Authors: Sumit Garg, MD; Ann Z McColgin, MD; Roger F. Steinert, MD

Introduction

The argon fluoride excimer laser, emitting ultrashort pulses at 193 nm, exactly etches the cornea with solely submicron harm to the adjoining nonablated stroma. The freshly ablated floor helps fast and secure re-epithelialization upfront of reformation of regular basal lamina complexes. Subsequently, the cornea may be recontoured by the excimer laser in vivo. The dominant medical curiosity on this laser has been for modification of the corneal optics, significantly for reshaping the cornea for the correction of refractive error. Superficial corneal opacities and irregularities are also handled by the laser as a therapeutic intervention termed phototherapeutic keratectomy (PTK). This chapter critiques the overall ideas, indications for, and medical outcomes of PTK.

Rules of the Excimer Laser

The time period excimer is a contraction of excited dimer. Excited dimers are molecules with little or no binding within the electron floor state however a extra intently sure higher vitality state. Uncommon fuel atoms work together with a halogen molecule when stimulated to the higher state by electrical discharge throughout the laser cavity. Excessive-power ultraviolet (UV) radiation is emitted because the sure higher state quickly dissociates to the bottom state. Developed in 1975, the excimer laser is used scientifically to carry out analysis in bodily chemistry and pump dye lasers, and industrially to etch supplies. Excimer laser emission has quick pulses, sometimes round 10 nsec, with a repetition fee of 1 to 50 Hz. Desk 1 lists the UV wavelengths obtained from widespread fuel mixtures.

Desk 1. Excimer laser emission. 

Gasoline Medium

Wavelength (nm)

F2

157

Xe2

170

ArF

193

KrCl

222

KrF

248

XeCl

308

XeF

351

 

Photobiology of Excimer Laser Corneal Ablation

Analysis within the early Eighties confirmed that excimer laser-generated UV gentle can exactly etch a wide range of polymers. Srinivasan and Leigh noticed that the irradiated molecules are damaged into small fragments which are ejected into the encircling environment, a course of referred to as ablative photodecomposition. Ablative photodecomposition of natural polymers is attributed to the polymer’s excessive absorption of quick UV radiation, confining the impact to close the floor, and the excessive vitality of every UV photon. At 193 nm, a single UV photon has an vitality of 6.4 eV, which exceeds the covalent bond power of many molecules. After bond breakage happens, intense stress in a confined quantity ejects the molecular fragments into the encircling environment.

Direct bond breakage by a high-energy photon is a photochemical laser–materials interplay. The relative contribution of photochemical and thermal mechanisms to the UV ablation of natural polymers is negligible. On the shortest laser wavelengths, reminiscent of 193 nm, the excessive photon vitality could lead to a purely photochemical means of ablative photodecomposition. At longer wavelengths, absorbed photon vitality results in a neighborhood rise in temperature, inflicting etching by means of a photothermal course of. At longer wavelengths, protein coagulation could happen adjoining to the ablation zone. Brief laser pulses delivered at a low repetition fee assist to restrict native heating.

Photochemical and photothermal results of excimer laser wavelengths on the cornea outcome from absorption by stable parts. Water poorly absorbs wavelengths between 193 and 293 nm. The carbon-nitrogen peptide bond is believed to be the supply of a robust protein absorption peak at roughly 190 nm. Many of the corneal amino acids are nonaromatic and account for collagen absorption that begins to rise at wavelengths of lower than 260 nm and significantly lower than 240 nm. Fragrant amino acids soak up extra strongly at wavelengths of better than 240 nm. A collagen absorption peak round 250 nm is attributed to a carbon-nitrogen enolized peptide linkage. The corneal glycosaminoglycans have related absorption spectra, with peaks round 190 nm and minimal absorption at 248 nm. Nucleic acids are restricted to the occasional keratocyte within the stroma however are extra necessary chromophores within the epithelium, with sturdy absorption at each 248 and 193 nm. Ascorbic acid, significantly concentrated in epithelial cells, has extra absorption at 248 nm than at 193 nm.

Mutagenesis and carcinogenesis are issues with UV radiation. Virtually all carcinogens have been proven to be mutagens. UV radiation-induced mutation parallels absorption by DNA. The low density of stromal keratocytes gives some safety towards carcinogenesis ensuing from stromal photoablation. In a number of research, 193-nm irradiation didn’t trigger mutagenic or carcinogenic mobile occasions. Nuss et al. examined unscheduled DNA synthesis, a measure of restore of pyrimidine dimers. In contrast with a management incision made with a diamond knife, unscheduled DNA synthesis didn’t improve after 193-nm linear ablation; in distinction, a statistically vital improve occurred after 248-nm irradiation.

Mechanisms for decreased toxicity at 193 nm embody absorption of that wavelength by protein surrounding the nucleus (protein defend), lack of cytotoxicity of DNA photoproducts produced by 193-nm gentle, DNA harm readily repaired by the cells, or such harm that mutagenic restore processes should not attainable. In a pores and skin mannequin, DNA harm and subsequent cytotoxicity had been least at 193 nm, intermediate at 308 nm, and biggest at 248 nm.

Corneal irradiation at 193 nm additionally ends in fluorescence between 295 and 425 nm. These emissions may very well be each mutagenic and cataractogenic; nonetheless, the extremely attenuated vitality of the fluorescence could not attain poisonous ranges.

