CHAPTER 6
Photoablation: Complications and Adverse Effects
Surface ablation techniques, including photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), are relatively safe and effective surgical procedures. As with all types of surgery, there are potential risks and complications. It is important to understand how to avoid, diagnose, and treat many of the complications of refractive surgery. Comprehensive ophthalmologists, as well as refractive surgeons, should be knowledgeable about these postoperative problems, given the increasing number of patients who undergo refractive surgery each year.
General Complications Related to Laser Ablation
Overcorrection
Myopic or hyperopic surface ablation typically undergoes some degree of refractive regression for at least 3–6 months. In general, patients with higher degrees of myopia and any degree of hyperopia require more time to attain refractive stability, which must be achieved before any decision is made regarding possible re-treatment of the overcorrection.
Overcorrection may occur if substantial stromal dehydration develops before the laser treatment is initiated because more stromal tissue will be ablated per pulse. A long delay before beginning the ablation after removing the epithelium in surface ablation or after lifting the flap in LASIK allows for excessive dehydration of the stroma and increases the risk of overcorrection. Controlling the humidity and temperature in the laser suite within the recommended guidelines should standardize the surgery and ideally improve refractive outcomes. Overcorrection tends to occur more often in older individuals because their wound-healing response is less vigorous and their corneas ablate more rapidly for reasons not fully understood. Studies reveal that older patients with moderate to high myopia have a greater response to the same amount of dioptric correction than younger patients do.
Various modalities are available for treating small amounts of overcorrection. Myopic regression can be induced after surface ablation by abrupt discontinuation of corticosteroids. Patients with consecutive hyperopia—that is, hyperopia that occurs when originally myopic eyes are overcorrected —and patients with myopia due to overcorrection of hyperopia require less treatment to achieve emmetropia than do patients with previously untreated eyes, as both are considered to have overresponded to the initial treatment. When re-treating such patients, the surgeon should take care not to overcorrect a second time. With conventional ablation, most surgeons will reduce the ablation by 20%–25% for consecutive treatments. For wavefront procedures, review of the depth of the ablation and the amount of higher-order aberration helps titrate the re-treatment.
Undercorrection
Undercorrection occurs much more commonly at higher degrees of ametropia because of greater severity and more frequent occurrence of regression. Patients with regression after treatment of their first eye have an increased likelihood of regression in their second eye. Sometimes the regression may be reversed with aggressive administration of topical corticosteroids. Topical mitomycin C, administered at the time of initial surface ablation, can be used to modulate the response, especially in patients with higher levels of ametropia. The patient may undergo a re-treatment after the refraction has remained stable for at least 3 months postoperatively. A patient with significant corneal haze and regression after surface ablation is at higher risk after re-treatment for further regression, recurrence of visually significant corneal haze, and loss of corrected distance visual acuity (CDVA; also called best-corrected visual acuity, BCVA). It is recommended that the surgeon wait at least 6–12 months for the haze to improve spontaneously before repeating surface ablation. In patients with significant haze and myopic regression, removal of the haze with adjunctive use of mitomycin C should not be coupled with a refractive treatment, as the resolution of the haze will commonly improve the refractive outcome. Undercorrection after LASIK typically requires flap lift and laser treatment of the residual refractive error after the refraction has remained stable for at least 3 months. In higher levels of residual refractive error, phakic intraocular lenses can also be offered as an option.
Optical Aberrations
After undergoing surface ablation or LASIK, some patients report optical aberrations, including glare, ghost images, and halos. These symptoms are most prevalent after treatment with smaller ablation zones (<6.0 mm in diameter), after attempted higher spherical and cylindrical correction, and in patients with symptoms prior to refractive surgery. These vision problems seem to be exacerbated in dim-light conditions when mydriasis occurs, although no correlation has been found between pupil size and optical aberrations. Wavefront mapping can reveal higher-order aberrations associated with these subjective complaints. In general, a larger, more uniform, and well-centered optical zone provides a better quality of vision, especially at night.
Night-vision complaints are often the result of spherical aberration, although other higher-order aberrations also contribute. The cornea and lens have inherent spherical aberration. In addition, excimer laser ablation increases positive spherical aberration in the midperipheral cornea. Customized wavefront-guided corneal treatment patterns are designed to reduce existing aberrations and to help prevent the creation of new aberrations, with the goal of achieving a better quality of vision after laser ablation.
Several studies have demonstrated that although the excimer laser photoablation causes the majority of post-LASIK change in lower-order and higher-order aberrations, the creation of the flap itself can also change lower-order and higher-order aberrations (Fig 6-1). Some studies have demonstrated that femtosecond lasers cause little or no change in higher-order aberrations, in contrast to mechanical microkeratomes. Pallikaris showed that LASIK flap creation alone, without lifting, caused no significant change in refractive error or visual acuity but did cause a significant increase in total higher-order wavefront aberrations.
Figure 6-1 Wavefront analysis depicting higher-order aberrations after laser in situ keratomileusis (LASIK), including coma
and trefoil. (Courtesy of Steven I. Rosenfeld, MD.)
Pallikaris IG, Kymionis GD, Panagopoulou SI, Siganos CS, Theodorakis MA, Pellikaris AI. Induced optical aberrations following formation of a laser in situ keratomileusis flap. J Cataract Refract Surg. 2002;28(10):1737–1741.
Tran DB, Sarayba MA, Bor Z, et al. Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes: potential impact on wavefront-guided laser in situ keratomileusis. J Cataract Refract Surg. 2005; 31(1):97–105.
Waheed S, Chalita MR, Xu M, Krueger RR. Flap-induced and laser-induced ocular aberrations in a two-step LASIK procedure. J Refract Surg. 2005;21(3):346–352.
Central Islands
A central island appears on computerized corneal topography as an area of central corneal steepening surrounded by an area of flattening that corresponds to the myopic treatment zone in the paracentral region (Fig 6-2). A central island is defined as a steepening of at least 1.00 D with a diameter of >1 mm compared with the paracentral flattened area. Central islands may be associated with decreased visual acuity, monocular diplopia and multiplopia, ghost images, and decreased contrast sensitivity.
Figure 6-2 Corneal topography findings of a myopic ablation (blue) with a central island (yellow) in the visual axis. (Courtesy of
Roger F. Steinert, MD.)
The occurrence of central islands has been reduced significantly through the use of scanning and variable-spot-size lasers and is now rarely encountered with modern laser technology. Fortunately, most central islands diminish over time, especially after surface ablation, although resolution may take 6–12 months. Treatment options such as topography-guided ablations may be helpful in treating persistent central islands.
Decentered Ablations
Accurate centration during the excimer laser procedure is important in optimizing the visual results. Centration is even more crucial for hyperopic than myopic treatments. A decentered ablation may occur if the patient’s eye slowly begins to drift and loses fixation or if the surgeon initially positions the patient’s head improperly; if the patient’s eye is not perpendicular to the laser treatment, parallax can result (Fig 6-3). The incidence of decentration increases with surgeon inexperience, hyperopic ablations, and higher refractive correction, due to longer ablation times. Decentration may be reduced by ensuring that the patient’s head remains in the correct plane throughout the treatment—that is,