LASIK Fundamentals, Surgical Techniques, and Complications - part 3 pps

36. PA Bloom, D Papakostopoulos, Y Gogolitsyn, JA Leenderz, S Papakostopoulos, RH Grey. Clinical and infrared pupillometry in central retinal vein occlusion. Br J Ophthalmol 1993; 77:75–80. 37. BS Wachler, RR Krueger. Agreement and repeatability of infrared pupillometry and the comparison method. Ophthalmology 1999;106:319–323. 38. M Colvard. Preoperative measurement of scotopic pupil dilation using an office pupillometer. J Cataract Refract Surg 1998;24:1594–1597. 39. EM Schnitzler, M Baumeister, T Kohnen. Scotopic measurement of normal pupils: Colvard versus Video Vision Analyzer infrared pupillometer. J Cataract Refract Surg 2000;26:859–866. 40. FW Price, DL Koller, MO Price. Central corneal pachymetry in patients undergoing laser in situ keratomileusis. Ophthalmology 1999;106:2216–2220. 41. AC Snyder. Optical pachometry measurements: reliability and variability. Am J Optom Phys- iol Opt 1984;61:408–413. 42. RB Mandell, KA Polse. 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Corneal thickness measurements with the Orbscan Topography System and ultrasonic pachymetry. J Cataract Refract Surg 1997;23:1345–1350. 49. LE Probst. LASIK instrumentation. In: J Machat, S Glade, L Probst, eds. The Art of LASIK. Thorofare, NJ: Slack, 1999, pp 73–78. 50. L Buratto, S Brint, M Ferrari. Surgical instruments. In: L Buratto, SF Brint, eds. LASIK principles and techniques. Thorofare, NJ: Slack, 1998, pp 35–68. 90 Ang and Azar 91 6 LASIK Indications, Contraindications, and Preoperative Evaluation RICHARD E. BRAUNSTEIN and MARC WINNICK Columbia University College of Physicians and Surgeons, Harkness Eye Institute, New York, New York, U.S.A. KENNETH A. GREENBERG Columbia University College of Physicians and Surgeons, New York, New York, U.S.A. A. INTRODUCTION Corneal refractive procedures are currently widely applied to correct ametropia. A successful refractive procedure is gauged by many criteria: safety, efficacy, predictability, and long-term stability. Laser-assisted in-situ keratomileusis (LASIK) is presently the most widely performed refractive procedure, but it is not appropriate for all patients. Optimal results are achieved through proper patient selection, education, examination, and consent. A complete understanding of the risks of the procedure and the effects of pre-existing ocular conditions are critical in selecting patients for surgery. The ability to recognize subtle ocular conditions through careful patient examination will reduce the likelihood of complications. Finally, helping patients have realistic expectations and the ability to say no to patients who clearly are poor surgical candidates are the keys to building a successful LASIK practice. This chapter will outline the present refractive indications for LASIK with special attention given to the preoperative evaluation and patient preparation prior to refractive surgery. Clinical tips for maximizing refractive outcomes and contraindications to LASIK will be discussed. B. INDICATIONS Laser in situ keratomilieusis may be considered for patients who are dependent on optical correction of various refractive errors who desire to reduce or eliminate their dependence on glasses or contact lenses. In the United States, FDA approval for LASIK is a confusing issue for surgeons and patients alike. The FDA does not approve procedures, it only ap- proves drugs and devices. How those drugs and devices are used after approval is up to physicians under a provision known as the practice of medicine. The FDA labels the devices and drugs according to the information provided by the developers and serves as a guide for their use. The specific approval guidelines for LASIK are different for each excimer laser manufacturer based on data from FDA clinical trials. The approval range of refractive errors is only one criterion of judging a prospective candidate, as patients who fall within the guidelines may not qualify for surgery owing to other conditions (e.g., thin cornea or flat cornea). The excimer laser is currently approved for a wide range of myopia, myopic astigmatism, and hyperopia. Many studies indicate that the predictability of LASIK decreases with increasing preoperative myopia and astigmatism (1). Hyperopia ranging up to 6 diopters is correctible, but at higher levels of attempted correction, predictability is again reduced (2). For hyperopia greater than 6 diopters, loss of