CLINICAL SELECTION CRITERIA FORWAVEFRONT-GUIDED LASIK
Mahipal S Sachdev
Consultant Ophthalmologist, Centre for Sight, New Delhi.
Philosophy of patient care
Many patients presenting for refractive surgery assessment may not have had a comprehensive ophthalmic examination in several years. It is not only essential to rule out conditions such as corneal dystrophies or retinal disorders prior to recommending refractive surgery, but it also represents an opportunity to educate the patient with regard to the need for regular routine eye examinations including a dilated fundus examination. Additionally, patients can be screened for disorders such as glaucoma. Patients with conditions that preclude them from refractive surgery are generally appreciative of the thoroughness and care taken in the assessment examination.
It is important when discussing refractive surgery options with patient that they are aware of how long a procedure has been done both historically and at the surgery centre. A frequent concern is - what types of long-term results have been established. Patients need to thoroughly understand that new procedures suffer from the lack of long-term follow-up. However, if encouraging results from studies have been published, the patient should be made aware of the same. Less than ideal candidates for present-day refractive procedures may be counseled into waiting until newer procedures undergo further study.
The surgeon must evaluate all aspects of the patient history, examination, motivation and expectations as well as clearly communicate the risks and benefits. Only when the patient has a full understanding of the technique and application to his/her individual case can he/she make a truly informed choice.
Refractive surgery assessment
The assessment begins with a thorough medical and ophthalmic history. The patient is first asked about the motivation for refractive surgery. Recreational, cosmetic or occupational needs, inability to wear contact lenses, or a good surgical outcome of an acquaintance motivates most people. On the Indian sub-continent, the social stigma attached to a visual aid is often the strongest motivational factor. Unrealistic motivational factors should be carefully identified, documented, and discussed with the patient. Occupation and hobbies are important to identify, as they may determine the amount of post-operative time off the patient is willing to take. Protective eyewear may still be required in some occupations and some candidates may require glasses for near work or night driving.
Systemic conditions such as diabetes mellitus, collagen vascular disorders or pregnancy may delay the healing process. Diabetics may be a risk for dry eye conditions and corneal epithelial adherence problems. Collagen vascular disorders can cause a severe dry eye and delayed corneal healing. Pregnancy and its accompanying hormonal changes may lead to an unstable refractive status translating into variable post-operative refractive errors.
A complete ocular history is obtained, including previous ocular infection such as herpetic corneal disease (reactivation of herpes may occur post-refractive surgery). A history of previous corneal, refractive or ocular surgery, glaucoma, recurrent epithelial erosion syndrome, retinal detachment or lattice degeneration, or previous trauma is elicited. Finally, the patientís current medications : antihistaminics and decongestants can exacerbate dry eye problems, immunosuppressants can delay wound healing, and oral contraceptive pills can cause changes in corneal curvature and refraction. Any known allergies to medications should be ascertained.
Prior to the first assessment, patients wearing contact lenses are asked to discontinue soft lenses for at least 48 hours and semi-soft contact lenses for one to two weeks. A careful refraction is performed and refined with binocular balancing. Patients in the presbyopic are group require reading glasses after refractive surgery. Some patients are given a monovision trial - one eye is corrected fully for distance vision and the other is left slightly myopic for close work. Some are given a contact lens simulation of the same over a few weeks to get used to the change in lifestyle. Occasionally amblyopia (lazy eye) is detected in one eye and documented. The patient makes an informed choice on whether to go ahead with the surgery.
Palpebral fissure size and deep-set eyes are noted as potential exposure problems that may be encountered during surgery. Binocularity testing is performed including stereopsis, cover / uncover testing and extraocular movements. It is important to identify latent squint conditions as a change in refractive state may decompensate it, resulting in a manifest squint.
Examination of the pupil size in dim and room light is carefully measured and recorded. Patients with large pupils may suffer potential untoward effects such as haloes, glare and loss of contrast sensitivity due to optical aberrations (wavefront sensing has been introduced to greatly reduce or eliminate such optical aberrations and improve night vision). Patients with large pupils and very large refractive errors are discouraged from undergoing refractive surgery because fully treating a large refractive error requires employment of a small optical zone of treatment, which may aggravate dim light symptoms. The alternative is to have an undercorrected eye with a large optical zone.
Computerized corneal topography is now considered an integral part of preoperative evaluation. Some wavefront guided laser systems require both, aberrometer readings as well as corneal topography maps. Topography is generally used to determine various corneal conditions, commonly regular and irregular astigmatism. Corneal warpage from contact lens wear results in a central irregular astigmatism that is accompanied clinically by an unstable refraction, decreased visual acuity, spectacle blur and reduced contrast sensitivity.  Severe warpage may be a contraindication for going ahead with refractive surgery even if the contact lenses are not worn over a long period. Clinical and pre-clinical keratoconus are absolute contra-indications to any form of refractive procedures at the present moment. The corneal status of patients with a history of past refractive surgery is very important. Techniques such as radial keratotomy leave corneas with irregular astigmatism and excimer laser procedures done without proper eye tracking may result in irregular or decentred treatments.
After these measurements applanation tonometry is carried out.  a change in the corneal curvature and thickness usually decreases the post-LASIK intraocular pressure readings. It is therefore important to calculate and carefully document the difference between the pre-op and post-op readings. A detailed slit lamp examination is done paying particular attention to eyelid margin condition (blepharitis), tear margin appearance (keratoconjunctivitis sicca), tear film break-up time, conjunctiva (pinguecula, pterygium) and the cornea (dystrophy, scars, previous infection or surgery, peripheral corneal vascularization, endothelial dysfunction). The pupils are dilated and the following carried out
The aberrometer measures the errors in the optical system. Typically five serial readings are taken and the three closest readings are averaged. These readings are loaded onto the laser software.
Examination of the media includes the crystalline lens for cataract formation and the vitreous for opacities.
A detailed retinal examination includes magnified binocular macular study and peripheral retinal examination (lattice degeneration, retinal holes).
Pachymetry (measurement of corneal thickness) is important for determining whether adequate corneal tissue exists for performing the procedure. This is particularly an issue in high myopes, whether large amounts of corneal ablation are required. There must be a residual stromal bed of 250 microns.  The laser software generally does not allow more than a certain amount of treatment to be carried out. The patient is advised alternative forms of refractive surgery if it is felt that the refractive error cannot be completely treated by LASIK. It also is an issue in cases of previous refractive surgery where adequate corneal tissue might not be available for treating the residual refractive error.
Thoroughness and attention to detail applies to all aspects of refractive surgery starting from the initial contact with the refractive surgery candidate. Patient assessment is critical to the procedure due to the presence of absolute and relative contra-indications. Individual variables with regard to motivations, expectations and visual requirements must be fully addressed. Fully examined and informed patients tend to have much more positive experiences and more realistic expectations.
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2. Emara B, et al. Correction of intraocular pressure and central corneal thickness in normal myopic eyes after laser in situ keratomileusis. J Cataract Refract Surg 1998; 24 : 1320-5.
3. Yaylali et al. Corneal thickness measurements with the orbscan topography system and ultrasonic pachymetry. J Cataract Refract Surg 1997; 23 (9) : 1345-50.