The 193 nm excimer laser is under investigation for the treatment of several classes of corneal diseases. The first proposed therapeutic application was treatment of infectious keratitis.1 Because of the limited corneal penetration depth of 193 nm light, penetration of organisms into adjacent intact stroma, and concerns about ablating an already thinned, ulcerated cornea, this technique attracted limited interest. Clinical investigations have turned to the smoothing of irregular surfaces and excision of superficial anterior stromal opacities.2-3
To smooth an irregular surface, the surgeon must employ strategies to debulk areas of pathology of varying shape and size. At the same time, the laser ablation process must smooth elevated irregularities while protecting adjacent and underlying normal tissue. !Tunning of the corneal stroma is undesirable from both standpoints of structural integrity and inciting stromal remodeling.4 Such remodeling may result in a hyperopic optical shift2,5 or, if eccentric, irregular astigmatism.
Figure 1: Case 1. Apical corneal nodule (A) preoperatively, (B) immediately postoperatively, and (C) 1 week after laser ablation.
Several treatment strategies are capable of removing irregularities while limiting ablation of deeper tissue. Laser spot size is restricted to the zone of pathology. Epithelium left in the base of an irregular area can serve to protect underlying stroma. Application of fluid that absorbs 193 nm irradiation can shield depressions in the irregular surface while allowing ablation of exposed, elevated pathology. Studies in our laboratory demonstrated that an artificial tear solution of intermediate viscosity yielded smoother surfaces than those obtained with high or low viscosity solutions or no solution at all in a model of mechanically-induced corneal surface irregularities.6
We report three cases that illustrate successful applications of excimer laser phototherapeutic keratectomy for treatment of superficial corneal pathology. Case 1 has been published previously.7 Ln each case, a different treatment strategy was employed to achieve the desired result. All procedures were performed with the Summit Excimed UV200 laser system (Summit Technology, Ine, Waltham, Mass) as part of the United States Food and Drug Administration-approved investigational device clinical trial. Laser parameters were as follows: radiant exposure 180 millijoules/cm2; frequency 10 Hz; and pulse duration 10 nanoseconds. All cases were performed under topical anesthesia with proparacaine or tetracaine.
Case 1. A 44-year-old woman with longstanding keratoconus developed a fibroplastic dense elevated nodule at the apex of the cone in one eye, resulting in reduced contact lens tolerance. Manual surgical superficial keratectomy was performed, but the nodule recurred within 3 months (Fig IA). The round nodule had a diameter of approximately 0.50 mm. Depth was difficult to measure but estimated at less than 30 µ.
On February 14, 1990, the nodule was removed using the excimer laser. The minimum available spot size of 1.00 mm was used and 242 pulses delivered. The adjacent uninvolved cornea was shielded with 1.0% methylcellulose and retained epithelium. Since the treatment goal was to flatten the elevated nodule, removal of epithelium was not necessary. Immediately after ablation (Fig IB), a small epithelial defect and slight depression of the previously nodular area is present. No attempt was made to remove the intrastromal portion of the nodule, because the treatment goal was only to restore a smooth contour for contact lens wear. The nodule was not judged to be optically significant.
Dexamethasone 0.1% drops were used four times daily for 1 month. Polymyxin/bactracin ointment was applied 4x daily for 1 week. One week after treatment, the surface contour of the cone apex was smooth without fluorescein stain, and minimal residual subepithelial anterior stromal haze from the remaining deep nodule was present (Fig IC). Contact lens wear was resumed 2 weeks later, and 6 weeks postoperatively lens wearing time had increased from 8 hours per day compared to 3 hours preoperatively. Visual acuity was unchanged at 20/25, although the patient reported less glare disability and felt that the quality of her vision had improved. After 9 months follow up, the nodule had not recurred and the patient continued to wear her contact lens successfully.
Case 2. A 32-year-old man developed anterior stromal scarring from recurrent bouts of herpes simplex keratitis, decreasing visual acuity from 20/ 20 to 20/70 unimproved by hard contact lens overrefraction (Fig 2A). Keratometry was 42.00/44.00 at 80°. Photokeratoscopy disclosed only slight distortion of central mires.
Because the depth of the scar appeared less than 50 µ in the visual axis, the decision was made to attempt excimer laser phototherapeutic keratectomy before proceeding to lamellar keratoplasty. Although no active infection had occurred for over 12 months, the patient was pretreated with Viroptic daily for several days. On September 4, 1990, the scar was excised with the excimer laser using 180 pulses with a 4.50-millimeter spot size and a 0diopter intended correction (disc ablation).
The epithelium was not mechanically debrided because of the smooth contour of the surface. The patient's head was moved approximately 0.50 mm laterally during the ablation to achieve a gradual "blended" contour of the ablation-zone margins. Upon completion of the ablation, slit-lamp biomicroscopy was performed to assess scar removal in the visual axis (Fig 2B).
