New Diagnostics in Corneal Ectatic Disease

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Keratoconus was first described in detail in 1854 as a chronic, noninflammatory ectasia of the cornea.1 Although it is the most common primary ectasia, keratoconus is relatively rare with a reported annual incidence of around 2 per 100,000 and prevalence of 54.5 per 100,000, though rates vary geographically.2–4 Keratoconus is characterized by corneal steepening, visual distortion, apical corneal thinning, and central corneal scarring. The corneal thinning induces irregular astigmatism and myopia, leading to mild or marked visual impairment.1 The disease is typically bilateral, although only 1 eye may be affected at the onset, and it can be highly asymmetric.5,6 Early in the disease, and in subclinical or forme fruste keratoconus, there may be minimal or no symptoms, whereas in advanced disease there can be profound visual loss.
The detection of subclinical or forme fruste keratoconus is one of the most important steps in the preoperative evaluation of a refractive surgery candidate. Postoperative iatrogenic keratectasia is a rare but dreaded complication following excimer laser corneal refractive surgery. It most commonly occurs after laser in situ keratomileusis (LASIK), but has been reported following photorefractive keratectomy and small incision lenticule extraction as well.7–9 It typically occurs after surgery in eyes with undetected forme fruste keratoconus or, much less frequently, pellucid marginal degeneration. Other risk factors include preoperative topographic abnormalities, low postoperative residual stromal bed thickness, younger patient age, low central corneal thickness, and percent tissue altered (PTA).6 Postoperative keratectasia induces similar clinical features to those seen in keratoconus, and can be equally, if not more debilitating.
A major challenge for any corneal or refractive surgeon is the detection of keratoconus at its earliest stages. Many studies have been performed and screening tools have been developed to help clinicians more accurately identify the earliest forms of this disease in hopes of preventing or lessening the risk of postoperative keratectasia.9,10 However, a major limitation of many of these studies is that they are designed to diagnosis frank keratoconus, rather than screen for forme fruste disease. In addition, different definitions of forme fruste keratoconus exist in the literature and there is no real consensus on which is best, creating further challenges when screening these patients.
Placido topography, scanning-slit tomography, Scheimpflug tomography, optical coherence tomography (OCT), and direct biomechanical testing have been utilized, alone or in combination, in an attempt to identify patients at risk for postoperative ectasia. However, no system is able to detect all cases and predict unequivocally the risk of developing keratectasia after LASIK. Combining data from newer technologies may aid in identifying patients at risk of developing postoperative ectasia.
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