Journal of Refractive Surgery

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Astigmatism Following Photorefractive Keratectomy for Myopia

Michael Goggin, FRCSI, FRCOphth; Kais Algawi, FRCS, MRCOphth

Abstract

ABSTRACT

BACKGROUND: Astigmatism following photorefractive keratectomy for myopia has been reported as stable as early as 2 to 3 months. The authors report 36 out of 60 consecutive eyes with variations in the cylindrical component of their refraction at 6 months after laser treatment.

METHOD: A standard photorefractive keratectomy was carried out on 60 consecutive eyes in 52 patients over a 7-month period. The manifest refraction of these eyes was followed for 6 months.

RESULTS: Thirty-six eyes demonstrated a change in the cylindrical element of their refraction manifested as a change in cylinder power or axis, or both. The mean pretreatment cylinder power in the group that underwent a change in the cylindrical element was significantly higher than the mean of the group where this did not take place. The mean cylinder power change was 0.75 diopters (D) and in 9 eyes this change was 1.00 D or more. The corrected and uncorrected postoperative visual acuities were the same in the two groups.

CONCLUSIONS: This observation implies meridional variability in the healing process of the anterior cornea following photorefractive keratectomy. [J Refract Corneal Surg. 1994;10:540-544.]

RESUME

INTRODUCTION: Il existe des comptes rendus d'astigmatisme stabilisé immédiatement dans les 2-3 mois après la kératectomie photo-réfractive. Ici les auteurs rendent compte d'opérations sur 36 de 60 yeux consécutifs avec des variations cylindriques dans leurs réfraction, 6 mois après l'opération.

METHODES: La kératectomie photo-réfractive fut réalisée dans 60 yeux consécutifs sur 50 patients pendant 7 mois. La réfraction manifeste fut suivie pendant 6 mois.

RESULTATS: Trente-six des yeux ont démontré un changement cylindrique, soit dans le pouvoir, l'axe, ou les deux. Le pouvoir cylindrique moyen pré-opératif fut d'une façon significative plus grande chez les yeux qui ont éprouvé un changement cylindrique en post-opératoire. Le changement de pouvoir cylindrique moyen fut de 0,75 D, et 9 yeux ont éprouvé un changement d'au moins de 1,00 dioptrie. Les acuités visuelles corrigées et non· corrigées furent semblables dans les deux groupes.

CONCLUSIONS: Cette observation laisse supposer une variabilité méridionale dans la guérison de la cornée antérieure après la kératectomie photo-réfractive. (Translated by Robert Mack, MD, Kansas City, Mo.)

SOMMARIO

PREMESSA: Dopo PRK per il trattamento della miopia la situazione astigmatica viene considerata stabilizzata tra il secondo e terzo mese postchirurgico. Gli autori riportano la variazioni della componente cilindrica della refrazione in 36 occhi su 60 consecutivi considerati, 6 mesi dopo il trattamento laser.

METODI: Una PRK tradizionale é stata effettuata su 60 occhi consecutivi di 52 pazienti in un periodo di 7 mesi. La refrazione manifesta di questi pazienti é stata sequita per un periodo di 6 mesi.

RISULTATI: 36 occhi hanno evidenziato una variazione della componente cilindrica della loro refrazione, sia come modifica del potere cilindrico sia dell'asse, o di entrambi i parametri. La media del potere cilindrico preoperatorio nel gruppo degli occhi che hanno manifestato un cambiamento nei parametri cilindrici é stata significativamente più elevata rispetto alla media del gruppo di occhi dove questo non si é verificato. La variazione media del potere cilindrico é stata di 0.75 D, ed in 9 occhi questa stessa variazione é stata di 1.0 D o più. La migliore acuità visiva naturale e corretta non ha mostrato significative variazioni in entrambi i gruppi.

Abstract

ABSTRACT

BACKGROUND: Astigmatism following photorefractive keratectomy for myopia has been reported as stable as early as 2 to 3 months. The authors report 36 out of 60 consecutive eyes with variations in the cylindrical component of their refraction at 6 months after laser treatment.

METHOD: A standard photorefractive keratectomy was carried out on 60 consecutive eyes in 52 patients over a 7-month period. The manifest refraction of these eyes was followed for 6 months.