spectacle-corrected visual acuity occurred in a significant number of eyes in some studies, and accuracy was sufficiently poor to advise against LASIK in these eyes (3). Approval for treatment of hyperopic astigmatism and mixed astigmatism is expected in the near future. C. CONTRAINDICATIONS Contraindications for LASIK include patients with systemic collagen vascular disease, im- munodeficiency, autoimmune disease, severe atopy, and diabetes mellitus, diseases all likely to affect corneal wound healing. Patients who have unstable refractive errors or significant pre-existing ocular pathology of the cornea or anterior segment including but not limited to scarring, severe dry eye syndrome, uncontrolled blepharitis, uveitis, or early cataract should also not have laser vision correction. Surgery should not be performed on women who are pregnant or nursing or on patients taking Amiodarone or Acutane. LASIK is contraindicated in eyes with a history of herpes simplex keratitis or herpes zoster oph- thalmicus. Special concern should be given to eyes with corneal neovascularization within 1 mm of the ablation zone and those patients with difficult anatomy including small orbital aperture, narrow interpalpebral fissure or deep-set eyes. Patients with keratoconus or keratoconus suspects should not have LASIK surgery. Finally, patients with signs of anterior basement membrane dystrophy may be better served with photorefractive keratectomy (surface ablation) rather than LASIK, as the likelihood of an intraoperative epithelial de- fect may create numerous postoperative management difficulties. The cornea should not be flattened to less than 33 D or steepened to greater than 52 D, as refractive outcomes in this range are less predictable (4). To determine the postoperative corneal curvature, the preoperative keratometry is reduced or increased by the amount of desired correction at the corneal plane. All candidates for LASIK procedures should have a stable refraction for at least 12 months prior to the procedure differing by no more than 0.50 diopter in manifest sphere or manifest cylinder. For hyperopic patients, manifest and cycloplegic refraction should not differ more than 0.75 diopter. 92 Braunstein et al. D. IATROGENIC KERATECTASIA LASIK alters both the shape and the structural integrity of the cornea. Preoperative evaluation involves determining what the resultant corneal curvature and residual stromal thickness will be, prior to proceeding with surgery. Preoperative corneal thickness is of particu- lar concern in preventing iatrogenic keratectasia. Post LASIK keratectasia results in a progressive central corneal steepening and myopic shift causing irreversible damage by one year. It occurs in approximately 1 in 1000 eyes (5). Preoperative assessment of corneal thickness, flap and resultant stromal bed thickness, and amount of desired ablation are essential to prevent such ectasias. The average thickness of the human cornea is 520 microns, and an average flap created by a microkeratome is from 130 to 160 microns. The keratectomy depth of the excimer laser ablation on average must not exceed 45 to 120 microns with 325 (conservative case) to 250 microns, respectively, left in the stromal bed (6). This sparks questions regarding the minimal thickness of residual stroma, which needs to be maintained to prevent keratectasia. Many LASIK surgeons currently employ a 250 micron limit to the residual stromal bed following LASIK, but this is only an average value, and the biomechanical constants of the human cornea vary over a wide range, so that the range of residual corneal thickness that would prevent keratectasia is unknown. Some advocate using a percentage of the corneal thickness as a minimal residual stromal thickness rather than an absolute number, given ultrasound pachymetry measurement errors and biomechanical considerations, such as the deeper stroma having less tensile strength compared to the anterior layers (6). This dilemma can ultimately be resolved when we are better equipped to measure the biomechanical constants of the cornea in vivo. Promising ap- proaches such as mechanical spectroscopy and measurement of birefringence of the cornea can assist in future determination of preoperative stromal bed thickness, which would be necessary to avoid iatrogenic keratectasia. The residual stromal bed following a LASIK procedure should be calculated prior to surgery based on a nonnomogram adjusted treatment of the refractive