Although KC has been well documented in Western populations and some Asian countries, seldom was the epidemiology study on KC done in Chinese populations. This study aimed to demonstrate the basic clinical profiles of KC in Chinese patients.
The mean age of diagnosis was 22.24 ± 6.18 years in our study, which is similar with most other Asian or Western studies (ranged from 20.00 ± 6.4 to 24.05 ± 8.97)17, 18. Published male and female ratios of KC varied from 1:1.1 to 2.5:1 in other Asian countries and 1:2 to 3:2 in Western countries13. Our research showed a gender ratio of 2.83:1, which is slightly larger than other Asian researches. The skewed sex ratios with high male majority in China may account for this.
Three (0.77%) patients revealed a positive family history of KC in our study, which is much lesser than the published rates in other countries that vary from 5–23.5%19, 20. The prospective research in Singapore showed 26.3% of their KC patients had asthma, while only 0.9% of patients had vasomotor rhinitis13. Instead of asthma, allergic conjunctivitis and allergic rhinitis were the most common diseases accompanying with Chinese KC patients we studied. And the incidence of atopy diseases among Chinese KC patients was 66.0%, which is much higher than that cited by other studies17, 21. Different populations, unknown hereditary and especially the environmental factors could be reasons for this. As the urbanization and industrialization in China is accompanied by bad air quality, and the prevalence of asthma in China has been increasing in recent years22, the impact of deteriorating environment on KC should not be underestimated.
The association between eye rubbing and KC has been validated for many years. In 2000, Bawazeer et al found that eye rubbing was the most significant risk factor for KC in their multivariate analysis23. In-depth studies have illuminated the underlying mechanisms between eye rubbing and KC, as the microtrauma of epithelium caused by eye rubbing may cause abnormal improved inflammatory factors, such as IL-6 and TNF-α, which may in turn contribute to apoptosis of keratocytes24. Thus, gradual corneal thinning is inevitable. In our research, more than 90% of Chinese patients rubbed their eyes. And 48.5% of patients mentioned the habit of frequent eye rubbing, similar to the 48.2% in the case series by CLEK study25. Naderan M. et al found that in their Iranian patients, the higher frequency of eye rubbing and positive family history were associated with a more severe clinical stage of KC26. Similarly, we found in Chinese patients aged over 21, frequent eye rubbing significantly aggravate the disease condition, with these patients having more serious Kmax and more terrible astigmatism at diagnosis than those who did not. However, no relationship was found between positive family history, allergic diseases and the severity of KC in our study. But the correlation analysis showed patients with atopy were more likely to rub their eyes frequently (P = 0.045). This is consistent with the conclusion of Bawazeer et al that atopy may contribute to KC most probably via eye rubbing provoked by itching. As allergic diseases and eye rubbing were quite common among our patients, while their KC family history was sparsely positive. We speculate that differences may exist in the pathogenesis of KC between China and foreign countries, with environmental factors contributing more among Chinese patients than genetic ones.
KC was typically considered as a progressive disease that usually stabilizes by the third or fourth decade of life27, our data suggested that Chinese patients aged 21–30 have steeper Kmax and thinner thinnest pachymetry compared with other age group also accord with this. And similar with studies performed by other races28, the severity of Chinese KC was more serious in male patients than females. According to the value of Kmax, the majority of our patients (79.9%) being diagnosed were in severe stage (༞52D). The proportion of patients with severe KC in our study was significantly larger than that published by other countries4, 25. We suppose the weak awareness of KC may lead to the serious condition of disease, as the majority of our patients never heard of KC and realized eye rubbing was an important risk factor for KC before diagnosis.
Similar with the research of Reena17, our study found patients with earlier age at onset and eye rubbing had an increased risk for corneal acute hydrops. Besides, we also found male patients with smoking habit were more likely to get hydrops. This may partly contradict with the study of Spoerl, which indicated that fewer were smokers in KC patients received CXL and postulated that by-products of cigarette may lead to cross-linking of collagen, thus prevent the progression of KC29. Since the methodologies of these two studies were very different and no research had unveiled the inherent mechanism between smoke and KC, the interpretation should be with caution. Since all patients with history of smoking revealed the habit of frequent eye rubbing, we assume the behaviour of smoking may be related to terrible emotion that would cause vigorous eye rubbing and further lead to the happen of corneal hydrops.
Despite positive outcomes, there are still some limitations of our study: First, this study was conducted in a 3A (Class Three/Grade A) hospitals in China, Hangzhou. Thus, compared with general population, the patients reported in our study may have more serious disease. Second, as the majority of our patients reside in ZheJiang province, our results may have geographical limitations and may not reflect the accurate profile of all Chinese KC patients. Overall, further research awaits a more detailed study with evaluation of a larger number of Chinese cases.