Laser-Cornea Interplay

In 1983, Trokel et al. first reported the managed etching of the cornea by an argon–fluorine (ArF) excimer laser. Puliafito and colleagues et al. in contrast the histopathologic results of linear cornea ablation at 193 and 248 nm. Each research discovered glorious preservation of regular corneal stromal microstructure adjoining to the ablation zone at 193 nm (Determine 1). The adjoining cornea remained optically clear. Excessive-power transmission electron microscopy confirmed a submicron zone of electron density instantly adjoining to the ablation.

 

Determine 1. Mild micrograph of a 193-nm, slit-like ablation in a bovine cornea. Dosage parameters: 20,000 pulses, 50 Hz, 125 mJ/cm per pulse, with a 10-νm masks (authentic magnification × 214). Puliafito CA, Steinert RF, Deutsch TF, et al: Excimer laser ablation of the cornea and lens: Experimental research. Ophthalmology. 92:741, 1985.

Kerr-Muir et al. first described a pseudomembrane that appeared to seal cells and mobile nuclei transected by the laser beam. In distinction, at 248 nm, disorganization of the collagen microstructure prolonged into the adjoining stroma for greater than 10 νm. The cornea instantly adjoining to the ablation confirmed a lack of transparency, indicative of thermal damage. Peyman et al. confirmed a major coagulative impact from 308-nm excimer laser radiation with induced corneal necrosis, stromal opacification, and endothelial cell harm.

Krueger et al. quantitated ablation charges. Etch depth per pulse plotted towards radiant vitality density generated sigmoid curves. The steep portion of the curve is roughly logarithmic; this discovering corresponds to observations in less complicated natural polymers. The inflection level of the curve is roughly 200 mJ/cm for 193 nm, which represents the vitality density yielding probably the most environment friendly ablation.

Puliafito et al. postulated that ablation on the inflection factors could also be clinically undesirable; on the inflection level small modifications in laser output might dramatically alter the ablation fee. Ablation at larger vitality densities, reminiscent of 400 to 600 mJ/cm per pulse, the place the ablation fee has the flattest slope, is likely to be preferable in conditions the place ablation fee is vital.

Krueger et al. noticed that for wavelengths larger than 193 nm, the brink for corneal ablation will increase because the laser repetition fee decreases. At 193 nm, the brink was fixed regardless of various repetition charges. That is per the photochemical principle of excimer corneal ablation, by which the buildup of warmth doesn’t play an necessary function at 193 nm and thermal mechanisms of ablation are necessary for ablation at longer wavelengths.

The impact of radiation density on corneal smoothness stays controversial. Fantes and Waring confirmed improved corneal floor uniformity with their scanning system at larger laser vitality density, particularly with 500 mJ/cm per pulse. In distinction, Berns and et al. noticed that such excessive ranges of vitality may be related to corneal floor ridges, endothelial harm, and small pit formation on the stromal floor contained in the linear ablation zones.

Endothelial toxicity in all probability is minimal, though endothelial results may be seen regardless of the practically full absorption of 193 nm photons inside 1 micron of tissue. Dehm et al. discovered that in linear ablations, 193-nm incisions to 90% of corneal depth produced endothelial membrane disruption much like that seen underlying diamond knife incisions on the identical depth. Frank lack of endothelial cells underlying the incision was noticed at 248 nm.

Hanna et al. noticed the looks of electron-dense granules within the endothelium after floor ablation of rabbit corneas at 193 nm. These electron-dense granules migrated by means of Descemet’s membrane over a number of weeks after which dissipated. The medical significance of this commentary is unknown however could also be associated to transient stress damage to the endothelium. Whereas ablating the superficial stroma, Zabel et al. measured pressures as excessive as 100 atm on the endothelium throughout ablation of the superficial stroma, however no frank disruption of the corneal endothelium occurred.

The fragments ejected from the corneal floor are seen a number of hundred nanoseconds after the laser publicity. For 193-nm irradiation, the particles journey at an preliminary velocity of roughly 400 m/sec for the primary 500 nsec, however then quickly decelerate. Particle ejection ends inside 5 to fifteen νsec because the decelerating particles type a mushroom-shaped cloud (Determine 2). Elevated publicity vitality results in will increase in each the plume measurement and preliminary velocity of the ejected fragments. Evaluation of the plume has recognized quite a few molecules that comprise between 10 and 20 carbon atoms.

 

Determine 2. Ablation plume created by 193-nm excimer pulse at 900 mJ/cm per pulse. Excessive-speed {photograph} obtained by illumination with Nd:YAG laser at 532 nm 50 msec after the excimer pulse struck the corneal floor (authentic magnification × 4.5). Puliafito CA, Stern D, Krueger RR, et al: Excessive-speed images of the excimer laser ablation of the cornea. Arch Ophthalmol. 105:1255, 1987.

Therapeutic Purposes of the Excimer Laser

The excimer laser has acquired consideration for its potential to reshape the corneal floor to appropriate ametropia. Though correction of postoperative refractive errors and anisometropia has been referred to as therapeutic, PTK sometimes refers to using the excimer laser for remedy of different corneal illness.

