Treating Refractive Eye Problems with LASIK
What does LASIK Stand for?
LASIK stands for Laser Assisted In-Situ Keratomileusis. It is a one of the techniques used in corneal or refractive eye surgery intended to correct astigmatism, myopia or nearsightedness and hyperopia or farsightedness. As the term suggests, it uses laser technology and the procedure represents current medical advances in radial keratotomy in treating vision problems. Refractive surgery is one of the viable alternatives to correcting such problems if you don’t want to bother wearing prescription contacts or eyeglasses that are the more common solutions. And LASIK, along with PRL (Photorefractive Keratectomy) represent the latest in refractive surgical technologies.
A Short History
The technique was made possible in an eye clinic in Bogota, Columbia by a noted local ophthalmologist Dr. Jose Barraquer sometime in 1950. He developed the world’s first microkeratome and the surgical procedure called Keratomileusis for cutting thin flaps in the cornea to alter its shape while retaining the right corneal mass unaltered to create long term prognosis. In 1970, the procedure was enhanced to develop radial keratotomy by Dr. Svyatoslav Fyodorov. It was in 1983 after the excimer laser was first patented in 1973, that another Columbian ophthalmologist Dr. Steven Trokel of the Edward S. Harkness Eye Institute published an article in the American Journal of Ophthalmology about the use of the lasers in refractive surgery. He along with other professionals founded the VSX Corporation that developed the excimer lasers.
Three years earlier, the Rangaswamy Serinivasari from the IBM Research Lab that discovered ultraviolet excimer laser as a suitable option in etching living tissue with precision never before possible and without thermal damage to adjacent tissues. He named the procedure as APD or Ablative Photodecomposition. Finally, the first human eye surgery was performed by Dr. Marguerite B. MacDonald in 1989 using a VSX laser system.
The LASIK procedure had been successfully performed in other countries before the US benefited from it. It was first patented as a “method for modifying corneal curvature” in the name of an Iranian American Ophthalmologist Dir. R. Gholam A. Peymen in June 1989, who is generally credited for the LASIK invention. The US-FDA only started excimer laser trials in that year and the LASIK concept was first taught to a few surgeons chosen by the FDA in 1992 by Dr. Ioannis Pallikaris who is credited for performing the first LASIK procedures on the human eye.
How is it Done?
The operation itself is straightforward, involving the creation of a flap on the cornea, folding it to remodel the tissue underneath with a leaser and then repositioning the flap to heal during the post operative period. It is faster and less painful than other laser-based refractive surgery such as PRK or Photorefractive Keratectomy. There are just several pre-operative preparations to do
- Preparation: Contact lens wearers are advised to stop wearing them in 5-21 days poor to surgery. Those wearing hard contacts are instructed to desist wearing them for a minimum of 6 weeks with another 6 weeks for every 3 years they have been worn. Before surgery, a pachymeter is used to determine the corneal thickness and a topographer to measure the contour. The information is used to calculate the location and amount of corneal tissue to be removed. Patients are usually prescribed with antibiotics prior to the procedure.
- As a local outpatient procedure, local anesthesia drops are applied as well as mild sedative like valium is administered.
- LASIK operation involves three stages. First is the flap creation where the ophthalmologist or eye surgeon uses a microkeratome or femtosecond laser to excise a flap of the corneal tissue ranging in thickness from 80 to 100 micrometers. It is folded back to reveal the corneal stroma or mid section and the process can cause discomfort. Second is the laser remodeling where the excimer laser (193nm) precisely vaporizes corneal stroma tissues to reshape it. And the last step is to reposition the reshaped stroma layer, checked for air bubbles and debris before leaving it for natural adhesion until healed. No suturing is needed.
- Postoperative Care: A course of anti-inflammatory and antibiotic medication is administered lasting for weeks after surgery. Patients are advised to sleep more often and use dark sunglasses, not to rub eyes and moisturize eyes with artificial tears that contain no preservatives.
92% to 98% of LASIK patients have expressed satisfaction with the results in the operation. A post operation analysis conducted by the American Society of Cataract and Refractive Surgery over the last 10 years ending in march 2008 revealed that 95.4% satisfaction rating of LASIK patients around the world.
Laser Vision Correction (LASIK) - Preoperative Considerations.
Preoperative evaluation for refractive surgery follows a structured sequence that includes patient interview followed by a complete ophthalmologic examination. The aim of preoperative evaluation is to answer three broad questions in addition to generating specific refractive data for the actual treatment: 1) Is it possible to safely perform refractive surgery in the patient; 2) What is the risk of possible complications, given the patient specifics; and 3) Is it possible to meet the expectations that the patient has from the surgery?
