Could Psoriatic Arthritis Be Easily Diagnosed from Current Suspicious Physical Findings in the Dermatology Clinic?

Psoriasis is a common, chronic, and recurrent inflammatory disease of the skin affecting 1%~3% of the population worldwide1. This disorder can be associated with a characteristic variant of inflammatory arthritis termed as psoriatic arthritis (PsA). According to a recent systematic review, 7%~26% of patients with psoriasis develop PsA, which manifests as pain and stiffness, swelling, and joint disability in several patterns2.

Some features distinguish PsA from other forms of arthritis. Unlike rheumatoid arthritis, the pattern of joint involvement in PsA is usually asymmetric and frequently involves the distal interphalangeal (DIP) joints. There is also no marker of PsA that is usually seronegative for rheumatoid factor. Importantly, >90% of PsA cases are associated with psoriasis3. For these reasons, it is generally accepted that PsA is a distinct entity from other specified arthropathies. There is a link between psoriasis and PsA; however, the relation between skin and joint manifestations in PsA patients is inconsistent, and shared genetic markers or disease mechanisms have not been definitely elucidated4. A better understanding of the development of the symptoms of PsA, including early warning signs, would be of great value.

Patients with PsA experience significant morbidities such as progressive joint destruction and functional disability, in addition to increased health-care costs5, 6, 7, 8. As PsA is preceded by cutaneous psoriasis in 80% of cases, dermatologists have an important role in screening psoriasis patients for PsA and assessing them before further evaluation9. A recent study has shown that up to 29% of psoriasis patients seen by dermatologists have undiagnosed PsA10. Additionally, it is important for dermatologists to evaluate psoriatic nail involvement because nail psoriasis is associated with a prolonged duration of psoriasis itself and a greater severity of skin and joint involvement, and may also be an indicator for the severity of PsA.

Several classification criteria have been developed because the diagnosis of PsA is challenging. In 2006, the criteria of the classification of psoriatic arthritis (CASPAR) study group were derived from an in-depth clinical, laboratory, and radiographic study of 588 PsA cases and 536 controls with rheumatoid arthritis, ankylosing spondylitis, or undifferentiated arthritis11. However, there is a possibility of overdiagnosing PsA when using the CASPAR criteria in that psoriatic patients with joint symptoms and nail involvement can be considered as having PsA.

The purpose of this study was to investigate the prevalence of PsA by using the CASPAR criteria, as well as its correlation with psoriasis severity and nail psoriasis severity. The diagnosis was based on physical examination alone. Our aim was to estimate the proportion of psoriatic patients with suspected PsA diagnosed by using the CASPAR criteria without laboratory tests and radiology imaging. Measuring joint involvement is one of the most essential evaluation before determining the presence and clinical patterns of PsA. Therefore, the study patients with peripheral joint symptoms were assessed by the Joint Count 66/68 (JC66/68), which allows a global analysis of inflammatory involvement in PsA by capturing a maximum number of peripheral joints and minimizing chances to miss affected one12. Additionally, we evaluated all the fingernails and toenails of the included psoriatic patients by calculating the nail psoriasis severity index (NAPSI) score.

The prevalence and clinical characteristics of PsA in patients with psoriasis vary greatly worldwide. In one study, the prevalence of arthritis among psoriatic patients was estimated to be up to 42% with the Moll and Wright critieria15. Generally, the prevalence among whites is estimated at 15%~30% in Western countries, which is a rather high prevalence compared with that in Asians. In this prospective cross-sectional study, we found that 13.5% of psoriatic patients fulfilled the CASPAR criteria for PsA. Our results are in line with several other studies in Asian populations16, 17, 18, 19. In addition, we found that hyperlipidemia and localized pustular psoriasis were significant predictors of PsA. Previously, it was reported that hyperlipidemia was associated with an elevated risk of psoriasis and psoriasis with PsA20. Our results also agree with those of a previous study, and the close association between localized pustular psoriasis and PsA may be due to the high prevalence of nail psoriasis in patients with localized pustular psoriasis20.

There are some possible reasons for the low prevalence of PsA in our study. First, the patients without PsA might have satisfied other requirements of the CASPAR criteria, such as a negative rheumatoid factor or positive radiographic imaging of psoriatic bone changes. Therefore, more psoriatic patients in the study population might fulfill the CASPAR criteria as patients with suspected PsA did not have blood tests or imaging tests for further evaluation. Secondly, the number of psoriatic patients with arthritis could be higher because patients with spondylitis or axial joint involvement were not evaluated because of the difficulty for dermatologists to assess those joints. Accordingly, the limitations of the use of the JC66/68 in diagnosing PsA should be considered as the axial/spinal joint evaluation is not part of the JC66/68 method for the joint evaluation of PsA.