error to be corrected. E. THE CONSULTATION VISIT Preoperatively, each patient must have a complete evaluation including a medical, surgical, and ocular history as well as an ocular examination. A general medical history with emphasis on the above-mentioned systemic diseases should be discussed and a medication list obtained. Past ocular surgery and any previous or existing ophthalmic conditions, such as glaucoma, dry eye, amblyopia, and past contact lens use, should be reviewed. An initial preoperative evaluation for LASIK in contact lens wearers should be performed at least two weeks after discontinuation of soft contact lenses or at least three weeks after discontinuation of soft toric, hard, or rigid gas permeable lenses. Patients wearing rigid lenses must demonstrate keratometric and refractive stability prior to treatment. It may be necessary to discontinue contact lenses in these patients at least one month for ev- ery decade of contact lens use. Previous glasses and contact lens prescriptions should be compared to the manifest refraction. F. VISUAL ACUITY AND REFRACTION Uncorrected and best corrected visual acuity should be assessed. A careful refraction is critical to maximizing refractive surgery outcomes. Different refraction techniques are appli- LASIK Indications, Contraindications and Preoperative Evaluation 93 cable to different refractive errors. When refracting the myope, a resolution-based refraction should be performed to avoid overcorrections. A resolution-based end point involves using the least minus lens to visualize the most letters. The refraction is not terminated at 20/20 if an additional 0.25 diopters yields several letters on the 20/15 line. The Jackson Cross cylinder is used to determine the maximal amount of cylinder. The correct axis of the cylinder is easier to determine in eyes with higher degrees of astigmatism. Occasionally, autorefraction may be helpful in finding the astigmatic axis. Care should be taken to ensure that the trial lens frame or phoropter is appropriately positioned and level to define the axis most accurately. Hyperopic manifest refraction should emphasize a “push plus” technique. Patients are encouraged to accept the most plus sphere to see the most letters. Cycloplegic refraction with 1% cyclopentolate should be performed on all patients because it eliminates ac- commodation. This is essential to identify the overminused myope and to uncover latent hyperopia. For patients with hyperopia, manifest and cycloplegic examination should differ by 0.75 diopter or less. If a large amount of latent hyperopia is identified or if there is a significant discrepancy between the manifest and cycloplegic refraction in a myope, a post cycloplegic manifest refraction should be performed with an emphasis on pushing additional plus power. Occasionally, glasses may be prescribed temporarily to help the patient accept the additional plus power prior to performing surgical correction. A clinical workup should include manual keratometry, a pupillary examination, and a slit lamp examination with emphasis on any lid margin inflammation, corneal epithelial disease, basement membrane dystrophy, or stromal scars consistent with prior keratitis. Tonometry should be performed on all patients and gonioscopic examination on all hyper- opes. Central corneal pachymetry readings should be performed on all patients. A careful, dilated fundus examination is performed to analyze the optic nerve and retina for any pathology with careful attention to the peripheral retina in highly myopic eyes that may be at risk for lattice degeneration. Identified retinal tears or large areas of lattice may require laser photocoagulation prior to performing LASIK (7). Special attention should be given to the pupillary examination in myopic and partic- ularly astigmatic patients. Pupil size should be measured in dim and in bright lighting conditions and recorded. Measurement can be made with a pupil gauge or with an infrared pupillometer. Patients with higher degrees of refractive error and larger pupils may be at greater risk for postoperative night vision disturbances, although this remains a subject of great controversy (3). G. CORNEAL TOPOGRAPHY Corneal topography is essential in all patients prior to refractive surgery. Topography is used to identify patients with corneal curvature abnormalities that are not apparent on slit lamp examination. True keratoconus is often easy to detect by clinical history and examination, but subclinical cases may only be apparent by corneal topography. Corneal topography is necessary to determine whether patients have contact lens related warpage and to help determine when a cornea is stable following contact lens discontinuation. We recommend that any patient who wears contact lenses have corneal topography repeated 1 week apart to ascertain stability prior to surgery. Topography is also used to verify postoperative results and complications such as decentrations, central islands, and irregular astigmatism. 