Despite the successful translation of corneal cross-linking (CXL) into clinical practice for the therapy
of progressive ectasia, keratoconus globally still remains a major reason for severe visual impairment
in the young population.
Keratoconus is characterized by a biomechanical weakening effect on the corneal stroma. This
disease is typically most aggressive in the first three decades of life, and then slows down to an arrest
in the fourth to fifth decade. The prevalence of keratoconus has first been investigated in the 1980s,
when Kennedy and colleagues detected keratoconus based on irregular retinoscopic reflexes and
irregular mires detected with keratometery.1 Their study was conducted in the US, and the prevalence
reported was 1 in 2,000 patients, which, by definition, is a rare disease. Despite the study being 30
years old, it remains the most cited prevalence rate of keratoconus.
However, a number of more recent publications indicate that the occurrence of this disease might be
considerably more frequent than previously estimated. This might be due to the following reasons.
Changes in corneal imaging techniques
Corneal imaging techniques have evolved since the mid-1980s. In 1985, Klyce and colleagues
introduced the concept of modern corneal topography with computer-assisted analysis of the
reflection image from Placido photokeratoscope images.2 About 20 years later, Scheimpflug imaging
technology allowed for a more detailed analysis of both the corneal anterior and posterior surface,
coining the term “corneal tomography.”3,4
Measurement of corneal biomechanics in vivo
Technology has advanced to help analyze corneal biomechanics and detect early forms of keratoconus
with a greater sensitivity. The Ocular Response Analyzer (ORA), originally developed by David Luce in
the mid 2000s, was the first of its kind to measure the corneal deformation based on an air puff.5
A few years later, high-speed dynamic Scheimpflug imaging allowed for a more detailed analysis
of the changes induced by an air puff.6 Now, new indices are being developed to allow for a better
characterization of keratoconus at an early stage.7 More recently, Brillouin microscopy, a means to
measure corneal biomechanics in a depth-dependent manner is promising and is in its first clinical
stages of development.8
Major geographical variations
The reported prevalence and incidence rates for keratoconus are varied. Geographic, environmental
and ethnic factors may be influencing the differing rates; furthermore, interrelations between these
factors are also yet to be determined.
Some of the recently published articles on the prevalence of keratoconus
were focused on the Middle Eastern population and have shown a
significantly higher prevalence rate than what has been previously
reported; their findings show a keratoconus prevalence rate of up to
1:40 subjects.9,10
Assessing the global prevalence of
keratoconus—the K-MAP study
The Light for Sight Foundation (Zurich, Switzerland), founded by Nikki
and Farhad Hafezi, fosters research initiatives to better understand
keratoconus.11 To draw a more accurate global prevalence rate of
keratoconus, Light for Sight has launched the global K-MAP study
(NCT03115710). This multi-center study will collect data from children
and adolescents in a standardized way and following one common study
protocol. We currently have involved 14 sites on five continents. The
clinical sites were carefully selected to be able to best represent different
geographical regions as well as climate differences. All clinical sites are
required to have the same diagnostic equipment in order to be able to
accurately compare data. Another consideration for selecting clinical sites
is if a patient has a suspicious cornea, the investigator would immediately
refer the patient to a corneal specialists for further diagnostics.
The pilot study was conducted in Riyadh, Saudi Arabia, using young subjects
and a known geographical region with higher prevalence rates (Middle
East); 1,044 eyes of children and adolescents were examined, and the
topographical data were assessed by two independent cornea specialists.
We found a prevalence rate of 4.79% (1:21 patients). The manuscript has
been submitted for publication.
In conclusion, keratoconus is more common than what is defined by the
criteria of a rare disease. The K-MAP study will allow for a more customized
geographical approach in the assessment of the need for early screening
(and treatment) in the young population to, once and for all, prove that
keratoconus should no longer be considered rare.