This part discusses the appliance of excimer laser ablation for recurrent erosions, remedy of floor irregularities, removing of superficial opacities, and remedy of problems after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). PTK has been used efficiently in pediatric and grownup illnesses.

Recurrent Corneal Erosions

Painful recurrent erosion syndrome, whether or not ensuing from trauma or anterior membrane dystrophy (Cogan’s dystrophy or map-dot-fingerprint dystrophy), outcomes from abnormalities within the epithelial basement membrane advanced. These sufferers often reply to topical lubrication remedy, hyperosmotics, bandage comfortable contact lenses, debridement of the epithelium and basement membrane, or anterior stromal micropuncture. Sometimes sufferers proceed to have painful recurrent erosions regardless of these measures, and superficial PTK may be healing on this setting.

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The ablated anterior corneal floor seems extremely supportive of secure re-epithelialization. Though utterly regular reformation of the basal lamina advanced, together with regular density of hemidesmosomes and anchoring fibrils, could take months to years, the fast and secure re-epithelialization that happens after ablation, with absence of punctate keratitis, staining defects, or signs of recurrent erosion, is spectacular.

The target of PTK within the remedy of recurrent erosions is to take away sufficient of the superficial Bowman’s layer to allow formation of a brand new basement membrane with adhesion buildings; subsequently, it’s the most superficial of the PTK procedures. The standard approach is to debride the unfastened epithelium manually. If the world of medical recurrent erosion is localized, the surgeon could discover that the epithelium is poorly adherent over most of or all of the corneal floor. With using a big spot measurement, reminiscent of 6.5 mm, the surgeon ablates the central superficial corneal floor.

Various strategies are used to reveal the peripheral cornea exterior the central zone. With the VISX laser, a smaller spot is manually scanned across the periphery by transferring the affected person’s head. Some smaller spot scanning lasers may be programmed so the scan will embody the periphery as a part of areas handled. In a retrospective evaluation of ten consecutive eyes van Westenbrugge confirmed small spot PTK utilizing the NIDEK EC-5000 excimer laser to have an 80% success fee.

The quantity of tissue ablated to maximise the impact with minimal refractive change has not been elucidated. Some investigators take away solely 2 to six microns, however others have discovered that removing of 10 microns ends in fewer recurrences. After the laser ablation, a drop of topical antibiotic, steroid, and typically a nonsteroidal agent is utilized, adopted by placement of a bandage contact lens. Comply with-up is required till epithelialization is full, then the contact lens is eliminated. Sufferers sometimes are instructed to proceed aggressive lubrication, together with nightly software of lubricating ointment, for a number of months after the process.

Medical research of PTK for recurrent erosions have been encouraging. Morad et al. handled 23 eyes for recurrent or persistent corneal erosion with PTK by ablating 2 to six microns of tissue after epithelial removing. Eighty-three p.c of sufferers remained had been symptom-free through the 12- to 60-month follow-up, and none skilled a major refractive change.

Cavanaugh et al handled 36 eyes for recurrent erosion within the setting of anterior basement membrane dystrophy, and located that 86.6% had no recurrence of signs after 12 months of follow-up. The typical refraction change was +0.72 +- 1.26 diopters (D), and a optimistic correlation was discovered between the variety of pulses utilized and induced refractive change.

Jain and Austin reported that of 77 eyes handled with PTK for recurrent erosions, 69% had been symptom-free after 6 to 55 months of follow-up and 100% had been freed from acute episodes after 1 or 2 remedies. Sufferers handled for recurrent erosions ensuing from trauma had a greater end result than these whose signs had been idiopathic or brought on by a basement membrane dystrophy. Larger success charges with PTK in recurrent erosion syndrome as a result of trauma in comparison with corneal dystrophies have been proven. Different research have proven a 60% to 100% treatment fee with a number of PTK remedies for recurrent erosions. Quantitative research point out that the basic well being of the corneal epithelium could also be improved after PTK.

Extra research are essential to find out the optimum quantity of tissue removing to attenuate each recurrences and vital refractive modifications. Sridhar et al. discovered that diamond burr sharpening was a minimum of as efficient though less complicated and less expensive than PTK for recurrent erosions. One other research concluded that intraepithelial PTK and alcohol delamination of the epithelium had been each efficient for trauma-related recurrent erosions the place ablation of Bowman’s layer or anterior stroma doesn’t appear essential.

PTK has efficiently diminished painful bullae in eyes with pseudophakic bullous keratopathy, and when mixed with amniotic membrane grafting, could lower the recurrence fee.

Irregular Floor

Floor irregularities can severely disrupt the optical efficiency of the anterior cornea. Sufferers could complain of diminished visible acuity and optical aberrations reminiscent of monocular diplopia, glare, or halos. Widespread causes of floor irregularities embody

  • Anterior corneal dystrophies, together with basement membrane dystrophy, Reis-Bucklers’ dystrophy, Schnyder crystalline corneal dystrophy, and anterior granular and lattice stromal dystrophies;
  • Elevated corneal scars, together with apical scars related to keratoconus; and
  • Degenerations, reminiscent of Salzmann nodule formation and band keratopathy. Irregular astigmatism can also happen after refractive surgical procedure.