94 Braunstein et al. H. DISCUSSION OF SPECIFIC CONDITIONS 1. The Keratoconus Suspect Keratoconus is a noninflammatory bilateral corneal ectasia that produces irregular astigmatism and leads to marked refractive error. Clinical signs differ depending on the sever- ity of the disease. External signs include Munson’s sign and the Rizzuti phenomenon. Slit lamp findings include a Fleischer’s ring, Vogt’s striae, stromal thinning and scarring, prominent corneal nerves, and epithelial nebulae. Retroillumination signs include scissor- ing on retinoscopy, which is often the first evidence of early keratoconus, and an oil droplet sign. Early in the disease, many corneas appear normal on slip lamp biomicroscopy. Sev- eral devices are currently available for detecting early keratoconus. Simple, inexpensive devices such as a handheld keratoscope can show egg-shaped or inferocentral compression of mires, which may be indicative of early keratoconus. Ultrasonic pachymetry to demonstrate central and peripheral corneal thickness has been studied, and although it is highly accurate and reproducible for measuring corneal thickness, its use has failed to identify a large percentage of patients with clinically obvious keratoconus, and it should only be used as corroborative evidence for the diagnosis of keratoconus (8). Computer-assisted videokeratoscopes, which generate color-coded maps and topographic indices, are an excellent tool for the diagnosis of keratoconus even when signs of the disease are not obviously apparent at the slit lamp. Three features of keratoconus are common to video keratography; a localized area of increased surface power, inferior–superior power asymmetry, and skewed steep radial axes above and below the horizontal meridian depicting irregular astigmatism. Much work has been done to quantify the minimal topographic criteria for diagnosing keratoconus. One method is pattern recognition and one specific topographic pattern, asymmetic bow tie with skewing of the radial axis above and below the horizontal meridian, was found by Rabinowitz in virtually 100% of patients with clinical keratoconus. This pattern could represent the earliest sign of irregular astigmatism and might be a reasonable cutoff point in the transition from normal topography to keratoconus (9). The second method, the use of quantitative videokeratography derived indices, may represent a more reproducible way to quantify keratoconus and its early pheno- types. Maeda and coauthors devised an expert system classifier, utilizing an analysis of eight topographic indices derived from the TMS videokeratoscope. A linear determinant function is used to determine a composite discriminant value for each map: the KPI. A KPI value greater than the optimum cutoff is classified as keratoconus (10). Rabinowitz de- scribed a KISA% index, a quantitative videokeratographic algorithm embodying minimal topographic criteria for diagnosing keratoconus. Using a combination of four indices: central K, an expression of central corneal steepening; the I-S value, an expression of inferior–superior dioptric asymmetry; the AST index, which quantifies the degree of regular corneal astigmatism; and the SRAX index, the skewed radial axial index, the KISA index is derived. Kisa% ϭ K ϫ I-S ϫ AST ϫ SRAX ϫ 100 (11). This has been shown to have excellent preliminary clinical correlation and can be used by surgeons to determine a patient’s chance of having early keratoconus. A single index with excellent clinical correlation would be optimal to depict early keratoconus and warn a refractive surgeon of potential intra- and postoperative risks and complications. The formulation of these indices are still being altered to improve accuracy and at present should be used with appropriate clinical correlation including slit lamp biomicroscopy, pachymetry, retinoscopy, and keratometry to determine the relative risk of laser refractive surgery. Patients with keratoconus or LASIK Indications, Contraindications and Preoperative Evaluation 95 those who are likely keratoconus suspects are not candidates for LASIK using present al- gorithms and treatment profiles and should be considered to have a progressive corneal disease. Patients with asymmetric bow tie astigmatism and inferior steepening who do not appear to have keratoconus may be treated surgically with appropriate informed consent. 