Amblyopia and anisometropic amblyopia
Amblyopia is defined as a decrease in visual acuity without evidence of organic eye disease, typically resulting from unequal visual stimulation during the period of visual development. The prevalence of amblyopia is 2%–4% of the US population; up to half of these cases represent anisometropic amblyopia. Patients with anisometropia greater than 3.00 D between the 2 eyes are likely to develop amblyopia that may be more resistant to traditional amblyopia therapy, such as glasses, contact lenses, patching, or atropine penalization therapy, partly because of the significant amount of induced aniseikonia.
Evaluation of a patient with amblyopia should include a thorough medical history to identify any known cause of amblyopia, a history of ocular disease or surgery, assessment of ocular alignment and motility, and a comprehensive anterior segment and retinal examination. Patients should be referred to a strabismus specialist when indicated. Preoperative counseling of a patient with amblyopia must emphasize that, even after refractive surgery, the vision in the amblyopic eye will not be as good as that in the nonamblyopic eye. The patient should also understand that BCVA will be the same, or nearly so, with or without refractive surgery.
Typically, refractive surgery is performed in this group of patients to treat high anisometropia or astigmatism in 1 eye or high refractive error in both eyes. Laser vision correction and phakic IOL implantation have been successfully performed in the more myopic, amblyopic eye in adult patients with anisometropic amblyopia. Some studies suggest that postoperative BCVA may even improve modestly compared with preoperative levels in a subset of adults who undergo refractive surgery. In a study examining phakic IOL implantation in patients with greater than 3.00 D of anisometropia, an average of 3 lines of vision were gained; 91% of eyes gained more than 1 line, and no eyes lost BCVA. This improvement in vision was attributed to an increase in magnification and a decrease in optical aberrations, rather than an actual improvement in the amblyopia.
Performing refractive surgery in the normal eye of the adult patient with amblyopia, however, is controversial. The decision to do so depends on many factors, including the level of BCVA in the amblyopic eye and the normal eye as well as the ocular alignment. To increase safety, unilateral surgery in the amblyopic eye followed by surgery in the nonamblyopic eye can be considered. However, ocular alignment deviation has been reported after unilateral LASIK for high myopia because of focus disparity causing esodeviation and impairment of fusion. In some cases, a preoperative contact lens trial may help in assessing this potential risk.
Consider a patient with anisometropic amblyopia whose vision is corrected to 20/40 with –7.00 D in the right eye and to 20/20 with –1.00 D in the left eye. This patient may be an excellent candidate for refractive surgery in the amblyopic right eye because he or she probably cannot tolerate glasses to correct the anisometropic amblyopia and may not tolerate contact lenses. Even if the post-LASIK UCVA were worse than 20/40 in the amblyopic eye, it would be better than the pre-LASIK UCVA of counting fingers.
If the postoperative UCVA in the amblyopic right eye improved to 20/40, the patient could be considered for laser vision correction in the left eye for –1.00 D. However, if the patient had presbyopia, some surgeons would discourage further intervention and discuss potential advantages of the low myopia. In a younger patient with accommodation, some surgeons would inform the patient of the potential risks associated with treating the better eye but would perform the excimer laser vision correction.
If BCVA in the amblyopic eye were 20/200 or worse, the patient would be considered legally blind if he or she were to lose significant vision after laser refractive surgery in the normal eye. In such cases, refractive surgery in the amblyopic eye may or may not offer much benefit, and refractive surgery in the nonamblyopic eye should be regarded as contraindicated in most cases. In the extenuating circumstances for which such surgery might be considered, the physician and patient should have an extensive discussion about the potential risks. Generally, if the patient would not be happy with the vision in the amblyopic eye alone in the event that something adverse happened to the better eye, then refractive surgery should not be performed on the better eye.
Persistent diplopia has been reported after bilateral LASIK in a patient with anisometropic amblyopia and a history of intermittent diplopia in childhood. Preoperatively, this type of patient can adjust to the disparity of the retinal image sizes with spectacle correction. Refractive surgery, however, can result in a dissimilar retinal image size that the patient cannot fuse, resulting in diplopia. This type of diplopia cannot be treated by prisms or muscle surgery.