In the present study, the prevalence of PsA was comparable to that reported 16 years ago, although the patients were further assessed with laboratory evaluation, joint radiography, and bone scintigraphy in the previous study conducted with Korean psoriatic patients21. This suggests that the prevalence of PsA had increased in recent decades, which, as suggested by other researchers, could be caused by environmental factors and improved diagnostic tools18.

For better PsA screening, several questionnaires have been developed. Most of these are relatively complicated and very time consuming for routine clinical use. Recently, a new and simple screening questionnaire consisting of five questions has been proposed to identify cases of PsA in a population of persons with known psoriasis. The five questions that form the basis of the PEST screening questionnaire demonstrated good sensitivity and specificity14. In general, a score of ≥3 indicates a high probability of having PsA14. In our study, the mean of the total number of positive responses to five questions was 1.2 in patients with PsA. This indicates that a cutoff value of 3 might be set too high for reliable screening of PsA, especially in Koreans.

In comparison analyses based on the presence or absence of PsA, the scores of PEST and PASI, and the BSA were significantly higher in patients with PsA than in those without. A recent meta-analysis suggested a trend for an association between high PASI and PsA risk in accordance with our results22. Overall, we can conclude that there is a close relation between the severity of psoriasis and PsA. Psoriatic nail lesions, which can also be indicative of severe psoriasis, occur in about 10%~80% of patients with PsA. In our study, all of the patients with PsA had nail psoriasis as patients had to have nail psoriasis to satisfy the CASPAR criteria without further laboratory or imaging investigations.

This is the first study, to our knowledge, to assess the relation between PsA and psoriatic nail involvement by using the NAPSI score. The mean NAPSI score was higher in patients with PsA; however, the difference was not statistically significant. As a result, the nail psoriasis severity was not significantly different. Interestingly, the prevalence of nail psoriasis in the study population was 85.5%, which is high compared with that reported in previous studies23, 24. Psoriatic nail involvement, which is one of the most important symptoms-based criteria, should be evaluated thoroughly by dermatologists. If dermatologists carefully evaluate the nails of psoriatic patients like the trend of calculating the NAPSI score, nail involvement of psoriasis may be found more frequently than expected. Therefore, it is questionable whether one or two nail pittings should be considered as nail psoriasis because minor pitting could be found in subjects without any dermatologic disease. If these minor nail changes are considered as nail psoriasis, it is possible for PsA to be overdiagnosed by fulfilling the CASPAR criteria. From a dermatologist's point of view, patients with suspected PsA may need to be evaluated in different ways for the correct diagnosis of PsA, although the CASPAR classification criteria is useful in practice for affirming PsA diagnosis. Accordingly, we suggest that the CASPAR criteria should be more specified by using nail psoriasis severity or the clinical features of nail psoriasis. Many authors demonstrated that oligoarthritis was the most common pattern of PsA, whereas some authors reported that polyarthritis was the most common. The different results between the studies were possibly due to the differences of arthritis severity among the study populations. By assessing the patients with careful joint examination by using the JC66/68, oligoarthritis was the most common pattern of PsA in our study (55.6%). The JC66/68 allows a global analysis of inflammatory involvement in PsA by capturing a maximum number of peripheral joints12. Therefore, we believe that our assessment method minimizes the chances of missing affected persons in terms of peripheral joint evaluation.

The limitations of this study include the lack of a validated assessment by blood tests or imaging tests for the PsA diagnosis in psoriatic patients with joint symptoms. Therefore, our finding may still be an underestimation overall. Nonetheless, this pragmatic study produces results that can be applied in routine dermatology settings where laboratory or imaging tests are not readily available. The results of the previous study by Mease et al. support our findings showing that clinical evaluation alone is often a sufficient basis for PsA diagnosis25.

Conclusively, the prevalence of PsA in patients with psoriasis in Korea from physical examination for PsA was 13.5%. Dermatologists can diagnose PsA from current physical findings by using the nonlaboratory items of the CASPAR criteria. The strong point of the criteria is that the diagnosis of PsA can be established in the presence of proper cutaneous and articular symptoms so dermatologists can enroll appropriate patients with suspected PsA into early clinical intervention. However, careful examination of the fingernails and toenails is of utmost importance because nail psoriasis status can strongly affect making the diagnosis of PsA in patients with suspected PsA on the basis of the CASPAR criteria.