Many sources of floor irregularity are additionally related to corneal opacities. Smoothing the optical floor alone typically considerably improves the affected person’s visible operate with out the hyperopic shift and potential scarring that may be related to the deeper ablations usually essential for removing of opacities.

Onerous contact lens refraction may be useful in figuring out what element of a affected person’s visible complaints is secondary to floor irregularity alone. Enchancment in visible acuity with a tough contact lens is attributable to irregularity, and the remaining imaginative and prescient impairment with the arduous contact lens in place is brought on by the opacity.

Irregularities should not intrinsically eliminated by publicity to repeated pulses of the excimer beam. Photons that encounter an irregular floor don’t discriminate between elevations and depressions, and tissue that’s eliminated largely replicates the unique irregularity (Determine 3). Methods have been devised to guard the deeper tissues (valleys) whereas exposing elevated pathology (peaks) to the ablating photons.

 

Determine 3. An irregular corneal floor ablated with none masking fluid will keep the irregular contour because the ablation continues into the deeper stroma. Kornmehl EW, Steinert RF, Puliafito CA: A comparability research of masking fluids for excimer laser phototherapeutic keratectomy. Arch Ophthalmol. 109:860, 1991.

Epithelial hyperplasia generally happens in areas of corneal melancholy. Performing transepithelial ablation, the place the thickened epithelium shields the valleys from the laser vitality, can make the most of the pure masking impact of the epithelium.

One technique for eradicating floor irregularities of Reis-Bucklers’ dystrophy includes ablating by means of the epithelium that’s intermixed with the irregular substance till the epithelium is basically absent. This absence is set by the disappearance of the fluorescence that happens throughout epithelial ablation. Additional ablation is then carried out together with masking fluids.

Generally used masking fluids are synthetic tear substances, which can be found in various viscosity. Kornmehl et al. in contrast synthetic tear substances of various viscosity with saline and a nonfluid management. In a mannequin by which the corneal floor initially was roughened with sandpaper, options of average viscosity yielded a smoother floor than extra viscous synthetic tears, and markedly higher outcomes than the nonviscous saline answer. All fluids outperformed the nonfluid management. A subsequent research discovered good outcomes with a preparation of 0.25% sodium hyaluronate.

Determine 4 exhibits the theoretic idea {that a} fluid with insufficient viscosity tends to run off, offering inadequate safety for the valleys, whereas a substance that’s too viscous drapes over the peaks, defending the elevations, the place ablation is desired.

 

Determine 4. (A) Ablation of an irregular floor with a fluid of insufficient viscosity won’t adequately defend the valley by which runoff can happen, and irregularities will persist as ablation progresses. (B) Ablation of an irregular floor with a extremely viscous fluid will probably be irregular due to the variable coating of various thicknesses of the fluid. (C) The perfect fluid has sufficient ultraviolet absorption and average viscosity to trigger ablation of the uncovered peaks whereas masking the underlying valleys. Kornmehl EW, Steinert RF, Puliafito CA: A comparability research of masking fluids for excimer laser phototherapeutic keratectomy. Arch Ophthalmol. 109:860, 1991.

Along with to the impact of viscosity, completely different options could have various effectiveness within the absorption of 193-nm photons. A skinny movie of fluid will need to have sufficient absorption to forestall ablation of the underlying tissue. Enhancing absorption at 193 nm could contribute to a more practical masking fluid for phototherapeutic keratectomy of an irregular floor.

A gel-like substance additionally may very well be utilized uniformly over the cornea and molded into a brand new, easy anterior floor. This gel can be ablated and would expose the peaks whereas defending the valleys with out the necessity for repeated software of viscous fluids. To be absolutely efficient, the ablation fee of the gel must match that of the pathologic tissue. Such a gel has but to be applied in medical apply.

Superficial Corneal Opacities

Beforehand, the one choices for removing of corneal opacities had been lamellar or penetrating keratoplasty. The excimer laser can take away some superficial opacities with out the necessity for extra invasive surgical procedure. Figures 5 and 6 present examples of corneal opacities handled with PTK, leading to marked enchancment in central corneal readability.

 

Determine 5. (A) Preoperative look of a affected person with the granular look of the Avellino variant of lattice dystrophy. Many of the opacities are superficial. (B) 4 days after excimer ablation, re-epithelialization has occurred. Many of the opacities are eradicated from the remedy zone. (C) One month after surgical procedure, dramatic enchancment in central corneal readability has occurred. Steinert RF: Therapeutic keratectomy: Corneal smoothing. In Thompson FB, McDonnell PJ [eds]. Coloration Atlas and Textual content of Excimer Laser Surgical procedure. New York: Igaku-Shoin, 1993:179.

 

Determine 6. (A) Herpetic anterior stromal scar earlier than excimer phototherapeutic keratectomy. (B) Look instantly after excimer phototherapeutic keratectomy, with discount of the stromal scarring. The sting of the 4.5-mm–spherical ablation zone is clear. (C) Three months after surgical procedure, an indirect slit illumination exhibits some persistence of the deeper herpetic scarring plus the addition of reactive reticular haze. Right now, nonetheless, acuity was 20/30 versus 20/80 earlier than surgical procedure.

