Binocular Vision Problems Associated With Refractive Surgery
It has been estimated that over 11 million laser-assisted in situ keratomileusis (LASIK) procedures have been performed in the United States, of which a peak of 1.4 million procedures were performed in 2007.1 Optometrists play a significant advisory role with patients before surgery, comanage patients undergoing refractive surgery, and care for the patient after surgery. A 2004 survey on refractive surgery revealed that 55.5% of surgeons comanaged patients, with another ophthalmologist or optometrist performing the preoperative and postoperative care.2
Despite improvements in technologies and the increased experience of surgeons, complications do occur. In the 2004 refractive error survey, the most commonly reported complications were dry eye, glare, diffuse lamellar keratitis, and epithelial ingrowth.2 Other authors have reported infection, scarring, cataract formation, and corneal decompensation.3 In a survey of complications in 101 dissatisfied patients seeking consultation after refractive surgery, Jabbur, Sakatani, and O’Brien4 reported that the most common subjective complaints were blurred distance vision (59%), glare and night vision disturbances (43.5%), and dry eyes (29.8%). Although strabismus, binocular vision problems, and diplopia do not appear among the most common complications of refractive surgery, a review of the literature indicates that these problems do occur after refractive surgery.4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23 Based on our experience with the underdetection of binocular vision disorders in the general population by ophthalmic clinicians, we suspect that the true prevalence of these problems is higher than indicated in the literature. Thus, in addition to advising and comanaging refractive surgery patients, optometrists must be prepared to deal with the binocular vision complications that occur as a result of refractive surgery.
This chapter reviews the literature on binocular vision-related complications of refractive surgery, recommends strategies for minimizing such complications preoperatively, and recommends treatment strategies to deal with such complications when they do occur.
Review of Refractive Surgery
LASER IN SITU KERATOMILEUSIS
LASIK is a lamellar laser refractive surgery in which the excimer laser ablation is done under a partial-thickness lamellar corneal flap. The procedure permanently changes the shape of the cornea with an excimer laser and a microkeratome or femtosecond laser. The microkeratome or femtosecond laser produces a hinged lamellar flap in the cornea. The microkeratome uses an oscillating blade to cut the flap after immobilization of the cornea. Microkeratome cuts can vary widely in depth, even with the same preset thickness. The femtosecond laser provides more accuracy in flap thickness than was available in previous methods and is less dependent on corneal curvature. As compared with surface ablation, LASIK results in earlier and faster improvement of uncorrected visual acuity (VA), causes less postoperative discomfort, and offers improved stability and predictability. With LASIK, however, there are risks of flap-related complications such as wrinkles, folds, and diffuse lamellar keratitis.24,25
SURFACE ABLATION TECHNIQUES
This type of laser refractive surgical procedure uses the excimer laser to ablate the most anterior portion of the corneal stroma. These procedures do not require a partial-thickness cut into the stroma, and thus leave a larger residual bed to retain the cornea’s biomechanical strength.25 However, ablation of the anterior stroma leads to a wound-healing response that might result in more stromal haze and more serious scarring than those resulting from LASIK. Recovery after surface ablation is slower and more painful than after LASIK. Methods of surface
ablation being used include photorefractive keratectomy (PRK), laser-assisted subepithelial keratectomy (LASEK), and epi-LASIK. These methods differ in the manner in which the epithelial layer is handled in the surgery.
ablation being used include photorefractive keratectomy (PRK), laser-assisted subepithelial keratectomy (LASEK), and epi-LASIK. These methods differ in the manner in which the epithelial layer is handled in the surgery.
Photorefractive Keratectomy
PRK is the second most popular laser vision correction procedure. Like LASIK, PRK uses an excimer laser to reshape the cornea. However, in PRK, instead of creating a corneal flap, the surgeon removes the corneal epithelium and then applies the excimer laser treatment to the surface of the eye. The epithelium might be removed in several ways, including excimer laser destruction, mechanical debridement with a surgical blade, abrasion with a brush, or the use of alcohol to loosen the epithelium. Because epithelium is removed, a large epithelial defect is present, and healing occurs by migration and division of surrounding epithelium. Although healing time is longer after PRK, it produces visual results that are comparable to LASIK. PRK is often the treatment of choice for patients whose corneas are too thin for LASIK.
Laser-Assisted Subepithelial Keratectomy
LASEK is another surface ablation procedure designed for patients whose corneas are too thin for LASIK. LASEK resembles PRK in that the surgeon does not create a LASIK-style flap in the first step of the procedure. Instead of removing the outer corneal epithelium completely, the LASEK surgeon replaces the epithelial sheet after the excimer laser treatment is completed. LASEK offers results that are similar to those achieved by LASIK, but (like PRK) it involves more postoperative discomfort and longer healing times. The epithelium is protected from blinks and eye movements by the use of a bandage contact lens.
Epi-LASIK
Epi-LASIK is yet another surface ablation procedure for patients whose corneas are too thin for LASIK. Epi-LASIK is very similar to LASEK, but instead of subjecting the eye to a dilute alcohol solution to loosen an epithelial flap, epi-LASIK uses a handheld device to create a hinged sheet of corneal epithelium. After the eye is reshaped with an excimer laser, this epithelial flap is returned to its original position and held in place with a bandage contact lens for several days. Epi-LASIK patients usually experience some postoperative discomfort, but reportedly less than that experienced by patients who have undergone PRK or LASEK.
