Ankylosing Spondylitis (AS)

Ankylosing spondylitis is primarily characterized by inflammation of the spine and large joints. This inflammation can lead to pain and stiffness, particularly in the lower back and hips. Over time, AS can cause the vertebrae to fuse, resulting in a loss of flexibility and a hunched-forward posture Mayo Clinic. Muscle inflammation in AS is less common but can occur, leading to muscle stiffness and pain. A study found that muscle changes in AS patients included reduced fiber size and altered muscle enzyme activity, which may contribute to muscle weakness and fatigue SpringerLink.

Psoriatic Arthritis (PsA)

Psoriatic arthritis is associated with psoriasis, a skin condition that causes red, scaly patches. PsA can affect any joint in the body and is often accompanied by swelling of the fingers and toes, known as dactylitis. Muscle inflammation in PsA can lead to muscle pain and weakness, which may be exacerbated by reduced physical activity due to joint pain Healthline. The presence of proinflammatory T cells in the synovial fluid of PsA patients suggests a robust immune response contributing to joint and muscle inflammation PubMed.

Key Differences

While both AS and PsA involve joint inflammation, AS is more likely to affect the spine, leading to potential fusion of the vertebrae. In contrast, PsA is often associated with skin symptoms and can affect a wider range of joints, including those in the hands and feet. Muscle inflammation in AS tends to be more related to structural changes in the spine, whereas in PsA, it is often linked to systemic inflammation and skin involvement Cleveland Clinic Cleveland Clinic.