Maloney et al. studied 232 sufferers handled with PTK for corneal visible loss (ensuing from opacities, floor irregularity, or each). Roughly 45% of sufferers at every follow-up go to skilled a two-line enchancment of best-corrected imaginative and prescient. Different research have revealed related outcomes. A German group used PTK to deal with dense superficial corneal scars in a small cohort of kids rising corneal transparency and visible acuity.

When evaluating a visually vital corneal opacity, it’s vital to evaluate its depth. In some circumstances, reminiscent of Reis-Bucklers’ dystrophy, the situation of the abnormality is restricted to the anterior stroma. In lattice or granular dystrophy, optically vital opacities within the visible zone should be examined and their depth assessed.

Submit-inflammatory scarring is tougher to evaluate as a result of the posterior extent of the opacity sometimes is often unwell outlined, and sometimes deeper than initially appreciated at slit-lamp examination. Partial removing of the opacity could also be sufficient for the affected person’s visible wants. An optical pachymeter may be helpful in measuring the depth of the opacity, though the opacity itself could preclude its correct use. Advances in anterior section optical coherent tomography (AS-OCT) have allowed improved analysis of the depth of involvement of pathology.

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AS-OCT may be helpful in surgical planning by offering correct dimensions of the pathology, which may information the variety of pulses utilized by the excimer laser. AS-OCT can simulate PTK in circumstances of stromal dystrophies, permitting the surgeon to decide on the depth of ablation, which might reduce undesired refractive unwanted side effects.

Visible impairment related to these problems usually outcomes from a mixture of the opacity itself and accompanying corneal irregularity, each of which should be handled to acquire an optimum visible outcome.

Relying on its trigger, the opacity could ablate at a unique fee than the encircling stroma. The hyaline materials in granular dystrophy ablates extra slowly than the encircling stroma. A floor that was easy earlier than surgical procedure can change into irregular after PTK if consideration isn’t given to sustaining a easy floor through the process.

It may be efficient to debride discrete elevated opacities reminiscent of Salzmann nodules or calcium deposits manually with a blade earlier than PTK after which use masking brokers to reveal the elevated remaining irregularities through the ablation to create or keep a easy optical floor.

Ablation often is carried out as a planar disc ablation alone or adopted by a hyperopic ablation. These approaches are supposed to cut back the hyperopic shift. Though the preliminary ablation could also be planar and should retain the unique anterior corneal curvature, transforming after laser remedy with new collagen formation and epithelial hyperplasia on the periphery of the remedy zone could lead to web flattening and a hyperopic shift of as much as +8.00 D in some circumstances.

Amm and Duncker reported that, of 45 sufferers handled, all refractions remained secure after PTK for recurrent erosions, whereas after remedy for corneal scars, anterior stromal dystrophies, or floor irregularities, 40.6% of sufferers developed a hyperopic shift, 9% developed a myopic shift, and 40.6% remained secure. Deeper ablations had been related to a better chance of hyperopic shift.

Katsube et al. proposed that the fluid-filled porous nature of the cornea accounts for a lot of the tendency for the hyperopic shift after PTK.

In assessing suitability for PTK, refractive standing is a significant element. Except the affected person is reasonably myopic earlier than surgical procedure, and both has bilateral illness or readily accepts contact lens correction within the fellow eye, the potential for unacceptable anisometropia is excessive. Nevertheless, postoperative use of a tough contact lens usually is of further profit by optically eliminating residual irregular and common astigmatism.

Along with the hyperopic shift, deeper ablation, significantly exceeding 50 to 100 µm, often is accompanied by haze that may change into visually vital at its excessive.

It’s unclear whether or not intensive steroid administration after laser remedy considerably retards scarring, and to what extent intensive steroid remedy results in a extra pronounced everlasting hyperopic shift by retarding wound therapeutic.

Briefly making use of topical mitomycin-C (MMC) instantly after the ablation could scale back scar formation. MMC used prophylactically has been proven to forestall vital recurrent haze in eyes present process PTK retreatment.

Though PTK could scale back the optical and floor disruption of anterior dystrophies, most dystrophies recur naturally, requiring a repeat PTK or transplantation.

Miller et al. reported a case of Reis-Bucklers dystrophy that skilled 2 fast recurrences after standard PTK, however had 1 12 months with out proof of recurrence after a 3rd PTK together with topical software of MMC.

Ayres et al. famous prevention of recurrence in areas handled with MMC, however recurrence exterior of the handled space. MMC needs to be used with warning, nonetheless, because it has been related to corneal and scleral melts, cataract formation, and endothelial cell loss.

Elsahn et al. described utilizing PTK for the remedy of subepithelial nodules in keratoconus sufferers who had been illiberal to contact lenses. They discovered that PTK may very well be used to deal with the nodules and enhance contact lens tolerance delaying attainable penetrating keratoplasty.

Taenaka et al. have proven PTK to be therapeutic for the remedy of acanthamoeba keratitis not attentive to medical remedy or corneal debridement. They report utilizing a mixture of PTK adopted by restricted keratectomy to take away nearly all of contaminated tissue. Anti-amoebic remedy was continued for a number of weeks. The affected person had no recurrences for the 33-month follow-up interval, with remaining best-corrected visible acuity of 20/20. This novel thought permits for a easy removing of stromal lesions, leading to superior postoperative visible acuity.

