There is a lack of fully comprehensive data on the worldwide prevalence of KC, and this has attracted interest in recent epidemiological studies. Studies conducted in refractive surgery clinics show higher KC prevalence values compared with nationwide studies [12, 16–18]. The Sistina Ophthalmology Hospital was the first institution in N. Macedonia offering modern corneal tomography (Pentacam) and has the highest volume of refractive surgery in the territory. Thus, the data from this unit can be regarded as a reliable estimate for the prevalence KC in a refractive surgery department.
Our findings are similar to results observed in studies carried out in Iran, Iraq, Saudi Arabia, but not more geographically closer regions [7–11, 19, 20]. We are aware of only two other studies on refractive surgery populations in the region around N. Macedonia. The prevalence of KC in these studies was 2.9% and 8.5% respectively, values much lower compared with the current study [19, 20]. However, the design of these other studies did not consider ethnicity and the age ranges in the sample groups were limited.
There is no nationwide screening program for KC in N. Macedonia. The only figure for the prevalence of KC in N. Macedonian population is 6.8 per 100,000 for the general population [14]. This was based on a study performed between 1998 and 2004 where patients were classified based on clinical examination and keratometry values in patients considering contact lens wear [14].
The prevalence of 12.2% in our study sample is much higher than the previous estimate reported for the Ljubic study [14]. The main reason for the difference may be due to the diagnosis, which today can be made with more sophisticated objective devices on clinically asymptomatic patients with good visual acuity.
Geographically, Macedonia is sandwiched between Northern Europe and the Middle East. A sunny and dry climate is cited as one of the predisposing environmental factors for KC development, and this could be the reason why our results correspond more with those quoted for the Middle East than Europe [4]. Another reason may be the multi-ethnicity of the population in Macedonia resulting in greater genetic diversity. For the sake of comparison, we were unable to find any reliable figures on the prevalence of KC in regions such as Greece, Bulgaria and Albania.
Gender
There are uncertainties concerning gender as a predisposing factor in the diagnosis of KC in the different refractive surgery populations around the world. Studies by Omer [17] and Talal et al[21] found the prevalence in males and females was about the same, but Kozomara et al[20] found slightly higher prevalence among males than females.
Our study found that males had a 4 times greater chance of being diagnosed with KC than females, and the prevalence values of 71.7% for males and 28.3% for females are very similar to the results of a more recent study by Bejdic et al [19].
The Valdez-Garcia study in Mexico found KC affected females twice as more frequently than males (66% vs. 33%)[22]. The prevalence of KC in an adolescent population in Jerusalem was 5 times more frequent in males, and contrary to this, a report from India claimed KC predominantly affected females [23, 24]. The exact age of onset remains unknown when KC is diagnosed. KC is often suspected when there is a major shift in astigmatism or, an unusual retinoscopic reflex is detected or, there is an increase in corneal and ocular higher-order aberrations or, uncertainties in subjective refraction occur. But to pinpoint when the changes that led to the detection of these phenomena commenced remains elusive. The time when diagnosis is made depends on many factors including the quality of the local healthcare system, the existence of nationwide screening and the willingness of patients to come forward.
The mean age in the cohort was 31.7 years and the difference between the genders was not significant. Compared to other similar studies, our patients were older. This may be due to longer lag between the manifestation of symptoms and final diagnosis.
KC was most frequently diagnosed in the age group 21–30 years of age (41.35%) and a total of 59.8% were younger than 30 years at the time of diagnosis. Patients’ age significantly affects the likelihood of diagnose, by lowering the odds of occurrence by 3.7% per year. This supports the previous findings that age is inversely correlated with the diagnosis of KC, the opposite of what is commonly associated with most chronic diseases [25, 26]. However, this finding opposes the result from Wills Eye hospital in Philadelphia where, over a single year, 40% of KC diagnoses were for patients aged above 50 [27]. Refractive surgery units tend to attract a younger population seeking a solution for their visual impairment. Secondary and tertiary centers, like the Wills Eye hospital, attract a much broader spectrum of patients with other general and/or long-term ophthalmic health issues.
Ethnicity
Compared to Caucasians, the incidence of KC in Asians living in the UK is 4.4 to 9.2 times higher in patients referred to secondary care [28–30].
Our study showed the occurrence of KC was 4.1 times greater among the attending Turkish patients compared with the Macedonians. Data for the incidence, or prevalence, of KC in the Turkish population is scant. Nevertheless, the prevalence in our study (40.8%) is much higher than 8.1% and 14.1% values noted more recently [31].
Turkish minorities have a tendency towards a high consanguinity rate of up to 90%, according to some sources, and this is associated with the accumulation of genetic factors linked to KC and other genetic conditions [32]. The total number of cases of Turkish origin in our sample was low (2.7%) and these were significantly younger than the Macedonians. Factors such as these may have contributed to the higher odds for KC according to some [30, 32]. The higher odds in the remaining groups cannot be reliably explained because the numbers were too low (< 1.3% of the total).
The similar odds for KC among Macedonians and Albanians was expected due to geographical and habitual proximity. The Albanian community in Macedonia is significantly larger than the Turkish and nurtures relationships with neighbouring Kosovo and Albania itself. Furthermore, there is no evidence of different rates of consanguinity between Albanians and Macedonians.