Refractive Surgical Problem

consultation section edited by Thomas Kohnen, MD

refractive surgical problem

A 67-year-old white woman with a preoperative subjective manifest refraction of -5.00 -1 .75 x 15 in the right eye and -5.25 - 1.50 x 155 in the left eye, a cycloplegic refraction of -5.25 -2.25 x 10 and -5 .75 -1.75 x 172, respectively, and a central corneal thickness of 498 and 477 IJ.m, respectively, had photorefractive keratectomy (PRK) (-5.00 -2.00 x 10) in the right eye with a VISX 20/20 excimer laser and an ablation zone of 6.0 mm. Ablation depth was 70 IJ.m using 300 pulses. Her postoperative course was uneventful except for central haze of 1.0, which gradually decreased over the following 7 months with topical steroid treatment. The uncorrected visual acuity (UCVA) of 0.7 did not improve with spectacles. The patient was very satisfied and happy with the result and requested treatment in the other eye. Photorefractive keratectomy was performed in the left eye using a wide-area-ablation laser with an optical zone (OZ) of 6.7 mm and an anti-central-island program . The laser was programmed to -5.50 -1.75 x 180. Ablation depth was 100.25 IJ.m using 350 pulses. The patient developed central haze of 1.0 in the left eye even though she had extensive postoperative topical steroid treatment. Best spectacle-corrected distance acuity 6 weeks postoperatively was 0.4 and refraction, +0.75 -1.25 x 75. Computerized videokeratography (CVK) at that time revealed a central island (Figure 1, left).

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Eight months after PRK, UCVA in the left eye was 0.2; best corrected visual acuity (BCVA) was 0.4 with -1 .00 -0.50 x 145, and cycloplegic refraction was 0.4 with -0.50 -0.50 x 145. Computerized videokeratography still showed a central island (Figure 1, right), and the central cornea still had a haze grade 0.5. Best spectacle-corrected visual acuity (BSCVA) is now 0.9 in the right eye with a correction of -0.25 diopter (0) and only a slight trace of haze. The patient is unhappy with her current vision in the left eye. What are your recommendations and treatment options?

• This 67-year-old woman presents an extreme example of a common complication of surface PRK; namely, a central island after the treatment in her left eye. The incidence of central islands after myopic PRK or photo astigmatic refractive keratectomy is between 30 and 70%1-4 in consecutive series in which CVK is used to detect it. When the definition of central island includes a magnitude of greater than 3.00 D in CVK height, the incidence is between 304 and 50%.5 The incidence and size are said to correlate with the dioptric amount of myopic correction attempted as well as the actual depth of tissue removed. 2

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Figure 1. (Kohnen) One month (Jeft) and 8 months (right) after PRK for myopia, CVK (TMS) showed almost unchanged , 3.0 mm central islands in the left eye.

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Central islands are said to be more common when a broad-beam excimer laser is used (as in this case), and may account for some astigmatism and loss of BCVA. I am unsure whether this patient lost BCVA as the preoperative visual acuity was not given. If BCVA was lost, a potential acuity meter reading and a hard contact lens overrefraction would help confirm that the cornea was the cause of the problem. Central islands are thought to be caused by the production of a vapor plume over the central cornea during the ablation, differences in hydration between the central and peripheral ablation zone, postoperative epithelial hyperplasia, or a combination, hence the use of a nitrogen blower or suction nozzle over the cornea during treatment to minimize the plume. The natural history of central islands is that they resolve spontaneously over 6 months after the initial treatment in 90% of cases. 2 Between 26 and 10%2 of cases may persist to 12 months but rarely longer. If one looks closely at the two CVK images in Figure 1, one can see that the height of the central island has decreased from about 6.00 to 4.00 D between the 1 and 8 month examinations. (The scales for the two examinations are not congruent.) Given this, and the natural history outlined above, I feel it is appropriate to wait at least 12 months before considering re-treatment. If there were a continued trend toward resolution, I would continue to wait. If resolution did not occur, I would elect to re-treat using PRK with a treatment zone corresponding to the residual island (usually about 3.0 mm). I believe that removing the epithelium in the photo therapeutic keratectomy (PTK) mode (a transepithelial ablation) can be done to use the epithelium as a masking agent, and I would elect to do this. I would program the laser with the full dioptric height of the island as indicated on the CVK, provided that I was convinced there was not a large amount of epithelial hyperplasia. I would attempt to treat over the region of the island as indicated on the CVK. Finally, I am unsure why a 6.7 mm OZ was used in this case. There was no suggestion of problems using a 6.0 mm zone in the other eye, and the 6.7 mm zone implies a deeper ablation, which is more likely to give rise to a central island. LAURENCE SULLIVAN, FRACO

