Psoriatic arthritis and rheumatoid arthritis share some common symptoms.
Both psoriatic arthritis (PsA) and rheumatoid arthritis (RA) are inflammatory joint conditions with a wide variability of clinical features, and both may involve multiple systems of the body. Both may affect the eyes, the skin, the joints, and the cardiovascular system. However, their differences go deeper and they require different treatments with a clear divergence in the expected response to various specific modes of therapy and prognosis.
The diseases are similar in that patients with both conditions complain of joint pain, swelling and tenderness over the proximal interphalangeal joints of the fingers and the metacarpophalangeal joints (in 70% and 80% of PsA and RA respectively). The microscopy often shows symmetrical involvement of the synovial membrane in both cases, though asymmetrical joint involvement is also common in PsA.
PsA and RA show deep underlying differences, such as:
PsA is an inflammatory disease affecting a few joints of the spine, the sacroiliac joints, and the peripheral skeleton; RA characteristically shows peripheral joint involvement.
PsA typically has a less severe clinical picture than RA, but axial skeletal involvement including sacroiliac arthritis with a strong resemblance to ankylosing spondylitis is typical. Thus it is a spondyloarthropathy. DIP arthritis is characteristic in PsA, seen in 20-60% of patients.
Type of Bone Erosion
The type of bone lesion in PsA is a cortical erosion of significant size with indistinct margins, often just outside the joint surface of the bone with evidence of new bone formation, whereas the RA lesion is well-defined and located at the joint margin.
Presence of Enthesitis
PsA is characterized by enthesitis leading to entheseal osteogenesis with synovitis, compared to synovitis alone in RA as seen with MRI and micro-CT scanning.
The pathogenesis of the conditions is distinct, with PsA being part of the seronegative spondyloarthropathies, while RA shows the presence of rheumatoid factor (RF) or anti-citrullinated protein antibodies (ACPA) in circulation.
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