LASIK Fundamentals, Surgical Techniques, and Complications - part 9 doc

19. IG Pallikaris, DS Siganos. Laser in situ keratomileusis to treat myopia: early experience. J Cataract Refract Surg 1997;23:39–49. 20. I Toda, Y Yagi, S Hata, S Itoh, K Tsubota. Excimer laser photorefractive keratectomy for pa- tients with contact lens intolerance caused by dry eyes. Br J Ophthalmol 1996;80:604–609. 21. A Loewenstein, I Lipshitz, D Varssano, M Lazar. Macular hemorrhage after excimer laser pho- torefractive keratectomy. J Cataract Refract Surg 1997;23:808–810. 22. AM Mansour, GK Ojeimi. Premacular subhyaloid hemorrhage following laser in situ ker- atomileusis. J Refract Surg 2000;16:371–372. 23. E Yavitz. LASIK study shows brimonidine provides neuroprotective effect. Ocular Surg News 1999;10:48. 24. S Nath. Study shows NFL safe after LASIK. Ocular Surg News 2000;11:35. 25. YY Tsai, JM Lin. Effect of laser-assisted in situ keratomileusis on the retinal nerve fiber layer. Retina 2000;20:342–345. 26. R Gurses-Ozden, ME Pons, C Barbieri, H Ishikawa, DF Buxton, JM Liebmann, R Ritch. Scan- ning laser polarimetry measurements after laser-assisted in situ keratomileusis. Am J Ophthal- mol 2000;129:461–464. 402 Adelman et al. 403 30 Management of Topographical Irregularities Following LASIK JEFFREY JOHNSON Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A. ROSELYN JEUN Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, U.S.A. DIMITRI T. AZAR Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston, Massachusetts, U.S.A. The importance of computerized corneal videokeratography in the evaluation and manage- ment of laser in situ keratomileusis (LASIK) patients cannot be overstated. While corneal topography is important in the preoperative stages of refractive surgery as a screening de- vice for corneal irregularities, including contact lens warpage and keratoconus, it is also necessary postoperatively, as mild corneal irregularities can often be detected only through topographical analysis. These corneal irregularities include central islands, ablation decen- tration, corneal ectasia, and other forms of irregular astigmatism. Despite their low inci- dence, these complications can lead to patient dissatisfaction, as image quality can be adversely affected. As frustrating as these cases can be to patients, they can be equally frus- trating to the refractive surgeon. Not all surgically induced topographical irregularities fol- lowing LASIK are amenable to further treatment, and depending upon the type of irregu- larity found, treatment options often vary considerably. Therefore, these complications must be approached cautiously, and a thorough understanding of the treatment options and the likelihood of success is essential. A. CENTRAL ISLANDS Central islands were first described following automated lamellar keratoplasty (ALK) and occur in varying frequency following PRK and LASIK (1–7). Kreuger and colleagues de- scribed central islands following excimer laser ablation as central areas of steepness at least 1.5 mm in diameter and 3.0 diopters in height noted with corneal topography (8). The def- inition has changed over the years and we now understand that any central area of steep- ening may induce irregular astigmatism and interfere with visual acuity (Fig. 30.1). While central islands tend to resolve in most patients following PRK (9,10), they may be more likely to persist following LASIK (11,12). One recent review showed an incidence of central islands of 5.7% following LASIK, and only 25% of those resolved over a 6 month period (13). Because of the limited regression that may occur in these patients, surgical in- tervention is often required. 1. Nonsurgical Management Prior to attempting surgical intervention, monitoring the patient for 6 months may be valu- able to determine if regression or progression of the central island has occurred. Partial im- provement in symptoms (shadows, double vision, decreased contract sensitivity, etc.), un- corrected visual acuity (UCVA), best spectacle corrected visual acuity (BSCVA), and topographical appearance (Figs. 30.2A and 30.2B) are all signs that further monitoring may be required. It is often difficult to differentiate between a true central island and corneal ectasia. The latter is less likely to show substantial improvement with time. In fact, corneal ectasia is often a progressive condition that may worsen if retreatment is attempted. Therefore if improvement in symptoms and central island appearance occur during this follow-up period, it is more likely the patient has a true central island as opposed to a psuedo–central island caused by corneal ectasia. 