LASIK

Choose the Best Candidates For Laser Vision Correction

Consider following U.S. Navy guidelines to evaluate patients and ensure the best possible outcomes.

By David J. Tanzer, M.D.

► Since the U.S. military deemed both PRK and LASIK acceptable surgical procedures for eligible personnel,¹ Navy surgeons such as myself have had the opportunity to improve quality of life and job performance for thousands of service men and women. To date, more than 150,000 members of the armed forces have undergone laser vision correction (LVC), according to military data. The procedures have been overwhelmingly successful, and patient satisfaction is incredibly high.

As you work toward earning your refractive surgery wings, I can share with you two elements of the military's refractive surgery program that have helped to ensure its success: use of the best technologies and thorough, up-to-date patient screening.

Cornerstones for Success

The military's LVC program is based on meticulous research. The goal of the research, which comprises more than 45 studies, is to conduct independent evaluation of the safety and efficacy of LVC. Some of the studies have been presented at national ophthalmic meetings.^2–4 Also, the research has addressed specific issues, such as visual recovery time, postoperative quality of vision and the effectiveness of LVC in the unique environments in which military personnel work. While these issues are of specific importance to the military, they're also crucial for your patients, each of whom has a unique set of pre-op visual characteristics and post-op visual needs.

Based on the body of research from the Department of Defense (DOD), we've concluded that a customized (wavefront-guided) ablation with a femtosecond laser-created LASIK flap yields the best quality of vision with the fastest visual recovery time. This procedure is being performed in aviators from all three branches of the military, as well as in NASA astronauts. While this combination of technologies is our preferred procedure, not every person who has an interest in LVC is an appropriate candidate. Therefore, thorough and up-to-date patient screening protocols are a necessary ingredient for success.

For patient screening to be thorough and current, it must reflect how LVC technologies have evolved. As technology has advanced from conventional to custom laser ablations, we're required to measure the eye differently, specifically with wavefront aberrometry. In addition, we have new corneal diagnostic modalities, which have allowed us to move beyond simple keratometry to placido disk topography and elevation-based measuring instruments for patient screening. The latter has allowed us to measure both the anterior and posterior cornea, as well as the anterior lens. Using these new corneal diagnostic technologies improves the safety of the procedures and our clinical outcomes.

Thorough Screening

At our Naval facility, refractive surgeons in training receive rigorous instruction in the use of advanced corneal diagnostics and in proper application of the most effective laser technologies. To ensure the best possible result, they perform a comprehensive clinical exam and evaluate each potential LVC patient. You may be able to adapt our guidelines that follow to improve outcomes in your own practice.

The more stable the refraction preoperatively, the higher the likelihood the results of surgery will be satisfactory.

■ Refractive stability. According to the FDA, before undergoing LVC, patients should have a change in refraction no greater than 0.50D in either sphere or astigmatism over two exams separated by 12 months.

■ Relevant history. Patients are asked whether they have a history of ocular or medical conditions that are absolute or relative contraindications to LVC. Questions cover history or symptoms of herpes simplex virus (HSV) keratitis, ocular trauma, dry eye contact lens intolerance, and personal and family history of keratoconus. We also inquire about history of diabetes, autoimmune disease, collagen vascular disease and dermatologic conditions, such as atopic dermatitis and keloid formation, all of which can impair healing. Surgeons repeat the patient history before proceeding with LVC. In addition, patients are screened during a clinical exam for various ocular conditions.

■ Review of medications. Because several types of topical and systemic medications may impair healing, they're generally acknowledged as relative contraindications to LVC. So patients are asked if they're taking corticosteroids, antimetabolites, isotretinoin, amiodarone and sumatriptan.

■ Refraction. Auto, manifest, cycloplegic and wavefront refractions are performed and compared. Results from each method should be similar, indicating refractive stability. The more stable the refraction preoperatively, the higher the likelihood the results of surgery will be satisfactory. Attempting to correct a shifting refractive error with LVC is like trying to hit a moving target and, therefore, shouldn't be done. Wavefront refraction data are also useful in determining levels of higher-order aberrations such as coma, which has been linked to the possibility of irregular astigmatism and keratoconus.

■ Low contrast visual acuity. Navy surgeons measure low contrast acuity using 5% and 25% contrast acuity charts. The measure isn't used as a factor in deciding who's a candidate for LVC. Instead, it's used as a means to quantify and compare the quality of vision before and after surgery.

■ Pupil size. Surgeons measure pupil size using aberrometry, topography and/or infrared scanner, and document the results. However, our surgeons don't base treatment decisions on pupil size, which remains a contentious issue in refractive surgery. DOD research has found that pupil size isn't predictive of the level of postoperative visual complaints.⁵

■ Intraocular pressure. If ocular hypertension or signs of glaucoma are found during the comprehensive clinical exam, surgeons perform visual fields and nerve fiber layer analysis to rule out pathology.

■ Slit lamp/clinical exam. Patients are screened during the comprehensive clinical exam to determine if the following conditions are present.

• HSV keratitis. Typically, this can be diagnosed based on the appearance of the cornea through the slit lamp. Surgeons also can perform laboratory studies if needed to confirm clinical suspicion.