The eye was patched, and therapy begun the next day with FML Forte qid and Viroptic qd. Reepithelialization occurred within 72 hours and, at 1 week, visual acuity was 20/30 with a refraction of + 7.50 - 1.50 x 180. (The corneal scarring had prevented an accurate preoperative refraction). Keratometry readings of 34.75/38.00 at 90° confirmed that a significant flattening effect had occurred. At 1 month, visual acuity was 20/20-2 and the refraction was unchanged, but keratometry had steepened to 37.50/40.50 at 80°. Slit-lamp examination disclosed mild (1 + ) reticular subepithelial haze in the ablation zone. FML Forte qid was continued. At 2 months, visual acuity was 20/40 with + 5.50 -2.50 X 5. Keratometry had steepened to 41.50/43.00 at 85°. Moderate (2 + ) reticular subepithelial haze was present.
Case 3. A 41-year-old Hispanic male underwent pterygium excision with conjunctival autograft on May 21, 1990 by another surgeon. Three months later, best corrected visual acuity was 20/80. Keratometry was uninterpretable. Photokeratoscopy disclosed distortion of superonasal mires consistent with severe irregular astigmatism (Fig 3A) that extended through the visual axis, even where the stroma seemed uninvolved. Slit-lamp examination showed widespread subepithelial scarring of Bowman's layer in the area of the pterygium excision (Fig 4A). Scarring in the visual axis did not appear deeper than approximately 20 µ.
Phototherapeutic keratectomy was performed on September 4, 1990. Mechanical debridement of epithelium over the area of pathology revealed numerous variably thickened irregularities of scarred Bowman's layer previously obscured by epithelial faceting. A 2.00-mülimeter spot was used to deliver 1575 pulses of laser energy to a linear, mildly elevated area of central scarring. The beam was moved over the surgical field by manual rotation of the patient's head. Methylcellulose 1% was repeatedly applied during the ablation to the depressed margins of the scar with the tip of a cellulose sponge to prevent ablation of less elevated adjacent stroma. Laser energy was delivered in increments of 300 pulses, with interval slit-lamp examinations to assess the progress of scar removal in the visual axis. In the periphery, a highly elevated ridge of scarring was present 1.00 to 2.00 mm central to the limbus. Due to the potential for subsequent surface irregularities and dellen formation, this area was treated with 1000 pulses using a 1.00-millimeter spot size. Again, the adjacent, less elevated stroma was shielded with 1% methylcellulose during treatment.
Figure 2: Case 2. Herpetic anterior stromal scar (A) before and (B) immediately after excimer phototherapeutic keratectomy.
One week later, visual acuity was 20/25-3 with + 1.00 -2.75 X 175. Keratometry was 39.25/43.50 at 85°. Photokeratoscopy demonstrated improved regularity of the central mire (Fig 3B). The linear stromal scar in the nasal visual axis noted preoperatively was much less prominent (Fig 4B). At 1 month, vision was 20/30 with +3.00 -2.00 xl70 and mild reticular subepithelial haze was noted. At 2 months, vision was 20/40 with + 3.50 - 2.75 ? 160. Mild to moderate (1 to 2 + ) reticular haze was visible throughout the treatment area and involved the visual axis.
Figure 3: Case 3. Photokeratoscopy of post-pterygium removal scar (A) before and (B) 1 week after excimer phototherapeutic keratectomy.
Especially in case 3, most of the 1575 pulses were delivered with small spot sizes to elevated areas and absorbed by methylcellulose without removing adjacent stroma. Without these techniques, the stroma would have been severely thinned, with secondary scarring and distortion.
The potential spectrum of corneal disease processes treatable with excimer-laser phototherapeutic keratectomy is quite broad. The advantages of this procedure are that it may provide a simpler and safer alternative to lamellar or penetrating keratoplasty in many patients. Lamellar keratoplasty has historically been employed in some of these situations, but with mixed results.8 The advantages and drawbacks of each technique are fisted in the Table.
Figure 4: Case 3. Appearance of scarring after pterygium excision (A) before and (B) 1 week after excimer phototherapeutic keratectomy.
Stark et al treated 10 patients with diverse corneal pathology, including lattice dystrophy, ReisBucklers' dystrophy, cornea scars, and disabling high myopia following penetrating keratoplasty.2 With 3 months minimum follow up, 90% showed improved visual acuity, although patients treated for high myopia exhibited marked regression of the refractive effect. Prolonged topical steroid therapy was not utilized. These patients were treated with a single spot size large enough to encompass all pathology without moving the surgical field. The epithelium was left intact. Although this approach requires minimal manipulation, more tissue removal may be required to achieve complete removal of the pathology. As no viscous fluids were used, surface irregularities present at the start of treatment or as a result of different ablation rates at varying points with the lesion would be preserved on the posttreatment surface.
Comparison of Phototherapeutic Keratectomy to Lamellar Keratoplasty
The cases reported here illustrate three possible methods for excising anterior corneal pathology with the excimer laser. As no two cases are identical, an individualized surgical treatment plan will most likely provide the best result for each patient. A general approach for developing a strategy for phototherapeutic keratectomy is given in Figure 5.
