Residency · Residency · Rheumatology

Psoriatic Arthritis

Introduction

Psoriatic arthritis is a chronic inflammatory arthritis associated with psoriasis, classified within the spondyloarthritis family. The prevalence ranges from 6 to 42 per 100,000 in the general population, and PsA affects approximately 30 percent of patients with psoriasis. The disease is remarkably heterogeneous, encompassing 6 clinical domains: peripheral arthritis, axial disease, enthesitis, dactylitis, skin involvement, and nail disease. In approximately 70 percent of cases, skin psoriasis precedes the onset of arthritis, while in 15 percent the two present concurrently, and in the remaining 15 percent arthritis precedes skin disease. Untreated PsA can be severely erosive and destructive, with arthritis mutilans developing in approximately 5 percent of patients.

Pathogenesis

The pathogenesis of psoriatic arthritis is driven by the IL-23/IL-17 axis, which is central to both skin and joint disease. TNF-alpha drives synovial inflammation and osteoclastogenesis, contributing to joint damage. Similar to axial spondyloarthritis, PsA features entheseal-driven pathology in which mechanical stress activates resident type 3 innate lymphoid cells and gamma-delta T cells at entheseal sites. Genetic associations include HLA-B27 for axial PsA, HLA-C*06:02 for skin psoriasis, and polymorphisms in IL-23R, TNFAIP3, and TRAF3IP2. The PDE4 pathway, characterized by elevated cAMP degradation in immune cells, is the molecular target of apremilast.

Classification

CASPAR Criteria (2006)

The CASPAR classification criteria require the presence of established inflammatory articular disease affecting joints, spine, or entheses, plus at least 3 points from the following: current psoriasis (2 points), history of psoriasis (1 point), or family history of psoriasis (1 point); nail dystrophy including pitting, onycholysis, and hyperkeratosis (1 point); negative rheumatoid factor (1 point); current dactylitis or history of dactylitis (1 point); and radiographic evidence of juxta-articular new bone formation excluding osteophytes (1 point). These criteria achieve a sensitivity of 91.4 percent and specificity of 98.7 percent.

Clinical Manifestations

Peripheral Arthritis Patterns

The most common pattern at onset is oligoarticular asymmetric arthritis involving fewer than 5 joints, affecting both large and small joints. A polyarticular symmetric pattern may mimic rheumatoid arthritis but is distinguished by DIP joint involvement and RF negativity. The DIP-predominant pattern, though classic for PsA, is less common at approximately 5 percent and is strongly associated with adjacent nail disease. Arthritis mutilans represents severe osteolysis producing the "opera-glass" deformity and the characteristic pencil-in-cup appearance on radiographs. The spondylitis-predominant pattern features axial disease with sacroiliitis and spondylitis.

Enthesitis

Enthesitis occurs in 30 to 50 percent of patients with PsA and distinguishes the disease from rheumatoid arthritis. Common sites include the Achilles tendon insertion, plantar fascia, patellar tendon, epicondyles, and greater trochanter. Ultrasound demonstrates tendon thickening, hypoechogenicity, Doppler signal indicating active inflammation, and enthesophytes.

Dactylitis

Dactylitis, or the "sausage digit," presents as diffuse swelling of an entire finger or toe and is primarily caused by flexor tenosynovitis rather than isolated joint swelling. It is present in 16 to 48 percent of PsA patients, more commonly affects the toes, and is predictive of radiographic progression.

Skin and Nail Disease

Plaque psoriasis is the most common cutaneous pattern, though the extent of skin disease does not correlate well with joint disease severity. Nail involvement is present in 80 to 90 percent of PsA patients compared to 40 to 50 percent of psoriasis patients without arthritis. Nail matrix disease produces pitting, ridging, and leukonychia, while nail bed disease causes onycholysis, oil-drop discoloration, and subungual hyperkeratosis. Scalp psoriasis is very common and may be the only skin finding. Inverse psoriasis in the intergluteal, axillary, and inguinal folds should be specifically sought.