404 Johnson et al. Figure 30.1 Central island occurring after LASIK. During this follow-up period, patient education regarding the occurrence and natural progression of central islands is critical in alleviating the frustration resulting from reduced quality of vision. After PRK, epithelial hyperplasia in the mid-peripheral area surrounding the central island and epithelial thinning overlying the central island may lead to complete topographical and visual improvement (10,14). Furthermore, treatment of residual central islands after PRK is less likely to induce corneal ectasia because of the greater residual stro- mal thickness present following PRK. Because LASIK is associated with a decreased ep- ithelial healing response as compared to PRK, central island regression after LASIK may occur more slowly and retreatment may carry greater risk, especially if the residual stromal bed is relatively thin. Topographical Irregularities Following LASIK 405 Figure 30.2 Improvement of central island over time. (A) Central island occurring immediately following LASIK with topographical improvement 6 months later. (B) Difference map showing re- gression of central island over the 6 month period. A B During this time, the patient may benefit from wearing a contact lens to improve vi- sual functioning. Often a rigid gas permeable contact lens is required. Standard design RGPs may be adequate with a base curve chosen slightly steeper than the postoperative central corneal curve (15). This will likely yield a fluorescein pattern with mild central pooling and mid-peripheral alignment (15–17). Mid-peripheral alignment should be the goal when fitting these lenses, as this will be the area of the cornea supporting the contact lens. If a satisfactory fit is not achieved due to the corneal shape change induced by LASIK, then specialty RGP designs may be required. Reverse geometry lenses, in which the cen- tral base curve is flatter than the surrounding steeper peripheral curves, are available (18). If the patient is intolerant to RGP contact lenses, a soft toric contact lens may be tried (19). A careful refraction is necessary to detect fully the amount of astigmatism that these central islands can create. Correcting a significant portion of this astigmatism in a soft toric contact lens may not provide the crisp, consistent visual acuity that an RGP lens could pro- vide, but overall functioning may be improved. The main concern when treating central islands with contact lenses is induced corneal irregularity. A rigid contact lens will likely bear on the central island, and this can lead to curvature and elevation changes. Therefore, if a patient is fitted with a contact lens while awaiting surgical intervention, removal of the lens for 2 to 4 weeks prior to further treat- ment is advisable. Serial topographies and repeat refractions should be performed every 2 weeks until stability is confirmed. 2. Surgical Intervention If a true central island does not regress to a satisfactory level, surgical intervention may be required. Prior to central island treatment, the power (in diopters) and diameter (in mm) of the island, as well as the patient’s manifest refraction, must be determined. The power of the central island can be determined by placing the cursor at the apex of the central island and then comparing this point to the average of four cardinal points at the base of the cen- tral island. These numbers can be determined by moving the cursor to the desired locations or simply printing out a numeric map and/or a profile map (which are available on most systems) (Figs. 30.3 and 30.4). To determine size of the central island, the grid overlay can be utilized where each box represents 1 mm in size. The power of the central island is then compared to the refraction, which is used to determine the desired correction. If the central island power matches the refraction, Munnerlyn’s equation (20) can be used to determine the number of laser pulses necessary for treatment: Ablation depth ϭ ᎏ S 2 3 * D ᎏ where S ϭ diameter of central island in millimeters and D ϭ desired correction in diopters. Therefore, if a central island 2.5 mm in diameter and 5 diopters in height has resulted in 5 D of induced myopia, then 10.41 ␮m (2.5 2 * (5/3)) of tissue ablation will be required. It is estimated for most lasers that each laser pulse removes 0.25 ␮m of tissue, and therefore 42 pulses would be required. If a discrepancy between the central island power and refraction is encountered, sev- eral authors have advocated using the central island power instead of refraction and over- compensating for the estimated height of the central island when using Munnerlyn’s equa- tion (i.e., multiply the central island height D by 1.5). This would be done in order to avoid 406 Johnson et al. Topographical Irregularities Following LASIK 407 Figure 30.3 Profile map showing a 5–6 diopter (approximately) central island. Figure 30.4 Numeric map showing same central island as in Fig. 3, confirming approximate height of 5 diopters. 