2. The Incipient Cataract The incipient cataract patient requires special consideration. Cataract extraction should be viewed as both a media clearing surgery and a refractive surgery. Progression of central visual axis opacity will decrease the best corrected visual acuity post laser and result in a patient unhappy with LASIK and in need of a second surgical procedure. It is therefore rec- ommended that the best corrected visual acuity be assessed preoperatively and the effect of the incipient cataract on visual acuity be determined. If it is believed that the cataract is significant or is progressing rapidly, laser vision correction should not be performed, and consideration may be given to cataract surgery, if appropriate. Although phacoemulsification can be performed on an eye that has had LASIK, the intraocular lens power calculation can be problematic, and a more reliable refractive outcome may be achieved with lens extraction and intraocular lens implantation (12). 3. The Dry Eye Patient Patients with a history of dry eye symptoms and contact lens intolerance frequently seek laser vision correction. Dry eye symptoms and corneal epithelial staining are common problems following LASIK. The mechanism of this disorder following surgery is not well understood, although alteration of corneal innervation following LASIK is the most likely cause. Evaluation of these patients includes assessment of tear film quality and breakup as well as corneal epithelial integrity. Patients with evidence of corneal staining should be treated preoperatively. Management may include topical lubricants, punctal occlusion, and occasionally oral doxycycline if the patient has evidence of meibomian gland dysfunction. Schirmers’ testing has not been shown to be predictive of postoperative dry eye problems following LASIK. 4. The Glaucoma Suspect If preoperative screening identifies a patient with elevated intraocular pressure or optic nerve head cupping, focal ischemia, or hemorrhage, a 24-2 Humphrey visual field is rec- ommended, and further testing or consultation is necessary to determine if indeed the patient has glaucoma. If the patient has glaucoma, LASIK should not be performed. Patients who are glaucoma suspects by family history, borderline elevated intraocular pressure, or mild disc asymmetry must be treated cautiously. If the patient does not have glaucomatous optic neuropathy, and a diagnosis of glaucoma is considered unlikely, they may be considered for laser vision correction. However, informed consent discussing the potential reduced value to tonometry measurement must be discussed with the patient. In general, the validity of applanation tonometry following laser vision correction is less accurate and ultimately may limit the ability to treat glaucoma in selected patients. Falsely low intraocular pressure measurements are obtained by applanation due to easy compressibility of the fluid-filled space between the corneal flap and the stromal bed and the direct relationship between corneal thickness and Goldman applanation tonometry (13). In addition, the intraoperative increase in intraocular pressure following application of the 96 Braunstein et al. suction ring during LASIK will further decrease optic nerve head perfusion pressure and can cause additional damage to an already susceptible glaucomatous optic nerve. 5. The Presbyope Many LASIK patients are presbyopes. Patients who already wear some reading correction are easier to counsel than patients who simply remove their glasses to read. Patients need to be informed that the procedure will not correct distance and near vision completely. An informed patient can help determine what would be best for them. Patients should be of- fered various options including distance vision in both eyes, monovision, or slight undercorrection of both eyes. If the patient is a successful contact lens wearer with monovision, this is likely to be the best route. If the patient does not wear lenses, a preoperative monovision contact lens trial can be used to demonstrate the effects of monovision to the candidate for refractive surgery (14). The add power and selection of the distance and near eye can also be determined with a contact lens trial. 