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LASEK in High and Low Myopia

Chris P.Lohmann, MD, PhD

University Eye Clinic Regensburg, Germany, The Rayne Institute,

St Thomas Hospital

London, England

David O’Brart, MD, Ann Patmore, BSC, and John Marshall, PhD

The Rayne Institute, St. Thomas Hospital

London, England

Christoph Winkler von Mohrenfels, MD, Bernhard Gabler, MD, and Wolfgang Herrmann, MD

University Eye Clinic

Regensburg, Germany

Refractive surgery is a constantly changing field with new technologies being developed and introduced routinely. Currently, excimer laser photorefractive keratectomy (PRK) and excimer laser in situ keratomileusis (LASIK) are the surgical procedures most commonly being used to treat myopia (1,2). PRK changes the corneal curvature by ablating part of Bowman’s layer and anterior corneal stromal tissue after removing the epithelium. In contrast, LASIK does not remove the epithelium, Bowman’s layer, or anterior stromal tissue but does remove deep stromal tissue after making a cut into the corneal stroma at 160 µm using a microkeratome.

Over the past 15 years, PRK has been the subject of intensive scientific and clinical research. Based on numerous clinical trials, which have shown PRK to be safe and effective for the correction of low to moderate degrees of myopia, approval of the procedure was granted by the Food and Drug Administration (FDA) in the United States in 1995. However, PRK requires corneal epithelial debridement before laser ablation. This is a major disadvantage of the procedure, because it leaves the patient with a significant amount of pain for 3 to 4 days postoperatively, and increases healing time with a loss of corneal transparency (1).

Recently, interest has grown in the technique of LASIK. This technique is based on the concept of lamellar corneal surgery postulated by Barraquer in 1949 (3). Since then, various modifications have been undertaken, but it met with limited success because of its complexity, poor predictability, and sight-threatening complications (4). With the introduction of both microkeratomes and the excimer laser, in-situ reshaping of the corneal surface with submicron accuracy became reality. Initially, it was postulated that by creating an intrastromal ablation, it might be possible to improve the outcome of higher degrees of myopia by avoiding the influence of the healing epithelium on stromal wound healing as seen in PRK. However, in clinical practices, LASIK can safely correct up to −10.0 D because of decreased predictability and stability at greater refractive errors

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(2). The advantages of the intrastromal excimer laser ablation over the surface are cited principally as minimal postoperative discomfort, a rapid recovery of visual function, and minimal disturbances in corneal transparency. There has been rapid uptake of LASIK by corneal surgeons and the perceived short-term advantages have raised the demand for this procedure by patients. Although very limited data on the long-term effects of LASIK were available, LASIK obtained FDA approval in 1999. It is of concern that both early and long-term complications of LASIK are not yet fully elucidated and certainly not held in appropriate regard by some ophthalmologists and patients. LASIK is technically more demanding, more invasive of a greater portion of the cornea, and, although very rare, the intraoperative and early postoperative complications have the potential to cause significant loss of visual function (5). The long-term effects of LASIK on the biomechanical properties of the cornea have not been investigated so far. Histological evidence and simple mathematical models suggest that the tensile strength and stability of the cornea may be considerably compromised in years after surgery (6). The first signs of this may be the clinical reports of iatrogenic keratectasias after LASIK (2). This is in contrast to the well-documented long-term safety and stability of PRK.

Laser epithelial keratomileusis (LASEK), first popularized by Camellin, is a new surgical procedure to correct refractive errors (7). This technique may combine the advantages and eliminate the disadvantages of both PRK and LASIK. LASEK is based on detachment of the corneal epithelium using an alcohol solution, creating an epithelial flap that is then repositioned after the laser ablation. The epithelium regenerates itself within a few days and, in the meantime, the existing flap protects the ablated corneal surface.

In this chapter, we report our results of LASEK for the treatment of myopia on 314 eyes with a maximum follow-up of 18 months.

