KEY CONCEPTS
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Epidemiological studies describe the distribution of disease, identify factors that influence that distribution, and measure the impact and morbidity of disease in a defined population.
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Population-based screening studies are the best methodology to assess the true prevalence of the disease.
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Modern imaging technology has made an earlier diagnosis of keratoconus easier, even before the loss of visual acuity.
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An inverse relationship between age and the keratoconus severity has been reported. Corneal collagen interfibrillar space decreases with age, and the collagen bundles become thicker, modifying the biomechanical properties of the cornea.
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The presence of estrogen, progesterone, and androgen receptors in epithelial corneal cells and keratocytes raises the possibility of a relationship between hormones and biomechanical properties of the cornea at different stages of life.
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Genetic and hereditary links to keratoconus have also been studied. Most familial keratoconus is autosomal dominant.
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Eye rubbing, allergy, and environmental factors also contribute to keratoconus pathogenesis.
Introduction
Epidemiological studies describe the distribution of disease, identify factors that influence that distribution, and measure the impact and morbidity of disease in a defined population. They can recognize vulnerable groups, helping to define more transparent and consistent designs to health programs and services. The introduction of cutting-edge technology, both in disease diagnosis and data analysis, has further refined the epidemiological process, allowing a more efficient formulation of health policies. In this sense, the technological revolution provided by new diagnostic imaging devices in assessing the cornea and anterior segment and by innovative artificial intelligence algorithms has significantly changed the way we identify and treat vision-threatening conditions such as keratoconus.
Incidence and Prevalence
Attempts to establish prevalence and incidence estimates of keratoconus in the population have shown great variability over the last century. Among the possible factors for this variability are the heterogeneity of types of epidemiologic studies and the lack of well-defined criteria for keratoconus definition.
Most of the prevalence studies have been carried out in hospitals or clinics, because of the ability to collect data. However, they usually underestimate the prevalence of the disease, as patients are commonly symptomatic, and the early and more subtle forms can be missed. One of the most commonly cited publications on the epidemiology of keratoconus is the study by in Minnesota, United States, who found a prevalence of 0.054% based on the clinical diagnosis made by the findings of scissoring movement in retinoscopy and keratometry. This number was similar to those reported in Finland and Denmark but much higher than those reported in Russia, at 0.0004%. and 0.0068% in Macedonia. More recently, a large-scale evaluation from the Netherlands mandatory health insurance database showed prevalence approximately fivefold higher than previous reports, at around 0.27%, or 1:375 patients ( Table 3.1 ).
| Author | Location | Age in Years | Sample Size | Incidence/ 100,000 | Prevalence/ 100,000 | Method | ||
|---|---|---|---|---|---|---|---|---|
| Tanabe et al. (1985) | Muroran, Japan | 10–60 | 2601 | 9 | Keratometry | |||
| Kennedy et al. (1986) | Minnesota, USA | 12–77 | 64 | 2 | 54.5 | Keratometry + retinoscopy | ||
| Ihalainen (1986) | Finland | 15–70 | 294 | 1.5 | 30 | Keratometry + retinoscopy | ||
| Gorskova and Sevost’ianov (1998) | Urals, Russia | 0.2–0.4 | Keratometry | |||||
| Pearson et al. (2000) | Midlands, UK | 10–44 | 382 | 4.5 – W 19.6 – A | 57 229 | Keratometry + retinoscopy | ||
| Ota et al. (2002) | Tokyo, Japan | 325 | 9 | Keratometry | ||||
| Georgiou et al. (2004) | Yorkshire, UK | 74 | 3.3 – W 25 – A | Clinical examination | ||||
| Assiri et al. (2005) | Asir, Saudi Arabia | 8–28 | 125 | 20 | Keratometry | |||
| Nielsen et al. (2007) | Denmark | NA | 1.3 | 86 | Clinical indices + topography | |||
| Ljubic (2009) | Skope, Macedonia | 2254 | 6.8 | Keratometry | ||||
| Ziaei et al. (2012) | Yazd, Iran | 25.7 ± 9 | 536 | 22.3 | Topography |
Population-based screening studies are the best methodology to assess the true prevalence of the disease. Cross-sectional surveys enroll people who volunteer to participate in the investigation. Selection bias may also occur, as diseased individuals might not be interested in participating. However, others who are not aware of the condition may volunteer, counterbalancing the possibility of a significant error. , The first cross-sectional study was published by Hofstetter in 1959. Twenty-five optometrists used Placido disk images to analyze 13,395 eyes and confirmed the diagnosis of the disease by the presence of oval or keratoconic-type pattern images. The estimated prevalence of keratoconus was 0.6% in the American population. The elliptical or “irregular” pattern was observed in 0.1% of patients between 0 and 19 years and 7.4% between 70 and 79 years old, with a preponderance in females.