PHAKIC INTRAOCULAR LENSES
The phakic intraocular lens (IOL) procedure is usually reserved for patients whose refractive error is too severe to be corrected by LASIK surgery or for patients whose corneas are too thin for LASIK, PRK, or epi-LASIK. In the phakic IOL procedure, a thin plastic lens is implanted inside the eye either in front of the iris or behind the pupil. Some refractive surgeons today believe that phakic IOLs are the procedure of choice for high myopia and high hyperopia. Advantages of phakic IOLs include the possibility to remove the implanted lens if problems arise or a change in the power of the lens is required, and the fact that no thinning of the cornea is required. Because the phakic IOL procedure is relatively new, no long-term studies are available to assess the procedure’s potential for complications such as cataract formation and the loss of endothelium cells. Still, phakic IOL implantation currently appears to be safe and effective for many patients who are not good candidates for LASIK.
The 2004 Refractive Surgery Survey revealed that LASIK was the current refractive surgery procedure of choice, although wave front-guided ablation was gaining in popularity.2 Surface ablation techniques such as PRK, LASEK, and epi-LASIK were also popular, as was implantation of phakic IOLs. In 2004, 71% of the refractive surgeons surveyed reported using PRK, 40.8% LASEK, and 17.4% phakic IOLs.2
Binocular Vision Complications of Refractive Surgery for Myopia, Hyperopia, and Astigmatism
A variety of binocular vision problems have been reported after refractive surgery, including aniseikonia related to induced anisometropia,5,8 decompensated esodeviation,5,7,9,10,26 decompensated exodeviation,5,13,14,16,17 and decompensated fourth nerve weakness.5,6,16 Kushner and Kowal5 completed a retrospective review of patients seen in two private strabismus practices and found 28 patients who experienced chronic monocular or binocular diplopia after refractive surgery. Their objective was to determine the causative mechanism for the diplopia of these patients, and they established five different causes, including complications associated with refractive surgery (i.e., ablation zone too small, overcorrection), previous use of prism, aniseikonia, monovision, and accommodative/convergence issues.
COMPLICATIONS ASSOCIATED WITH THE REFRACTIVE SURGERY
Creation of an intralamellar flap during LASIK procedures increases the risk of intraoperative complications such as an irregular flap and a dislodged flap. Other complications can also occur, such as scarring, an ablation zone that is too small, and a decentered treatment zone. These problems were all implicated as causes of diplopia in Kushner and Kowal’s5 case series. In two cases, patients reported monocular diplopia caused by scarring and a small ablation zone. Other surgical complications are related to an undesirable refractive endpoint. For example, surgically induced anisometropia can cause induced aniseikonia, or a unilaterally blurred retinal image can cause diplopia (Case 24.1), and leaving an accommodative esotrope with residual hyperopia can lead to a decompensated esotropia (Case 24.2).
Case 24.1 Surgically Induced Anisometropia Causing Aniseikonia
HISTORY
Chris, a 26-year-old accountant, presented with complaints of eyestrain, blurry vision, and diplopia after about 20 minutes of reading. He had had these problems for several months since having LASIK for myopia and had been referred for evaluation. He had been told in the past that one eye occasionally turned out, but, in his previous visits to eye doctors, no treatment other than glasses had been recommended. His medical history was negative, and he was not taking any medication.
EXAMINATION RESULTS
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Pupils were normal; all external and internal health tests were negative; the deviation was comitant; and color vision testing revealed normal function.
DIAGNOSIS
Analysis of the results of accommodative testing revealed a normal amplitude, facility, and accommodative response. The NRA and PRA were both low; given the normal accommodative function, these findings reflect a problem with fusional vergence. The findings for both positive fusional vergence (PFV) and negative fusional vergence (NFV) were reduced on direct measures and indirect tests of fusional vergence. The low NRA, PRA, and reduced BAF results suggested problems with fusional vergence. These findings, and the significant exophoria at both distance and near, led to the conclusion that the most likely cause of Chris’s symptoms was a previously existing vergence disorder that was aggravated by the blur of the left eye after LASIK.
TREATMENT
Chris was prescribed a toric contact lens for the left eye and given home vision therapy procedures to practice. We followed the sequence outlined in Chapter 10 (Table 10.7); 8 weeks of home vision therapy with two visits of in-office vision therapy were necessary to achieve a successful result.
OUTCOME
At the end of therapy, Chris reported that he was once again able to read for as long as he wanted without any discomfort, provided that he wore his contact lens. He is currently considering whether to seek enhancement for the left eye.
Case 24.2 Residual Hyperopia Causing Decompensated Esotropia
HISTORY AND SIGNIFICANT CLINICAL FINDINGS
A 28-year-old woman presented with a complaint of eyestrain, blurred vision, and inability to concentrate after 10 minutes of reading. These problems had been bothering her since she had had LASIK for hyperopia 4 months prior. The pre-LASIK refraction was +3.75 D in each eye; she reported that she had worn these lenses since childhood, when her eyes “crossed at times.” The current (post-LASIK) refraction was +1.00 D OD and OS; the unaided distance phoria was 6 esophoria; and the unaided near phoria was 10 esophoria (IPD = 58 mm). The calculated AC/A ratio in this case is 7.4:1. Near point testing through subjective refraction revealed the following:
|
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
