmatism. Arch Ophthalmol 1991;109:710-13.
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