PTK doesn’t impair the result of subsequent penetrating keratoplasty when surgical intervention is required.

Therapy of PRK and LASIK Issues

The main curiosity within the excimer laser has been within the alteration of the anterior corneal contour for the correction of refractive errors. Enhancements within the strategies and instrumentation for PRK and LASIK have minimized the chance of great visible loss related to these procedures. Nevertheless, irregular astigmatism or scarring secondary to the ablation, the affected person’s therapeutic response, or flap problems can nonetheless happen.

Epithelial hyperplasia and hypoplasia (Determine 7) usually reduce the irregularities over time, and superficial haze after PRK usually slowly fades. In some circumstances, lack of best-corrected imaginative and prescient, multiplopia, or different optical aberrations can persist and have to be managed surgically. PTK may be useful within the administration of those problems.

 

Determine 7. Regular thickness of the epithelium is seen over the untreated cornea (proper). Hyperplasia of the epithelium within the ablation zone (left) roughly doubles the unique corneal thickness, resulting in restoration of a easy corneal contour (authentic magnification × 160).

Central Islands

Central islands, often outlined as a minimum of 1.00 D of central steepening within the presence of general flattening on corneal topography, are sometimes evident after PRK and fewer generally after LASIK (Determine 8). Sufferers with symptomatic central islands could expertise delicate lack of best-corrected visible acuity, multiplopia, ghost pictures, halos, glare, and elevated astigmatism. The refraction often is mildly myopic due to the central steep zone.

 

Determine 8. Corneal topography of a central island after a toric ablation. Be aware central steepening throughout the in any other case flattened ablation zone.

Stopping central island formation requires adherence to the producer’s suggestions. These embody “anti-island” ablation remedies the place further pulses are delivered to the middle of the ablation zone. Therapy could also be interrupted a number of instances to dry the central stroma with a cellulose surgical sponge.

Most central islands resolve or change into optically insignificant by 6 months after PRK surgical procedure. Histologic documentation of the therapeutic mechanism of central islands stays unavailable. A typical speculation is that the central island represents unablated stromal tissue, and that the “healing” consists of a thickening of the epithelium within the midperiphery surrounding the island, which regularly envelops and covers the island.

In sufferers with symptomatic central islands after PRK, it’s prudent to attend a minimum of 6 months earlier than contemplating retreatment. Some topographically evident central islands don’t trigger signs and needs to be ignored. When signs are current and unchanged after 6 months, and the topography and refractive standing are secure, remedy needs to be thought of.

After LASIK , in distinction, floor transforming is minimal. If a symptomatic central island is secure for a minimum of 2 months, relifting the flap and treating the central island within the stromal mattress may be thought of.

Two remedy choices for central islands after PRK have been advocated. Within the first remedy choice, the central epithelium is eliminated with a blade, exposing the central stromal island. The height of the island is then handled with both a PRK algorithm whose optical zone is about to the diameter of the central island and whose dioptric correction matches the peak of the island or a PTK algorithm set to the width of the island and the variety of pulses calculated to create the dioptric flattening wanted (roughly 12 pulses per diopter). In both case, the dioptric correction is predicated on the peak of the island as detected on an early topography, often the 1-month topographic evaluation. By 3 to six months, the island could look like much less elevated as a result of partial hyperplasia of the encircling epithelium; subsequently, retreatment will probably be incomplete.

The second remedy choice is to retain the epithelium and use a PTK disc ablation equal to the biggest diameter of the island. The primary space of stroma to be uncovered is on the peak of the island if epithelial hyperplasia has occurred across the central island. This may be visualized because the “breakthrough” of the cobalt blue fluorescence that happens with ablation of the epithelium; fluorescence readily visualized when the room lights and microscope lights are shut off. Ablation by means of the epithelium progressively exposes the island, whereas the thicker peripheral epithelium shields the encircling stroma. When the central nonfluorescent stromal island reaches the width of the topographic island because it was seen at 1 month, the island ablation is full. If the breakthrough of the transepithelial ablation doesn’t happen centrally however moderately concurrently all through the optical zone, PRK or PTK pulses are employed, calculated as beforehand described.

For each strategies, after removing of the tissue island, aggressive topical steroids needs to be used till secure therapeutic and secure refractive standing are achieved to discourage any central scarring.

After LASIK, central islands are handled by lifting the pre-existing flap and ablating the central stroma with a diameter and depth akin to the width and peak of the island on the topographic map. Choice is to program a myopic correction for the dioptric worth of the peak of the island on topography. The width is programmed to match the topographic diameter of the island.

Meals and Drug Administration restrictions in america could not permit the surgeon to pick out a small optical zone in myopic corrections, the surgeon utilizing a broad-beam laser can both manually cease the publicity when the ablation is noticed to achieve the specified measurement, or extra exactly, apply the variety of pulses that correspond to a sure diameter. The laser producer can provide a nomogram interrelating dioptric correction, diameter, and variety of pulses.

Corneal Haze After PRK

After PRK, subepithelial corneal haze often develops in a number of weeks, peaks inside 1 to 2 months, after which resolves spontaneously through the subsequent 6 to 12 months (Determine 9). Haze is extra outstanding in eyes after treating larger levels of correction, particularly when utilizing smaller ablation zones. Haze is usually asymptomatic, being extra noticeable to the examiner than the affected person. Haze can contribute to lack of distinction sensitivity and even lack of best-corrected visible acuity.