PATIENTS AND METHODS

Patients

Three hundred fourteen myopic eyes (187 patients) with a mean age of 28.7 years (range 19 to 42 years) were enrolled in this study between August 2000 and February 2002. Patients with pre-existing ocular pathology, sicca syndrome, diabetes, or connective tissue disorders were excluded from this study. Preoperative spherical refraction was between −2.00 and −10.00 diopters (D). All eyes had less than 1.0 D of refractive cylinder. A detailed ocular examination was performed on each patient preoperatively, including subjective and objective refraction, corneal topography, measurements of pupil diameter under mesopic conditions, Goldmann applanation tonometry, slit-lamp biomicroscopy, and funduscopy. Patients were asked to remove hard contact lenses at a minimum of 4 weeks before surgery and soft contact lenses at a minimum of 4 days. Patients were examined postoperatively on day 3, day 7, and thereafter at 1, 3, 6, 12, and 18 months.

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Heidelberg, Germany) (Fig. 1a). The trephine is designed to create a 280-degree epithelial incision, leaving a blunt section of 80 degrees at the 12-o’clock position for the formation of a hinge. The trephine was placed centrally on the pupillary axis and downward pressure of the trephine was evenly applied to the blade and slight rotation of the blade (approximately 5 degrees in both directions) was used to create the incision.

2.A 9.0-mm alcohol solution cone (Geuder, Heidelberg, Germany) was placed on the corneal surface encircling the epithelial incision (Fig. 1b). This cone was filled with 20% ethanol (in distilled water) and left for 20 seconds. The cornea was then dried and thoroughly washed with balanced salt solution (BSS) to remove all remaining alcohol.

3.To create the epithelial flap, the precut margin of the epithelium was lifted using the sharp side of the epi-peeler (Geuder, Heidelberg, Germany) and the epithelial flap was gently detached and folded up at its hinge at the 12-o’clock position using the blunt, large side of the epi-peeler (Fig. 1c).

4.A Chiron Technolas 117 excimer laser (221 eyes) or the Summit Apex excimer laser (93 eyes) were used to perform the laser ablation (Fig. 1d). The maximum diameter of the laser beam was 7.0 mm for the Chiron Technolas laser and 6.5 mm for the Summit laser. In all eyes, we have aimed for emmetropia.

5.After the laser ablation the entire cornea was flooded with BSS and the epithelial flap was repositioned using the blunt side of the epi-peeler (Geuder, Heidelberg, Germany) (Fig. 1e).

6.The cornea was covered with a Bausch & Lomb PureVision soft contact lens for 3 days to secure the epithelial flap.

Postoperative Management

Ofloxacine and dexamethasone eyedrops were used five times daily for 3 days postoperatively. After the removal of the bandage contact lens on day 3 after surgery, patients were switched to dexamethasone eyedrops three times daily for 3 weeks and daily for 1 week. Artificial tears (carbomer 2.0 g) were administered immediately after the surgery and were used five times daily for 4 weeks.

Assessment of Postoperative Pain

The first 50 patients were asked to complete a “visual analogue pain chart,” which consisted of a series of horizontal lines 10 cm in length with “no pain” written at one end and “worst pain imaginable” at the other. At each assessment period patients were asked to make a vertical mark across the given line in a position that best represents the severity of pain they were experiencing. Time “0” hours was taken as the time of surgery. Over the next 3 days, patients recorded their pain score initially at 30-minute intervals for 6 hours, then, when awake, every hour for 3 days.

Assessment of Corneal Haze

Corneal haze was graded clinically at each visit based on the following scale:

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0: no haze

0.5: trace, just perceptible

1:easily seen on slit-lamp biomicroscopy

2:moderate haze

3:pronounced haze; iris details still visible

4:severe haze, iris details obscured

Assessment of Halos and Glare

Because postoperative halos and glare are reported by many patients as a problem of night vision after PRK and LASIK, we have measured the size of halos and glare preoperatively and at 3 months postoperatively in 20 patients (20 eyes) with a preoperative myopia between −5.0 and −6.0 D using objective tests described previously (8,9).