When confronted with an elevated focal lesion, as in ease 1, tissue loss can be minimized by use of a small laser spot size and application of viscous fluid to the adjacent normal epithelium, providing a shielding effect. This approach is probably more precise and less traumatic than a conventional method of surgical excision, as evidenced in this case by the lack of recurrence 9 months later.
When pathologic changes are extensive but evenly distributed such as in case 2, ablation through intact epithelium with a large spot size is useful. Gentle rotation of the head with the patient looking straight ahead allows the surgeon to manually "massage" the ablation zone and blend the edges, much like one might smooth a rough surface with a piece of sandpaper, lì, after the ablation has begun, it is noted that surface irregularities are increasing or more prominent than initially perceived, a fine layer of viscous fluid may be applied and the procedure continued.
If multiple areas of abnormal tissue are interspersed by relatively normal cornea, a stepwise approach is used. The epithelium is first debrided in the afflicted areas and the underlying Bowman's layer and anterior stromal surface inspected. Varying laser spot sizes and repeated focal applications of viscous fluid permit the removal of foci of elevated lesions without inducing excessive localized or widespread corneal ectasia and resultant problems with healing and corneal topography. These principles of therapy are well illustrated in case 3.
Postoperative pain control and topical steroid therapy are important management issues following excimer phototherapeutic keratectomy. Eye pain is often severe enough during the 1st 24 hours after surgery to require oral narcotic analgesia. Patients should be forewarned of these symptoms and treated with a strong cycloplegic agent, pressure-patching, and a prescription for several days' supply of a potent oral analgesic such as oxycodone, meperidine, or a synthetic morphine derivative.
It is advisable to continue topical steroids for 2 months or longer if there is evidence of active stromal remodeling such as a myopic shift (after an initial hyperopic shift) of increasing subepithelial haze formation. Many investigators favor the use of FML Forte (fluorometholone 0.25%) because this compound achieves adequate corneal penetration but lower levels elsewhere within the anterior segment than prednisolone or dexamethasone. Thus, the risk of intraocular pressure elevation or cataract formation with prolonged use is theoretically reduced.910
Excimer laser phototherapeutic keratectomy holds great promise as a powerful surgical technique, but is clearly still in an investigational phase. Many issues remain unresolved including the role of postoperative pharmacologic modulation of wound healing, accurate deterrnination of the differences in ablation rate between various pathologic tissues (eg, calcium, scars of different density, and age), and selection of the optimal materials to assist ablation of irregular corneal surfaces (eg, viscous agents versus ablatable molded solids such as collagen or plastics).
Figure 5: Treatment strategy for phototherapeutic keratectomy ot superficial scars and iRegularities.
Two FDA-approved phototherapeutic keratectomy clinical treatment protocols are currently underway at approximately 10 sites nationwide. Over 100 patients have been treated so far, and by late 1991, sufficient data should be available to determine the efficacy of this procedure for many superficial pathologic conditions of the cornea. Results are promising to date, and excimer phototherapeutic keratectomy may become an integral and essential component of the corneal surgeon's armamentarium.
1. Serdaveric O, Darrell RW, Krueger RR, et al. Excimer laser therapy for experimental Candida keratitis. Am J Ophthalmol. 1985;99:534-538.
2. Stark WJ, Gilbert ML, Goodman GL, et al. Phototherapeutic keratectomy preliminary report. Invest Ophthalmol Vis Sci. 1990;(suppl)3 1:243.
3. Durrie D, Hunkeler J, Thompson V, et al. Preliminary report of therapeutic superficial keratectomy with the excimer laser. Proceedings of the Third International Congress on Laser Surgery of the Cornea. Refract Corneal Surg. In press.
4. Fantes FE, Hanna KD, Waring GO, et al. Wound healing after excimer laser keratomileusis in monkeys. Arch Ophthalmol. 1990;108:665-675.
5. Taylor DM, L'Espérance FA, Del Pero RA, et al. Human excimer laser lamellar keratectomy: a clinical study. Ophthalmology. 1989;96:654-664.
6. Kornmehl EW, Steinert RF, Puliafito CA. A comparative study of masking fluids for excimer laser phototherapeutic keratectomy. ArcA Ophthalmol 1991;109:860-863.
7. Steinert RF, Puliafito CA. Excimer laser phototherapeutic keratectomy for a corneal nodule. Refract Corneal Surg. 1990;6:352.
8. Hamilton W, Wood TO. Inlay lamellar keratoplasty. In: The Cornea. Kaufman HE, Barron BA, McDonald MB, et ai, eds. New York, NY: Churchill-Livingstone; 1988: 683-696.
9. Leibowitz HM, Kupferman A. Penetration of fluorometholone into the cornea and aqueous humor. Arch Ophthalmol. 1975;93:245.
10. Stewart RH, Kimbrough RL. Intraocular pressure response to topically administered fluorometholone. Arch Ophthalmol. 1979;97:2139.
Comparison of Phototherapeutic Keratectomy to Lamellar Keratoplasty