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Determine 9. Average central corneal haze 3 months after photorefractive keratectomy.

Vital subepithelial haze could enhance with rising the frequency of topical steroids. If visually vital haze persists with out enchancment after 6 to 12 months, surgical intervention could also be indicated. Typically haze formation is accompanied by regression, in order that repeat excimer ablation can concurrently deal with each the regression and the haze. Retreatment in sufferers with vital haze yields much less predictable outcomes than retreatment of sufferers with regression within the absence of haze.

Seiler et al. used a mixed PTK-PRK strategy to deal with 21 undercorrected eyes with corneal haze and located scar recurrence in 4 eyes (19%).

Pop and Aras discovered that sufferers with better haze had a 40% fee of overcorrection. Retreatment needs to be directed towards lowering haze to a clinically insignificant stage moderately than eliminating it.

A transepithelial strategy usually is utilized in PTK mode with a big ablation diameter. With the PRK mode, it has been beneficial to appropriate solely 50% to 60% of the myopia to keep away from overcorrection. Using MMC has been proven to lower the quantity of haze. PTK has been efficiently mixed with amniotic membrane transplants in two circumstances of extreme haze following PRK.

Decentered Ablations

Decentered ablations can happen with PRK or LASIK secondary to preliminary improper positioning, or lack of affected person fixation with drift of an initially centered remedy (Figures 10 and 11). In the course of the ablation, the affected person’s head should be stabilized, sometimes by the surgeon’s fingers, and the affected person and surgeon should monitor the fixation beams.

Decentered ablations are extra widespread after correction of upper levels of refractive error, in all probability secondary to the longer fixation time required. They’re extra prone to be symptomatic with using smaller optical zones, as a result of the sting of the ablation is then extra prone to be throughout the pupillary margin. Delicate decentrations could also be asymptomatic, but when better than 1 mm, are sometimes related to glare, halos, monocular diplopia, and sometimes lack of best-corrected visible acuity.

 

Determine 10. Corneal topography of a decentered ablation secondary to preliminary misalignment.

 

Determine 11. Corneal topography of decentered ablation secondary to drifting of the affected person’s fixation after an initially well-aligned remedy. The affected person has 20/20 uncorrected imaginative and prescient on this eye however is considerably bothered by glare and halos.

Decentrations are troublesome to deal with; nonetheless, some success has been discovered utilizing the excimer laser. In a single strategy, the epithelium is retained over the topographically flattened zone to behave as a masks. The epithelium exterior the ablation zone is eliminated mechanically or with the excimer laser in PTK mode.

A ridge on the border of the remedy zone may be ablated with a small optical zone PTK. A PRK remedy then is carried out both centrally or decentered 180 levels away from the primary ablation, with the optical energy guided by topography.

The already flattened remedy zone is protected by the pre-existing epithelium and, if essential, different masking brokers reminiscent of high-viscosity synthetic tear fluids or a methylcellulose sponge.

Pallikaris described the remedy of decentered ablations with out using the laser by utilizing a limbal stress-free incision or arcuate keratotomy oriented 180 levels reverse the decentration. Research are essential to find out whether or not these incisions lead to higher outcomes in contrast with retreatment with the excimer laser.

Decentration often ends in great amount of the excessive order aberration often known as coma. With the medical introduction of wavefront-guided optics for main remedy, therapeutic purposes will doubtless be developed that will probably be a extra exact and efficient methodology for correcting decentration.

LASIK Flap Issues

When performing the keratectomy for LASIK, a flap of uniform thickness with a easy interface is crucial. When a skinny, incomplete, or buttonholed flap is created, the flap needs to be changed with out making use of the ablation. Irregular astigmatism and scarring with lack of finest corrected-vision or optical aberrations nonetheless can happen and are troublesome to deal with. PTK probably can be utilized to deal with the irregular floor in these circumstances.

Kapadia and Wilson reported a case of a buttonholed flap that healed with superficial scarring related to monocular diplopia and glare. Transepithelial PTK adopted by PRK for partial remedy of the refractive error was carried out with decision of the optical aberrations.

A case of a buttonholed LASIK flap with epithelial ingrowth, was efficiently handled with PTK together with MMC with out haze formation or recurrence of epithelial ingrowth at 24 months.

Leu and Hersh used PTK on the underside of the flap and on the stromal mattress to resolve a case of recurrent diffuse lamellar keratitis (DLK) unresponsive to traditional remedy; ablation of inciting toxins was hypothesized to be the idea for the decision.

PTK might help resolve recurrent erosions and anterior basement membrane irregularities after LASIK, much like managing main recurrent erosions as beforehand mentioned.

Flap Striae

Giant macrofolds that happen due to flap slippage within the early postoperative interval needs to be handled with rapid repositioning. Lengthy-standing macrofolds, are immune to repositioning and handbook smoothing, as are microstriae that disrupt the anterior corneal optics (Figures 12 and 13).

 

Determine 12. Slit-lamp retroillumination photomicrograph of microstriae. Reproduced with permission from Steinert RF, Ashrafzadeh A, Hersh PS: Outcomes of phototherapeutic keratectomy within the administration of flap striae after LASIK. Ophthalmology. 111:740, 2004.