The halo test (8) was performed using a computer and a high-resolution monitor under mesopic conditions. The patient fixated on a bright white light source, 40 minutes of arc in diameter, located in the center of the otherwise dark monitor screen. A small white spot acted as a cursor and was moved centripetally until it coincided with the perceived edge of the halo. The movement of this cursor was restricted to 12 radii at 30-degree intervals, passing through the center of the halo source. When the patient determined that the cursor location was coincident with the halo edge, the cursor’s location was recorded by pressing the mouse button. The cursor then moved to the next meridian. The position of the halo edge was recorded at all 12 meridia, and the area within these points was calculated in square millimeters.

Glare caused by forward light scatter was measured using a two-part test in which visual contrast was measured first with a central test stimulus generated on a highresolution monitor (9). This stimulus flickered at 7.5 Hz between the background level and the match level. The patient adjusted the contrast between the match luminance and the background to minimize flicker by means of buttons on the computer keyboard. Then the test was repeated with a bright, annular light source flickering in counterphase and surrounding the central test stimulus. This “straylight” provided an additional luminance for the forward scatter of light.

RESULTS

Surgical Experience

With the exception of three eyes, the preparation of the epithelial flap was easy and without any complications. In one eye, a central buttonhole was created during the preparation of the epithelial flap. In the other two eyes, the epithelium was very adherent and the preparation required additional exposure to the ethanol for 10 seconds.

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Clinical Examination

Some dead superficial epithelial cells were seen underneath the bandage contact lens; however, no erosion of the epithelium was noted using fluoresceine dye at the third postoperative day. In most eyes, a slight epithelial edema was noted at day 3 before removing the contact lens. Throughout the postoperative period, we have not seen any instability of the epithelium and patients did not report any signs of epithelial breakdown. However, one patient (one eye) lost the contact lens on day 1 after the surgery. This led to a loss of the epithelial flap.

Postoperative Pain

None of the patients reported severe postoperative pain that is usually experienced after PRK. During the time the bandage contact lens was placed, most patients reported slight ocular discomfort. On the Visual Analogue Pain Chart, the maximum level of pain was 3.6. This maximum was reached between 3 and 5 hours after the surgery. The pain intensity declined thereafter to a level of 2 until removal of the contact lens. After removal of the contact lens, no pain was reported by all patients.

Postoperative Refraction

In all eyes, we aimed for emmetropia. Postoperative refraction was measured for the first time at the 1-week follow-up. At this stage, all eyes were between plano and +1.25 D. In the next few months, the refraction changed slightly. All eyes were within +0.50 and −0.75 after 3 months. For analysis, we divided the eyes into two groups: the first group included all eyes with a correction of up to −6.00 D. The second group consisted of all eyes with preop refraction (SE) between −6.25 and −10.00 D.

A total of 189 eyes underwent corrections up to −6.00 D (SE). Fourteen (14) eyes had 18 months of follow-up and all were within +0.50 and −0.75 D. Fifty-eight eyes reached the 12-month follow-up (±0.50 D=84% and ±1.00 D=97%), and 117 eyes reached the 6- month follow-up (±0.50 D=81% and ±1.00 D=98%) (Fig. 2a and 2b).

We started to treat myopia over −6.00 D (SE) 10 months after we began treating lower myopes, follow-up data for corrections between −6.25 D and −10.00 D are therefore available for only up to 12 months. Out of a total of 87 eyes in this group, seven eyes reached the 12-month follow-up and these eyes were within +0.50 and −0.50 D. Twentynine eyes have reached the 6-month follow-up (±0.50 D=86% and ±1.00 D=90%), and 51 eyes have reached the 3-month follow-up (±0.50 D=90% and ±1.00 D = 98%) (Fig. 3a and 3b).