Axial PsA

Axial PsA follows inflammatory back pain criteria similar to axial SpA but has several distinguishing features. Sacroiliitis can be unilateral, unlike the bilateral pattern characteristic of ankylosing spondylitis. Syndesmophytes tend to be non-marginal, bulky, and asymmetric, contrasting with the marginal, thin, symmetric syndesmophytes of AS. HLA-B27 is positive in approximately 50 percent of axial PsA compared to 90 percent in AS.

<image>A clinical features composite illustration of psoriatic arthritis showing the six domains. (1) Hands: DIP joint swelling with adjacent nail changes (pitting, onycholysis, oil-drop sign) and dactylitis of a finger ("sausage digit"). (2) Feet: Dactylitis of a toe and Achilles enthesitis with tendon thickening at calcaneal insertion. (3) Skin: Plaque psoriasis on extensor elbow surfaces with well-demarcated, erythematous, silvery-scaled plaques. (4) Spine: X-ray showing unilateral sacroiliitis and non-marginal, bulky asymmetric syndesmophytes. (5) Hand X-ray: Pencil-in-cup deformity at DIP joint with adjacent periosteal new bone formation. (6) Nail close-up: Detailed view of pitting, onycholysis, and subungual hyperkeratosis. Label each domain clearly.</image>

Diagnostic Evaluation

Laboratory

Rheumatoid factor is negative in 85 to 90 percent of PsA patients, and RF negativity is itself part of the CASPAR criteria. Anti-CCP is positive in only 5 to 10 percent, substantially lower than in RA. ESR and CRP are elevated in approximately 40 to 50 percent. HLA-B27 should be checked when axial symptoms are present. Uric acid may be elevated because psoriasis increases purine turnover, creating the possibility of coexistent gout.

Imaging

Plain radiographs reveal the distinctive combination of erosions and new bone formation within the same joint, a hallmark that distinguishes PsA from RA. The pencil-in-cup deformity of arthritis mutilans, periosteal reaction producing fluffy periostitis, DIP erosions, and asymmetric sacroiliitis are characteristic findings. MRI demonstrates bone marrow edema, enthesitis, the flexor tenosynovitis underlying dactylitis, and synovitis. Ultrasound is valuable for detecting enthesitis with Power Doppler positivity, synovitis, tenosynovitis, and erosions, and nail ultrasound can reveal thickened nail bed and loss of the normal trilaminar structure.

Disease Activity Assessment

No single gold standard measure exists for PsA disease activity given the disease's heterogeneity. The DAPSA (Disease Activity in Psoriatic Arthritis) score incorporates the swollen joint count, tender joint count, patient pain and global VAS, and CRP, with remission defined as 4 or less. Minimal Disease Activity (MDA) requires meeting 5 of 7 criteria, while Very Low Disease Activity (VLDA) requires all 7. The TICOPA trial supported tight control using an MDA target for improved outcomes.

Management

Treatment Principles

A treat-to-target approach, validated by the TICOPA trial, and a domain-based treatment selection strategy guided by the GRAPPA 2021 and ACR/NPF 2021 guidelines are the cornerstones of PsA management. Both skin and joint disease should be considered in therapeutic decisions.

Conventional DMARDs

Methotrexate at 15 to 25 milligrams weekly is the most commonly used csDMARD and helps skin and peripheral joints, though the MIPA trial did not meet its primary endpoint and the SEAM-PsA trial showed MTX monotherapy was inferior to etanercept. Critically, methotrexate does not help axial disease, enthesitis, or dactylitis. Leflunomide at 20 milligrams daily, supported by the TOPAS trial, is an alternative. Sulfasalazine at 2 to 3 grams daily has modest peripheral arthritis benefit without skin benefit.

