Stability of refraction

Stability of refraction needs to be confirmed. The candidate should be at least 18 years old and the refractive status history (previous spectacle prescriptions) should be checked. Frequent changing of spectacles or contact lenses may sometimes indicate keratoconus.

Contact lens (CL) wearers

Soft CL wearers should stop using their lenses for at least 1 week prior to evaluation and surgery. The contact lens flattens the cornea and an examination conducted immediately after lens removal might underestimate myopia. For hard CL wearers, the corresponding period should be double, i.e. 2 weeks minimum. In some cases of corneal warpage syndrome, the patient may need to abstain from wearing contact lenses for up to 3–5 months. Topographies obtained even 2 months after discontinuation of contact lens use may give the erroneous impression of keratoconus in hard contact lens wearers or in patients suffering from warpage syndrome; however, topographies of the same eyes were absolutely normal after 4 months had elapsed.

History-taking should include both ophthalmic and systemic history. Many patients are poor or ineligible candidates because of topical or systemic disease. On the other hand, some topical or systemic conditions and diseases are considered relative or absolute contraindications for laser keratorefractive surgery.

Naturally, ophthalmic examination should always be complete and meticulous and the results recorded.

Ocular contraindications

These may be divided into active, residual, and recurrent ocular disease. The following are examples of each respectively:

Systemic contraindications

• Diabetes is not a contraindication for keratorefractive surgery, however it should be discussed with the patient that:

Healing is generally suboptimal and may be delayed.

Refraction is usually not stable so it would not be known whether a suboptimal refractive result is caused by regression or under- or over-correction.

Diabetics develop cataracts earlier than non-diabetics.

Diabetics have higher susceptibility to infection. This may be aggravated by the steroids that may be required postoperatively.

Unaided visual acuity (UVA)

This is a measure of efficacy and predictability. Near UVA is a test for macular function and is essential in presbyopes. −2.0 diopter myopes in their 50s may have never realized their presbyopia. Correcting patients to plano in both eyes renders them unable to read without reading spectacles. The patient should be made aware of their reading problem by being asked to read while wearing their distant correction or their contact lenses. In this situation, monovision or presbyopic laser correction could be offered. Patients who have jobs requiring the constant use of near vision should be informed prior to surgery about the possibility of over-correction and the consequences. These patients may be offered correction of their eyes in different sessions.

Best corrected visual acuity (BCVA) – manifest refraction

This is possibly the most important factor to evaluate as its postoperative value is the safety of a procedure. The phrase ‘gain of BCVA’ is anecdotal. It is caused by the magnification of the image on the corneal plane compared with the minification of a moderate to high myopic spectacle minification of image leading to a lower recorded BCVA that is lower than the actual one. In cases of myopia over −6.50 diopters or hyperopia over 4.0 diopters, or whenever the best spectacle-corrected VA is less than 20/20, soft or, preferably, hard contact lens over-refraction should be used to avoid minification or magnification of image respectively.

No laser keratorefractive surgery should be performed without assessing the cornea with topography. Corneal topography and aberrometry are essential preoperatively and Chapter 1 of this book is dedicated to this subject. Contrast sensitivity testing is not usually routinely assessed but used only for academic purposes.

Ocular motility

Postoperative ptosis3 has been reported and, according to various studies, is in the range of 0.4%. It is usually temporary and is caused by prolonged use of steroids, usually after PRK. Should the patient present with any ptosis preoperatively, photos should be taken and kept on file for medicolegal reasons.

Slit-lamp examination

Slit-lamp examination of the anterior segment is mandatory to rule out disease or other pathology. Lids and adnexae should be examined and any infectious disease or blepharitis, etc. should ideally be treated prior to surgery. The cornea should be evaluated for clarity, scars and the presence of dystrophies or degenerations, especially keratoconus.

Subclinical keratoconus is diagnosed by corneal topography but, for some refractive surgeons, it is not necessarily a contraindication for LASIK or PRK. Recent studies suggest that in some mild cases treatment may be possible following collagen cross-linking with riboflavin to stabilize these corneas. However, corneas with a central corneal thickness of less than 500 µ are considered high risk for keratectasia. In any case, should correction of such corneas be attempted, the patient should be informed that refraction may end up suboptimal, that the condition may deteriorate and that corneal transplant may become necessary.

The anterior chamber must be clear and free of any active iritis. The crystalline lens should be clear. It is always advisable to discuss with a moderately myopic (e.g. 8.0 D) presbyope the possible advantage of an alternative correction with clear lensectomy.

Most LASIK candidates are contact lens wearers and some peripheral corneal neovascularization almost always exists. In severe cases, intraoperative conjunctival hemorrhage is anticipated during flap creation, the suction-cutting of the mechanical or femto second laser microkeratome, especially in large flaps and/or small eyes. This is usually not a serious complication, but it might decrease the surgeon’s visibility during surgery and the conjunctival hemorrhage may produce anxiety during the first 3–4 postoperative weeks (i.e. the period required for its absorption).

