Laser intrastromal keratomileusis for high myopia and myopic astigmatism.

BACKGROUND: Laser intrastromal keratomileusis (LASIK) is an evolving technique which enables high degrees of myopia (> 8.0 dioptres) and myopic astigmatism to be corrected. This paper describes initial experience with this procedure. It also details the methodology, the results, the problems encountered, and discusses retreatment procedures.
METHODS: 51 eyes (48 primary cases and three retreatments) underwent LASIK for simple myopia or compound myopic astigmatism. After the keratotomy was fashioned with a Chiron corneal shaper, the ablation was performed with either a Summit or Meditec excimer laser. The actual preoperative astigmatism ranged from -0.5 D to -6.0 D (in the astigmatic myopic LASIK (AML) series), while the range of preoperative myopia in the combined myopic LASIK (ML) and AML series was -8.0 D to -37.0 D. Of the ML cases, group 1 (-8.0 to 15.0 D (dioptres)), group 2 (> -15.0 to -20.0 D), and group 3 (> -20.0 D) had mean preoperative myopia values (spherical equivalent) of -11.26 D, -16.84 D and -27.78 D. The same groupings (1, 2, and 3) for the AML cases had respective values of -9.702, -17.4, and -23.08. In the AML series the mean preoperative astigmatism was -2.109 D. Follow up ranged from 8 to 27 months (mean 15.8 months). Six of the cases required retreatment.
RESULTS: There was a reduction in best corrected visual acuity (BCVA) (of 1 Snellen line) in seven of the primary cases (14.5%) (three in the ML group and four in the AML group), and in one of the retreatment cases. The BCVA improved in 28 cases (58%) in the primary treatment group. The mean correction attempted (spherical equivalent) for the ML groups 1, 2, and 3 was 10.51 D, -14.5 D, and -27.78 D, versus a mean correction achieved of -9.445 D, -15.625 D, and -21.571 D. Similarly, for the AML groups, attempted correction values were -9.702 D, -17.4 D, and -23.08 D, while the values achieved were -6.95 D, -51.425 D, and -15.708 D. Regression was minimal and stabilisation of the refractive result was achieved in all groups, except group 3 of the ML series, by the 3 month examination period. The mean postoperative astigmatism in the AML series was -0.531 D. Vector analysis of the AML series showed that the mean surgically induced astigmatism was +0.93 D. The most common complication encountered was undercorrection, which occurred in 35 cases-23 cases in the ML group and 12 cases in the AML series. Twenty eight per cent of the ML cases, and 25% of the AML cases were within plus or minus 1.5 D of the attempted refraction.
CONCLUSION: For the correction of high myopia and myopic astigmatism, LASIK results in less postoperative pain and relatively little subepithelial haze compared with high myopic photorefractive keratectomy. Furthermore, a stable refraction and reasonably predictable outcome occurs much earlier. High myopia up to -37.0 D can be corrected, albeit with some limitations at the extremes of myopia-in terms of the amount of myopia correctable; this represents a limitation of the technique. Retreatment is a technically straightforward and effective way to treat undercorrection. Undercorrection, the main complication seen in our series, should become less common when the ablation algorithms are further refined.