• Lid disease. Patients are examined for signs of lid disease. If signs are present, physicians prescribe treatment. For anterior lid margin disease, such as blepharitis, the doctors may prescribe warm compresses, a daily lid hygiene regimen and, depending on severity, a topical antibiotic ointment. Surgeons consider a course of oral doxycycline for posterior lid margin conditions, such as meibomian gland dysfunction. Patients are examined again in a month to determine if treatment should continue or if it's time to proceed with LVC surgery.

• Dry eye. Physicians check patients for signs of dry eye, including a poor or nonexistent tear meniscus, conjunctival injection and corneal surface changes, such as punctate epithelial erosions in the palpebral fissure. If signs are present, physicians treat patients aggressively. Treatment may include nonpreserved artificial tears, topical cyclosporine and, if lid disease has been ruled out, punctal occlusion. Surgeons examine patients again in a month to determine if treatment should continue or if it's time to proceed with LVC surgery.

• Corneal scars. Surgeons screen patients closely for corneal scars, which may be caused by previous radial keratotomy, ocular trauma or several other factors. The presence of corneal scars will influence which refractive procedure is most appropriate.

• Cataract. Navy surgeons also perform the comprehensive clinical exam to rule out cataract.

■ Dilated fundus exam. As with the measurement of intraocular pressure, if the dilated fundus exam reveals any potentially glaucomatous abnormalities, doctors perform full glaucoma testing. If they observe any retinal abnormalities, they refer patients to a retinal specialist to guide their decision-making. For example, the presence of a peripheral retinal hole could theoretically lead to a detachment due to the increased pressure during LASIK. A retinal specialist may be able to treat the hole, allowing the patient to safely undergo LASIK. Or, the refractive surgeon could perform a surface procedure, such as PRK, instead of LASIK.

■ Topography. Corneal topography, both placido disk and elevation-based, is performed to alert the surgeon to abnormalities in corneal curvature or shape, such as irregular astigmatism, or signs of ectatic disease, such as keratoconus and pellucid marginal degeneration. Most surgeons won't proceed with LVC in these situations. The latest generation of placido-based devices includes sophisticated software that assesses the cornea qualitatively and quantifies the level of abnormality. Scheimpflug and slit scanning elevation-based devices allow doctors to view the anterior and posterior surfaces of the cornea. This is important because it's been postulated that keratoconus may first emerge on the posterior surface.

■ Pachymetry. Corneal thickness is a critical measurement in the preoperative LVC evaluation. To avoid ectasia, an adequate amount of stromal bed must remain after LASIK flap creation and ablation.

LVC Decision-making

All of the clinical data from the preoperative screening is crucial when deciding if a patient can safely undergo LVC with positive results, which type of procedure is best and how much correction to provide, but some pathologies or abnormalities may disqualify a patient altogether. Certain conditions, such as lid disease or dry eye, may be treatable so the patient can undergo any type of LVC, while others may prompt you to use an LVC procedure other than custom, femtosecond laser-assisted LASIK.

For example, a mechanical microkeratome may be preferable to the femtosecond laser for a patient with a corneal scar. It's possible that the gas bubbles created by the femtosecond laser could migrate through the scar and exit through the corneal epithelium.

In addition to the clinical data from the preoperative screening, you must consider the patient's age, occupation and expectations of the procedure. Taking age into account may involve considering a monovision result for presbyopic and prepresbyopic patients.

In appropriate patients, monovision is an effective way to provide quality uncorrected vision at distance and near. Trial contact lenses should be given to patients to ensure they can tolerate this effect.

With wavefront-guided femtosecond LASIK, all eyes in this study were 20/20 or better by 2 weeks post-op.

Taking age and occupation into account may involve planning the laser ablation for a slight overcorrection in a young, myopic pilot who flies day and night. Given the ability of the young eye to accommodate, slight overcorrection may allow clear vision in light or dark situations. In contrast, such an overcorrection wouldn't be advisable for a 45-year-old who may not accommodate as well.

For a younger, hyperopic patient, you should take into account the difference between the manifest and full cycloplegic refraction to increase the probability of a successful outcome. Most younger hyperopes function with a varying amount of accommodative tone (less manifest hyperopia). Correcting the full cycloplegic refraction may result in an unhappy patient, unless you can relax that accommodative tone. This process can take many months as progressively stronger (hyperopic) eyeglasses or contact lenses are used to relax the accommodation.

For a presbyopic hyperope, accommodative tone is less of an issue; treating the full cycloplegic refraction usually is recommended. As mentioned previously, you should discuss monovision options with all presbyopic LVC candidates.

Finally, you need to understand what a patient expects from an LVC procedure. Patients who expect more than what LVC can deliver aren't good candidates. You must obtain informed consent, ensuring each patient has considered and accepted the risks, and knows that a perfect result cannot be guaranteed. Patients must be fully aware that other vision correction options are available, including eyeglasses and contact lenses.

Keys to Success

Success with LVC depends on many factors. It begins with a motivated patient being evaluated in a systematic and comprehensive manner, using the latest diagnostic modalities.

It's furthered by applying the optimal surgical technology, which, based on a thorough evaluation and understanding of the patient's expectations, has been determined to be the best option.

Finally, success requires educating the patient about all vision correction options available, all risks possible and the proper postoperative care. nMD