RESULTS: Thirty-six eyes demonstrated a change in the cylindrical element of their refraction manifested as a change in cylinder power or axis, or both. The mean pretreatment cylinder power in the group that underwent a change in the cylindrical element was significantly higher than the mean of the group where this did not take place. The mean cylinder power change was 0.75 diopters (D) and in 9 eyes this change was 1.00 D or more. The corrected and uncorrected postoperative visual acuities were the same in the two groups.

CONCLUSIONS: This observation implies meridional variability in the healing process of the anterior cornea following photorefractive keratectomy. [J Refract Corneal Surg. 1994;10:540-544.]

RESUME

INTRODUCTION: Il existe des comptes rendus d'astigmatisme stabilisé immédiatement dans les 2-3 mois après la kératectomie photo-réfractive. Ici les auteurs rendent compte d'opérations sur 36 de 60 yeux consécutifs avec des variations cylindriques dans leurs réfraction, 6 mois après l'opération.

METHODES: La kératectomie photo-réfractive fut réalisée dans 60 yeux consécutifs sur 50 patients pendant 7 mois. La réfraction manifeste fut suivie pendant 6 mois.

RESULTATS: Trente-six des yeux ont démontré un changement cylindrique, soit dans le pouvoir, l'axe, ou les deux. Le pouvoir cylindrique moyen pré-opératif fut d'une façon significative plus grande chez les yeux qui ont éprouvé un changement cylindrique en post-opératoire. Le changement de pouvoir cylindrique moyen fut de 0,75 D, et 9 yeux ont éprouvé un changement d'au moins de 1,00 dioptrie. Les acuités visuelles corrigées et non· corrigées furent semblables dans les deux groupes.

CONCLUSIONS: Cette observation laisse supposer une variabilité méridionale dans la guérison de la cornée antérieure après la kératectomie photo-réfractive. (Translated by Robert Mack, MD, Kansas City, Mo.)

SOMMARIO

PREMESSA: Dopo PRK per il trattamento della miopia la situazione astigmatica viene considerata stabilizzata tra il secondo e terzo mese postchirurgico. Gli autori riportano la variazioni della componente cilindrica della refrazione in 36 occhi su 60 consecutivi considerati, 6 mesi dopo il trattamento laser.

METODI: Una PRK tradizionale é stata effettuata su 60 occhi consecutivi di 52 pazienti in un periodo di 7 mesi. La refrazione manifesta di questi pazienti é stata sequita per un periodo di 6 mesi.

RISULTATI: 36 occhi hanno evidenziato una variazione della componente cilindrica della loro refrazione, sia come modifica del potere cilindrico sia dell'asse, o di entrambi i parametri. La media del potere cilindrico preoperatorio nel gruppo degli occhi che hanno manifestato un cambiamento nei parametri cilindrici é stata significativamente più elevata rispetto alla media del gruppo di occhi dove questo non si é verificato. La variazione media del potere cilindrico é stata di 0.75 D, ed in 9 occhi questa stessa variazione é stata di 1.0 D o più. La migliore acuità visiva naturale e corretta non ha mostrato significative variazioni in entrambi i gruppi.

The healing process following photorefractive keratectomy for myopia dictates the final refractive error. Considerable variation in the error occurs in the early recovery period.1,2 It has been our observation that a proportion of eyes have a variation of the astigmatic element of their refractive error following excimer photorefractive keratectomy for myopia. This report describes 36 eyes out of 60 in which either a spherical refractive error before laser was replaced with a new astigmatic error, or an astigmatic error underwent a change of power and/ or axis, by 6 months after laser.

METHOD

Sixty eyes in 52 consecutive patients were followed up for 6 months or more following excimer laser photorefractive keratectomy for myopia using a Summit Technology ExciMed UV200LA argon fluoride laser system with one operator (M. O.K.). This instrument produces pulsed laser radiation at a wavelength of 193 nm and a fluence of 180 mJ/cm2 at the corneal surface. The patient's fixation is ensured by the use of a single light emitting diode (LED) fixation lamp coaxial with the laser beam.

All eyes had automated and subjective noncycloplegic refraction before treatment. For those using contact lenses, a period of at least 1 week following removal of contact lenses was allowed to elapse before final measurements were taken. In only 17 eyes was contact lens wear constant before the treatment. Contact lens use was intermittent or non-existent (usually due to intolerance) in all others. Most eyes also had a corneal topographical map constructed using an EyeSys Laboratories corneal topography system. The mean age of the patients was 31 years (±8.38), range 20 to 61 years. The mean sphere was -4.73 diopters (D) before laser, range -12.5 D to -1.25 D. The mean cylinder, before laser, was 0.40 D, range 0.00 D to 3.50 D.

