Psoriatic arthritis (PsA) is a chronic inflammatory joint disease where a person’s immune system attacks normal tissues in the body, especially the skin and joints. This causes overgrowth of skin cells as well as inflammation in the peripheral joints and entheses, leading to structural damage.1 PsA occurs in up to 30% of patients who have psoriasis and is often progressive, especially if not treated. Though PsA generally occurs in patients who already have psoriasis, it may occur as the first sign of the disease, making differentiation from other types of arthritis, such as RA, difficult.2
A number of studies have examined the prevalence of PsA in various countries. Prevalence estimates in the US range from 0.06% to 0.25% while prevalence estimates in Europe range from 0.05% in Turkey and the Czech Republic to 0.21% in Sweden. Only a few reports of the prevalence of PsA in South America and Asia exist, and they suggest that the prevalence is lower in these regions (eg, 0.07% in Buenos Aires and 0.02% in China). However, it is believed that the low prevalence of PsA in China may be due to underdiagnosis.3
Differentiating inflammatory arthritides from one another can be difficult. Some physical differences, such as small vs large joints and pattern of joint involvement, may be helpful, but are overlapping in multiple etiologies. Laboratory studies, including rheumatoid factor (RF), can aid in this diagnostic dilemma as a rule out for other arthritides.4
Plain radiographs can show joint destruction.5 Other radiologic studies such as computed tomography and magnetic resonance imaging may also be helpful in diagnosis.6 Further study with ultrasound has been shown to greatly aid in early diagnosis because it can reveal early signs of PsA, such as soft tissue changes.4,5
Ultrasound findings for differentiation of psoriatic arthritis from rheumatoid arthritis. A. Short-axis view of palmar plate inflammation. FT, flexor tendon; MH, metacarpal head; PP, palmar plate. B. Dorsal long view of enthesitis of the extensor tendon from a distal interphalangeal joint in a patient with psoriatic arthritis. DIP, distal interphalangeal; S, DIP synovitis; asterisk (*), enthesophyte; double asterisk (**), extensor tendon demonstrating thickening, hypoechogenicity, and loss of fibrillar architecture; triple asterisks (***), extensor tendon with insertional Doppler.7
Signs & Symptoms
Signs and symptoms of PsA may include:8,9
Complications of PsA include joint deformity (including arthritis mutilans, which involves marked bone resorption or osteolysis), uveitis, cardiovascular disease, and impaired function, all of which can lead to reduced quality of life.
Known predisposing factors to developing PsA are genetic factors, psoriasis, environmental factors, and immune-mediated inflammation caused by overexpression of proinflammatory cytokines.9
In addition, a recent cohort study has shown that not only can obesity contribute to PsA development, but weight loss in an obese patient with psoriasis may decrease the risk for PsA.10
Disease & Clinical Trial Tools
Accurate and reproducible assessments of disease activity in PsA are critical to the understanding of disease progression and assessing the effectiveness of various therapies. Emerging clinical trial information often utilizes novel disease measures not commonly used in clinical practice.11 This section will provide details of some disease indexes used in clinical practice and clinical trials.
The American College of Rheumatology (ACR) response criteria, a composite measure developed in RA clinical trials, has been used to assess peripheral joint involvement in PsA. Although ACR20, ACR50, and ACR70 do not assess skin, spine, or entheseal domains, they reflect the extent of improvement in disease activity, as shown in the table on the right.
ACR response is determined by assessing the percentage improvement in the number of tender and swollen joints along with three of the following: patient global assessment, physician global assessment, functional questionnaire, pain score, and erythematosus (ESR) or C-reactive protein (CRP). ACR20 response indicates at least 20% response, while ACR50 and ACR70 indicate at least 50% and 70%, respectively.
A group of 60 experts evaluated 40 patient profiles to determine the criteria for classification of patients as achieving minimal disease activity (MDA).13
A validated endpoint, MDA indicates that patients satisfy 5 of 7 of the following criteria:12,14
A composite measure to assess disease activity in PsA, the Psoriatic Arthritis Disease Activity Score (PASDAS).15 Download this PASDAS PDF for score calculation.
A composite measure to assess disease activity in PsA, the Disease Activity Index for Psoriatic Arthritis (DAPSA) is a simple sum, making it easy to perform in clinical practice. To calculate DAPSA, add the tender joint count (TJC), swollen joint count (SJC), patient pain score (VAS pain), patient global assessment of disease activity (PtGA), and C-reactive protein (CRP) level.