Patients are advised to discontinue wearing contact lenses at least two weeks prior to the preoperative evaluation and to schedule up to two hours for the preoperative evaluation examination. Contact lens wear should be discontinued three weeks prior to evaluation for wavefront-guided refractive surgery. Since cycloplegic refraction is performed as part of the examination, patients are advised that they may be unable to read for 6 to 12 hours and are advised against driving by themselves during this time.
A detailed history forms an important part of the patient selection process. The purpose is to identify patients who are either not expected to have a good postoperative outcome or not expected to be satisfied with the procedure.
Patients less than 18 years of age and females who are pregnant or breastfeeding cannot have refractive surgery. Patients engaged in sports in which blows to the face and eyes are a common occurrence (boxing, wrestling or martial arts), or in occupations that have a greater likelihood of producing trauma or injuries (armed forces, police or secret service) may have refractive surgery but are usually offered PRK or LASEK as alternatives to LASIK. Since refractive surgery may cause loss of best corrected visual acuity, loss in contrast sensitivity or higher order aberrations, patients should check with their prospective employers about the qualifying refractive criteria. Some employers require contrast sensitivity testing and Glare disability measurement in addition to determining uncorrected snellen visual acuity after the refractive surgery procedure.
Expectation from the surgery
The goal of refractive surgery is to reduce the dependence on glasses and contact lenses in a safe and effective way. Postoperative vision can be invariably improved further with additional optical correction. In presbyopes, additional near vision correction is required after adequate distance vision correction. Patients who expect perfect distance vision or presbyopes who expect equally good distance and reading vision may not be satisfied with the surgical outcome.
There are multiple criteria by which the efficacy and safety of refractive surgical procedures can be measured. The most commonly cited metrics are those of uncorrected and best-corrected Snellen visual acuity, with the former serving as a measure of efficacy and the latter as a measure of safety. In addition, the magnitude of the residual refractive error serves as a measure of efficacy and precision of the correction technique. The National Eye Institute Refractive Quality of Life (NEI-RQL) instrument has been developed to measure vision-targeted health-related quality-of-life issues relating to refractive error and refractive error correction. Using the NEI-RQL questionnaire and its scoring algorithm, it is possible to detect changes in health-related quality of life in response to different methods of refractive error correction. Based on the NEI-RQL questionnaire, McDonnell et al have reported that presbyopia is associated with worse vision-targeted health-related quality of life compared with younger subjects with emmetropia. Monovision correction of presbyopia is related to some improvements in health-related quality of life, but it is still worse than that for younger subjects with emmetropia in several areas. If monovision is suggested as an option, then a two week trial of contact lens monovision is given prior to refractive surgery to determine if the patient accepts the compromises inherent in the monovision strategy. One in four patients will fail to adapt to monovision.
The refraction should be stable over at least 1 year. Patients in whom refraction has changed considerably over the past 1 year (more than 0.5 diopters), are poor candidates for surgery.
Ocular and Medical History
Refractive surgery is contraindicated in patients with history of Herpes simplex or Herpes zoster ophthalmicus. Reactivation of Herpes virus has been reported in the postoperative period. Refractive surgery is not performed in patients who have keratoconus and in those who are on Accutane therapy. Relative contraindication for refractive surgery includes patients who have glaucoma, patients who are glaucoma suspect, or have ocular hypertension or those who have a history of uveitis. If the patient has a history of prior refractive surgeries, particularly radial or astigmatic keratotomy, then additional refractive procedures (PRK or LASIK) are associated with unpredictable refractive outcomes and greater potential complications.
Certain medical conditions, such as autoimmune diseases (e.g., lupus, rheumatoid arthritis), immunodeficiency states (e.g., HIV) and diabetes may prevent proper healing after a refractive procedure and are therefore also considered relative contraindications. If the patient has a history of keloid formation, then PRK or LASEK is to be avoided. Although safety with PRK has been reported in keloid formers, LASIK may be safer in such patients given the minimal wound healing response.
A complete ophthalmic examination is performed. This includes recording uncorrected snellen visual acuity, visual acuity with present glasses, dry manifest refraction and wet manifest refraction (after cycloplegia with 1% cyclopentolate eye drops). An autorefractor may be used to provide an estimate of the patient's refractive error. Manifest refraction is performed using techniques to prevent accommodation (fogging technique). The strongest plus lens (convex) in hyperopes and the weakest minus lens (concave) in myopes that allows best visual acuity is determined. Cross cylinder technique is used to determine the strongest cylinder and the axis that allows best visual acuity. Cycloplegic refraction is important to uncover pseudomyopia due to spasm of accommodation and latent hyperopia. Based on the currently approved indications, the extent of myopia and astigmatism determine the choice of excimer laser as well as whether wavefront-guided surgery is an option.
