Introduction

Spondyloarthritis (SpA) is a group of heterogeneous diseases involving inflammation of the spine, sacroiliac joint, peripheral joint, and entheses1,2. SpA is composed of Ankylosing spondylitis (AS), reactive arthritis, psoriatic arthritis, inflammatory bowel-associated spondyloarthritis, and undifferentiated spondyloarthritis. In recent years, spondyloarthritis has been classified as axial spondyloarthritis (ax-SpA) and peripheral spondyloarthritis (peripheral-SpA) due to their main clinical manifestation of the disease. Ax-SpA patients have axial joint predominate manifestations involving the spine and sacroiliac joint. Ax-SpA has 2 clinical categories; Radiographic ax-SpA or ankylosing spondylitis (AS) which has plain radiographic changes of the sacroiliac joint according to the New York (NY) classification criteria, and the other is Non-radiographic ax-SpA which does not have radiographic change according to the NY classification.

The prevalence of radiographic ax-SpA or AS varies among countries and regions but is usually less than the prevalence of rheumatoid arthritis3. A prevalence of 0.12% or equivalent to 120 per 100,000 population for Thai AS patients was reported by Chaiamnauy P in 19984. While this study provides valuable data on the prevalence of musculoskeletal disorders, including AS, in a rural Thai population, certain limitations should be acknowledged. The sample may not be fully representative of the entire Thai population, particularly those residing in urban areas. At present, as the electronic medical record of the patient is widely used over the country using the ICD 10 record system, it is an opportunity to evaluate the real prevalence of this disease in Thailand to develop a policy for case management in the future. This study aims to define the incidence and prevalence of AS in Thailand in the year 2017–2020.

Method

A descriptive epidemiological study was performed by using the database of the Information and Communication Technology Center, Ministry of Public Health to identify adult (aged 18 years or greater) patients who were diagnosed with ankylosing spondylitis (M45) from 1 Jan 2017 to 31 Dec 2020.

Information resources and statistical analysis

The database of the Information and Communication Technology Center, Ministry of Public Health, included health data from inpatient and outpatient visits from almost all hospitals in Thailand, except for some university hospitals in Bangkok and private clinics. The diagnosis was coded using the International Classification of Diseases, Tenth Revision (ICD-10). The epidemiologic data of the patients with a primary diagnosis of M45 ankylosing spondylitis were extracted. The incidence and prevalence of AS will be analyzed with their 95% confidence interval (CI). The analyzed data were reported from an overall national perspective and regionally (namely, the northern, eastern, northeastern, southern, and central regions). The categorical data were presented as numbers and percentages while the continuous data were presented as means and standard deviation (SD), or median and interquartile range (IQR). All data analyses will be performed using Statistical software. The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Ethics approval and consent to participate

The Human Research Ethics Committee of Khon Kaen University reviewed and approved the study per the Helsinki Declaration and the Good Clinical Practice Guideline (HE651589). The need for informed consent was waived by the Human Research Ethics Committee of Khon Kaen University because of the nature of the database of the study. Participant privacy was protected by anonymized data and kept confidential. The study methods were performed following the Helsinki Declaration statement.

Results

A total of 42, 933 patients were identified as having the diagnosis of M45 from 1 Jan 2017 to 31 Dec 2020. The number of new cases of AS in 2018, 2019, and 2020 were 6784, 6805, and 6791 cases respectively as shown in Table 1. The total Thai populations in 2018, 2019, and 2020 were 65,406,320 persons, 65,557,054 persons, and 65,421,139 persons respectively. Therefore, the incidence of AS in 2018, 2019, and 2020 were 10.37 (95% CI 10.13–10.62), 10.38(95% CI 10.14–10.63), and 10.38(95% CI 10.14–10.63) per 100,000 person-years respectively. The prevalence and incidence of AS across different years, age groups, genders, and regions are shown in Table 2.

Table 1 Data of AS cases from 2017 to 2020 (n = 42,933) categorized by age, gender, and area of habitat.
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Table 2 Prevalence and incidence of AS per 100,000 person-years with 95% CI in Thailand from 2017 to 2020 categorized by age, gender, and area of habitat.
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The prevalences of AS across the country of Thailand are shown in Fig. 1. AS were found most in the northeastern and southern regions of Thailand, as it had the highest prevalence rates, particularly in 2017, where the northeastern region reported a prevalence of 24.6 per 100,000 and the southern region reported 30.0 per 100,000.

Figure 1
figure 1

Distribution of prevalence and incidence of AS. *(A); prevalence case, (B, C, and D); incidence cases.

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Discussions

In this study, the prevalence of AS was conducted by using information from the National Information and Communication Technology Center, The Ministry of Public Health database that covered approximately 90% of the general Thai population. The missing data accounted for 0.9% of the records in the database, referring to certain demographic details such as age and sex that were not available. Despite this small gap, the database provided extensive coverage and was representative of the national population. This study utilizes the country's largest database to investigate the prevalence and incidence of AS in Thailand.

