Refractive surgery in children

Introduction

Excimer laser refractive surgery for high refractive error associated with amblyopia has been performed for over two decades in children, with excellent visual acuity and refractive results and minimal complications. Intraocular refractive procedures have also been performed in children for high refractive errors that are outside the range for excimer laser procedures for over a decade, also with excellent visual and refractive outcomes and few complications. Excimer laser refractive surgery has been performed for accommodative esotropia with reasonable results.

Conventional amblyopia therapy consists of the following:

• 1.

  Clearing the ocular media of obstruction of the visual axis, such as corneal leukoma, cataract, or vitreous hemorrhage.

• 2.

  Correcting significant refractive errors either with spectacles or contact lenses.

• 3.

  Encouraging use of the amblyopic eye through occlusion or pharmacologic and/or optical penalization of the fellow eye (see Chapter 73 ). This conventional therapy is successful in the majority of children with amblyopia.

There are, however, several subsets of children with amblyopia that often fail this standard therapy. These groups are:

• 1.

  Children with severe isoametropia (bilateral high ametropia) who are spectacle or contact lens non-compliant or intolerant.

• 2.

  Children with severe anisometropia who are non-compliant or intolerant of spectacle and contact lens wear.

• 3.

  Children with high ametropia, either anisometropia or isoametropia, who have other special circumstances such as craniofacial anomalies, ear deformities, or neck hypotonia that preclude the proper use of refractive correction.

There are many reasons for poor compliance with spectacles or contact lenses. Spectacles for the treatment of extreme myopia or hyperopia can cause prismatically induced optical aberrations, a narrow visual field, and social ostracism due to unattractive thick lenses. Group 1, above, consists primarily of children with neurobehavioral abnormalities and intellectual disability. They have severe tactile aversion to anything touching their face and thus refuse to wear glasses or contact lenses in spite of being functionally blind without the refractive correction. Their visual impairment may impede their attention and social interaction, exacerbating pre-existing behavioral and social problems, and further interfere with the development of normal communication skills. This group is mostly comprised of former premature infants with a history of severe retinopathy of prematurity and high myopia, children with genetic mutations, or children with autism spectrum disorder, obsessive compulsive disorder or severe attention deficit hyperactivity disorder. In group 2, spectacle-induced aniseikonia and anisconvergence impede stereopsis and binocular vision and may cause asthenopia.

Contact lenses, more than spectacles, improve the quality of vision, reduce the minification effect of high myopic spectacles, give better contrast sensitivity, and reduce social discomfort. Contact lenses, however, are often impractical in children due to difficulty with their insertion and removal, cost, intolerance, non-compliance, and frequent loss.

In the past, no other treatment options existed. The result was variable levels of visual impairment in the affected eye(s) depending on the severity of the refractive error. If the condition was bilateral, severe blur-induced amblyopia was the result. Refractive surgery, just as in adults, reduces refractive error in these children and thus reduces or eliminates these other related issues. Additionally, it greatly improves their quality of life.

A paradigm shift is needed when we think about the management of severe refractive error in children. Untreated high refractive error in young children can result in severe levels of blur-induced amblyopia akin to that found with a dense congenital cataract or corneal leukoma. We should therefore approach this form of amblyopia as aggressively as we would other treatable causes of form vision deprivation. Refractive surgery is effective in treating highly amblyogenic levels of refractive error in children with few complications when standard therapy fails.

Lastly, refractive surgery may prove to be a viable alternative in older children with accommodative esotropia if longer-term follow-up in children who have undergone refractive surgery for this indication demonstrates stability of ocular alignment. Some recent studies have shown refractive surgery to be effective though the sample sizes in the studies were small.

Types of refractive surgery used in children

Both corneal and intraocular procedures reduce refractive error. Today, corneal procedures are performed with the excimer laser and include photorefractive keratectomy (PRK), laser-assisted subepithelial keratectomy (LASEK) (these two procedures together will henceforth be referred to as advanced surface ablation [ASA]), and laser-assisted in situ keratomileusis (LASIK). ASA can be used to treat up to 10–12 diopters of myopia, 6 diopters of hyperopia, and 4 diopters of astigmatism. These numbers are typically reduced by about one-third for treatment using LASIK.