In patients who have hyperopic or mixed astigmatism, although the final optical result may be the same, different strategies of correcting the hyperopic or mixed astigmatism vary in the depth, profile, and amount of tissue ablation. Azar et al demonstrated that the combined use of hyperopic spherical and myopic cylindrical corrections to treat hyperopic astigmatism or mixed astigmatism incurs the greatest amount of central and peripheral corneal tissue ablation and, therefore, may be the least optimal therapeutic alternative. The hyperopic spherical correction removes tissue from the entire periphery of the optical zone and the myopic cylindrical correction ablates tissue both centrally and peripherally. For hyperopic and mixed astigmatism, using a combination of plus or minus spherical and hyperopic cylindrical corrections appears to best preserve corneal tissue. In patients who have high astigmatism, axis alignment during surgery is an important issue. A 5 degree axis misalignment results in 17% less effect in astigmatism reduction. If the axis is misaligned by 30 degrees, there is no reduction in the magnitude of the cylinder.
The pupil size is measured under low light (mesopic) conditions (less than 5 lux) preferably with an infrared pupillometer such as the Colvard Pupillometer (Oasis, Glendora, CA). The average mesopic pupil diameter has been reported to range from 4.0-8.0 mm. If the dark-adapted pupil diameter is more than 6 mm, it is considered to be a large pupil for a 6-mm diameter LASIK or PRK treatment. In low-light, a large pupil will allow light from the untreated cornea (outside the 6-mm diameter treatment zone) to create glare or a halo effect around the viewed image. This is especially true for patients who have high astigmatism as the minor axis of an elliptical astigmatic treatment is smaller than the ablation diameter, and in patients with high preoperative refractive errors. Therefore, an assessment of preoperative pupil sizes and the attempted level of both the spherical equivalent (SE) and astigmatic correction may be useful in identifying patients who may be at risk of developing glare and halos after photoastigmatic refractive keratectomy (PARK). It is likely that larger diameter ablation zones will reduce the incidence of glare in patients with large pupils, however this needs to be balanced by the increased ablation depth for larger diameter treatments. Although several anecdotal and case series reports as well as ray tracing models have linked the relationship between the ablation zone and low-light (mesopic) pupil size to night vision problems after laser vision correction, recent reports did not find a correlation between preoperative pupil sizes and the long-term persistence of symptoms of glare and halos. For mild to moderate myopia with low astigmatism, large mesopic pupils were not found to be predictive of glare and halo after LASIK treatment.
The intraocular pressure (IOP) is measured using applanation tonometry (Goldmann tonometer or Pneumotonometer). If the patient has ocular hypertension, then baseline glaucoma workup should be performed. This includes a visual field test and optic nerve pictures as well as glaucoma service consultation. Goldmann applanation tonometry is the standard for routine measurements of intraocular pressure (IOP). There are several sources of error in applanation tonometry, including central thickness and structural rigidity of the cornea. Central thickness is related to the rigidity of the cornea and has an impact on the force required to flatten the area that is used to estimate IOP in applanation tonometry. Several reports have shown that Goldmann applanation tonometry overestimates IOP in patients with thicker corneas and underestimates it in those with thinner corneas. Several published reports have confirmed that postoperative IOP readings after corneal refractive surgery for myopia as well as hyperopia are reduced. The reduced IOP after excimer laser refractive surgery is considered to be due to false low IOP reading by Goldmann applanation tonometry due to a thinner post operative cornea rather than a real decrease in IOP. In contrast to Goldmann applanation tonometry, pneumotonometry measures the IOP more reliably after laser in situ keratomileusis.
Corneal ablation of approximately 90 microns reduces Goldmann applanation tonometry readings by 3.0 mmHg after LASIK surgery. This decrease equals approximately 0.2 mm Hg/10 �m of stromal ablation. As opposed to treatment for higher degree of myopia, in mild to moderate myopic treatments LASIK has little influence on IOP readings obtained with a Goldmann applanation tonometer. Therefore, in patients with high myopia who undergo LASIK, nomogram adjustment or the use of a constant correction factor may help to calculate true IOP after refractive corneal surgery.