408 Johnson et al. Figure 30.5 (A) Central island appearance prior to surgical intervention. (B) Central island fol- lowing initial (conservative) surgical intervention with partial resolution of island. A B undercorrection (21,22) and the laser would be set to PTK mode. In the example above, the number of laser pulses would increase from 42 to 63. While this is only a minor increase in the number of pulses, our strategy is to approach these cases in a conservative manner and limit treatment to PRK mode based upon reproducible refraction, if available. Further CI treatment can be applied in the future if undercorrection of the island occurs (Fig. 30.5A and 30.5B). Several caveats must be considered when determining the required refractive correc- tion. If the central island is visible on elevation-based topography and the estimated num- ber of pulses determined using Munnerlyn’s equation correlates with the manifest refrac- tion, then eliminating the central island may lead to satisfactory improvement of UCVA after a single treatment. More likely, the irregular astigmatism and decreased BSCVA as- sociated with the CI may have created a variable refraction that is not extremely accurate. If the central island’s power and diameter predicts a significantly different number of laser pulses as compared to the manifest refraction, then the treatment may require a two-step ap- proach. The central island should be treated based upon the refractive data and the pre- scription monitored during follow-up visits. If this process topographically eliminates the central island, yet a refractive error still exists, then further ablation may be required after corneal stabilization is established 3 to 6 months later. Again, these ablations should be performed in PRK mode owing to the smooth abla- tion profile that PRK creates. While PTK allows for the ablation zone diameter to be pro- grammed in 0.1 mm steps from 2.0 to 6.5 mm, it may create abrupt treatment edges. In PRK mode, the treatment should be halted when the ablation diameter reaches the predetermined diameter of the central island (which is determined by monitoring the number of pulses us- ing a wide area ablation laser). In the future, topography-linked lasers may lead to improved outcomes when treat- ing central islands (23,24). The ability to link the topographical data directly to a scanning spot laser may eliminate or reduce the estimation errors that can occur with current treat- ment equations. Furthermore, a topography-linked system could aid in centering the treat- ment directly over the central island. Ideally, such treatments would allow for topographi- cal analysis to be performed once the LASIK flap has been retracted, so that the true curvature of the stromal bed could be obtained (25). Trials are currently underway utiliz- ing topography-linked laser treatments, although early results have been somewhat equiv- ocal (26). Wave front analysis will also become more important in the future when assessing central island position and optical effect. Currently, a patient with a central island is mon- itored based upon subjective symptoms and elevation-based topographical analysis. As wave front technology becomes more commonplace, we will be able to understand better the amount of distortion that a specific central island is creating. This may allow for im- proved monitoring of the patient, as it is a further objective measurement of possible re- gression of the central island. 3. Prevention While no single etiology has been proven, several preventative strategies for central islands do exist. First, newer laser ablation profiles (scanning lasers) seem to have reduced the in- cidence of central islands (27). Secondly, many laser companies have developed algorithms in which a series of laser pulses is initially applied to the central stromal bed. These pulses are applied just prior to the actual prescriptive ablation and can be performed in PRK or PTK Topographical Irregularities Following LASIK 409 mode. This anti-island software is available on all of today’s commercially available wide- area ablation excimer lasers. Also, when performing highly myopic treatments, intraopera- tive wiping of the stromal bed is often recommended to reduce the risk of fluid and cellular debris accumulation. This buildup can adversely affect the uniformity of the laser ablation and has been implicated in the formation of central islands. Minimizing the time between flap creation and laser treatment is also important in order to prevent localized differences in stromal bed hydration levels. Finally, proper laser maintenance and strict adherence to calibration will help prevent equipment errors that may be contributing to this problem. B. DECENTRATION Ablation decentration can occur following LASIK owing to misalignment of the laser treat- ment over the patient’s entrance pupil or from involuntary eye movement of the patient dur- ing laser treatment (Fig. 30.6) (28,29). Decentration can lead to patient symptoms of monocular diplopia, shadows, glare, and reduced contrast sensitivity (both high and low contrast) (30). These complications are often due to induced irregular astigmatism and are associated with a loss of best spectacle corrected visual acuity. While slight decentra- tion typically will cause no patient complaints (31), significant decentration can be problematical. Ablation decentration should be differentiated from intraoperative treatment drift, a complication that can occur if the surgeon attempts to compensate intraoperatively for an observed decentration (Fig. 30.7) (31). Treatment drift can result in an uneven ablation profile with a flatter treatment zone shifted peripherally. If the treatment drift is due to 410 Johnson et al. Figure 30.6 Nasal decentration of ablation following LASIK. Figure 30.7 Treatment drift is illustrated (left) with a contrasting illustration of a centered laser treatment after PRK (right). The two patients were treated with similar number of pulses for similar preoperative refractive errors, but ended with different topographical maps. The phenomenon of intraoperative treatment drift is best illustrated with tangential topography and is best avoided using a tracking laser. (From Ref. 31.) [...]