Types of arthritis Explain psoriatic arthritis

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types of arthritis classification 2024 overview

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Types of Arthritis

Arthritis is a broad term covering over 100 conditions involving joint inflammation or degeneration. The primary classification divides arthritis into noninflammatory and inflammatory categories.

Classification of arthritis: noninflammatory vs inflammatory types

Arthritis Classification - Frameworks for Internal Medicine

Noninflammatory Arthritis

These conditions cause joint damage without significant immune-mediated inflammation:

Type	Key Features
Osteoarthritis	Most common; cartilage degeneration; age-related
Traumatic arthritis	Post-injury joint damage
Hemarthrosis	Bleeding into the joint (e.g., hemophilia, trauma)
Charcot joint (neuropathic)	Joint destruction from loss of pain sensation
Osteonecrosis	Bone death due to vascular insufficiency
Hypertrophic osteoarthropathy	Periosteal new bone formation; often paraneoplastic
Acromegaly	GH excess causing cartilage and bone overgrowth
Hemochromatosis	Iron deposition in joints

Inflammatory Arthritis

Characterized by synovial inflammation, warmth, swelling, and stiffness. Subdivided by joint count:

Monoarticular (1 joint)

Crystal arthropathy: Gout (monosodium urate - needle-shaped, negatively birefringent crystals) and Pseudogout/CPPD (rhomboid-shaped, weakly positively birefringent crystals)
Acute infectious (septic) arthritis - S. aureus and N. gonorrhoeae are most common
Chronic infectious arthritis (Lyme disease, TB, fungal)

Oligoarticular (2-4 joints)

Psoriatic arthritis (early/asymmetric form)
Reactive arthritis
Juvenile idiopathic arthritis (pauciarticular)
Early spondyloarthropathy

Polyarticular (5+ joints)

Rheumatoid arthritis (RA) - symmetric, RF+, erosive
Psoriatic arthritis (symmetric polyarthritis subtype)
Ankylosing spondylitis - axial predominant
Systemic lupus erythematosus (SLE)
Reactive arthritis (formerly Reiter's syndrome)
Enteropathic arthritis (IBD-associated)
Juvenile idiopathic arthritis (polyarticular)

Psoriatic Arthritis (PsA)

Definition and Epidemiology

Psoriatic arthritis is an inflammatory seronegative spondyloarthropathy that occurs in approximately 1 in 4 patients with psoriasis (up to 40% in some series). It can precede, follow, or occur simultaneously with skin disease - the extent of cutaneous psoriasis correlates poorly with arthritis development. Age of onset ranges from 30 to 55 years, with equal sex distribution for peripheral disease; psoriatic spondylitis has a slight male preponderance. Obesity is a significant risk factor. - Goldman-Cecil Medicine

Pathobiology

The synovitis histology resembles other spondyloarthropathies - notably, rheumatoid factor is absent (seronegative), distinguishing it from RA
Genetic associations are complex: HLA-Cw6 is dominant in psoriasis; HLA-B27 and HLA-B39 are associated with sacroiliitis and axial involvement
30-50% of patients with PsA are HLA-B27 positive
No specific etiologic agent has been confirmed; the pathogenesis involves dysregulated T-cell activation, TNF-alpha, IL-17, and IL-23 pathways

Clinical Subtypes (Moll and Wright Classification)

Five patterns are recognized, per Andrews' Diseases of the Skin:

Subtype	Frequency	Features
1. Asymmetric oligoarthritis	~70%	Swelling and tenosynovitis of one or a few hand joints - most common form
2. Asymmetric DIP involvement	~16%	Distal interphalangeal joints + nail damage
3. Symmetric polyarthritis	~15%	Mimics RA; "clawhand"; but RF negative
4. Psoriatic spondylitis	~2-4%	Axial disease (sacroiliitis, syndesmophytes); can be sole manifestation
5. Arthritis mutilans	~5%	Severe osteolysis; highly destructive; marked disability

Key Clinical Features

Dactylitis ("sausage digit") - seen in 40-50% of patients; diffuse swelling of an entire digit; associated with more severe radiographic joint damage.

