Clinical patterns
Important clinical features
Prevalence
Asymmetric oligoarthritis
≤4 joints involved
Asymmetric in distribution
Most common
~20–55% [5]
Symmetric polyarthritis
≥5 joints involved
Symmetric in distribution
Similar to rheumatoid arthritis
~20–62% [5]
Predominant DIP involvement disease
Typically DIPs are involved
May or may not have concurrent nail involvement [16]
Distribution similar to DIP OA
Axial spondyloarthritis
Sacroiliitis as well as other areas of spinal involvement
Similar to other seronegative SpA
May or may not be associated with peripheral PsA
Can be asymptomatic
Arthritis mutilans (destructive)
Rare
Asymmetric
Severe osteolysis of involved joints resulting telescoping of digits
Severe disease
~5% [2]
Dactylitis
Inflammation of IP joints and surrounding tendon sheath resulting in classic sausage-shaped swelling
Characteristic feature
Most common in 3rd and 4th toes
Can be acute or chronic
Often associated with severe disease with bone erosion
40–50% [1]
Enthesitis
Most commonly Achilles tendinitis and plantar fasciitis
Other less common areas: iliac crest, around the patella, epicondyles, etc.
30–50%
Classification criteria of peripheral psoriatic arthritis (CASPAR = classification for psoriatic arthritis)
Patient with inflammatory articular diseases (joints, spine, or enthesitis) with ≥3 points from the following five categories: | |
---|---|
Criterion | Points |
1. Evidence of psoriasis | |
Current psoriasis (examined by dermatologist or rheumatologist) | 2 |
Personal history of psoriasis (provided by patients or physician) | 1 |
Family history of psoriasis (1st or 2nd degree relative) | 1 |
2. Psoriatic nail dystrophy (onycholysis, pitting, or hyperkeratosis) present currently | 1 |
3. Negative test for RF by any test except latex (ELISA and nephelometry preferred) | 1 |
4. Dactylitis | |
Current dactylitis | 1 |
History of dactylitis (according to a rheumatologist) | 1 |
5. Radiographic features of juxtaarticular new bone formation (ill-defined ossification near joint margin, excluding osteophytes) | 1 |
Laboratory Features
There are no definitive laboratory tests for psoriatic arthritis. Inflammatory markers (i.e., erythrocyte sedimentation rate and C-reactive proteins) can be elevated on polyarticular diseases but remains normal in ~60% of patients. Absence of rheumatoid factor (RF) and cyclic citrullinated peptide (CCP) antibodies is seen in 95% of cases and is helpful to distinguish from RA, especially when PsA presents in a similar fashion. HLA-B27 is present in ~25% of patients.
Radiological Features
Plain X-Ray
Musculoskeletal Ultrasound (MSKUS)
MSKUS is particularly helpful in detecting enthesitis in PsA as it shows entheseal thickening, changes in echogenicity of tendons (hypoechoic), and increased power Doppler signals indicating increased vascularity. In addition, it shows erosions and enthesophytes around the entheses. It can also show combination of synovitis and tenosynovitis that leads to dactylitic changes of a digit. Studies further suggest MSKUS is more sensitive than in clinical examination to detect synovitis [18, 19]. This could be helpful to detect subclinical or preclinical synovitis in patients with skin psoriasis.
Magnetic Resonance Imaging (MRI)
Resolution of MRI is excellent to detect bone marrow edema seen at the site of entheseal inflammation, especially in acute sacroiliitis or other areas of spinal involvement (Fig. 7.6b). MRI is also employed to detect focal bony erosions, synovitis of peripheral joints, as well as entheseal involvement (such as Achilles tendinitis and others).
Differential Diagnosis
Differential diagnosis of PsA and distinguishing features
Clinical features | |||||
---|---|---|---|---|---|
Features | PsA | RAa | ReAb | OAc | Gout |
Pattern of joint involvement | Asymmetric (more common) and symmetric | Symmetric | Asymmetric | Asymmetric or rarely symmetric | Asymmetric or rarely symmetric (esp. polyarticular disease) |
Number of joints involved | Oligoarticular most common. Polyarticular less common | Polyarticular | Oligoarticular | Monoarticluar or oligoarticular | Monoarticluar or oligoarticular |
Unique joint involved | DIP | Proximal (never DIP) | DIP (associated with Heberden’s nodes) | ||
Dactylitis | Unique feature (common) | Not seen | Present but uncommon | Not seen | Confused with an acute gout flare |
Skin involvement | Psoriasis | Rheumatoid nodules | Keratoderma blennorrhagicum | No associated skin disease | Tophi |
Axial (spinal and SIJ) | Present in ~50% cases (inflammatory) | Uncommon and never involves SIJ | Present in 100% cases (inflammatory) | Common (degenerative etiology) | Absent |
Lab features | |||||
RF and CCP | Only <5% cases | Present | Absent | Absent | Absent |
Hyperuricemia | Can be present as an associated comorbidity | Less common association | Less common | Comorbidity | Unique feature |
Radiological features | Erosion and pathological new bone formation (enthesophytes) | Erosion but no pathological new bone information | Enthesophytes are seen | Osteophytes | Erosions (juxtaarticular) |
Comorbidities
In addition to the skin and joint manifestations, psoriatic arthritis is associated with a variety of comorbidities that are associated with increased morbidity and excess mortality. Patients have higher prevalence of traditional cardiovascular risk factors (i.e., hypertension, hyperlipidemia, diabetes mellitus, and obesity) as well as increased prevalence of myocardial infarction and stroke that is independent of these risk factors [20–23]. There is also an increased prevalence of inflammatory bowel disease (particularly Crohn’s disease) as well as ophthalmologic manifestations, uveitis being the most common [24–26]. Furthermore, these patients also suffer from higher rates of depression and anxiety, which may impact psoriatic clinical outcomes [27, 28].
