Figure 2-4 Images of epithelial basement membrane dystrophy. Epithelial map changes can be obvious (A) or more subtle
(B). Arrows show geographic map lines. (Part A courtesy of Vincent P. deLuise, MD; part B courtesy of Christopher J. Rapuano, MD.)
The anterior chamber, iris, and crystalline lens should also be examined. A shallow anterior chamber depth may be a contraindication for insertion of certain phakic intraocular lenses (PIOLs) (see Chapter 8). Careful evaluation, both undilated and dilated, of the crystalline lens for clarity is essential, especially in patients older than 50 years. Surgeons should be wary of progressive myopia due to nuclear sclerosis. Patients with mild lens changes that are visually insignificant should be informed of these findings and advised that the changes may become more significant in the future, independent of refractive surgery. They should also be told that IOL power calculations are not as accurate when performed after keratorefractive surgery. In patients with moderate lens opacities, cataract extraction may be the best form of refractive surgery. Some surgeons give patients a record of their preoperative refractions and keratometry measurements along with the amount of laser ablation performed and the postoperative refraction.
Kim TI, Kim T, Kim SW, Kim EK. Comparison of corneal deposits after LASIK and PRK in eyes with granular corneal dystrophy type II. J Refract Surg. 2008;24(4):392–395.
Moshirfar M, Feiz V, Feilmeier MR, Kang PC. Laser in situ keratomileusis in patients with corneal guttata and family history of Fuchs’ endothelial dystrophy. J Cataract Refract Surg. 2005;31(12):2281–2286.
Dilated Fundus Examination
A dilated fundus examination is also important before refractive surgery to ensure that the posterior segment is normal. Special attention should be given to the optic nerve (glaucoma, optic nerve drusen) and peripheral retina (retinal breaks, detachment). Patients and surgeons should realize that highly myopic eyes are naturally at increased risk of retinal detachment (see Chapter 10), unrelated to refractive surgery.
Packard R. Refractive lens exchange for myopia: a new perspective? Curr Opin Ophthalmol. 2005;16(1):53–56.
Ancillary Tests
Corneal Topography
The corneal curvature must be evaluated. Although manual keratometry readings can be quite informative, they have largely been replaced by computerized corneal topographic analyses. Several different methods are available to analyze the corneal curvature, including Placido disk, scanning- slit-beam, rotating Scheimpflug photography, high-frequency ultrasound, and ocular coherence tomography techniques. (See also the discussion of corneal topography in Chapter 1.) These techniques image the cornea and provide color maps showing corneal power and/or elevation. Patients with visually significant irregular astigmatism are generally not good candidates for corneal refractive surgery. Early keratoconus, pellucid marginal degeneration (Fig 2-5), and contact lens warpage should be considered possible causes of visually significant irregular astigmatism. Irregular astigmatism secondary to contact lens warpage usually reverses over time, although the reversal may take months. Serial corneal topographic studies should be performed to document the disappearance of visually significant irregular astigmatism before any refractive surgery is undertaken.
Figure 2-5 A corneal topographic map of the typical irregular against-the-rule astigmatism found in eyes with pellucid marginal degeneration. Note that the steepening nasally and temporally connects inferiorly. (Courtesy of Christopher J. Rapuano,
MD.)
Unusually steep or unusually flat corneas can increase the risk of poor flap creation with the microkeratome. Femtosecond laser flap creation theoretically may avoid these risks. When keratometric or corneal topographic measurements reveal an amount or an axis of astigmatism that differs significantly from that determined through refraction, the refraction should be rechecked for accuracy. Lenticular astigmatism or posterior corneal curvature may account for the difference between refractive and keratometric/topographic astigmatism. Most surgeons will treat the amount and axis of the refractive astigmatism, as long as the patient understands that after any future cataract surgery, some astigmatism may reappear (after the astigmatism contributed by the natural lens has been eliminated).
Pachymetry
Corneal thickness should be measured to determine whether it is adequate for keratorefractive surgery. This procedure is usually performed with ultrasound pachymetry; however, certain non– Placido disk corneal topography systems can also be used if properly calibrated. Most newer systems
can provide a map showing the relative thickness of the cornea at various locations. The accuracy of the pachymetry measurements of scanning-slit systems decreases markedly for eyes that have undergone keratorefractive surgery. Because the thinnest part of the cornea is typically located centrally, a central measurement should always be taken. The thickness of the cornea is an important factor in determining whether the patient is a candidate for refractive surgery and which procedure may be best. In a study of 896 eyes undergoing LASIK, the mean central corneal thickness was 550 µm ± 33 µm (range, 472–651 µm). It has been suggested that an unusually thin cornea (beyond 2 standard deviations) indicates that the patient may not be ideal for any refractive surgery. Many surgeons would not consider LASIK refractive surgery if the central corneal thickness is less than 480 µm, even if the calculated residual stromal bed (RSB) is thicker than 250 µm. If LASIK is performed and results in a relatively thin RSB—for example, around 250 µm—future enhancement surgery that further thins the stromal bed may not be possible. If there is a question of whether endothelial integrity is causing an abnormally thick cornea, specular microscopy may be helpful in assessing the health of the endothelium.
Price FW Jr, Koller DL, Price MO. Central corneal pachymetry in patients undergoing laser in situ keratomileusis. Ophthalmology. 1999;106(11):2216–2220.
Wavefront Analysis
Wavefront analysis is a technique that can provide an objective refraction measurement (see also discussion of this topic in Chapters 1 and 5). Certain excimer lasers can use this wavefront analysis information directly to guide the ablation, a procedure called wavefront-guided, or custom, ablation. Some surgeons use wavefront analysis to document levels of preoperative higher-order aberrations. Refraction data from the wavefront analysis unit can also be used to refine the manifest refraction. If the manifest refraction and the wavefront analysis refraction are very dissimilar, the patient may not be a good candidate for wavefront treatment. Note that a custom wavefront ablation generally removes more tissue than does a standard ablation in the same eye.
Calculation of Residual Stromal Bed Thickness After LASIK
A lamellar laser refractive procedure such as LASIK involves creation of a corneal flap, ablation of the stromal bed, and replacement of the flap. The strength and integrity of the cornea postoperatively depend greatly on the thickness of the RSB. RSB thickness is calculated by subtracting the sum of the flap thickness and the calculated laser ablation depth from the preoperative corneal thickness. For example, if the central corneal thickness is 550 µm, the flap thickness is estimated to be 140 µm, and the ablation depth for the patient’s refraction is 50 µm, the RSB would be 550 µm – (140 µm + 50 µm) = 360 µm thick. When the surgeon determines the RSB, the amount of tissue removed should be based on the actual intended refractive correction, not on the nomogram-adjusted number entered into the laser computer. For example, if a patient with –10.00 D myopia that is being fully corrected, the amount of tissue removed is 128 µm for a 6.5-mm ablation zone for the VISX laser. Even if the surgeon usually takes off 15% of the refraction for a conventional ablation and enters that number into the laser computer, approximately 128 µm of tissue will be removed, not 85% of 128 µm.
Most surgeons believe the RSB should be at least 250 µm thick. Others want the RSB to be greater than 50% of the original corneal thickness. If the calculation reveals an RSB that is thinner than desired, LASIK may not be the best surgical option. In these cases, a surface ablation procedure may be a better option because no stromal flap is required; this results in a thicker RSB postoperatively.