6. The Low Myope For peripresbyopic and presbyopic lower myopes (Ϫ1.00 through Ϫ3.50 diopters), we frequently treat the dominant eye first and postoperatively determine the correction for the non- dominant eye, if any. Some patients happily tolerate large amounts of anisometropia, while others are intolerant of as little as 0.50 diopters. The refractive goal needs to be tailored to the requirements of the individual patient. Patients who spend most of their time doing near work without glasses preoperatively need to understand that correcting their distance vision will necessitate the use of reading glasses. Often, patients have a preconceived notion that surgery will allow them to throw away their glasses. Occasionally, patients misunderstand or are misinformed and are disappointed in their postoperative spectacle dependence for near work when they did not require glasses preoperatively. 7. The Very High Myope Patients with high degrees of myopia are usually very motivated as they have greater visual disability. A preoperative evaluation in high myopes must include a careful dilated retinal examination using indirect ophthalmoscopy to search for peripheral retinal pathology as well as a careful macular exam. Preoperative retinal consultation is of value to determine if existing retinal pathology requires any treatment prior to surgery. Additionally, with higher degrees of myopia, there is an increase in the loss of BSCVA as well as reduced accuracy. Patients need appropriate preoperative counseling to help understand these risks. 8. The Hyperope LASIK, at present, shows great promise for effective and stable correction for mild to mod- erate hyperopia. A gonioscopic examination should be performed prior to LASIK in all hyperopic patients and a prophylactic peripheral iridotomy may be indicated for an occlud- able angle. Although there is no cause and effect relation between hyperopic LASIK and acute angle closure glaucoma, patients with hyperopia and narrow angles should be fol- lowed for the possibility of angle closure at a later time (15). The ablation profile for hyperopia extends out to 9 mm requiring a large corneal flap. Patients with a history of contact lens wear and corneal neovascularization may have significant bleeding with LASIK surgery, and potential complications of bleeding should be discussed and included in the consent prior to surgery. LASIK Indications, Contraindications and Preoperative Evaluation 97 9. The Astigmat A preoperative evaluation of the astigmat is essential for successful refractive surgery. Astigmatism has a directional component and a magnitude, and sources of astigmatism can be corneal or lenticular. As the magnitude of cylinder decreases, the ability to measure pre- cisely the cylinder axis also decreases. Refractive, keratometric, and topographic cylinder occasionally do not match in axis or magnitude, but treatment is based on refractive cylinder. When disparity occurs, proceed cautiously and ascertain stability by repeating measurements over time. I. PATIENT SELECTION Selecting the appropriate candidate for refractive surgery requires an understanding of the patient’s expectations, desires, and disposition in addition to a clinical ocular examination. A candidate must have the ability to understand the risks and benefits of LASIK and be able to give informed consent. The patient must be able to tolerate the procedure and have the ability to lie flat without difficulty, to tolerate topical anesthesia, and to fixate steadily and accurately for the duration of the procedure. Patients must be targeted toward their differing visual needs. A myope prior to laser vision correction enjoyed close and clear visual space and will be disappointed if overcorrected postoperatively. Some presbyopes desire monovision, while others would rather wear spectacle correction. Patients who demand spectacle independence all the time and have unrealistic expectations of laser vision correction should be avoided in addition to all patients with the above-mentioned systemic diseases or ocular pathology that otherwise jeopardize the efficacy, safety, and stability of LASIK. J. LASIK CONSENT A patient’s understanding of the possible postoperative complications and adverse symp- tomatology is an essential