False low IOP readings pose the risk of delaying the diagnosis of future glaucoma in patients who undergo refractive surgery. The inability to recognize glaucoma early may result in irreversible vision loss in patients who are glaucoma suspects and those who are steroid responders. Patients with ocular hypertension with falsely low IOP may not be subjected to the same degree of glaucoma suspicion and testing as they otherwise would have received.
Several methods are available to obtain reliable and reproducible measurements of corneal thickness. The most commonly used approach is ultrasonic pachymetry. Ultrasonic pachymetry is an efficient and accurate way to measure corneal thickness; however, the probe must touch the corneal surface and topical anesthesia is thus required. Its accuracy is dependent on the perpendicularity of the probe's application to the cornea and reproducibility relies on precise probe placement on the corneal center. It may be difficult to accurately locate the same point of measurement in serial examinations. Over the past few years, new instruments have been developed to measure corneal thickness. The Orbscan II corneal topography system (Bausch & Lomb) is an optical scanning-slit instrument that provides topographic analysis and pachymetric measurements of the cornea. Scanning-slit topography requires the patient to fixate for 1.0 to 1.5 seconds. The SP-2000P specular microscope (Topcon Corp.) is a noncontact optical instrument that provides pachymetric measurements and specular microscopy simultaneously. The central corneal thickness measurements are higher with Orbscan than with ultrasonic pachymetry. This disparity between instruments can result from their distinct methodologies. The noncontact Orbscan system measures the hydrated mucous component of the tear film over the cornea; contact ultrasonic pachymetry does not. Thus, Orbscan readings are higher than ultrasonic readings and require the use of the acoustic equivalent correction factor (0.92). If the cornea is unusually thick (>600 microns) or thin (less than 500 microns) as measured with ultrasonic pachymetry, then an Orbscan could be performed to confirm the measurements.
The safety goal during LASIK procedure is to leave a central bed beneath the microkeratome flap that will allow corneal stability and prevent bulging or ectasia. While the minimum safe bed thickness is not known with certainty, it is thought to be at least 250 microns, and many surgeons recommend leaving 275 or 300 microns. Apparently the flap itself does not contribute to stability of central corneal curvature. If the cornea is thin (less than 500 microns), then PRK or LASEK may be preferable to LASIK. Given that at least 250 microns of bed is left untreated to prevent the possibility of postoperative ectasia, and given that the corneal flap is about 160 to 180 microns in thickness, only limited extent of treatment is possible in patients with thin corneas. Since the actual flap thickness after a microkeratome cut may vary, the patient is advised that intraoperative pachymetry performed after the corneal flap is fashioned, will provide a better estimate of how much treatment can be performed. The average flap thickness does not predictably follow the manufacturer's label due to instrument variability and other operative factors. The standard deviation of flap thickness by subtractive pachymetry ranges from 16 to 30 microns.
Although slit lamp examination of the endothelium using specular reflection will reveal presence of guttata, Specular Microscopy may be performed to assess the endothelial cell morphology and density in patients who have a thick cornea (>600 microns). Fuchs corneal endothelial dystrophy has been associated with poor flap adhesion and corneal decompensation.
Eye dominance is determined by the hole-in-the-card test or by viewing a distant object through a gap between the outstretched hands. Usually the non-dominant eye is operated first. The second eye (dominant eye) may be operated either on another day (usually within a week) or on the same day as the first eye. Patients who have high ametropia in both eyes may complain of disorientation and uncomfortable vision if sequential eye treatments (on different days) are performed. A contact lens over the untreated eye may help during the period in between treatments. Alternatively such patients may be offered simultaneous (same day) treatment for both eyes. As long as the patient does not have unusual risk factors, bilateral simultaneous surgery appears to be safe. Although bilateral simultaneous refractive surgery offers the advantage of convenience to patients, theoretical concerns have been expressed regarding this strategy. Opponents of simultaneous refractive surgery argue that a catastrophic complication could occur bilaterally if simultaneous surgery is performed. These complications include infectious keratitis in the central cornea, entry of the microkeratome into the anterior chamber, corneal melting following a persistent epithelial defect, or macular hemorrhage. With sequential surgery, the outcome in the first eye can be assessed; if a catastrophic complication occurs, the surgeon may then choose not to operate on the second eye. The theoretical advantages of sequential bilateral LASIK include increased predictability for the second eye, especially when the surgical plan for the second eye can be modified given the events and refractive results of the first eye, relative to the biologic response to treatment. Sequential bilateral LASIK surgery may offer greater flexibility of available options to the patient for the second eye surgery based on the outcome of the first eye. Patients can establish satisfaction with vision in the first eye and confirm absence of glare, halos, or night driving problems. Patients may choose to decline a second surgery based on their comfort level of the first eye and may wait further technological developments in refractive surgery, including fellow eye correction with a different modality, such as a phakic IOL. Proponents of simultaneous refractive surgery argue that catastrophic unilateral complications of refractive surgery are very rare, so bilateral catastrophic complications are sufficiently rare that the risk is acceptable. If one eye is predisposed to complications, the other eye is likely to be similarly predisposed. For example the risk of overcorrection or haze after PRK may be related to some unknown healing property of the cornea. For this reason, although sequential surgery allows assessment of complications in the first eye, it probably does not in general reduce the risk of a complication in the second eye. Waring et al found no clinically important differences in the refractive outcomes, visual outcomes and intraoperative complications between simultaneous and sequential surgeries that were performed in a prospective, randomized manner. Gimbel et al also reported simultaneous bilateral LASIK to be as safe and effective as sequential surgery.