Pathogenesis
PsA results from a complex interplay of genetic and environmental factors. Twin studies in psoriasis showed a threefold higher concordance rate in monozygotic twins than dizygotic twins [29]. Furthermore, prevalence of psoriasis and psoriatic arthritis in first-degree relatives of patients with PsA is much higher than general population [30]. Unlike RA, psoriasis and PsA are associated with major histocompatibility complex (MHC) class I alleles. HLA-C∗w6 has been found in 60% patients with psoriasis, but the association is not strong in arthritis. Association of PsA with HLA-B∗08, B∗27, B∗38, and B∗39 have been observed. Additionally, genome-wide association studies have observed association of PsA with single nucleotide polymorphisms (SNP) of IL23 receptor (IL23R), IL12A, IL12B, nuclear factor κB (NF-κB), and protein tyrosine phosphatase (PTPN22) [31–33]. These findings highlight the importance of heritability but risk is substantially lower than monogenic diseases and thereby indicates environmental role in PsA pathogenesis.
Similar to Koebner’s phenomenon observed in skin psoriasis [34], a deep Koebner’s phenomenon (trauma to deep tissues or entheses) is thought to play a role in PsA [35, 36]. Severe skin disease, nail psoriasis, genital psoriasis, and obesity are among other environmental triggers that play a role in initiation of PsA [37].
T cells, especially CD8+ and CD4+ Type 17 (Th17) play a pivotal role in PsA pathogenesis. Macrophage-/monocyte-derived tumor necrosis factor-α (TNF-α) in synovial fluid plays a critical role, as evident by therapeutic efficacy of TNF-α inhibitors in psoriasis and PsA. More recent work suggests entheseal sites being the initial sites of initial inflammation leading to synovitis of psoriatic arthritis. A novel population of T cells (CD3+CD4−CD8−) resident to entheses responds to circulatory IL23 [38] and elaborates IL17 and IL22. Synovia of PsA patients are particularly enriched with Th17 cells, and downstream complex effect of IL23/Th17 axis leads to joint inflammation and erosion [39]. A recent novel murine model (STAT3 overexpression) of PsA further highlights the role of Th17 cell types on PsA pathogenesis [40].
Similar to other subtypes of seronegative SpA, gut-joint axis plays an important role. Up to 70% patients with PsA were shown to have subclinical or microscopic gut inflammation. Moreover, intestinal microbiome is presumed to regulate immune system via Th17 pathway—a key cell population in PsA pathogenesis. Perturbation of intestinal microbiota (specifically lower relative abundance of Akkermansia and Ruminococcus) was shown to be associated with PsA [41].
Treatment
Medications used in the treatment of psoriatic arthritis
Medication | Joints | Skin | Radiography | Side effects |
---|---|---|---|---|
++ | + | NA | GI upset, hepatic effects | |
+ | − | NA | GI upset, renal effects | |
+ | − | NA | GI upset, neutropenia | |
PDE4 inhibitor: | ++ | + | NA | GI upset, weight loss |
TNFi | ||||
Adalimumab [50] | +++ | ++ | ++ | Infections, demyelination, lymphoma, nonmelanoma cancer |
Certolizumab [51] | +++ | ++ | ++ | Infections, demyelination, lymphoma, nonmelanoma cancer |
Etanercept [52] | +++ | + | ++ | Infections, demyelination, lymphoma, nonmelanoma cancer |
Golimumab [49] | +++ | ++ | ++ | Infections, demyelination, lymphoma, nonmelanoma cancer |
+++ | +++ | ++ | Infections, demyelination, lymphoma, nonmelanoma cancer | |
IL17 inhibitors | ||||
Ixekizumab [54] | +++ | ++++ | + | Candida infections, injection site reactions |
Secukinumab [55] | +++ | ++++ | + | Candida infections |
IL12/23 inhibitor: Ustekinumab [53] | +++ | ++++ | + | Infections |
JAK inhibitor: Tofacitinib [57] | +++ | + | NA | Infections, zoster, increased LDL |
Abatacept [58] | ++ | + | + | Infections, headache |