Psoriatic arthritis (PsA) is a chronic inflammatory disease linked to the autoimmune skin condition psoriasis, where the body’s immune system mistakenly attacks healthy tissue. This condition primarily targets the musculoskeletal system, causing pain, swelling, and stiffness in and around the joints. While many people with PsA report what feels like deep muscle pain, the direct answer to whether the disease causes muscle inflammation is generally no. PsA does not typically cause myositis, which is the inflammation of the muscle belly itself, but it does inflame the structures that connect muscles to the skeleton, creating a painful sensation often mistaken for a muscle ache or strain.
Psoriatic Arthritis vs. Muscle Tissue
Psoriatic arthritis is classified as a type of inflammatory arthritis, meaning its primary site of attack is the synovial joint, which includes the joint capsule, cartilage, and the connective tissues surrounding it. The immune response in PsA focuses on these areas, leading to joint damage and limited mobility. This differs fundamentally from inflammatory myopathies like dermatomyositis, where the disease process specifically attacks the large muscle fibers closest to the core of the body, such as those in the hips and shoulders.
The key distinction lies in the tissue being attacked by the immune system. In PsA, the pathology is centered on the joints and the points where ligaments and tendons anchor to the bone, not the contractile tissue of the muscle itself. This focus on connective tissue and bone/joint structures means that PsA is an issue of the mechanical support system, rather than a primary muscle fiber disease. Symptoms of PsA typically begin with joint stiffness and swelling, particularly in the extremities, rather than the profound muscle weakness that characterizes true myositis.
Enthesitis and Tendinitis: The Mimics of Muscle Pain
The most frequent source of pain that mimics a muscle ache in PsA is enthesitis, which is the inflammation of the entheses—the sites where tendons and ligaments insert directly into the bone. The immune-driven inflammation of these anchor points is a hallmark feature of psoriatic disease, affecting up to 50% of patients. When these structures become inflamed, the resulting pain can radiate or feel like a deep, pulling ache, easily misinterpreted by the patient as a strained muscle.
Common locations for enthesitis include the Achilles tendon where it attaches to the heel bone, the plantar fascia on the sole of the foot, and around the elbows and ribs. The inflammation at the Achilles insertion, for example, can cause severe heel pain that feels like a constant strain on the calf muscle during movement. Similarly, tendinitis, which is inflammation of the tendon itself, can cause localized pain near a joint that is often localized near the associated muscle groups. A classic example is dactylitis, sometimes called “sausage digit,” which is an inflammation of all the soft tissues, including the entheses and tendons, throughout an entire finger or toe.
Secondary Causes of Muscle Aches
While PsA does not directly cause primary muscle inflammation, the chronic nature of the disease creates several indirect pathways that lead to genuine muscle aches, known as myalgia. The persistent joint pain and stiffness lead to a protective reduction in physical activity, which causes deconditioning over time. This inactivity results in muscle atrophy and weakness, so when a person with PsA does exert themselves, their muscles ache much sooner than they otherwise would.
The body often attempts to compensate for painful or damaged joints, leading to altered biomechanics and posture. If a knee joint is painful, a person may shift their gait, causing undue stress and strain on compensating muscle groups in the hip or back. This chronic overuse of muscles attempting to stabilize an unstable joint results in actual muscle soreness and spasms. Furthermore, the systemic inflammatory burden of PsA often contributes to profound, pervasive fatigue that is not relieved by rest, and this systemic fatigue frequently manifests as a generalized, flu-like muscle soreness throughout the body. The coexistence of conditions like fibromyalgia, which causes widespread musculoskeletal pain, is also common and contributes to this generalized muscle aching.
Managing Musculoskeletal Pain in PsA
The most effective way to manage the complex musculoskeletal pain associated with PsA is to treat the root cause, which is the underlying systemic inflammation. This often involves the use of disease-modifying anti-rheumatic drugs (DMARDs) and biologic therapies, which work to calm the overactive immune system and reduce inflammation in the joints and entheses. By controlling the primary disease activity, the pain from enthesitis and secondary muscle strain can be significantly reduced.
Non-steroidal anti-inflammatory drugs (NSAIDs) can be used to provide targeted relief for localized pain and inflammation, though they do not modify the disease course. Physical therapy plays a role in addressing the secondary causes of muscle pain by focusing on strengthening deconditioned muscles and correcting altered movement patterns. Therapists help patients regain strength and flexibility through low-impact exercises, which reduces the strain placed on compensating muscle groups. Targeted measures like applying heat or ice can also help soothe localized muscle aches and tension that result from chronic pain and poor posture.