Enthesitis - inflammation at tendon/ligament insertions; present in 30-50%; most common sites are the Achilles tendon and plantar fascia.

Nail changes - onycholysis, subungual keratosis, pitting, and "oil drop" staining; highly associated with DIP joint involvement.

Axial disease - asymmetric sacroiliitis (vs. bilateral in ankylosing spondylitis); nonmarginal, chunky syndesmophytes (vs. marginal "bamboo spine" in AS).

Radiological Features

X-rays of psoriatic arthritis showing pencil-in-cup deformity and dactylitis in feet and hands

X-rays of the feet (A) and hands (B) in psoriatic arthritis - Harrison's Principles of Internal Medicine (22nd Ed, 2025)

Characteristic radiographic findings that distinguish PsA from RA include (Andrews' Diseases of the Skin, Goldman-Cecil):

"Pencil-in-cup" deformity - tapering/whittling of phalanges with cupping of proximal ends; classic for arthritis mutilans/DIP disease
Acrosteolysis - erosion of terminal phalangeal tufts
Bony ankylosis - fusion in a ray distribution
"Fluffy" periosteal new bone formation at erosion sites
Asymmetric sacroiliitis (vs. bilateral in AS)
Relative sparing of MCP/MTP joints (predilection for DIP/PIP joints)
Soft tissue swelling (especially in dactylitis)

Diagnosis - CASPAR Criteria

The Classification of Psoriatic Arthritis (CASPAR) criteria require:

Inflammatory articular disease (joint, spine, or entheseal), plus
At least 3 points from 5 categories (current psoriasis = 2 pts; history of psoriasis = 1 pt; nail changes = 1 pt; negative RF = 1 pt; dactylitis = 1 pt; juxta-articular new bone formation = 1 pt)

Specificity: 98.7%, Sensitivity: 91.4% - Harrison's Principles of Internal Medicine (22nd Ed)

Differential diagnosis includes RA, seborrheic dermatitis (skin), dyshidrotic eczema, fungal infection, keratoderma blennorrhagicum.

Treatment

Management follows a treat-to-target approach. (Goldman-Cecil Medicine, Andrews' Diseases of the Skin)

Step 1 - Symptomatic relief

NSAIDs - first-line for musculoskeletal symptoms; used at highest tolerable doses

Step 2 - Conventional DMARDs (for swollen joints, structural damage, or extra-articular features)

Methotrexate (up to 25 mg/week)
Sulfasalazine (up to 3 g/day)
Leflunomide
Cyclosporine / Tacrolimus

Note: Conventional DMARDs are NOT effective for axial disease

Step 3 - Biologic DMARDs (if conventional DMARDs fail or active disease persists after ~8 weeks of NSAIDs)

Drug Class	Agents
Anti-TNF	Infliximab, Etanercept, Adalimumab, Golimumab, Certolizumab
Anti-IL-17A	Secukinumab, Ixekizumab
Anti-IL-12/23	Ustekinumab (45 mg SC at 0 & 4 wks, then q3 months)
Anti-IL-23 (p19)	Guselkumab (100 mg SC at 0 & 4 wks, then q8 weeks)

Step 4 - JAK inhibitors (oral targeted synthetic DMARDs)

Tofacitinib 5 mg twice daily
Upadacitinib 15 mg once daily

Adjuncts

Apremilast (PDE4 inhibitor) - useful for mild-moderate disease
Local glucocorticoid injections for specific joint flares
Topical corticosteroids for mild skin disease
Rest, splinting, passive motion for joint protection

Screening before biologics/JAK inhibitors: Latent TB, Hepatitis B, Hepatitis C testing required before initiating therapy.

Comorbidities

Patients with psoriasis and PsA have increased risk of:

Metabolic syndrome (2.9x more frequent in psoriasis patients)
Cardiovascular disease (hypertension in ~35%, hyperlipidemia in ~30%)
Myocardial infarction (especially in younger patients with severe disease)
Rheumatoid arthritis, Crohn's disease, ulcerative colitis
Hodgkin's and cutaneous T-cell lymphoma
Depression and psychosocial impairment

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