Melbourne, Australia 1282

References 1. Schimmick ]K, Telfair WB, Munnerlyn CR, et al. Corneal ablation profilometry and steep central islands. ] Refract Surg 1997; 13:235-237 2. McGhee CN], Bryce IG. Natural history of central topographic islands following excimer laser photorefractive keratectomy. ] Cataract Refract Surg 1996; 22:1151-1158 3. Schmidt-Petersen H, Seiler T. "Central islands"-an early postoperative complication after photo refractive keratectomy. Klin Monatsbl Augenheilkd 1996; 208:423-427 4. Levin S, Carson CA, Garrett SK, Taylor HR. Prevalence of central islands after excimer laser refractive surgery. ] Cataract Refract Surg 1995; 21:21-26 5. Krueger RR, Saedy NF, McDonnell Pl. Clinical analysis of steep central islands after excimer laser photorefractive keratectomy. Arch Ophthalmol 1996; 114:377-381 6. Lin DT. Corneal topographic analysis after excimer photorefractive keratectomy. Ophthalmology 1994; 101: 1432-1439

• The initial history raises several interesting points bearing on the major issue of the central island. One wonders about the decision to proceed with PRK in the second eye when the UCVA of only 0.7 (approximately 20/30) could not be improved with spectacles, despite the patient's perception of being "very satisfied and happy." The second issue is the decision to expand the OZ from 6.0 to 6.7 mm without a history of symptoms related to the original OZ size. Extensive clinical experience has demonstrated that central islands are rare with wide area ablation zones of 5.0 mm or less; they become common although not universal at 6.0 mm and extremely common and troublesome at 6.5 mm or larger. Even though the laser used was stated to have an anti-island program, it was inadequate for this patient. A final caveat is that Placido-image-based topography units may erroneously diagnose a central depression as an elevation. The myopic shift in this patient is a strong indication that this is a true elevation rather than a depression, which should cause hyperopia. Although the history repeatedly mentions haze, the loss of BCVA is almost certainly solely the result of the elevated central island. In the case presented, we now have 8 months follow-up. Central islands generally improve over time, and most do not need treatment if 6 or 12 months of follow-up are provided. My usual practice is to consider re-treatment after 6 months if there is no evidence of improvement; in the current case, the principal difference from the 6 week to the 8 month topography is the additional connection of

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the island to the periphery at 4 o'clock, probably representing secondary epithelial thickening or new collagen deposition. The island itself is virtually unchanged. Re-treatment is therefore indicated. In the absence of specific reliable data on the true elevation of the island, I base re-treatment on a calculation from the Munnerlyn formula. The diameter of the island in millimeters is squared, multiplied by the dioptric elevation, and divided by 3, giving the amount of tissue that must be removed. Most clinical excimer lasers remove approximately 0.25 IlID per pulse. Therefore, multiplying the thickness value by 4 gives the number of pulses necessary to bring the island down to the level of the surrounding ablation zone. In the topography images supplied, the diameter is not given, nor do we have the specific values for the dioptric value of the peak of the island compared with the surrounding blue area. The color-coded bars have a large range. It would appear, however, that the island may have approximately 5.00 D of elevation and a diameter of approximately 3.0 mm. In that case, the elevation might be as much as 15 1lID, which would require 60 pulses to treat. I have never seen an island this large or high, and I would err on the side of conservatism. I would apply only 30 or 35 pulses with a 3.0 mm OZ and allow the eye to heal, assessing the effect before possibly giving further treatment. Before the island itself is ablated, it is necessary to remove the epithelium. I prefer a transepithelial ablation, monitoring the fluorescence of the epithelium with all the lights off. The OZ should be slightly larger than the island, such as 4.0 mm using PTK. Typically, the epithelium will break through at the peak of the island first, which would be visualized as a nonfluorescent (black) spot. The laser is stopped as soon as the first breakthrough is visualized. The counter is then noted, and further PTK pulses can be applied up to the total number per the above calculation. Or, the laser can be reprogrammed for PTK with the number of calculated pulses. The eye is treated using the same regimen as after routine PRK. I apply a bandage soft contact lens and use oxyfloxacin (Ocuflox®) and fluorometholone (FML®), both four times a day, until re-epithelialization (usually 48 hours because of the smaller treatment zone). After re-epithelialization, all medication is discontinued. In particular, steroids are avoided. After stable

re-epithelialization, one would expect a reduction or complete elimination of the residual myopia because central islands typically result in a myopic shift in the refraction caused by the elevation of the central island. ROGER F. STEINERT, MD Boston, Massachusetts, USA