component of an informed consent. Patients should be given a copy of the FDA brochure provided from the excimer laser manufacturer citing the results of clinical trials using the specific laser. Additional statistics may be provided regarding the doctor’s own data and experience. A detailed informed consent document is often helpful in highlighting many of the known side effects and complications of LASIK surgery. Undercorrection, overcorrection, and induced astigmatism and the possibility of additional surgery should be discussed. Complications that could lead to a loss of best corrected visual acuity include but are not limited to irregular flaps, irregular astigmatism, haze, scarring, infection, central islands, striae, and epithelial ingrowth; these must be explained to the patient in a manner that they can understand. Many patients do not appreciate that they do risk a loss in best corrected visual acuity and possibly blindness due to an infection or retinal vascular event. Patients who require excellent night vision (e.g., truck drivers) should be cautioned prior to LASIK surgery regarding their risk of night vision impairment. Night vision impairment has been reported by Guell and Muller in 23% of patients at 6 months postoperatively, and other re- ports of night halos occurred in up to 30% of eyes at 6 months (16). Halos around lights and night vision impairment appear to decrease with larger ablation zones and smaller pupil size and often improve with time (17). Finally, patients should be advised of the possibility of dry eye symptoms, which may affect vision following laser vision correction. Prior to surgery, the patient should be given an opportunity to read all of the materials provided and to have all of their questions answered by their surgeon. 98 Braunstein et al. REFERENCES 1. T Salah, GO Waring III, A El-Maghraby, K Moadel, SB Grimm. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996;121: 143–155. 2. S Esquenazi, A Mendoza. Two year follow-up of laser in situ keratomileusis for hyperopia. J Refract Surg 1999;15:648–652. 3. MC Arbelaez, MC Knorz. Laser in situ keratomileusis for hyperopia and hyperopic astigmatism. J Refract Surg 1999;15:406–414. 4. DJ Salchow, ME Zirm, C Stieldorf, A Parisi. Comparison of objective and subjective refraction before and after laser in situ keratomileusis. J Cataract Refract Surg 1999;25:827–835. 5. T Seiler. Iatrogenic keratectasia: academic anxiety or serious risk? J Cataract Refract Surg 1999;25:1307–1308. 6. T Seiler, K Koufala, G Richter. Iatrogenic keratectasia after laser in situ keratomileusis. J Re- fract Surg 1998;14:312–317. 7. TP Werblin. Barraquer Lecture 1998. Why should refractive surgeons be looking beyond the cornea? J Refract Surg 1999;15:357–376. 8. YS Rabinowitz, K Rasheed, H Yang, J Elashoff. Accuracy of ultrasonic pachymetry and videokeratography in detecting keratoconus. J Cataract Refract Surg 1998;24:196–201. 9. YS Rabinowitz. Keratoconus. Surv Ophthalmol 1998;42:297–319. 10. N Maeda, SD Klyce, MK Smolek. Comparison of methods for detecting keratoconus using videokeratography. Arch Ophthalmol 1995;113(7):870–874. 11. YS Rabinowitz, K Rasheed. KISA% index: a quantitative videokeratography algorithm embodying minimal topographic criteria for diagnosing keratoconus. J Cataract Refract Surg 1999;25:1327–1335. 12. B Seitz, A Langenbucher. Intraocular lens calculations status after corneal refractive surgery. Curr Opin Ophthalmol 2000;11:35–46. 13. J Najman-Vainer, RJ Smith, RK Maloney. Interface fluid after LASIK: misleading tonometry can lead to end-stage glaucoma. J Cataract Refract Surg 2000;26:471–472. 14. MM Hom. Monovision and LASIK. J Am Optom Assoc 1999;70:117–122. 15. M Paciuc, CF Velasco, R Naranjo. Acute angle closure glaucoma after hyperopic laser in situ keratomileusis. J Cataract Refract Surg 2000;26:620–623. 16. JL Guell, A Muller. Laser in situ keratomileusis (LASIK) for myopia from Ϫ7 to Ϫ18 diopters. J Refract Surg 1996;12:222–228. 17. SG Farah, DT Azar, C Gurdal, J Wong. Laser in situ keratomileusis: literature review of a de- veloping technique. J Cataract Refract Surg 1998;24:989–1006. LASIK Indications, Contraindications and Preoperative Evaluation 99 [...]