There has been a great interest in attempting to identify patients at risk for post-LASIK corneal ectasia before surgery. It is now known that corneas that share similarities with ectatic corneas (keratoconus or pellucid marginal corneal degeneration) are at higher risk for this complication.
1 –4 Thus, efforts have been concentrated on using available specular topography, tomography, or biomechanical studies to recognize keratoconus (KC) in its earliest stages.
5 –12 KC is a noninflammatory progressive localized thinning and protrusion of the cornea. The progressive nature of the disease makes it easily recognized in its advanced stages. However, there is a persistent ambiguity regarding the exact definition of a suspect keratoconus (KCS) cornea, and there are no widely accepted criteria for categorizing an eye as KCS.
13 –15 It has been debated whether the first detectable sign of KC, consequently defining the KCS category, is a localized steepening seen with Placido corneal topography
15 –19 or a slight bowing of the posterior corneal surface detected by tomography.
9,14,20 Current biomechanical parameter (corneal hysteresis and corneal resistance factor) studies have shown that KCS corneas differ significantly from normal and KC corneas, but the results have been of little clinical value until now.
5 An important practical task for clinicians is to improve the sensitivity of their screening methods for identifying patients with mild manifestations of KC and prevent iatrogenic keratectasia. Even if only one eye is affected initially, KC is an asymmetric progressive disorder that will ultimately affect both eyes. The reported frequency of unilateral KC among all KC patients varies, depending on the methods used for diagnosis. The estimated prevalence of unilateral KC ranges from 14.3% to 41% in studies in which only clinical parameters were considered.
21 –23 In more recent studies, the reported frequencies based on computerized videokeratography diagnostic techniques ranged from 0.5% to 4%.
18,24 Thus, the incidence of true unilateral KC is very low, and results in some studies have suggested that, if patients are observed for a sufficient period, signs of keratoconus will develop in the opposite eye.
23,24 This conclusion was reached mainly because both eyes in unilateral KC have the same genetic makeup, and therefore the less-affected eye is also thought to have KC,
15 considering that KC is genetically described as a model of autosomal dominant transmission with complete penetrance but incomplete expression.
25 –28 Therefore, the term forme fruste KC (FFKC), first proposed by Amsler in 1961
29 and then adopted by Klyce et al.,
15 should be used to define the contralateral eye in unilateral KC, the forme fruste being “an incomplete, abortive, or unusual form of a syndrome or disease.”
29