Preoperative Evaluation for LASIK Surgery
The preoperative LASIK evaluation is an extremely important and structured examination to determine if a patient is a good candidate for LASIK surgery. This examination is one of the most comprehensive ocular exams done today.
Chief Complaint and History of Present Illness
When a patient first presents for the consideration of LASIK surgery, it is important to know why they have chosen LASIK, and what are their goals of surgery? Once these questions are answered, the ophthalmologist will have a better idea of the type of surgery (distance vison, monovison, etc.) and the patients expectations. The key to a happy patient after LASIK surgery is whether or not the patient’s expectations were met. For example, a patient who dislikes their contact lenses and wants to see better without them, has normal expectations which should be accomplished after surgery. A patient, on the other hand, who wants better vision than they already have with their contact lenses, may not have their expectations met, and may be disappointed with the outcome of surgery. A patient with unrealistic expectations is a poor refractive surgical candidate.
Past Medical History/Past Ocular History
Obtaining a thorough medical and ocular history is important to look for diseases that potentially can affect the outcome of LASIK surgery. Systemic diseases such as diabetes, autoimmune diseases, and collagen vascular diseases, history of cheloids may alter wound healing. Ocular conditions such as ocular herpes or peripheral keratitis may reactivate after LASIK surgery. Patients with history of strabismus surgery may also experience a recurrence after LASIK surgery. Patients that have high risk conditions must be informed of the serious risks involved. If surgery is considered in these patients, than more information needs to be gathered before surgery is performed.
A thorough list of medications is important, including any over the counter drugs, as many drugs can affect the outcome of LASIK surgery. Medications such as ImitrexTM, a migraine drug, can alter wound healing in the cornea, and my need to be stopped around the time of surgery. AccutaneTM, a medication to treat severe acne, may result in ocular surface drying. Over the counter medications such as antihistamines may exacerbate dry eye problems after LASIK surgery.
Past Surgical History
Any previous eye surgeries, such as childhood strabismus, are important to know. Often times, patients will forget or not think a surgery in childhood is important information, unless asked by the doctor.
The use of alcohol and cigarettes especially in excess may alter healing after LASIK surgery. Also important in the social history are lifestyle questions such as hobbies, work, etc., which give understanding about the type of vision that is important to the patient. Questions about high risk activities such as contact sports are valuable since there may be a higher risk of flap complications.
Certain diseases may run in families which if present, may be a risk factor for the patient who is seeking LASIK surgery. For example, having a family member who underwent a corneal transplant, may give the suspicion for the possibility of keratoconus, and should be carefully screened.
Both uncorrected and corrected visual acuities are taken.
A thorough manifest refraction is completed initially, and checked against a cycloplegic refraction. Having both refractions will help identify patients with latent hyperopia, as well, help guard against overminusing myopic patients.
Complete Eye Examination
A complete eye examination is done, including tonometry and a posterior dilated exam. Special attention is paid to the anterior segment of the exam. Lid examination is important to look for signs of meibomian gland dysfunction. If lid disease is present, pretreatment is necessary for the patient. Next, attention is focused on the conjunctiva and cornea, investigating for signs of dry eyes and punctate keratitis. Staining with vital dyes, including fluorescein and lissamine green are important to quantify the amount of dry eyes. Next, it important to look for any corneal irregularities/abnormalities, such as anterior basement membrane dystrophy, that may interfere with LASIK surgery. Corneal scaring or vascularization may signal old infections, in which a herpetic infection could be a possibility. Anterior chamber and iris examination should be inspected for any signs of old iritis, or iris atrophy which again could signal the possibility of old herpetic disease, or other inflammatory diseases. The posterior cornea should be inspected for signs of endothelial dysfunction such as guttae. The crystalline lens should be carefully examined for any signs of cataract formation. If cataracts are present, then LASIK surgery may not be indicated, and other alternatives may be discussed. A complete dilated fundus examination is necessary. In highly myopic patients, it is especially important to perform a peripheral examination to look for tears and holes, which may need treatment by a retinal specialist before performing LASIK surgery. However, no studies have found an increase risk of retinal problems such as retinal detachments after LASIK.
Testing in the LASIK evaluation process is vital, and without proper testing, a patient cannot be adequately screened.
Dry Eye Testing
Ocular dryness already present in patients undergoing LASIK surgery, will have a higher risk of post-operative dryness. If ocular dryness is detected, than treatment is started promptly. Tear break up time measures the quality of the tear film. If abnormal, lid hygiene and other treatments for blepharitis are initiated. Schirmer testing detects tear film quantity. If abnormal, than tear substitutes, punctal plugs, nutrient supplements (fish oil or flax), cyclosporine and/or lifitegrast treatments are begun. Patients will need continual follow up and re-evaluation until dryness is improved, before LASIK can be performed. New treatments, such as tear osmolarity tests are becoming more popular in the refractive surgery patient. There are several other tests for tear film stability that can be obtained ranging from OQAS™ II (Optical Quality Analysis System), Non-invasive tear break-up time (NIBUT) (e.g. Oculus Keratograph 5M, Medmont E300 topographe, Lipid layer assessment at the slitlamp biomicroscope, using specular reflection and a qualitative grading scale and Meibography – imaging with photography (e.g. Oculus Keratograph 5M).
Contrast Sensitivity Testing
Contrast sensitivity tests may be performed by some doctors to better understand the functional vision of the patient, beyond just the Snellen chart (a high contrast chart). Tests such as the Hamilton-Veale Contrast Sensitivity Test and the Mars Letter Contrast Sensitivity Test are two examples of testing used today.
Evaluating the pupil size, especially in the dark, will allow better informed consent. Pupil size can be measured accurately with special devices such as an infrared pupillometer. Patients with larger pupil sizes may be at risk for glare and halos after LASIK surgery. However, the association between pupil size and postoperative complaints is still debated.
Topography, Tomography and Corneal Biomechanics
Corneal topography, or computerized videokeratography, has had an unquestionable role in the development of refractive surgery. The term topography is derived from the Greek words topos (place) and graphein (to write). Although topography has been classically related to the study of geographical elements, such as the Earth’s surface shape and features, corneal topography is a method of front or anterior corneal surface imaging. The term tomography is derived from the Greek words tomos (a cut or section) and graphein (to write). Tomography is a method for mathematically calculating a 3-D structure of a solid organ. Thus, the term corneal tomography should be used for the diagnostic characterization of the front and back surfaces of the cornea, along with thickness mapping. Several technologies for tomography, including horizontal slit scanning, rotational Scheimpflug imaging, arc scanning with high-frequency ultrasound, and anterior segment optical coherence tomography, are available in various commercial instruments. To improve the sensitivity and specificity of screening protocols for refractive surgery candidates, corneal characterization should go beyond front surface curvature and single-point central thickness. A tomographic approach is essential. Tomography combines anterior and posterior corneal elevation with complete pachymetric data to reconstruct corneal architecture in three dimensions. Interpreting such an enormous amount of data is a complex task. Refractive surgeons should be aware that the goal of screening refractive surgery candidates for their risk of ectasia should not be limited to the detection of mild or subclinical forms of ectatic diseases (keratoconus and pellucid marginal degeneration); it should also include the assessment of a relatively normal cornea’s susceptibility to biomechanical failure.
Wavefront testing is used to detect higher order aberrations that may degrade vision. Standard LASIK procedures cannot treat patients with significant amounts of higher order aberration, which may leave them with unwanted visual symptoms after surgery. Wavefront guided LASIK may be the better choice to reduce higher order aberrations, subsequently giving the patient a better visual outcome.