 

Determine 13. “Negative staining” sample of fluorescein within the tear movie due to disruption by microstriae. Reproduced with permission from Steinert RF, Ashrafzadeh A, Hersh PS: Outcomes of phototherapeutic keratectomy within the administration of flap striae after LASIK. Ophthalmology. 111:740, 2004.

Steinert et al. reported glorious outcomes with PTK used to deal with these irregularities. Determine 14 schematically exhibits the idea of the PTK remedy of striae. The approach, as carried out with the VISX broad-beam laser, is to make use of the PTK mode for planar ablation with the maximal 6.5 mm zone. With the tracker engaged, 200 pulses are utilized to the cornea, performing transepithelial ablation. The epithelium acts as a masking agent for the PTK. The elevated ridges are uncovered first the place the epithelium is thinnest whereas the valleys between the ridges, the place the epithelium is hyperplastic, are protected (Determine 15).

 

Determine 14. Schematic diagram of the discount in optical disturbance of striae by transepithelial phototherapeutic keratectomy. Reproduced with permission from Steinert RF, Ashrafzadeh A, Hersh PS: Outcomes of phototherapeutic keratectomy within the administration of flap striae after LASIK. Ophthalmology. 111:740, 2004.

 

Determine 15. Sequential operative pictures of the epithelial fluorescence sample seen throughout phototherapeutic keratectomy (PTK) for microstriae. (A) Preliminary perforation over most elevated area of striae. (B) With successive pulses, epithelial fluorescence over most elevated striae disappears. (C) With additional pulses, the epithelium between the striae recedes, with additional discount in fluorescence. (D) On the finish of the transepithelial section of the PTK, many of the epithelium is eliminated, with solely minimal residual fluorescence within the areas of thickest epithelium between the striae. Reproduced with permission from Steinert RF, Ashrafzadeh A, Hersh PS: Outcomes of phototherapeutic keratectomy within the administration of flap striae after LASIK. Ophthalmology. 111:740, 2004.

Subsequent, the tracker is turned off, and the ring illuminator is turned on, usually revealing a considerable amount of striae ensuing from the removing of the partially masking epithelium.

At this level, the surgeon gently debrides any remaining epithelium, stroking with a minimally moist surgical spear sponge or, if wanted, a back-tilted surgical blade within the route away from the hinge.

Extra laser pulses are utilized briefly bursts of 5 or so pulses, with a wipe of masking fluid between every burst. A medium viscosity synthetic tear often is your best option. The cornea ought to look barely moist however not with a lot fluid that the striae disappear. If the masking fluid bubbles, or the laser pulses sound like a boring thud moderately than a snap, an excessive amount of fluid is current.

The PTK process stops each time the cornea seems to be smoother, however in any case, not more than 100 additional pulses (300 complete) are utilized, with a purpose to keep away from a significant hyperopic shift. Used with this system, the standard hyperopic shift is about 1 D on common. PRK may be utilized after PTK to appropriate any vital optical error. This will probably be much less correct than main PRK as a result of variable impact of the PTK, nonetheless. No optically vital haze was noticed within the revealed sequence. This system resulted in secure outcomes and minimal problems at 2 years follow-up.

Topography-Primarily based Ablations

Regardless of the PTK strategies out there, remedy of irregular astigmatism stays suboptimal. Positive floor irregularities may be handled utilizing current epithelium or masking brokers to focus on the raised areas. Broad floor irregularities that always trigger irregular astigmatism after refractive surgical procedure or penetrating keratoplasty should not simply handled on this style. New expertise by which topographic information are included instantly into laser software program will probably be useful for the remedy of all sorts of astigmatism.

Wiesinger-Jendritza et al. reported a sequence of 23 eyes handled with LASIK utilizing topography-based ablations for irregular astigmatism after penetrating keratoplasty, trauma, or earlier excimer laser surgical procedure. They discovered that 61.9% had been partially corrected and 19.4% had been absolutely corrected after the remedy. Enhancements on this approach will doubtless be seen as topographic evaluation turns into included into the laser for real-time suggestions on topographic modifications through the ablation.

Vinciguerra and Camesasca have reported good outcomes with a topography-based customized therapeutic remedy of extremely aberrated eyes. In 52 eyes, the proportion with finest spectacle-corrected visible acuity (BSCVA) of 20/30 or higher elevated from a preoperative fee of 59% to 88% at one postoperative 12 months, and BSCVA of 20/15 was reached by 25% of eyes in comparison with none preoperatively.

Conclusion

Phototherapeutic keratectomy is a vital choice for sufferers with painful recurrent erosions. PTK gives sufferers an alternative choice to lamellar or penetrating keratoplasty for the correction of corneal opacities and floor irregularities. Excimer lasers with real-time corneal topography suggestions included into the system will doubtless present dramatic enhancements within the potential to realize a easy corneal floor for sufferers with irregular astigmatism, invaluable within the remedy of sufferers with irregular astigmatism after penetrating keratoplasty and former refractive surgical procedure. Topography-based excimer remedies could permit the surgeon to deal with common astigmatism extra exactly and reshape each corneal floor to realize the optimum corneal contour for finest visible outcomes.

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