Psoriatic arthritis (PsA) occurs in around 20% of psoriasis patients as a systemic, polymorphic disease of the joints that varies in presentation and clinical course. (Zachariae et al., 2003; Goldenstein-Schainberg et al., 2012; Alinaghi et al., 2018). Skin involvement is not required to establish the diagnosis. PsA is diagnosed based on specific joint symptoms (such as enthesitis, present in 60–80%, and dactylitis, present in 30%) and serology (usually negative for rheumatoid factor and cyclic citrullinated peptide) (Coates & Helliwell, 2017).
Prevalence of PsA differs with age. While the overall pooled proportion of PsA among psoriasis patients in a meta-analysis of 266 studies was 19.7% (95% CI, 18.5%–20.9%), this dropped to 3.3% (95% CI, 2.1%–4.9%) among patients <18 years of age (Alinaghi et al., 2018).
Prevalence of PsA also differs with ethnicity (Alinaghi et al., 2018).
Professor Andrew Blauvelt describes why psoriatic arthritis is an important factor to consider in treatment selection.
Genetics and psoriatic arthritis
It remains to be established why some patients with psoriasis develop PsA whilst others do not, but there are genetic differences between purely cutaneous psoriasis and PsA (O'Rielly et al., 2019).
Although the most dominant genetic effect for both psoriasis and PsA exists within the MHC region located on chromosome 6p21.3 (Rahman et al., 2012), dedicated GWAS and meta-analyses of psoriasis and PsA reveal differences in their genetic architecture and provide insights into the pathogenetic similarities and differences between cutaneous psoriasis and PsA (Stuart et al., 2015). Putative functional coding variants in TYK2 and TRAF3IP2 within psoriasis susceptibility genes have been strongly associated with cutaneous psoriasis and PsA (O'Rielly et al., 2019).
Genomics and serological factors may predict treatment response in tumour necrosis factor inhibitors in PsA. Detecting the genetic variants that create a predisposition to psoriatic disease may also be valuable in predicting response to biological therapy (O'Rielly et al., 2019).