Slit lamp examination of the anterior and posterior segments of the eye is performed. Specifically, the presence of blepharitis is noted and treated if present prior to surgery to decrease the risks of infection and interface inflammation following surgery. The presence of superficial punctuate keratitis (SPK) may be due to dry eyes. A Schirmer test is performed. Wetting less than 5 mm in 5 minutes is consistent with severe dry eye disease. Since after the refractive surgery the dry eye disease will most likely worsen, the patient is counseled about this possibility. A punctual plug may be placed prior to or immediately after surgery. Corneal epithelial basement membrane dystrophic changes increase the risk of epithelial sloughing at surgery and later epithelial ingrowth and diffuse lamellar keratitis, and may be an indication for PRK rather than LASIK. The presence of clinical signs of keratoconus are noted. These include corneal thinning, Fleisher's ring and Vogt's striae. The Massachusetts Eye & Ear Infirmary method of keratoconus classification is a useful method to detect keratoconus suspects.
Dilated fundus examination is performed using slit lamp biomicroscopy (using a +78 D or +90D lens) to examine the central fundus and indirect ophthalmolscopy (using a +20 D lens) to examine the retinal periphery. If peripheral retinal degenerations (lattice degeneration or atrophic retinal holes) are present, retina consultation may be sought. Retinal detachment has been reported in a few cases 2 to 9 months after LASIK procedure.
Corneal Topography and Wavefront analysis
Computerized corneal topography examination is an important part of the preoperative evaluation. It can detect irregular astigmatism, whether from contact lens warpage or other causes, which, if significant, is a contraindication to LASIK The average Sim K is noted. The central keratometry number is used to choose the diameter of the flap cut (9.5 mm if 41 D or less and 8.5 mm if 48 D or more). Flat corneas are associated with small microkeratome flaps and free caps, and steep corneas are associated with flap buttonholes. Central keratometry flatter than 35 or 36 D or steeper than 50 D after LASIK is said to be associated with a decrease in quality of vision.
Corneal topography is used to screen for keratoconus or asymmetrical steepening, which may be associated with unpredictable refractive outcomes and progressive ectasia after LASIK. Patients with positive keratoconus screening tests have higher anterior and posterior elevation on Orbscan II topography. Orbscan II topography system may be also help in identifying patients who are potentially at high risk for developing ectasia after LASIK. Inferior corneal steepening, sometimes designated as forme fruste keratoconus, is a topographical finding in corneas that appear normal on slit-lamp biomicroscopy. Mathematical indices to detect subtle keratoconus topographically have been developed. If the Inferior-Superior (I-S) value is more than 1.4, keratoconus is suspected and should be ruled out. Any inferior steepening should raise the suspicion of forme fruste keratoconus. Refractive surgery in such patients may lead to keractasia. In order to exclude the possibility that the inferior steepening may be due to contact lens warpage, a repeat topography is performed after 2 additional weeks of discontinued contact lens wear. Contact lens warpage induced steepening will reduce on subsequent topography examinations whereas forme fruste keratoconus steepening will remain unchanged. Wavefront analysis is performed to determine the extent of higher order aberrations and the data is used to perform wavefront-guided LASIK or PRK.
Choice of Refractive Surgical Procedure
The majority of refractive surgery procedures that are performed for low to high myopia and low to moderate hyperopia use the excimer laser. These procedures include: Laser in Situ Keratomileusis (LASIK), Photorefractive Keratectomy (PRK), Photoastigmatic Keratectomy (PARK) and Laser Epithelial Keratomileusis (LASEK). These procedures are discussed in details in the subsequent chapters. Incisional refractive surgical procedures are used infrequently. These procedures include: Radial Keratotomy (RK) and Astigmatic keratotomy (AK). Progressive hyperopic shift and structural weakening of the cornea are some of the concerns with these procedures. Other refractive surgery procedures include Intacs for treatment of low myopia, and Conductive Keratoplasty (CK) and Laser Thermokeratoplasty (LTK) for low to moderate hyperopia.