PRK and LASEK are surface ablations, with minor differences between them, that permanently change the shape of the cornea by ablating (via vaporization) tissue from the anterior corneal stroma, just under the corneal epithelium.

• 1.

  In PRK, the corneal epithelium is removed and Bowman's membrane and anterior corneal stroma are ablated with laser.

• 2.

  In LASEK, the epithelium is not removed, but an alcoholic solution is used to loosen the epithelial cells; the surgeon folds the epithelial layer out of the treatment field, performs the laser ablation on Bowman's membrane and anterior corneal stroma, and then replaces the epithelial layer.

• 3.

  In LASIK, a partial-thickness corneal flap, composed of epithelium, Bowman's membrane, and anterior stroma is created using either a mechanical microkeratome or a femtosecond laser microkeratome. A hinge is left on one side of this flap. The flap is folded back and the laser ablation is performed on the deeper corneal stroma. The LASIK flap is then repositioned in place over the treated corneal stroma. The flap is held in position after the procedure by natural adhesion.

Current intraocular refractive procedures reduce refractive error by changing the refractive power of the eye and include phakic intraocular lenses (phIOLs), refractive lens exchange (RLE), and clear lens extraction (CLE). These procedures are used to treat higher refractive errors that fall outside the treatment parameters for the excimer laser, or in cases where the cornea is too thin for an excimer laser procedure. Phakic IOL procedures can add or reduce refractive power. In these procedures, an intraocular lens (IOL) is placed into the anterior or posterior chamber, thus preserving the natural crystalline lens. Anterior or posterior chamber phIOLs can be used to treat extremely high myopia if the anterior chamber is deep enough to tolerate the lens (≥3.2 mm). It is rare that a highly hyperopic eye has a deep enough anterior chamber depth to be able to have a phakic IOL procedure. One significant advantage of phIOL over other intraocular refractive procedures is the preservation of accommodation.

The other intraocular procedures that change refractive power are refractive lens exchange (RLE) and clear lens extraction (CLE). They are technically identical to pediatric cataract surgery except the crystalline lens being removed is clear. In RLE, an appropriately powered IOL is placed in the eye after removing the crystalline lens; in CLE, the eye is left aphakic. Currently, both corneal and intraocular refractive procedures are utilized “off-label” in children and are not approved by the Federal Drug Administration (FDA) in the United States.

Safety of ASA versus LASIK

Box 72.1 outlines the risks of ASA and LASIK. While LASIK has been shown to be effective in children to correct refractive error, ASA has several advantages. First, no corneal flap is created, therefore, there is no risk of flap loss, epithelial in-growth, or flap striae as with LASIK. In LASIK, the flap is held in position by natural suction; however, the flap never heals completely to the posterior stroma and can be lifted indefinitely. Second, since ASA is performed on the surface of the cornea, the remaining posterior stroma is thicker, affording less risk of late keratectasia. Because most children who fail standard amblyopia therapy and are treated with excimer laser procedures require large excimer treatment doses, the depth of the corneal ablation is greater than in most adults. Keratectasia following ASA in adults is extremely rare and has never been reported in children. A recent study on corneal stability 5+ years after pediatric keratectomy demonstrated no topographic signs of keratectasia or corneal haze with stable refractive error. The main long-term risk of ASA is corneal haze, but, in the author's experience, it is uncommon and usually mild when it occurs. It is even less common now with the addition of topical mitomycin C immediately following the ASA. Corneal haze is now almost exclusively encountered in a child when the topical steroid (fluorometholone or loteprednol) is discontinued too soon. Topical steroid must be used for 6 months following PRK to minimize the risk of this serious complication. Corneal haze can be further reduced by limiting ablation treatments to within the FDA-approved (in the USA) parameters and by the child taking vitamin C 250–500 mg daily for a year after the procedure.