• This patient had two different excimer laser ablation profiles. In the right eye, a 6.0 mm diameter ablation using 300 pulses to achieve an ablation depth centrally of 70 IlID was performed for a spherical equivalent of -6.00 D. In the left eye, for a spherical equivalent of -6.37 D, a 6.7 mm diameter ablation zone was used, with 350 pulses to achieve a central ablation depth of 100.25 1lID. Eight months after surgery, the results in the right eye were excellent, whereas in the left eye with a spherical equivalent of -1.25 D, the BCVA is 20/50. Videokeratography shows a small central island in the left eye, surrounded by a larger area of central steepening. The patient's BCVA is reduced from a preoperative level of presumably 20/20 to a postoperative level of 20/50, and yet the left central cornea has very mild haze. Excluding the possibility of lenticular opacity or retinal change, we must ascribe the decreased vision to the presence of the central island in the left eye. Since 8 months have elapsed and the patient has not had a significant improvement, it is best to perform a repeat transepithelial PRK in the left eye with a spherical goal of 1.00 D or less. Less than 1.00 D can be achieved by programming the laser to correct 1.00 D and stopping the ablation prematurely. Before performing the repeat PRK' however, I would assess the lens and macula and perform a pinhole acuity measurement with or without the presence of a hard contact lens to assure myself that the decreased vision was due solely to the cornea. Qualitative assessment of the keratometry mires and the dilated red reflex will help the examiner feel confident that the decreased vision is, in fact, the result of the central island. It is interesting that anti-island software was used and the patient still developed a significant central island. The larger the ablation diameter, the greater the chance that the acoustic shock wave will "push" water from the corneal stroma into the area of the ablation. Since the anti-island software was probably designed

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for 6.0 mm diameter ablations, it was probably inadequate for preventing a central island in this patient. PERRY S. BINDER,

MD

San Diego, California, USA

• First, given the age of the patient and the prolonged topical steroid treatment, it is important to evaluate the lens and intraocular pressure (lOP). Only if the lens is really clear and there is no lOP change should one consider re-treatment. In the clinical description, the preoperative BCVA is not specified; however, it is reasonable to assume it was 20/20 in both eyes. Therefore, the patient has two complaints-unsatisfactory UCVA and decreased BSCVA-caused by the combined effects of corneal haze, irregular astigmatism (central island related), and undercorrection. My suggestion would be to perform a corneal retreatment. The choice of technique is related to corneal clarity. If a mild subepithelial scar is present (haze 0.5), PTK using a masking fluid agent (hyaluronic acid 0.4%) is the best choice to regularize the corneal profile. The VISX 20/20 laser should be set for the largest beam diameter using a PTK program. After the epithelium is manually removed, some protuberances on the stromal surface are usually visible. The procedure is completed when the corneal surface is smooth and intraoperative videokeratography is regular. On the contrary, if the cornea is clear, a new PRK with a small ablation area (4.0 to 4.5 mm) should be performed, centering the treatment over the pupil. The laser will be programmed for the entire subjective manifest refraction of the patient (-1.00 -0.50 X 145). At the end, based on corneal surface regularity and intraoperative videokeratographic data, it could be beneficial to perform PTK, always using a masking fluid agent. After the treatments, I suggest using homatropine 1% drops, norfloxacin 0.3% drops every 6 hours for 5 days, and then fluorometholone 1% drops three times per day, tapering and titrating the steroids in the follow-up on the basis of refraction and corneal clarity. Although the patient may end up with low hyperopia, UCVA and BCVA should improve. LEOPOLDO SPADEA,

MD

Rome, Italy 1284

• I recommend that the central island be removed with a PRK program that allows ablation of a PRK element with the diameter and power of the island as determined topographically. I would make no effort to correct the apparent undercorrection as removal of the island will change the refraction. A hard contact lens overrefraction may be obtained to prove that the irregular corneal surface is the cause of the reduced visual acuity. STEPHEN