Intracorneal ring segments (ICRS) or Intacs inserts (Addition Technology, Inc., Fremont, CA) are approved for patients who have -1.00D to -3.00D of myopia with 1.00D or less of astigmatism. Intacs inserts are two tiny inserts made of polymethylmethacrylate (PMMA). Each Intacs inserts segment has an arc length of 150 degrees. The degree of myopic correction is determined by the thickness of the Intacs inserts; the thicker the Intacs inserts the greater the amount of correction achieved. Intacs inserts can be removed or replaced. A problem sometimes associated with Intacs is corneal flattening in the meridian of the corneal incision (against-the-rule astigmatic shift).
Intacs have been used in patients with keratoconus and in those with keratectasia. Excimer laser refractive surgery procedures (LASIK and PRK) are not used in treating keratoconus patients because of poor refractive predictability and poor stability. Corneal reinforcing procedures, including epikeratophakia, are also not widely used in treating keratoconus patients because of the unpredictability of results. Intacs may benefit patients with keratoconus because this procedure does not weaken the central and paracentral cornea. Instead, it changes the shape and power of the central cornea by an arc-shortening effect. Asymmetric Intacs implantation can improve both uncorrected and best spectacle-corrected visual acuity and can reduce irregular astigmatism in keratoconus. In keratoconus, unlike in the standard myopic technique, a thicker ring segment is placed inferiorly, and a thinner segment is placed superiorly to preferentially flatten the inferior cornea. Intacs may also be used as a mechanical device to alter the biomechanical properties of the cornea for the correction of iatrogenic keratectasia after LASIK for myopia. Intracorneal ring segment implantation improves uncorrected visual acuity and best spectacle-corrected visual acuity in patients with post-LASIK ectasia.
Laser thermal keratoplasty is a thermal technique to shrink peripheral corneal collagen and thereby steepen the central cornea. LTK may be performed in patients with +0.75 to +2.50 diopters of hyperopia with no more than 1.0 diopters of astigmatism. The Hyperion LTK system (Sunrise Technologies International, Inc., Fremont, CA) is a holmium:YAG laser. Two concentric rings of eight spots of laser energy is applied to the periphery of the cornea to gently heat the corneal collagen and steepen its shape. The hyperion delivers eight simultaneous spots (0.6 mm) on the cornea in a circular pattern; one ring at 6 mm diameter and one ring at 7 mm diameter for a total of 16 spots. LTK has been reported to provide predictable refractive outcomes for low hyperopia with tendency for regression.
Conductive Keratoplasty (Refractec, Inc. Irvine, CA ) builds upon the principles of thermokeratoplasty, using radiofrequency (RF) energy to reshape the cornea and therefore adjust its refractive characteristics. CK may be performed for low to moderate hyperopia (between +0.75 and +3.00 diopters). To perform the procedure, a handpiece with a Keratoplast Tip delivers controlled RF energy directly to the corneal stroma in a ring pattern. Conductive Keratoplasty creates a purse-string effect that steepens the central cornea through a ring of application spots around the periphery of the cornea. Several studies have concluded that the depth of shrinkage determines the degree of corneal correction. CK seems to be safe, effective, and stable for correcting low to moderate spherical hyperopia in patients 40 years old or older. Uncorrected visual acuity, predictability, and stability are as good as or better than those obtained with other techniques used to correct hyperopia.
Refractive Surgery in Presbyopia, High Ametropia or Pediatric Population
Monovision for Presbyopia
Monovision is a method of presbyopic correction whereby the dominant eye is usually corrected for distance vision and the nondominant eye corrected for near. This approach has been used successfully with contact lens correction This technique has also been applied in refractive surgical corrections for pre-presbyopic and presbyopic patients. A comprehensive review of the contact lens literature showed a mean monovision (MV) success rate of 76%. Patients who were dissatisfied with monovision had strong sighting preferences, significant reduction in stereoacuity with MV, minimal interocular blur suppression and large esophoric shifts with MV. Patient satisfaction after monovision refractive surgery ranges from 72% to 86%. The changing patient preferences and the spectrum of possible visual outcomes after refractive surgery make a clinically important departure from contact lens MV. Some of the patients who preoperatively request full correction may unintentionally achieve MV. Some presbyopic patients have effective MV because of postoperative multifocal corneas. Myopic patients who are over-corrected and then receive the opposite enhancement may be more likely to have multifocal corneas.