The pupil

It is always preferable that the laser ablation zone covers the scotopic pupil, as it is best measured with the automated infrared scotopic pupillometry, e.g. procyon pupillometer. The Zywave wavefront sensor with the fixating target turned off using the study settings and light conditions provided measurements of scotopic pupil diameter that were closest to the reference values (Procyon). With other devices (Colvard pupillometer, Zywave aberrometer with the fixating target switched on, Wasca aberrometers and Orbscan topographer), the difference was statistically significant4. When small ablation zones, i.e. those not sufficiently covering the scotopic pupil, are used, the prismatic effects of the reshaped corneal optics are liable to induce postoperative glare and halo phenomena.

Schirmer test

Schirmer testing is performed to avoid treating cases with xerophthalmia, something that may delay healing. It is important both in PRK5,7 and in LASIK6,7. However, some preservatives used in drugs (even artificial tears) following LASIK may accentuate symptoms of xerophthalmia. The patient should be aware that artificial tears may be needed in excess of 3–6 months postoperatively and that, in some cases, permanent plugs may be required.

Cycloplegic refraction

One drop of cyclopentolate 1% is instilled into the eyes and repeated 5 minutes later. Refraction is determined about 30–40 minutes later, when optimal cycloplegia is obtained. Contact lens over-refraction should be used in cases mentioned earlier.

Cycloplegic refraction is very important in young hyperopes who may present with spasm of accommodation and the difference between manifest and cycloplegic refraction is high. Differences of up to six diopters have been noted. Ideally, no discrepancy should exist between manifest and cycloplegic refraction; however, should such a discrepancy exist, such as in hyperopes, the standard rule in treatment is that the amount of sphere treated is that measured with the cycloplegic refraction – corrected to the corneal plane – while the amount of cylinder, as well as its axis is that determined with manifest refraction.

When cycloplegic refraction is lower than manifest, the patient is asked to recheck his or her manifest refraction at a later date. The spasm is usually caused by over-correction in the spectacles or contact lenses. Very good fogging should be used in determining the manifest refraction.

Ultrasound or OCT central pachymetry is performed. Orbscan pachymetry is unreliable, especially in post-LASIK eyes. Provided that there is no contraindication from the examination, and once the central corneal thickness and the scotopic pupil diameter are known, ablation depth (i.e. the amount of corneal tissue to be removed) may be calculated. The golden rule is that after the flap is made and ablation is effected, the remaining corneal bed thickness should be no less than 250 µm.

Intraocular pressure measurement (IOP)

Following PRK or LASIK, the gold-standard Goldmann applanation tonometer (GAT) underestimates the IOP. Glaucoma is a vision-threatening disease and diagnosis may be missed in a patient who has undergone LASIK. Currently, it seems that the Pascal dynamic contour tonometer8 (PDCT) is the only tonometer that accurately records the IOP in post-LASIK or -PRK eyes. Its measurement is not affected by corneal curvature or thickness. Corneal abnormalities seem to influence GAT IOP measurements more than PDCT IOP measurements9.

Glaucoma is not an absolute contraindication for LASIK. Except for the possibility of inaccurate IOP measurements with a common tonometer, another concern is the period for which steroids are prescribed postoperatively. Many glaucoma patients are steroid responders, therefore steroids may destabilize the IOP. The corneal epithelium may also be subject to the effects of an anti-glaucomatous drug-related epitheliopathy.

Fundus examination

Thorough retinal examination is conducted prior to any LASIK surgery. Most candidates are either moderate or higher myopes. Any peripheral retinal degenerations, holes, tears, breaks, etc. are treated with Argon laser prior to LASIK. If there is any macular pathology, it should be photodocumented and fluorescein angiography performed. It is most important that patients understand that correction of their refractive error is optical and not anatomical, and that, as myopes, they will always be more liable to retinal detachment than the natural emmetrope.

One or both eyes?

It is preferable to treat both eyes in the same session. This allows for fewer follow-up visits and avoidance of temporary aniseikonia produced if a bilateral refractive error is treated in different sessions. However, if any intraoperative complication occurs in the first eye that may jeopardize the patient’s vision in this eye, albeit temporarily, operation on the fellow eye should be postponed.

Intended correction

What is the intended correction? Should one aim for plano refraction? This is something requested by most patients. Many candidates at the age of declining accommodation have never recognized their reading problems and these candidates may benefit from monovision. A bilateral −2.0 diopter, 50-year-old patient may be best served with a unilateral myopic correction. In cases where monovision is intended, the dominant eye is usually corrected for distant vision and the non-dominant for near, unless otherwise requested by the patient.

Amount of refractive error – limits of LASIK

Despite the patient’s eyes being eligible for LASIK surgery, i.e. normal topography, adequate corneal thickness, small scotopic pupil, there is a general trend to lower the limits of keratorefractive surgery. Many surgeons post their upper limit to as low as −6.0 D for myopia and +4.0 D for hyperopia, suggesting alternative techniques (phakic IOLs or clear lensectomy10,11) for higher errors.

Informed consent

The informed consent form should contain at least the following:

The patient should understand the possibility of:

All the above should be recorded in a simple language/format comprehensible to the patient. Women should also sign that they are not pregnant or lactating.

The consent should never be signed with the patient under cycloplegia.