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Sakatani K, Jabbur NS, O’Brien TP. Improvement in best corrected visual acuity in amblyopic adult eyes after laser in situ keratomileusis. J Cataract Refract Surg. 2004; 30(12):2517–2521.
Refractive surgery in children
In children, refractive surgery is controversial because their eyes and refractive status continue to change. Additional studies on the growing eye and the long-term effect of excimer laser treatment and phakic IOLs on the corneal endothelium and lens are needed to better assess the outcome of refractive surgery in children. Consequently, these procedures are typically regarded as investigational.
However, the literature is replete with reports of the successful performance of PRK, LASEK, LASIK, and phakic IOL implantation in children, mostly 8 years and older, when conventional therapies have failed. Most of these children underwent treatment for anisometropic amblyopia in the more myopic eye. In these studies, refractive error was decreased and visual acuity was maintained or improved in moderately amblyopic eyes. Refractive surgery did not improve BCVA or stereopsis in older children with densely amblyopic eyes. The limited effect on visual acuity was generally attributed to the fact that the children were beyond amblyogenic age. In 1 study, general anesthesia was used during performance of PRK in 40 children, aged 1–6 years, who were unable to wear glasses or contact lenses for high myopia or anisometropic amblyopia from myopia. Patients were treated for existing amblyopia, and mean BCVA improved from 20/70 to 20/40. The study found that posttreatment corneal haze developed in 60% of eyes. Most patients demonstrated “increasing corneal clarity” within 1 year, although 2 of 27 patients required PTK for the corneal haze. Regression of effect was attributed to a vigorous healing response and the axial myopic shift associated with growth.
Several studies have reported successful implantation of phakic IOLs in children with high anisometropia and amblyopia. This technique eliminates the previously mentioned corneal-wound-healing problems associated with keratorefractive procedures and may be considered when the refractive error is high and other traditional methods of amblyopia therapy have failed. Depending on the type of phakic IOL, however, other potentially serious complications may ensue, including progressive corneal endothelial cell loss, cataract formation, pupillary block glaucoma, and persistent inflammation, as well as the usual risks associated with intraocular surgery. Thus, phakic IOLs should be considered investigational in children. Larger clinical trials are necessary to adequately evaluate the safety and efficacy of this technique in this age group. Furthermore, these patients should be monitored for endothelial evaluation through the years.
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Uncorrected hyperopia can stimulate an increase in accommodation, leading to accommodative convergence. Esotropia arises from insufficient fusional divergence. Traditional treatment includes correction of hyperopia with glasses or contact lenses and muscle surgery for any residual esotropia (see BCSC Section 6, Pediatric Ophthalmology and Strabismus). While glasses or contact lenses are being worn, the esotropia is usually not manifest. As a child ages, the hyperopia typically decreases, with concomitant resolution of the accommodative esotropia. If significant hyperopia persists, glasses or contact lenses continue to be needed to control the esotropia.
Before refractive surgery, it is important to perform an adequate cycloplegic refraction (using cyclopentolate, 1%) on patients younger than 35 years who have intermittent strabismus or phoria. Accurate refraction is necessary to avoid inducing postoperative hyperopia. Otherwise, the postoperative hyperopia may result in a new onset of esotropia with an accommodative element.
Several studies performed outside the US report the use of PRK and LASIK for adults with accommodative esotropia. In one of the studies, orthophoria or microesotropia was achieved after LASIK for hyperopia in accommodative esotropia in a series of 9 patients older than 18 years. A second study demonstrated a reduction in the mean esotropia of 21 prism diopters (Δ) prior to LASIK to 3.7Δ after surgery. However, another study of LASIK in accommodative esotropia in patients aged 10–52 years found that 42% of these patients had no reduction in their esotropia.
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Hoyos JE, Cigales M, Hoyos-Chacón J, Ferrer J, Maldonado-Bas A. Hyperopic laser in situ keratomileusis for refractive accommodative esotropia. J Cataract Refract Surg. 2002;28(9): 1522–1529.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.