Several reports have considered ocular dominance in relation to monovision in the presbyopic patient.(Ref. 4) Many advantages have been suggested in correcting the dominant eye for distance vision: (1) correcting the dominant eye to maximize performance of visual tasks requiring spatial perception; (2) correcting the left eye for increased driving safety; (3) correcting the less myopic eye to decrease the peripheral blur during distance vision; and (4) correcting the dominant eye for the most commonly used viewing distance to maximize blur suppression. Correcting the dominant eye for distance has become the convention in achieving monovision. However, Jain et al reported a high level of satisfaction patients who had crossed MV.3 Crossed monovision can be created both intentionally and unintentionally. In certain situations correcting the nondominant eye for distance vision may be elected if that eye is significantly more myopic than the dominant eye. This decision may be helped by a trial of crossed monovision contact lenses prior to the surgical correction. Given the possibility of relative unpredictability of refractive results in the presbyopic age group, crossed monovision may be encountered during refractive surgical procedures. Some patients who request full correction in both eyes may end up with crossed monovision if the dominant eye correction is less than expected.
Conversely, in patients who desire bilateral equivalent undercorrection, an overcorrection in the nondominant eye can produce crossed monovision. In a scenario of planned conventional monovision, if the first treatment overcorrects for near vision in the nondominant eye, the remaining options are to match full correction in the other eye or to cross monovision by leaving the other eye untreated or slightly undercorrected. Similarly, if the first treatment in planned conventional monovision undercorrects for distance vision in the dominant eye, the options are to retreat in the dominant eye striving for full correction, or to cross monovision by correcting the other eye for distance vision. One final scenario is the creation of crossed monovision by retreatment of a severely undercorrected nondominant eye that is originally intended for near vision.
The correction of high myopia (more than -10D) remains a challenge. Depending on the corneal thickness, LASIK alone may or may not be feasible. Higher levels of myopia cannot be treated with deeper excimer laser ablations because enough corneal stromal tissue must remain to guard against keratectasia and refractive instability. Current recommendations suggest a minimal residual stromal bed thickness of 250 microns. This precludes the application of highly myopic ablations, especially in patients with thinner corneas.
Surface ablation, phakic intraocular lenses (IOLs) and clear lens extraction are reasonable alternatives to LASIK for high myopia. Each of these procedures has specific limitations. Surface ablation (PRK or LASEK) may cause corneal scarring in eyes with high myopia, although corneal scarring can be reduced by mitomycin-C treatments or prophylaxis. Mitomycin-C (MMC) is an antibiotic derived from Streptomyces caespitosus. Its alkylating properties enable it to cross-link thereby inhibiting DNA synthesis. Rapidly dividing cells are preferentially sensitive to the effects of MMC. Majmudar et al report a series of eight eyes of five patients who had central subepithelial fibrosis after RK or PRK.35 In this series, treatment with MMC proved effective in preventing recurrence of the fibrosis when combined with superficial keratectomy. A sterile, 6-mm circular sponge (Merocel "Corneal Light Shield") soaked in MMC (0.02%) was applied to the corneal surface for 2 minutes. In PRK for high myopia, the prophylactic use of single dose intraoperative MMC 0.02% solution applied topically with a soaked microsponge placed over the ablated area and maintained for 2 minutes after PRK produced lower corneal haze rates than those in the control group. However, given the potential for vision-threatening side effects after MMC use, when it is used as a prophylactic agent to prevent corneal scarring, the approach should be to limit the contact between MMC and the patients' corneas to the minimum time, concentration, and surface area necessary to be effective. To achieve this goal, Jain et al have suggested the use of annular ring of MMC to avoid central corneal toxicity.
Depending on their design, phakic IOLs are placed into either the anterior or posterior chamber, where they sit in front of the crystalline lens. Phakic IOLs include: 1) anterior chamber angle-fixated phakic IOL (Nuvita, Bausch & Lomb), 2) iris-claw phakic IOL (Artisan, Ophtec USA), and 3) posterior chamber phakic IOL (ICL, Staar Surgical). Early cataract formation has been observed with phakic IOLs. With clear lens extraction, accommodation is lost. Azar et al have described a 3-step surgical technique combining LASIK and INTACS to treat high myopia. In this procedure, the residual myopia after a maximal LASIK treatment is corrected using INTACS as it does not remove further stromal tissue. Bioptics (phacoemulsification with IOL implantation followed several months later by LASIK) for correction of moderate to high myopia and hyperopia, with astigmatism, is yet another procedure for treating high ametropia and adjust final outcomes.
