Refractive Errors and Refractive Surgery

In the United States and other industrialized societies, the prevalence of myopia is increasing. Increased time spent outdoors appears to be protective against myopia in children. Increased levels of near work are less of a risk factor than previously believed.

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Studies from around the world have confirmed that that the incidence of microbial keratitis has not been reduced with the introduction of new lens types (including the newest highly gas-permeable silicone hydrogel lenses), and that overnight wear of any contact lens is associated with a higher risk than daily wear.

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Overnight wear, regardless of contact lens type (including the newest highly gas-permeable silicone hydrogel lenses), increases the likelihood of corneal infection. Although there are lenses approved by the FDA for extended wear, this and other risks, benefits, and alternatives should be presented to patients for whom this mode of contact lens wear is being considered.

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Environmental risk factors and hygiene practices, such as no-rub cleaning, topping off (reuse) of solutions, contaminated lens cases, exposure to tap water, wearing contact lenses in hot tubs, and changes in water supply are risk factors for the increases in Acanthamoeba and fungal keratitis in association with contact lens wear in the past decade.

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Hydrogen peroxide systems may be superior to preserved disinfecting solutions in reducing pathogen binding and in cysticidal disinfection, but they require more complex care regimens.

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Daily disposable contact lenses are a good option for reducing the likelihood of complications associated with soft contact lens wear.

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Presbyopia can be managed by using eyeglasses or contact lenses (soft, rigid gas-permeable, aspheric bifocal or multifocal). Surgical management of presbyopia includes keratorefractive surgery, corneal inlays, or intraocular lens implantation (multifocal, accommodative, and extended depth of focus lenses).

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As data from published studies fail to demonstrate a relationship between pupil size and the quality of postoperative vision, the importance of pupillometry in the preoperative workup remains controversial.

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It is recommended that keratorefractive surgery patients be provided with a record (K Card) or that the ophthalmologist maintain a record that lists information about the patient’s eye condition, including preoperative keratometry readings and refraction as well as stable postoperative refraction. The K Card should be available if the patient requires cataract surgery or future eye care.

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Excimer ablations that result in very thin residual stroma increase the risk for ectasia. For LASIK procedures, 250 μm has been suggested as a safe residual stromal bed thickness, but there is no absolute value that guarantees that ectasia will not occur. Although surgeons do not agree on a figure, they do agree that when ectasia risk is assessed, many factors should be considered. Abnormal topography is the most significant risk factor for postoperative ectasia. In the context of normal preoperative topography, percentage of tissue altered (PTA) higher or equal to 40% has been associated with higher ectasia risk. Other hypothesized risk factors include thin preoperative central corneal thickness, younger patient age, and higher attempted corrections.

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Persistent diffuse lamellar keratitis (DLK) that is unresponsive to corticosteroids should prompt consideration of microbial keratitis or interlamellar fluid due to increased intraocular pressure (IOP), intraocular inflammation, or endothelial decompensation. For moderate to extensive DLK, the interface should be irrigated sooner rather than later to minimize stromal loss and changes in refractive correction.

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Contraindications to corneal laser refractive surgery include:

• Unstable refraction
• Abnormalities of the cornea (e.g., keratoconus or other corneal ectasias, thinning, edema, interstitial or neurotrophic keratitis, extensive vascularization)
• Insufficient corneal thickness for the proposed ablation depth
• Visually significant cataract
• Uncontrolled glaucoma
• Uncontrolled external disease (e.g., blepharitis, dry eye syndrome, atopy/allergy)
• Uncontrolled autoimmune or other immune-mediated disease
• Unrealistic patient expectations