No eyes with ocular surface disorders or collagen vascular disorders were treated.

All eyes were treated under topical anesthetic (amethocaine) and had pilocarpine 1% instilled before laser therapy. Sedation was not required in any case. Following patient fixation training, the corneal epithelium was removed, using a Beaver blade, over a central 5- to 6-millimeter diameter circular area. Pulsed laser was then applied to a central 5millimeter diameter zone, the number of pulses dependent on the amount of myopia. No complications of the treatment were encountered.

Following treatment, all eyes had Maxitrol ointment instilled (dexamethasone 0.1%, neomycin sulfate 0.35% and polymyxin B sulfate 6000 iu/g) and a double pad applied for at least 24 hours. Twentynine eyes received topical hyaluronic acid (Healon) at the same time as the Maxitrol and 31 did not. All eyes were reviewed on the first posttreatment day, at 1 week, and 1 month following laser treatment and subsequently attended at monthly intervals, when they were examined, had non-cycloplegic retinoscopy and/or automated refraction, and a manifest refraction was carried out. Spectacle-corrected and unaided visual acuity was assessed. A topographical map of the cornea was constructed at the 3and 6-month reviews, using a video-keratoscope, to double check the optically measured change in corneal topography and confirm central placement of the laser therapy. A change in cylinder was deemed to have occurred only if the difference in measurements for spectacle-corrected visual acuity was 0.50 D and/or 10° or greater, because of the difficulty in establishing reproducible results between observers below this level. Following laser, all eyes were continued on topical Maxitrol ointment four times a day. This was usually changed, after 1 week, to dexamethasone 0.1% drops, fluorometholone 0.1% drops, or betamethasone 0.1% drops administered three to four times a day on an empirical basis in the absence of a substantial consensus in the literature regarding the optimal regime following photorefractive keratectomy. Intraocular pressure (IOP) was measured at each visit and pressures over 21 mm Hg (as a result of corticosteroid response) were treated with topical antihypertensives. Eyes regressing in the first 8 weeks were started on dexamethasone drops as the regression was diagnosed. Medication was usually terminated in the fourth month after laser treatment.

RESULTS

Sixty consecutive eyes in 52 patients (26 male, 26 female) were assessed at 3 and 6 months following excimer laser photorefractive keratectomy. Of these, 36 (60%) eyes had a change in the astigmatic element of their refraction at the 6-month review. The change could be categorized in one of four ways: a change from a spherical refraction before laser to an astigmatic refraction after laser (12 eyes, 20%), an increase in the power of a preexisting cylinder after laser (4 eyes, 6.7%, two of which also had a change of axis), a decrease of the preexisting cylinder power (12 eyes, 20%, 10 to zero and the remaining two with a change of axis), or a change of cylinder axis without a change in power (8 eyes, 13.3%, 5 without a change of rule and 3 with a change of rule). The mean power change (in 28 eyes) was 0.75 D (range, 0.50 D to 1.75 D). The axis of new cylinders was predominantly with-the-rule (8 out of 12), 2 others being againstthe-rule and 2 oblique. Too few axis changes in preexisting cylinders took place to comment on any pattern, 90° to 75° and 80° to 110° accompanying a decrease in cylinder power, and 90° to 180° and 90° to 110° accompanying an increase in cylinder power.

Table

Table 1Refractive Error (D) in 36 Eyes Before and at 6 Months After Photorefractive Keratectomy (PRK)

Table 1

Refractive Error (D) in 36 Eyes Before and at 6 Months After Photorefractive Keratectomy (PRK)

An astigmatic change in corneal topography was significantly associated with the existence, before laser, of an astigmatic refractive error (p = .0004, Yates corrected chi square test). Those with an astigmatic error, before laser, had a relative risk of this sort of change of 2.29 (Taylor Series 95% confidence interval, 1.43 to 3.67) and, consequently, the mean cylinder power before laser in the group that underwent a change (0.58 D) was significantly higher than the mean in those who did not (0.13 D) (p = .0004 Kruskal-Wallis test). The mean sphere, before laser, was higher in the group that underwent change (-5.22 D versus -4.00 D), but this did not achieve statistical significance, with ? = .071 (Kruskal-Wallis test). A summary of the refractive errors before, and at 6 months after photorefractive keratectomy in the group that underwent change, appears in Table 1.

