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6 DISCUSSION
The data presented in this register-based study brings important and novel information on the time trends in IA incidences during the first 15 years of this millennium. It also describes IA patients’ early DMARD and pain medication, especially opioid use between the years 2009 and 2015. With the utilization of the unique and nationwide SII register databases, which allow the inclusion of basically all Finnish early IA patients that have started on DMARDs (a total number of almost 60,000 patients), this study provides a broad picture of the overall disease burden of IAs in Finland.
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the last 5-year periods grew by 12%, meaning almost 450 more diagnosed IA patients on average per year, thus increasing the burden of patients on rheumatology clinics.
Between the single years 2000 and 2014, the mean yearly number of incident IA patients grew even more, by 23%. The utilized national registers comprehensively include early IA patients that are diagnosed by rheumatologists and evaluated to require DMARDs. This supports the reliability of the current results. It is unlikely that the true incidence of IAs would be lower than the current estimates, which may in fact exclude the mildest cases. The possible other reasons for the observed growth in the incidence rates can only be speculated. It is possible that the number of new true cases in some IA groups may have risen, or more of them may have visited a rheumatologist. The attention on early diagnosis and treatment may have resulted in DMARDs being prescribed for milder cases that might not have even been diagnosed in the past and consequently, more certificates have been filed and more reimbursements granted than in the past. The increased awareness of the diseases and progressed diagnostic tools can influence the observed incidences, thus both increasing and reducing the numbers. Also, the ageing of the population, especially of the baby boom generation (those born between 1946 and 1964) have probably caused a rise in the crude incidences of some IA groups. Although the IA incidence rate was clearly higher in women than in men, it increased more rapidly among men compared to women between 2000 and 2015, thus perhaps the gender gap will somewhat level off in the future.
A decrease in the mean age at diagnosis among women was found, mostly caused by the rising number of patients in those diagnosis groups that have contracted such an illness at a younger age. Due to the growing incidence of IAs, the declining age at diagnosis in some IAs, the need for life-long monitoring and treatment, and the longer overall life expectancy of the population, the burden caused by IAs on the health care system has been amplified. This burden has affected especially rheumatologic clinics, which are primarily in charge of diagnosing and treating IA patients. In Finland, it has been estimated that there will be a threatening shortage of working-aged rheumatologists in the future (Rellmann 2016), thus taking care of the demands of rheumatology care will be challenging. A study from the USA, based on emergency department visits, hospitalizations, and mean charges from visits involving arthritis and other rheumatic conditions, also discovered the total burden of IA to be increasing (Han et al. 2016).
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The incidence of seropositive RA did not change significantly, while that of seronegative RA declined. Since the scoring system in the new ACR/EULAR classification criteria for RA emphasizes the significance of RF or ACPA and more joint involvement is required at diagnosis, the fulfilment of the criteria for seronegative RA is harder; thus seronegative arthritis may more frequently than before be categorized as UA or PsA. This may explain why the incidence of seronegative RA declined whereas that of UA and PsA increased in this study. Also, the rising PsA incidence may be due to the advanced education and the rheumatologists’ better awareness of the typical PsA signs (e.g. family history or nail changes) (Wilson et al. 2009).
The incidence of axSpA rose in both genders during the 15-year observation period. Better diagnostic resources (e.g. the availability of MRI) have probably affected this increase. The 2009 ASAS classification criteria for axSpA has raised concerns about increasing overdiagnosis (van der Linden and Khan 2016), but as long as clinicians remember to separate classification criteria from diagnostic criteria, this should not become a problem. Nevertheless, it is most likely that in this study, the incidence of axSpA is rather underestimated than overestimated, since the early and mild cases were not included due to the methods used. The increasing proportion of women in the axSpA group noted in this study may be explained by the growing number of nr-axSpA patients, since the sex distribution is known to be more balanced in nr-axSpA compared to AS (Sieper and Poddubnyy 2017).
A receding trend in the incidence of SLE was visible, which may be explained, e.g. by a decline in real disease cases or a decline in the prescription of DMARDs (for mild cases). The increase in the use of intravenous therapies administered in the hospitals (e.g. RTX, belimumab) is a possible but unlikely explanation, since those patients have usually tried conventional therapies first and are thus found on the SR register.
The group under the ICD-10 code of M35 includes separate diagnoses ranging from SS to overlap syndromes, UCTD, and PMR. Unfortunately, it was impossible to distinguish between these specific diseases from the register data. In addition, SS patients not needing DMARDs and most PMR patients treated only with PRD are not found in the SR register. However, it was decided to keep the M35 group in the analysis since it included a rather high number of patients (regardless of the possible underestimation), who are treated with DMARDs and thus burden the health care system.
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At its best, an epidemiologic incidence study could help to better understand the factors that play roles in the initiation of IAs. However, in the absence of clinical and health behaviour data, this study provides no clear explanations for these points.