Psoriatic Arthritis

Psoriatic arthritis (PsA) is a unique, clinically heterogeneous type of inflammatory arthritis associated with skin psoriasis. Psoriatic arthritis affects the joints as well as surrounding structures such as the tendon area that inserts onto bone (enthesitis), tenosynovitis of the entire digits (dactylitis), or it can cause nail changes such as pitting or onchonylysis . These disease manifestations distinguishes it from other types of inflammatory arthritis such as rheumatoid arthritis. Symptoms commonly overlap with to some degree with several other conditions including axial involvement or IBD associated arthritis, which together form a family of diseases termed seronegative spondyloarthritis. Early diagnosis and treatment can relieve pain and inflammation, which may achieve disease remission and prevent progressive joint involvement and damage.
Epidemiology, Etiology, and Risk Factors

Estimates of PsA prevalence in the general population vary from 0.3% to 1%, with a higher prevalence of 6% to 42% in patients with psoriasis.[1-3] The prevalence of spondyloarthropathies, of which PsA is a subset, has been estimated at 1% to 2% of the population. Psoriatic arthritis is more common in those aged 30 to 50 years, and it occurs nearly equally in both sexes. Prevalence studies also suggest a geographic variation, with a higher incidence in people of Northern European descent and a lower incidence in those of Japanese descent.[4-5]

Genetic factors play a role, as evidenced by the presence of a strong familial association and links with several major histocompatibility complex (MHC) class I alleles. Up to 40% of patients with PsA have a positive family history of psoriasis or arthritis.[6]

The greatest risk factor for PsA is having psoriasis. Estimates of PsA among those with psoriasis vary from approximately 5% to 30%.[5] Most patients first present with cutaneous involvement, although arthritis predates cutaneous psoriasis in nearly 10% of cases.[7,8] Psoriasis features associated with higher risk of PsA are scalp lesions, nail dystrophy, uveitis, and intergluteal/perianal skin lesions.[3,8]

The severity and course of PsA varies, ranging from mild symptoms to severe irreversible inflammatory joint destruction. Many patients initially present with oligoarthritis (one to four joints) then progress to polyarticular (five or more joints) involvement. Aggressive disease is more common in patients who exhibit polyarticular or erosive PsA at presentation, extensive skin involvement, strong family history of psoriasis, and disease onset before 20 years of age.

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Pathophysiology

Multiple factors contribute to the pathogenesis of PsA, including genetic, environmental, and immunologic factors. The exact cause has not been identified, and the concomitant pathogenic connection between the skin and joints is not clear. There is evidence that activated T cells are present in both skin and joint tissue. It is likely that cytokines such as the tumor necrosis factor (TNF)-alpha play a role in guiding the inflammatory process that leads to cartilage and bone degradation as well skin inflammation. Proinflammatory cytokines involved with skin and joint disease also may be linked to adverse cardiovascular and metabolic outcomes associated with PsA.

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Signs and Symptoms

Patients with PsA present with pain, swelling, stiffness, and tenderness of the joints, limiting motion. The disease has a heterogeneous presentation including monoarthritis, oligoarthritis, or polyarthritis as well as spondylitic variants. There is a growing awareness that some patients with PsA have an axial component to their disease, primarily affecting the spine, hips, and shoulders.

Five different subtypes of PsA have been described (Table 1), and they can overlap. Most patients present with monoarthritis or asymmetric oligoarthritis. Some can present with a symmetric arthritis similar to rheumatoid arthritis and some may have predominantly the axial or spondylitis joints affected. Arthritis mutilans is a more rare, painful, and rapidly destructive type of PsA characterized by deforming arthritis, especially of the hands, and by resorption of phalangeal bones.
Table 1: Psoriatic arthritis: Subtypes.
Subtypes Percentage of cases
Distal interphalangeal joint-predominant arthritis 10%
Symmetric polyarthritis-predominant arthritis 5%-20%
Asymmetric oligoarthritis or monoarthritis 70%-80%
Axial disease predominant spondylitis and/or sacroiliitis 5%-20%
Arthritis mutilans Rare
destructive arthritis of the third digit
Figure 1: Click to Enlarge
onychomycosis of the toenail
Figure 2: Click to Enlarge
nail pitting and onycholysis
Figure 3: Click to Enlarge

Other associated musculoskeletal features include enthesitis and dactylitis (Figure 1). Dactylitis, which usually presents as a sausage-like inflammation of the entire finger or toe, is a hallmark feature of PsA, although it is also seen in other spondyloarthropathies including reactive arthritis. Enthesitis, or inflammation at the tendon or ligament insertion into bone, is a common feature of spondyloarthropathies, including PsA. Common presentations of enthesitis include the Achilles tendon insertion site and the plantar fascia insertion to the calcaneus. Inflammation of the eye, which is seen more frequently in males who are also HLAB27 positive, can also be associated with PsA.[9]

Cutaneous psoriasis usually precedes the onset of joint symptoms. Skin changes may not be reported by the patient, requiring scrutiny of hidden areas such as the scalp, umbilicus, ears, and perianal skin areas. Psoriatic nail pitting and onycholysis (Figures 2 and 3) are additional features often correlated with distal interphalangeal joint involvement.

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Diagnosis
Clinical diagnosis

The diagnosis of PsA is primarily established by the presence of characteristic signs and symptoms in both the skin and joints. There are no laboratory test values that will provide a definitive diagnosis. Given the heterogeneity of disease presentation, diagnosis can be complex.

Classification criteria have been introduced to help diagnose PsA. The Classification of Psoriatic Arthritis (CASPAR) criteria provide a simple and highly specific assessment tool for PsA.[10] The diagnosis requires patients to have inflammatory arthritis plus at least three points from a list of disease manifestations in order to classify them into the PsA group (Table 2).
Table 2: Classification criteria for PsA.
Diagnosis of PsA requires patients to have inflammatory articular disease (joint, spine, entheseal) plus a total of at least three points from these five features:
Skin psoriasis that meets one of these three criteria:
Evidence of current psoriasis defined as psoriatic skin or scalp disease (2 points)
Personal history of psoriasis (1 point)
A family history of psoriasis in a first- or second-degree relative (1 point)
Psoriatic nail dystrophy, such as onycholysis, pitting, or hyperkeratosis (1 point)
Dactylitis, either present or past, as documented by a rheumatologist (1 point)
Negative test result for presence of rheumatoid factor (1 point)
Radiographic evidence of juxtaarticular bone formation, distinct from osteophytes (1 point)

Classification criteria for axial and peripheral spondyloarthropathy have been developed by the Assessment of SpondyloArthritis International Society (ASAS) criteria.[11] Both classification sets include the presence of psoriasis, and there is overlap between the two. The presence of inflammatory arthritis, inflammatory back pain, enthesitis, uveitis, family history, and radiographic abnormalities should prompt evaluation for PsA. It is important to diagnose PsA early so that treatment can quickly relieve pain and inflammation and prevent irreversible joint damage.
Differential diagnosis

PsA should be distinguished from other inflammatory arthropathies such as rheumatoid arthritis (RA.) Enthesitis, dactylitis, and distal interphalangeal (DIP) involvement help differentiate PsA from RA. Other spondyloarthropathies, including ankylosing spondylitis, reactive arthritis, and enteropathic arthritis may have similar clinical presentations. Crystal-induced arthritis can coexist with PsA, but it can be distinguished from PsA by synovial fluid crystal analysis. Dermatologists can help confirm psoriasis and to comanage therapeutic decisions for both skin and joint involvement.