Astigmatic change was not significantly associated with regression of the refractive error into the myopic range, the degree of anterior stromal haze after laser (grade 23 or less in all but one case where grade 3 was noted), the sex or age of the patient, the side undergoing photorefractive keratectomy, constant contact lens use before laser or padding of the eye for more than one day. There was no significant difference in the amount of laser delivered to each group and the mean spherical error following laser of the two groups did not differ significantly.

Table

Table 2List of Variables Assessed for Association With Astigmatic Change Following Photorefractive Keratectomy

Table 2

List of Variables Assessed for Association With Astigmatic Change Following Photorefractive Keratectomy

Table 2 contains a summary of the variables described above. Significance was measured by the chi square test for categoric variables, the ANOVA test for comparison of means in normally distributed data, and the Kruskal-Wallis test for nonparametric data.

Centration of the ablated zone was assessed by videokeratography, measuring the distance between the center of the treated zone and the center of the entrance pupil. The center of the treated zone was established, where the flattened area was not circular, by finding the intersection of the shortest and longest diameters of the flattened area. The mean decentration in the group undergoing change at 6 months was 0.384 mm, the mean in the group where change did not occur was 0.382 mm. This difference was not significant. The maximum decentration that occurred was 0.93 mm in one case.

Comparison of refractive errors before and at 3 and 6 months after photorefractive keratectomy reveals that of the 36 eyes that had astigmatic change at 6 months by comparison with their examination before photorefractive keratectomy, 21 (58%) had change occurring both in the first 3 months and between 3 and 6 months.

With regard to the unaided visual acuity after laser, in this series 33 eyes achieved 6/12 or better in the group with astigmatic change (91.7%, ? = 36) and 22 (91.7%, ? = 24) in the group without change, at 6 months. If those eyes with a myopic error before laser treatment of greater than 6.00 D are excluded, all those in the group with astigmatic change achieved 6/12 or better, and all but one achieved 6/12 or better in the group without change. In the former group, 12 reached the same level of acuity as their spectaclecorrected acuity before laser, and 11 in the latter. These differences are not statistically significant.

Following photorefractive keratectomy, because of the variety of pharmacological regimes used after the first week, it was not possible to assess their role on an individual basis in the processes described. However, a proportion of eyes, both with and without astigmatic change, had ceased receiving medication between 1 and 3 months following photorefractive keratectomy. Once again, there was no significant difference between the two groups in this regard. Six eyes, three eyes in each group, demonstrated a rise in IOP in response to topical corticosteroid. Of the 29 patients who received topical hyaluronic acid immediately after laser, 17 had a change in the cylindrical element of their refractive error, and of the 31 who did not receive it, 19 had such a change. This difference is not significant.

DISCUSSION

The healing process taking place in the anterior corneal stroma dictates the final refractive error following photorefractive keratectomy. Stabilization of this process has been noted from as early as 2 months following treatment4 for those with lower refractive errors. It has been our observation that a considerable proportion (60%) of patients following routine application of excimer laser photorefractive keratectomy for myopia, by a standard technique (and achieving good ablation centration), have a change in the cylindrical element of their manifest refraction during the first 6 months. This observation is directly contrary to previous publication.4'5 Early publications would suggest that at 6 months these changes are likely to be stable;1'2·6 however, the authors have observed change after this time.

Three eyes undergoing photorefractive keratectomy, exposed preoperatively to paraformaldehyde, have had an increase in astigmatism reported following laser. The change was attributed to corneal remodeling following the chemically induced keratitis.7 The findings reported in this study would suggest that at least some of the change may have occurred even without exposure to formaldehyde.

The commonest forms of astigmatic change in this study have been the development of a new cylindrical element where the refraction before laser had been spherical, and a decrease in cylinder power. Axis change in preexisting cylinders and an increase in cylinder power occurred less commonly. It is our clinical impression that the patients who were least satisfied with their unaided visual acuity were those whose eyes experienced a change in axis.

The one predictive feature for astigmatic change following photorefractive keratectomy, detectable during the pretreatment work up, was the presence of an optically detectable astigmatic refractive error. The technique of photorefractive keratectomy delivery was standard and unvarying and there were no differences in any of the vital statistics or demographics of the patients in each group.