Biologic Therapies

TNF inhibitors are the first-line biologic for most PsA domains, with adalimumab, etanercept, infliximab, golimumab, and certolizumab all approved and achieving ACR20 responses of approximately 55 to 65 percent. They are effective across all domains and inhibit radiographic progression. IL-17 inhibitors including secukinumab, ixekizumab, and bimekizumab are effective for all domains including axial disease, with ixekizumab demonstrating superiority to adalimumab for skin clearance in the SPIRIT H2H trial; caution is warranted as they may worsen or unmask IBD. Ustekinumab (IL-12/23 inhibitor) is effective for peripheral arthritis and skin but has less robust axial data. IL-23 inhibitors including guselkumab and risankizumab are effective for peripheral PsA with superior skin clearance but notably failed in axial SpA trials. Abatacept has modest efficacy in PsA. JAK inhibitors including tofacitinib and upadacitinib are effective for PsA, with upadacitinib showing superiority to adalimumab for some endpoints in the SELECT-PsA trials.

PDE4 Inhibitor

Apremilast at 30 milligrams twice daily after titration is an oral option with modest efficacy, achieving ACR20 responses of approximately 40 percent in the PALACE trials. Its advantages include oral administration, no laboratory monitoring requirement, and the absence of traditional immunosuppression. Side effects include diarrhea, nausea, weight loss, and headache, and depression screening is recommended.

Domain-Based Treatment Selection (GRAPPA 2021)

The GRAPPA 2021 guidelines recommend domain-based treatment selection. For peripheral arthritis: MTX followed by TNFi, IL-17i, IL-23i, or JAKi. For axial disease: NSAIDs followed by TNFi, IL-17i, or JAKi (not MTX, not IL-23i). For enthesitis: NSAIDs followed by TNFi, IL-17i, IL-23i, or JAKi (not MTX). For dactylitis: NSAIDs or glucocorticoid injection followed by TNFi, IL-17i, IL-23i, or JAKi (not MTX). For skin: topicals followed by MTX or apremilast, then TNFi, IL-17i, IL-23i, or JAKi. For nails: topicals followed by TNFi, IL-17i, or IL-23i, with nails responding slowly over 6 to 12 months.

<image>A domain-based treatment algorithm for psoriatic arthritis presented as a matrix/grid. Columns represent the six PsA domains: peripheral arthritis, axial disease, enthesitis, dactylitis, skin, and nails. Rows represent treatment classes: NSAIDs, csDMARDs (MTX, LEF, SSZ), apremilast, TNFi, IL-17i, IL-12/23i, IL-23i, JAKi, and abatacept. Use green checkmarks for effective, red X for not effective/not recommended, and yellow dash for limited/modest evidence. Highlight the first-line and second-line recommendations per GRAPPA guidelines for each domain. Include a note that IL-23i is effective in peripheral PsA but NOT in axial SpA.</image>

Key Clinical Pearls

• Nail dystrophy adjacent to DIP arthritis is highly suggestive of PsA over RA
• Dactylitis is pathognomonic for SpA and is primarily due to flexor tenosynovitis, not just joint swelling
• MTX helps peripheral arthritis and skin but is NOT effective for axial disease, enthesitis, or dactylitis
• IL-17 inhibitors are the best choice when both significant skin and joint disease coexist (especially axial)
• IL-23 inhibitors work in peripheral PsA but have failed in axial SpA; mechanism unclear
• Psoriasis increases uric acid turnover; gout and PsA can coexist - aspirate joints when in doubt

References

1. Taylor W, et al. Classification criteria for psoriatic arthritis: development of new criteria (CASPAR). Arthritis Rheum. 2006;54(8):2665-2673.
2. Coates LC, et al. Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA): updated treatment recommendations for psoriatic arthritis 2021. Nat Rev Rheumatol. 2022;18(8):465-479.
3. Singh JA, et al. 2018 ACR/NPF Guideline for the Treatment of Psoriatic Arthritis. Arthritis Rheumatol. 2019;71(1):5-32.
4. McInnes IB, et al. Secukinumab in psoriatic arthritis (FUTURE 2). Lancet. 2015;386(9999):1137-1146.
5. Deodhar A, et al. Guselkumab in patients with active psoriatic arthritis (DISCOVER-2). Lancet. 2020;395(10230):1126-1136.

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