We identified AS patients from the ICD-10 code to find the prevalence and incidence of AS during 2017–2020. The prevalence of AS in 2017 was recorded as only 20.4 (95% CI 20.0–20.3) cases per 100,000 individuals. Between 2018 and 2020, we identified new cases of AS numbering 6784, 6805, and 6791, respectively. Consequently, the incidence rates for AS were recorded as 10.4 (95% CI 10.1–10.6), 10.4 (95% CI 10.1–10.6), and 10.4 (95% CI 10.1–10.6) during these years. The prevalence of AS varied across age groups, with the highest prevalence observed among individuals aged 50 to 69 years (Table 1). We can note a consistent rise in the number of new cases each year, which will eventually accumulate and contribute to changes in prevalence in the short future.

AS prevalences vary among countries and depend upon the time and the survey methods4,5,6,7,8,9. Dean et al. reported that the global prevalence of ankylosing spondylitis was 23.8 per 10,000 population in Europe, 16.7 in Asia, 31.9 in North America, 10.2 in Latin America, and 7.4 in Africa6. The prevalence among Thais seemed comparable to Europeans but slightly higher than in other Asians and much higher than in Latin Americans and Africans. Various factors may contribute to the difference in disease prevalence, including disparities in methodology, data reliability, and ethnic groups. When compared to the prior study in Thailand in 19984, the prevalence of our study was lower than that in the previous study. The difference in prevalence might be explained by variations in the study methods, levels of healthcare accessibility, and disease awareness. In the past, AS was not widely recognized due to the absence of diagnosis criteria that provided the sensitivity for AS diagnosis compared to the present criteria. Hence, the prevalence of AS might change in the future if more sensitivity criteria are developed.

Age at diagnosis

When examining the age of diagnosis among patients, it was observed that the majority of individuals were diagnosed after reaching the age of 50. Less than 10% of diagnoses were made in individuals below the age of 30. the peak age at diagnosis was identified to be between 50 and 59 years during the years 2018–2020. This observation may suggest a delay in diagnosing AS among patients. Therefore, there is a pressing need for the dissemination of knowledge to both physicians and society to mitigate delays in diagnosis. The higher incidence of AS in individuals aged > 50 may be due to delayed diagnosis, as supported by clinical data. Studies, including a meta-analysis by Zhao et al.10, indicate that AS often remains undiagnosed for many years due to its insidious onset and variable early symptoms, especially in patients with milder initial presentations. However, there was a possibility of diagnostic errors when using ICD-10 codes, especially if the coding was not done by physicians.

Gender

The data regarding gender distribution revealed a higher prevalence of female patients in this study, contrasting with existing literature where males are usually more prevalent. Although the prevalence of HLA B27 is similar between men and women, AS tends to affect more men than women, with an approximate ratio of 2:1, although this ratio can vary significantly between studies11, However, recent evidence from the literature suggests an increasing prevalence of AS among females12,13,14, Walsh et al. reported that the prevalence of AS from January 2006 to December 2016 was 0.04% in 2006 and 0.09% in 2016. In 2006, around 40% of cases were female, whereas, in 2016, the proportion of females increased to over 47%14.

Interestingly, the prevalence of ankylosing spondylitis (AS) among females was comparable to that among males, with rates of 22.4 per 100,000 person-years in females compared to 18.3 per 100,000 person-years in males in 2017. This trend persisted in subsequent years, with similar incidence rates observed in 2018 (females: 11.8 per 100,000 person-years, males: 8.9 per 100,000 person-years), 2019 (females: 11.6 per 100,000 person-years, males: 9.1 per 100,000 person-years), and 2020 (females: 11.7 per 100,000 person-years, males: 9 per 100,000 person-years). These findings suggest a consistent pattern of AS prevalence among females comparable to that among males over the four years. (Table 2). Our findings support the trends in the epidemiology of AS concerning gender and suggest increased awareness of AS among women.

Habitat

When examining patients based on their region of residence, it was observed that the southern and northeastern areas of Thailand exhibited the highest prevalence of AS.

Figure 1 depicts the prevalence and incidence of AS in each province of Thailand. Each province is labeled with a color representing its respective prevalence or incidence rate. The explanation of these differences might be influenced by the degree of urbanization, as access to healthcare is generally easier in urban areas compared to rural areas, and also other factors such as differences in frequency of HLA B27 in different areas and environmental factors. Hwang reported that environmental factors can elevate the risk of developing ankylosing spondylitis, although their precise mechanisms remain unclear11. These factors comprise exposure to specific infections, toxins, and pollutants; joint injuries; alterations in colon bacteria; and inflammation of the bowel. Further research in this area is warranted to gain a deeper understanding of the role these factors play in the development of ankylosing spondylitis.

Our study's strength lies in the utilization of the country's largest database to investigate the prevalence and incidence of AS in Thailand. This comprehensive database offers extensive information for our analysis. A major limitation of this study is the potential misdiagnosis of AS. The lack of access to medical records to confirm the AS classification criteria to confirm the diagnosis. These factors highlight the need for better dissemination of diagnostic knowledge to physicians and to perform future research to prioritize the validation of AS diagnoses recorded in the national database.

Conclusion

AS appears to be relatively rare among the Thai population, with a prevalence of 20.4 per 100,000 populations in 2017. The incidence rates remained consistent, averaging approximately 10.4 per 100,000 person-years between 2018 and 2020. The peak age at diagnosis was identified to be between 50 and 59 years. These results underscore the importance of heightened awareness and early detection strategies for AS.