Interestingly, no differences in visual outcome were demonstrable, so the clinical significance of the findings remains to be seen. Certainly, there was no association with regression. This may explain why this kind of finding has not previously been demonstrated in studies reporting on outcome at 6 months or more.1·2 The lack of effect on visual outcome of this phenomenon is surprising; the mean change in cylinder power was 0.75 D. But since this is so, this observation has more implications for the understanding of the dynamic processes that occur during the healing phase rather than for the final therapeutic outcome or for the understanding of crosssectional pathophysiological studies previously published.8·9 It implies that there is meridional variation in the healing process. Should there have been some variation from standard of laser fluence or beam profile leading to "astigmatic" ablation, the astigmatic change would most likely have been stable from the time of treatment. However, in 58% of the eyes that underwent such change this change was a continuous process during the follow-up period.

Furthermore, routine beam profile analysis by polymethylmethacrylate (PMMA) block and gelatin film ablation (as recommended and assessed by the laser system manufacturer), carried out each time the laser system was switched on, revealed no such variation. During the period of the study, a maximum of five treatments were carried out before the beam profile was re-assessed.

Manipulation of the factors that affect healing will be necessary to refine and improve the visual outcome in photorefractive keratectomy.

Several agents are available to manipulate these processes - the most commonly used are corticosteroids. The advent of topically applied cyclo-oxygenase inhibitors may, in the future, allow more selective inhibition of prostaglandin activity. These more selective agents may have different effects on the processes compared to corticosteroids and these differences need to be investigated.

In summary, a series of eyes demonstrating changes of the cylindrical element of their refractive error at 6 months following photorefractive keratectomy for myopia are presented. This represented over half those treated over the same period. The only significant correlate was the presence of a cylindrical refractive error before laser. The long-term significance of this new observation remains to be elucidated since it does not appear to affect the visual outcome at 6 months. It implies meridional variability in the healing process in the cornea in the early period following laser and suggests caution in the selection of patients with non-spherical pretreatment refractive errors for photorefractive keratectomy.

REFERENCES

1. Sher NA, Chen V, Bowers RA, Franz JM, Brown DC, Eiferman R, Lane SS, Parker P, Ostrov C, Doughman D. The use of the 193-nm excimer laser for myopic photorefractive keratectomy in sighted eyes. A multicentre study. Arch Ophthalmol. 1991;109:1525-1530.

2. Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy. 18 month follow-up. Ophthalmology. 1992;99:1209-1219.

3. Fantes F, Hanna K, Waring GO III, Pouliquen Y, Thompson K, Saivodelli M. Wound healing after excimer laser keratomileusis (photorefractive keratectomy) in monkeys. Arch Ophthalmol. 1990;108:665-675.

4. McDonald MB, Liu JC, Byrd TJ, Abdelmegheed M, Andrade HA, Klyce SD, Vamell R, Munnerlyn CR, Clapham TN, Kaufman HH. Central photorefractive keratectomy for myopia. Partially sighted and normally sighted eyes. Ophthalmology. 1991;98:1327-1337.

5. McDonald MB, Franz JM, Klyce SD, Beuerman RW, Vamell R, Munnerlyn CR, Clapham TN, Salmerón B, Kaufman HH. Central photorefractive keratectomy for myopia. The blind eye study. Arch Ophthalmol. 1990;108:799-808.

6. Wilson SE, Klyce SD, McDonald MB, Liu JC, Kaufman HH. Changes in corneal topography after excimer laser photorefractive keratectomy for myopia. Ophthalmology. 1991;98:1338-1347.

7. Pallikaris IG, Tsilimbaris MK, Papatzanaki ME, Goles KG, Síganos DS. Paraformaldehyde-índuced keratitis after photorefractive keratectomy. Am J Ophthalmol. 1992;114: 339-344.

8. Sheih E, Moreira H, DArcy J, Clapham TN, McDonnell PJ. Quantitative analysis of wound healing after cylindrical and spherical excimer laser ablations. Ophthalmology. 1992;99:1050-1055.

9. Rawe G?, Zabel RW, Tuft SJ, Chen V, Meek KM. A morphological study of rabbit corneas after excimer laser keratectomy. Eye. 1992;6:637-642.

Table 1

Refractive Error (D) in 36 Eyes Before and at 6 Months After Photorefractive Keratectomy (PRK)

Table 2

List of Variables Assessed for Association With Astigmatic Change Following Photorefractive Keratectomy

10.3928/1081-597X-19940901-11

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