Psoriatic Arthropathy

Psoriatic arthritis (PsA) is a chronic, seronegative inflammatory arthritis associated with psoriasis, belonging to the spondyloarthritis family, characterised by enthesitis, dactylitis, axial disease, nail disease and skin psoriasis.

It affects roughly 20–30% of people with psoriasis; overall prevalence is about 0.1–0.25% of the population, with an equal sex distribution (unlike RA).

Peak onset is 30–50 years, but elderly-onset PsA (after 60) is recognised and characteristically has more acute onset, higher inflammatory markers, more polyarticular and erosive disease, and more constitutional upset — and is frequently misdiagnosed as late-onset RA, PMR or gout.

In about 85% the skin disease precedes the arthritis (often by around 10 years); in 10–15% arthritis comes first or they appear together — so always look for hidden psoriasis in the scalp, retroauricular area, umbilicus, natal cleft and nails.

Genetic: strong familial clustering; HLA-B27 associates with axial disease, HLA-C*06 with skin-predominant disease, and HLA-B38/B39 with peripheral arthritis.

Immunological: the IL-23 / Th17 axis is central — IL-23 drives Th17 and IL-17-producing resident entheseal cells; TNF-α, IL-17A/F and IL-22 mediate synovitis, enthesitis and new bone formation.

The enthesis is the primary site of disease (unlike the synovium in RA); mechanical stress ("deep Koebner phenomenon") at entheses triggers inflammation, explaining dactylitis and the pattern of joint involvement.

Uniquely, PsA shows both erosion and new bone formation — periostitis, enthesophytes, ankylosis and, at the extreme, arthritis mutilans.

(i) Asymmetrical oligoarthritis (≤4 joints) — classically described as the commonest at presentation.

(ii) Symmetrical polyarthritis — RA-like but seronegative, with DIP involvement; the commonest pattern in established disease.

(iii) Distal interphalangeal predominant — strongly associated with nail disease of the same digit.

(iv) Spondylitis and sacroiliitis — often asymmetrical, with inflammatory back pain and chunky non-marginal paravertebral ossification.

(v) Arthritis mutilans — rare, severely destructive, with telescoping "opera-glass" digits from osteolysis.

Enthesitis — Achilles tendon insertion, plantar fascia, lateral epicondyle, patellar tendon and iliac crest; heel pain in an older adult with psoriasis is enthesitis until proved otherwise.

Dactylitis ("sausage digit") — uniform swelling of the whole digit from combined tenosynovitis, synovitis and soft-tissue oedema; occurs in about a third and marks more severe disease.

Nail disease in up to 80–90% of those with arthritis: pitting, onycholysis, subungual hyperkeratosis, oil-drop sign, splinter haemorrhages, ridging — the single most useful clue when skin disease is inapparent.

Extra-articular: anterior uveitis (often insidious, bilateral and posterior, unlike the acute unilateral uveitis of ankylosing spondylitis), inflammatory bowel disease, and aortic root disease.

Comorbidity burden — important in geriatrics: a high prevalence of metabolic syndrome, obesity, type 2 diabetes, hypertension, fatty liver, hyperuricaemia and gout, and a significantly increased cardiovascular risk; also depression and osteoporosis. These comorbidities dictate drug choice and must be actively screened.

Seronegativity: rheumatoid factor and anti-CCP are typically negative — but a low-titre RF may be present in older adults from age alone, and about 5–10% of PsA is anti-CCP positive (marking more erosive disease).

ESR and CRP may be raised, but are normal in up to half despite active disease; serum urate is often high, which confuses the picture with gout.

Radiographs: the hallmark is erosion with adjacent new bone formation — the "pencil-in-cup" deformity, acro-osteolysis, fluffy periostitis, ankylosis, asymmetrical sacroiliitis and non-marginal syndesmophytes. Absence of periarticular osteopenia helps distinguish it from RA.

Ultrasound with power Doppler and MRI detect enthesitis, dactylitis and early sacroiliitis before radiographic change.

CASPAR classification criteria (score ≥3 in the presence of inflammatory articular disease): current psoriasis (2 points), personal or family history of psoriasis (1), psoriatic nail dystrophy (1), negative rheumatoid factor (1), current or past dactylitis (1), and juxta-articular new bone formation on radiograph (1). Specificity approaches 99%.

Screening tools such as PEST and PASE are used in dermatology clinics to detect PsA early; MDA (minimal disease activity) and DAPSA are the treat-to-target measures.

Differential diagnosis in the elderly: late-onset rheumatoid arthritis, erosive/nodal osteoarthritis of the DIP joints (Heberden's nodes — no inflammation, no nail change), gout and CPPD, reactive arthritis, and PMR.

General: patient education, smoking cessation, weight reduction (obesity worsens disease and blunts drug response), structured exercise, physiotherapy for enthesitis and axial disease, occupational therapy and assistive devices, plus aggressive cardiovascular and metabolic risk management.

Peripheral arthritis: NSAIDs for mild disease (used sparingly in the elderly); local corticosteroid injection for mono/oligoarthritis; then csDMARDs — methotrexate (also treats skin), leflunomide, sulfasalazine (not for skin or axial disease); avoid ciclosporin in older adults.

Inadequate response: escalate to biologic or targeted synthetic DMARDs — TNF inhibitors (adalimumab, etanercept, infliximab, golimumab, certolizumab), IL-17 inhibitors (secukinumab, ixekizumab, and the IL-17A/F inhibitor bimekizumab), IL-12/23 inhibitor ustekinumab, IL-23 inhibitors (guselkumab, risankizumab), JAK inhibitors (tofacitinib, upadacitinib), the TYK2 inhibitor deucravacitinib, the PDE-4 inhibitor apremilast, and abatacept.

Domain-directed choice (GRAPPA): for axial disease, NSAIDs, physiotherapy and simple analgesia are strongly recommended, as are a JAK inhibitor, TNF inhibitor or IL-17 inhibitor — csDMARDs do not work for axial disease. For enthesitis, strongly recommended options include a TNF inhibitor, an IL-12/23, IL-17 or IL-23 inhibitor, a JAK inhibitor or a PDE-4 inhibitor. For dactylitis, GRAPPA strongly recommends a TNF inhibitor, an IL-12/23, IL-17 or IL-23 inhibitor, a JAK inhibitor or a PDE-4 inhibitor.

Geriatric prescribing considerations: apremilast is attractive in frail patients for its safety profile but causes diarrhoea and weight loss; JAK inhibitors are restricted in those over 65 with cardiovascular risk factors or a smoking history (ORAL Surveillance); IL-17 inhibitors are avoided in inflammatory bowel disease and predispose to candidiasis; all biologics require latent tuberculosis and hepatitis screening, which is essential in India, plus vaccination and infection vigilance.

Treat-to-target aiming for minimal disease activity, with therapy tapering considered in sustained remission; surgery (synovectomy, arthroplasty, fusion) for established joint damage.

Prognosis: around 20% develop severe destructive disease; poor prognostic markers are polyarticular onset, raised ESR/CRP, early radiographic damage, dactylitis and nail disease. Excess mortality is largely cardiovascular.