In patients with high hyperopia (more than +6D) excimer laser refractive surgery results are unsatisfactory. Phakic IOLs and clear lens extraction are the available options.
Pediatric Refractive Surgery
Pediatric patients who have considerable anisometropia, high ametropia, or refractive accommodative esotropia and are at risk for developing amblyopia are treated with either spectacles or contact lenses, often accompanied by some form of occlusion therapy. This approach, however, may fail as a result of poor compliance. Children who refuse to wear their corrective lenses or who are intolerant to them could, theoretically, be candidates for refractive surgery that may help to prevent or limit the development of amblyopia. Primack et al reviewed the literature on pediatric refractive surgery and found five reports regarding 44 pediatric eyes that underwent either PRK or LASIK for the correction of myopic anisometropia. Although an improvement in visual acuity (BSCVA and UCVA) was reported after following these procedures, more formal studies are necessary before the ultimate safety and efficacy of refractive surgery is established in pediatric patients.
Refractive surgery in patients with prior corneal surgery
Refractive unpredictability after penetrating keratoplasty (PK) is common. Refractive anisometropia and high postoperative astigmatism can compromise the patient's return to normal binocular function despite a clear and compact corneal graft. LASIK or PRK may be considered as a therapeutic option in post-PK patients in whom conventional optical methods of correction have failed. Large refractive errors, anisometropia not successfully corrected with spectacles, and cases of contact lens intolerance may be considered for refractive surgery. The primary goal of PRK or LASIK after PK is resolution of sufficient myopia and astigmatism to allow spectacle correction of the residual refractive error. Return to binocularity and optimized best-corrected visual acuity with spectacles or contact lens is the true endpoint for success with refractive surgery after PK. The UCVA remains a secondary goal with refractive surgery after PK, whereas UCVA is clearly the primary objective of cosmetic refractive surgery. LASIK has been used more commonly to treat myopia or myopic astigmatism as compared to hyperopia or hyperopic astigmatism after PK. The interval between PK and LASIK has not been precisely established but is generally accepted to be 2 years after PK and after all sutures have been removed for at least 6 months. Most studies report an improvement in UCVA, BCVA and a return to binocularity following refractive surgery. In general, any of the complications seen after primary LASIK surgery can also occur after LASIK following penetrating keratoplasty. The risk of damage to the corneal transplant (graft rejection) or the graft-host wound interface (wound dehiscence) is a feared complication in refractive surgery after PK.
Patients who have had radial keratotomy (RK) may have unsatisfactory vision either due to residual myopia or due to the progressive hyperopic shift. Once stability of refraction has been achieved, in both these circumstances refractive surgery (LASIK or PRK) may be considered. Patients with lower original and residual myopia achieve better visual outcomes after PRK than those with higher myopia. The amount of myopic correction achieved using RK is not predictive of the amount of myopic correction using PRK. With LASIK after RK, there are concerns regarding the flap integrity being compromised by the radial incisions as well as the risk of dehiscence of old RK incisions. With PRK after RK there is concern regarding the development of significant corneal haze and scarring in the visual axis. The reason for the risk for increased scarring is hypothesized to be the presence of fibroblasts and myofibroblasts in the vicinity of radial keratotomy incisions that mount an aggressive wound healing response following PRK.
Patient Education and Informed Consent
An important part of the preoperative evaluation concerns with informing the patient regarding potential risks and limitations of the surgery as well as providing a copy of the consent form for the patient to read, understand and sign.
Complications of refractive surgery are infrequent, in general occur in less than 5% patients, but nonetheless could cause permanent vision sequelae.45 Some of the potential complications include: undercorrection, overcorrection, additional surgeries, glare, haloes, flap complications46 (in LASIK), infection, inflammation, need for glasses after surgery, loss of best corrected vision, loss of vision and eye. The likely risk of these complications based on the results of the eye examination are discussed with the patient. A video demonstration of the procedure helps to further provide clarity about these issues.
It is important to maintain the records of preoperative refractive data. If the patient needs cataract surgery in the future, it becomes very difficult to calculate the correct power of intraocular lens needed if data prior to refractive surgery is unavailable.
Refractive surgery is an elective surgery that is performed to enhance the quality of life in patients who are dependent of glasses or contact lenses. A carefully performed preoperative examination helps to select individuals in whom a satisfactory outcome can be reasonably expected. Patients who have a high risk of developing potential complications or who have unrealistic expectations from the surgery are advised against having refractive surgery.