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Postoperative Visual Acuity

Immediately after surgery, uncorrected visual acuity (UCVA) was 20/40 in all treated eyes. At day 7, all eyes had a UCVA of 20/40 or better. At 6 months, UCVA was 20/40 or better in 97% and 20/20 or better in 67%.

Postoperative best corrected visual acuity (BC V A) was equal to or better than preoperative BCVA in 73% for corrections up to −6.00 D and 50% for corrections between −6.00 and −10.00 D; 24% (for corrections up to −6.00 D) and 37% (for corrections between −6.00 and −10.00 D) of the eyes improved by one Snellen line. In contrast 9% (for corrections up to −6.00 D) and 13% (for corrections between −6.00 and −10.00 D) of the eyes decreased by one line. No eye lost more than one line of BCVA (Fig. 5a and 5b).

Postoperative Corneal Haze

None of the eyes showed any significant subepithelial haze usually seen after PRK at any postoperative stage (Fig. 6a and 6b). Mean corneal haze was 0.16 (SD 0.18) for corrections up to −6.00 D and 0.19 (SD 0.17) for corrections between −6.00 and −10.0 D. Only a single eye, which lost the epithelial flap caused by the loss of the contact lens developed significant haze of +1.

Postoperative Halos and Glare

On direct questioning, only two patients (two eyes) noted halos around bright light sources at night. In all treated eyes the magnitude computerized measurements of halo was postoperatively greater than before the surgery. Objective halo measurements showed that there was an increase in size postoperatively compared to preoperatively from 430 mm2 (mean, SD ±238 mm2) to 1900 mm2 (SD ±563 mm2). No correlation was observed between halo size and degree of preoperative refractive error, but a significant correlation was found with preop size of the pupil under mesopic conditions.

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measurements of glare caused by forward light scatter expressed as percentage contrast between the luminance of the background and match stimuli showed that without stray light, only marginal differences existed between the preoperative and postoperative readings. However, in the presence of the glare source, we noted an increase in postoperative glare compared to preoperative levels. Objective glare measurements confirmed that there was an increase in glare postoperatively caused by forward light scatter compared to preoperatively. In the presence of the glare, source the preoperative mean value was 7.8% (SD ±2.8%) compared to 11.8% (SD ±6.9%) at 3 months postoperatively.

CONCLUSIONS

LASEK results in very good refractive outcomes for the treatment of myopia between −2.00 and −10.00 D. The postoperative refraction remains stable during the 12-month follow-up. Visual recovery after LASEK is relatively fast with the usual immediate postoperative uncorrected visual acuity of 20/40. After 1 week, most eyes have uncorrected visual acuity of 20/25 or better. No eye lost more than one line of Snellen acuity. There was no or minor postoperative corneal haze. Compared to PRK, ocular discomfort after LASEK was tolerated much better. Both subjective and objective tests of glare and halos after LASEK are comparable to PRK and LASIK. Because LASEK does not require stromal cut, problems in the long-term biomechanical instability of the cornea are avoided.

REFERENCES

1.American Academy of Ophthalmology. Ophthalmic procedure preliminary assessment: excimer laser photorefractive keratectomy (PRK) for myopia and astigmatism. Ophthalmology 1999; 106:422–437.

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8.Lohmann CP, Fitzke FW, O’Brart DPS, Kerr Muir M, Marshall J. Halos: a problem for all myopes? A comparison between spectacles, contact lenses, and excimer laser photorefractive keratectomy. Refract Corneal Surg; 1993; 9:S72–S75.

9.Lohmann CP, Fitzke FW, O’Brart DPS, Kerr Muir M, Timberlake GT, Marshall J. Corneal light scattering and visual performance in myopes: a comparison between spectacles, contact lenses, and excimer laser photorefractive keratectomy. Am J Ophthalmol; 1993; 115:444–453.