... imaging system paths, rays can be intersected in three-dimensional space to compute the x, y, and z coordinates of the surface (32 ) Studies showed that the PAR system was both accurate and reproducible and had the ability to image irregular, deepithelialized, and keratectomized corneas (33 36 ) On the other hand, the use of fluorescein staining and limited measurement points (1400) are among the major... Opt 1990;10 :33 36 MJ Collins, B Brown, KJ Bowman Contrast sensitivity with contact lens corrections for presbyopia Ophthal Physiol Opt 1989;9: 133 – 138 KA Lebow, JB Goldberg Characteristics of binocular vision found for presbyopic patients wearing single vision contact lenses J Am Optom Assoc 1975;46:1116–11 23 110 14 15 16 17 18 19 20 21 22 23 24 Ambati et al GS Rubin Contrast sensitivity and glare testing... undilated conditions Perez-Santonja et al found that contrast sensitivity 1 month after LASIK decreased significantly only at low and intermediate spatial frequencies (3 and 6 cycles/degree) ( 23) By 3 months, there were no significant differences in contrast sensitivity at all spatial frequencies compared to baseline Wang et al found that LASIK eyes recovered contrast sensitivity by 3 months as well, a significant... Refract Surg 1998;24:1 83 189 Z Wang, J Chen, B Yang Comparison of laser in situ keratomileusis and photorefraactive keratectomy to correct myopia from Ϫ1.25 to Ϫ6.00 diopters J Refract Surg 1997; 13: 528– 534 8 Corneal Topography and LASIK Applications LI WANG and DOUGLAS D KOCH Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, U.S.A DIMITRI T AZAR Massachusetts Eye and Ear Infirmary, Schepens... is an indispensable tool for refractive surgeons for preoperative screening, surgical planning, assessment of surgical outcomes, detection and management of complications, and refinement and development of surgical techniques This chapter reviews the basic principles of CVK, the recognition of corneal topographic patterns, and the role of corneal topography in refractive surgery A PRINCIPLES OF CORNEAL... lens, a black and white (B&W) camera (TechnoMed utilizes a color camera and color ring system), a video frame grabber, and a computer system The number, thickness, color, and position of the rings relative to each other vary from system to system Most systems can be divided into “near design” (Tomey TMS, TechnoMed C-Scan, and Keratron) and “distant design” (EyeSys Corneal Analysis System-2000, Humphrey... Lomb) uses a scanning slit beam and direct stereotriangulation to measure the anterior corneal surface, the posterior corneal surface, and the anterior iris and anterior lens surfaces (Fig 8 .3) It employs a calibrated video and a forty scanning slit beam system (20 from the left and 20 from the right) to measure independently the x, y, and z locations of several thousand points on each surface with... horizontal and vertical lines spaced about 0.2 mm (200 microns) apart and computes elevation data based on the distortion of the grid The PAR system requires that a small amount of fluorescein be placed in the tear film, and the images are collected using standard fluorescence-based photography Image acquisition is rapid and relatively insensitive to focusing From the known geometry of the grid projection and. .. of LASIK Refractive surgeons must be aware of more than spherocylindrical thin-lens first-order optics in order to make informed decisions The following chapters will cover additional preoperative considerations that are necessary to optimize LASIK outcomes and improve patient satisfaction REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 American Academy of Ophthalmology Ophthalmology, Optics, Refraction, and. .. instantaneous, and refractive maps: (A) circular, (B) circular with central bow tie, (C) circular with central irregularity, (D) symmetric bow tie, (E) asymmetric bow tie, and (F) irregular pattern Corneal Topography and LASIK Applications C D Figure 8.5 Continued 121 122 E F Figure 8.5 Continued Wang et al Corneal Topography and LASIK Applications 1 23 (Table 1) The circular with central irregularity and irregular . eds. LASIK principles and techniques. Thorofare, NJ: Slack, 1998, pp 35 –68. 90 Ang and Azar 91 6 LASIK Indications, Contraindications, and Preoperative Evaluation RICHARD E. BRAUNSTEIN and MARC. 1997; 23: 134 5– 135 0. 49. LE Probst. LASIK instrumentation. In: J Machat, S Glade, L Probst, eds. The Art of LASIK. Thorofare, NJ: Slack, 1999, pp 73 78. 50. L Buratto, S Brint, M Ferrari. Surgical. 19 93; 77:75–80. 37 . BS Wachler, RR Krueger. Agreement and repeatability of infrared pupillometry and the comparison method. Ophthalmology 1999;106 :31 9 32 3. 38 . M Colvard. Preoperative measurement of scotopic