An important population-based study including 4667 subjects was conducted in central India. Using keratometric values of more than 48 diopters (D) as a cutoff, the authors found a prevalence of keratoconus of 2.3%. Considering that keratometry measures only the central corneal power, it is possible that cases of inferior ectasia were missed. Another similar study carried out in Beijing, China with 3468 people reported a prevalence of 1% of corneas with more than 48 D, using optical low coherence reflectometry biometry of the right eyes. An investigation of French army recruits using corneal topography found a prevalence of keratoconus of 1.2%.
Other prevalence studies, including population-based surveys from Asia and the Middle East using corneal topographic and tomographic values, found a higher prevalence of keratoconus in these parts of the world, ranging from 0.9% to 3.3% ( Table 3.2 ). In 2018 performed a cross-sectional, observational, multicenter study collecting data from 522 pediatric patients from 6 years to 21 years of age who were seen at multiple nonophthalmic emergency departments in Saudi Arabia. Bilateral corneal measurements were performed using a Scheimpflug corneal tomography system. Two masked examiners established the diagnosis of keratoconus using both objective and subjective screening criteria. Final keratoconus prevalence was 4.79% (95% confidence interval [CI]: 2.96 – 6.62) or 1:21 patients, the highest reported so far.
| Author | Location | Age in Years (Mean) | Sample Size | Prevalence/ 100,000 | Method | Sampling Method | ||
|---|---|---|---|---|---|---|---|---|
| Hofstetter (1959) | Indianapolis, USA | 1–79 | 13345 | 120 | Placido disk a | Rural volunteers | ||
| Santiago et al. (1995) | France | 18–22 | 670 | 1190 | Topography | Army recruits | ||
| Jonas et al. (2009) | Maharashtra, India | >30 (49.4 ± 13.4) | 4667 | 2300 | Keratometry a | Rural volunteers (eight villages) | ||
| Millodot et al. (2011) | Jerusalem, Israel | 18–54 (24.4 ± 5.7) | 981 | 2340 | Topography | Urban volunteers (one college) | ||
| Waked et al. (2012) | Beirut, Lebanon | 22–26 | 92 | 3300 | Topography | Urban volunteers (one college) | ||
| Xu et al. (2012) | Beijing, China | 50–93 (64.2 ± 9.8) | 3166 | 900 | Optical low coherence reflectometry a | Rural + urban volunteers | ||
| Hashemi et al. (2013) | Shahrud, Iran | 50.83 ± 0.12 | 4592 | 760 | Topography | Urban volunteers from random cluster | ||
| Hashemi et al. (2013) | Tehran, Iran | 14–81 (40.8 ± 17.1) | 426 | 3300 | Topography | Urban volunteers (stratified cluster) | ||
| Gordon-Shaag et al. (2015) | Haifa, Israel | 18–60 (25.05 ± 8.83) | 314 | 3180 | Topography | Urban volunteers (one college) | ||
| Hashemi et al. (2014) | Mashhad, Iran | 20–34 (26.1 ± 2.3) | 1073 | 2500 | Topography | Urban volunteers (stratified cluster in one university) | ||
| Torres Netto et al. (2018) | Riyadh, Saudi Arabia | 6–21 (16.8 ± 4.2) | 1044 | 4790 | Rotational Scheimpflug corneal tomography system | Patients who were seen at emergency rooms for nonophthalmic appointments at four locations in the Kingdom of Saudi Arabia |
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