Acuity, Astigmatism and Other Measures Ineffective in Keratoconus Screening Programs

Normal values of central keratometry were established in teenagers without keratoconus during this study, with Kmax ranges from 38.0D to 49.0D; these values indicate physiologically steep corneas can be present in the absence of keratoconus, while keratoconus can also be present in “normal” curvature eyes. Photo: Brian Chou, OD. Click image to enlarge. Although a rarer corneal condition, keratoconus is progressive and can cause permanent loss or decrease of vision. Consequently, community-based screenings are one way to diagnose and intervene early in its course. One recently published study had researchers collect data from a high school population in Kenya that was screened for the condition, aiming to elucidate reliable cutoff values to facilitate early disease identification vs. normative ranges.The prospective investigation included 3,051 participants aged 13 to 25 (mean age 17.4), and were classified into three groups: non-keratoconus, keratoconus suspect and keratoconus. Just 1.7% were found to have keratoconus. Of those, one-third (31.4%) had a binocular visual acuity (VA) of ≤0.2logMAR; median VA was worse, spherical equivalent refractive error was more myopic and astigmatism was higher in keratoconus than non-keratoconus groups. Unfortunately, between the three groups, considerable overlap remained when comparing these parameters, making it difficult to establish a reliable cutoff.More clearly, asymmetry was found in a large proportion of those with keratoconus. Most diagnostically reliable was the use of scissor reflex on retinoscopy, maximum anterior corneal curvature and presence of astigmatism, individually or collectively.Importantly, the authors discuss in their paper on the work for Clinical and Experimental Optometry that along with the one-third of students with keratoconus having a binocular vision of 0.2logMAR or better, more than half had no significant refractive errors; this is indicative of keratoconus being asymptomatic early in the disease course. In real-world settings, this means that school screening programs for uncorrected refractive errors that rely on VA, autorefraction and subjective refraction may miss up to 50% of keratoconic patients. What’s more, taking advantage of measuring monocular compared to binocular VA confers the advantage of identifying more individuals with the disease; this is because of the high asymmetry often seen, resulting in students not realizing visual loss may have occurred until their better eye worsens, hence delayed diagnoses.It was found that, upon retinoscopy, 66.0% of keratoconic eyes had a scissor reflex, which the researchers suspect may be due to the refraction at the apex of the cone in keratoconic eyes being slightly myopic when compared with the periphery, resulting in the reflex. The authors succinctly described in their paper the most important takeaway from their study results: “When screening in schools and using a combination of all three tests—retinoscopy, keratometry and refraction—students with a scissors reflex on retinoscopy or Kmax >46.5D or astigmatism >2.50D should be referred for corneal topography.”They add that “whilst VA, refractive error, astigmatism and Kmax on their own are not good predictors for the presence of keratoconus and should not be used as independent parameters, short-term changes in these parameters should trigger a referral for corneal tomography as keratoconus can progress.”Click here for the journal source. Rashid ZA, Mashige KP, Moodley VR. Validity of keratoconus screening tests. Clin Exp Optom. July 22, 2025. [Epub ahead of print]. This article was developed by the editorial staff in conjunction with experts in the field. In the process, AI may have been among the editorial tools used to meet the goals of human editors, who approved all content.