Excimer laser photorefractive keratectom y resulting more m y opia postoperativel y than preoperativel y

Purpose: To present three patients whoexperienced excessivemyopic regression which resulted in more myopia after excimer laser surg�ry. Methods: Ali patients, white males, 31to35 years old, were treáted with the VISX 20/20 excimer laser. Results: Patient 1 had a manifest refraction of -5.00 diopters before PRK and 9 months later it was -4.25 +0.50 X 65°. Six months after retreatment it was -6.00 diopters and the patient had a corneal haze grade 3. Patient 2 had a manifest refraction of -12.25 + 2.25 x 1 ()<> before surgery and -14.25+1.00x105° afterwards with a corneal haze grade 3. Patient 3 had a manifest refraction of -5.75 +0.25 x 25° and four monthsaftêr surgery it was -6.25 with a corneal haze grade 2. • ... Conclusion: The corneal healing process after excimer is riofwêll understood and patients should be informed about the remo te possibility that their myopia may increase after PRK and that they may develop a significant haze.


INTRODUCfION
Worldwide excimer laser has been used to correct refractive errors . The results are very encouraging, and for myopia up to -6.00 diopters (D) more than 90% of the eyes have an uncorrected vision (UCV) of 20/40 or better 1• However, a few unexpected results occur. Corneal wound healing affects the refractive outcome after photor�fractive keratectomy (PRK) and the variability in the wound healing response of individual eyes cannot be controlled.
We report three eyes of three patients that experienced excessive myopic regression, which resulted in more myopia postoperatively than they had preoperatively, and important corneal haze after excimer laser surgery. The scale used for grading haze was as follows: O: clear cornea, possibly with faint haze; 1: mild reticulated haze not affecting refraction; 2: moderate haze, refraction possible but difficult; 3: opacity prevents refraction, anterior chamber easily viewed; 4: opacity impairs view of ante rior chamber and 5: anterior chamber not visible 2• Exc imer laser photorefra ctive keratectomy resulting in more myopia postoperatively than preoperatively

Case 1
A 32-year-old white man underwent a photorefractive keratectomy on his left eye. He had 20/400 UCV and 20/20 with a manifest refraction of -5.00 diopters on this eye. Kerato metric readings were 46.00/46.75 at 90º and central corneal thickness was 0.526 mm. The surgery was unremarkable and 0.3% ciprofloxacin hydrochloride and 0. 1 % diclofenac sodium drops were instilled in the immediate postoperative period. Cornea healed in 4 days and 0.1 % fluorometholone (FML) drops were prescribed 3 times a day (TID).

Case 2
A 35-year-old white man underwent excimer laser photore fractive keratectomy on his right eye. Preoperatively, UCV was counting finger, the manifest refractive error was -12.25 +2.25 x 100º yielding a 20/20 vision, keratometric readings were 44. l 0/46.60 at 90º and ultrasonic central corneal pachy metry was 0.555 mm. The surgical procedure was unremarkable and on the immediate postoperative period a bandage soft contact lens was placed on the eye. Ciprofloxacin hydro chloride (0.3%) and 0. 1 % diclofenac sodium drops were prescribed four times a day. On the 5th postoperative day, cornea had reached full reepithelialization and unpreserved artificial tears were used thereafter.
On his third month follow-up visit, UCV was counting finger, BSCV was 20/400 with a manifest refraction of -4.00 182 -ARQ. BRAS. OFTAL. 61(2), ABRIL/1998 diopters. He had corneal haze graded 2 and keratometric values were 43.00/44. 10 at 175º. Central corneal pachymetry was 0.572 mm. One percent prednisolone acetate was given every two hours for two days and four times a day for the following month . The patient experienced no improvement and at the fifth month examination his UCV was counting finger and BSCV 20/60 with a manifest refraction of -14.25 + 1.00 x 105º. Corneal haze was graded 3 and K-readings were 4 7 .00/49 .90 at 80º. Corneal thickness was 0.560 mm. Figure 1 shows videokeratography from pre and postope rati ve period (1, 3, and 5 months) for patient 2.

Case 3
A 31-year-old white man underwent an excimer laser surgery. Preoperatively he had UCV of counting finger and with a manifest refraction of -5.75 +0.25 x 25° his vision was 20/ 20. Keratometric readings were 44.25/44.87 at 92º and ultra sound central corneal pachymetry was 0.541 mm. The surgical procedure was unremarkable. He was given 0.3% ciprofloxacin hydrochloride and 0. 1 % diclofenac sodium drops four times a day and a bandage soft contact lens was placed on the eye till reepithelialization. On the 4th postoperative day his cornea was healed and unpreserved artificial tears were prescribed.
At one month examination, his UCV was 20/30 and his BSCV was 20/25 with a manifest refraction of +0.50 +0.75 x 140º. Corneal clarity was graded 1 and K-readings were 39.00/ 39.50 at 75º. Central corneal thickness was 0.508 mm. He was Bottom left -Three months after the procedure, the area of ablation showed marked irreg ularity and steepening consisting with tissue addition. Sim K was 46.1/41 .9 x 170º. Bottom right -Five month postoperatively, continued tissue addition was evident, again with marked surface irregularity. At this time Sim K's had increased to 49.9/47.0 x 172º. Topographies of patient 1 and 3 were normal before surgery and evoluted similarly to patient 2 after the surgery. kept on artificial tears .
Ali cases were treated with the VISX excimer laser, model Twenty/Twenty (Santa Clara, CA). Videokeratographies were performed pre and postoperati vely and showed tissue addition and surface irregularity in ali cases at the last examination.

DISCUSSION
Two major areas are involved in the success of excimer laser corneal surgery : technical qualities of the laser system and corneal response to trauma. After years of intense research, excimer laser systems adjustments were performed and technical qualities improved. Less progress was made in the field of corneal wound healing after laser.
ln the early days of laser corneal surgery, it was expected that the minimal damage induced by the laser would not stimulate stromal keratocytes to migrate into the surgical site and secrete new extracellular matrix. After years, studies have shown that patients have a wide range of haze and regression 1• 2• 3• Following trauma, a sequence of events is activated to restore the damaged area. Two major systems are involved in the healing process: degradation and remova! of damaged tissue, and synthesis of cellular and extracellular elements. A lack of balance between these two processes may lead to abnormal healing5.
To the present moment, it is not well established which factors regulates corneal healing. Start signals are recognized like growth factors 5, proteins from the system plasminplasminogen 4 and different types of interleukins 6• 7• Less recognized are stop signals. Therefore, corneal healing process after laser is not well understood and individual variations occur.
These three cases presented here are interesting because the refractions after laser corneal surgery show an increase in the myopic power comparing to preoperative values. This could be caused by corneal epithelium or stroma hyperplasia or an association of both. The healing epithelium elaborates cytokines such as Interleukin-3 which stimulates keratocytes to turn into fi broblast and elaborate new extracellular matrix 7 causing regression and haze formation.
The effect of topical corticosteriods on refractive outcome and corneal haze after photorefractive keratectomy is contro versial. Cases 1 and 3 were not initially treated with steroids because of previous study showing that topical use of this drug had no effect on refractive outcome and corneal haze 8 . Our current researches involve in vitro response of keratocytes to excimer laser and tear analysis in the sense of predicting which are the good candidates to this surgery and thus increasing the predictability of this surgical procedure. ln the meantime, patients should be informed of the remote possibility that their myopia may actually increase after PRK.