... keratomileusis to correct myopia of Ϫ6.00 to Ϫ 29. 00 diopters J Refract Surg 199 6;12:575–584 MC Knorz, B Wiesinger, A Liermann, V Seiberth, H Liesenhoff LASIK for moderate and high myopia and myopic astigmatism Ophthalmology 199 8;105 :93 2 94 0 MC Knorz Broad-beam versus scanning-beam lasers for refractive surgery Ophthalmic Practice 199 7;15:142–145 B Wiesinger-Jendritza, MC Knorz, P Hugger, A Liermann Laser... keratectomy Ophthalmology 199 4;101:1432–14 39 CNJ McGhee, IC Bryce Natural history of central topographic islands following excimer laser photorefractive keratectomy J Cataract Refract Surg 199 6;22:1151–1158 SE Wilson LASIK: management of common complications Cornea 199 8;17:4 59 467 L Probst Response In: T Kohnen, ed Consultations section: refractive surgical problem J Cataract Refract Surg 199 8;24:1177–1178 Y... the cornea prior to treatment, and that is what we are currently working on to improve the results in the rare cases D RESULTS OF TOPOLINK IN NORMAL EYES In a prospective, noncomparative case series, we operated on 203 eyes of 203 patients between January 199 9 and July 199 9 Results were presented at the American Academy of Ophthalmology Annual Meeting in Orlando 199 9 and accepted for publication in... for moderate and high myopia and myopic astigmatism Ophthalmology 199 8;105 :93 2 94 0 MC Knorz, A Liermann, V Seiberth, H Steiner, B Wiesinger Laser in situ keratomileusis to correct myopia of Ϫ6.00 to Ϫ 29. 00 diopters J Refract Surg 199 6;12:575–584 RR Krueger, NF Saedy, PJ McDonnell Clinical analysis of steep central islands after excimer laser photorefractive keratectomy Arch Ophthalmol 199 6;114:377–381... Ophthalmology 199 8;105:612–6 19 S Levin, CA Carson, SK Garrett, HR Taylor Prevalence of central islands after excimer laser refractive surgery J Cataract Refract Surg 199 5;21:21–26 A El-Maghraby, T Salah, GO Waring III, S Klyce, O Ibrahim Randomized bilateral comparison of excimer laser in situ keratomileusis and photrefractive keratectomy for 2.50 to 8.00 diopters of myopia Ophthalmology 199 9;106:447–457... astigmatism using topography based flying-spot-mode excimer laser photoablation Am J Ophthalmol 199 8;125:252–256 B Wiesenger-Jendritza, MC Knorz, P Hugger, A Liermann Laser in situ keratomileusis assisted by corneal topography J Cataract Refract Surg 199 8;24:166–174 Topographical Irregularities Following LASIK 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 4 19 C Moser, J Kampmeier, P McDonnell, D Psaltis... 20/200, and spectacle-corrected visual acuity was 20/25 We performed a TopoLink LASIK One month after surgery, uncorrected visual acuity was 20/20 and refraction was plano Corneal topography showed no irregularities (Fig 31.3 regular astigmatism) Figure 31.2 Pre- and postoperative topographic maps and differential map of patient 1 (penetrating injury with ir- 424 Knorz ter PKP and RK) Figure 31.3 Pre- and. .. Cataract Refract Surg 199 8;24:166–174 MC Knorz, B Jendritza Topographically-guided LASIK to treat corneal irregularities Ophthalmology 2000;107:1138–1143.z 32 Management of Flap Complications in LASIK MANOLETTE R ROQUE and SAMIR A MELKI Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A DIMITRI T AZAR and EMILY YEUNG Massachusetts Eye and Ear Infirmary, Schepens... were available for follow-up In the low myopia group, 96 .1%, and in the high myopia group, 75% were within Ϯ0.50 D of emmetropia Uncorrected visual acuity was 20/20 or better in 82.4% of the low myopia group and in 62.5% of the high myopia group, 20/25 or better in 98 .0% of the low myopia group and in 70.0% of the high myopia group, LASIK and TopoLink for Irregular Astigmatism 4 29 and 20/40 or better in... of the low myopia group and in 95 .0% of the high myopia group In low myopia, spectacle-corrected acuity at the higher levels improved as compared to preoperative values, and 13.7% (n ϭ 7) had a spectacle-corrected visual acuity of 20/12.5 or better, and 47.1% (n ϭ 24) saw 20/15 or better after TopoLink LASIK as compared to the preoperative values of 5 .9% (n ϭ 3) and 37.3% (n ϭ 19) , respectively Differences . island appearance prior to surgical intervention. (B) Central island fol- lowing initial (conservative) surgical intervention with partial resolution of island. A B undercorrection (21,22) and. topographic islands following excimer laser photorefractive keratectomy. J Cataract Refract Surg 199 6;22:1151–1158. 11. SE Wilson. LASIK: management of common complications. Cornea 199 8;17:4 59 467. 12 topographic central islands following re- fractive surgery. J Cataract Refract Surg 199 8;24:464–470. 15. LB Szczotka, M Aronsky. Contact lenses after LASIK. J Am Optom Assoc 199 8; 69: 775–784. 16. I