L. TROKEL, MD

New York, New York, USA

• The central island in the left cornea of this patient (or any central island that does not regress spontaneously) can be successfully treated with a PTK strategy pioneered by Paolo Vinciguerra, MD, in Italy. This PTK technique involves the manual removal of the epithelium and the use of hyaluronic acid 0.4% as a masking fluid, with the laser set in a 9.0 mm diameter PTK mode. The masking fluid allows step-by-step ablation of the central island without creating new irregular transition zones. Between steps, the patient is taken to a corneal topography unit and monitored intraoperatively as the height of the central island decreases with each PTK step. Any standard corneal topography system can be used for this intraoperative monitoring. This PTK strategy can be very time consuming but is the only means of obtaining optimal optical conditions after complications such as central island or other irregularities, which impair post-PRK (or post-LASIK) BCVA. Many refractive surgeons have attempted to correct central islands by treating only the relevant zone (in this case, 3.0 mm of the persistent central island). However, such an approach is fraught with pitfalls because standard corneal topography algorithms tend to exaggerate the diameter of the zone involved. In this case, the central island may have an actual diameter smaller than 3.0 mm (e.g., 2.5 mm). If a 3.0 mm zone is ablated, the surgeon may be treating too large an area, creating new, poor transition zones and new optical aberrations. Because videokeratography will likely supply an inaccurate diameter and that diameter will be used to calculate ablation depth, a further confounding factor enters the equation. If the central island diameter is actually smaller than that derived from videokeratography, the planned ablation

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based on the inaccurate diameter assumption will be too deep. This will further compound the re-treatment error by creating an additional multifocal lens in the center of the cornea, with the center of the re-treated area becoming flatter than its periphery. In contrast, the PTK strategy with its intraoperative monitoring is independent of such false assumptions because it can adapt itself to the real-life anatomic conditions that cause the optical problem. DANIEL EpSTEIN, MD, PHD

Zurich, Switzerland

• I had one patient who was not satisfied with his vision because of haze and irregular astigmatism caused by a central island. This patient, who was in his 50s, had had PRK in 1990 with an ExciMed UV200 LA laser (Summit) and a 4.5 mm OZ. His preoperative refraction was -7.00 D. In the beginning, his UCVA was good and he was satisfied with the result. However,

severe haze developed 2 months postoperatively and continued. Regression occurred soon after even with continuous steroid therapy. His BCVA decreased to 0.4 with a refraction of -5.00 D. I believe this case resembles the one presented here. In my patient, I performed PTK with a Chiron Technolas laser in 1994 with the intention of making the surface smoother. The OZ was 5.0 mm, and 500 pulses were used. The patient's BCVA improved to 0.9 with a refraction of -1.00 D. The severe, ringshaped haze remained around the PTK area, but the patient was satisfied after this second treatment. In conclusion, I would suggest performing PRK with a 4.5 mm OZ. About 200 to 300 pulses without epithelial removal are recommended. The patient may become slightly hyperopic but to improve BCVA, this disadvantage must be accepted. KIMIYA SHIMIZU, MD

Tokyo, Japan

edited by Samuel Masket, MD

A 22-year-old man seeks your opinion regarding the following problem. According to his history, he had bilateral cataract surgery for familial cataract when he was approximately 10 years old. The left eye had extracapsular cataract extraction (ECCE) without intraocular lens (IOL) implantation. The right eye, operated on second, had a posterior chamber IOL (PC IOL) placed in the sulcus after ECCE with a "can-opener" capsulotomy. The left eye did well with a contact lens until the patient sustained a retinal detachment that required several repairs, including vitrectomy. Currently, the eye is aphakic and the retina attached; however, best corrected visual acuity is only 20/200. The right eye maintained 20/20 corrected acuity with the IOL but also required retinal reattachment, which was successful. Recently, 12 years after the original cataract procedure, the IOL dislocated inferiorly within the sulcus. The PC lens was removed and replaced with an anterior chamber IOL (AC IOL) that measured 13.5 mm in loop length. The surgeon noted that the lens appeared small but seemed stable at the time of surgery. However, the patient observed fluctuating vision, the surgeon observed that the lens was short, and both have requested an alternative evaluation.

Examination of the right eye revealed that the corneal diameter was large, measuring 14.5 mm in horizontal, "white-to-white measurement". In addition, the lens appeared short, was tilting forward above, and was riding somewhat low (Figure 1); pseudophakodonesis was present. The cornea appeared healthy with respect to the endothelium. The iris was normal, although the pupil

Figure 1. (Masket) The AC IOL appears short, is tilting forward above, and is riding somewhat low.

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