A doctor will work to differentiate psoriatic arthritis from other spondyloarthropathies (such as ankylosing spondylitis, reactive arthritis, and nteropathic arthritis), as well as other conditions to which it closely relates, including rheumatoid arthritis, gout (a.k.a. gouty arthritis), and, to a lesser extent, osteoarthritis.
Because there are no lab or imaging tests that can definitively diagnosis psoriatic arthritis, a keen understanding of the following clinical features of the disease—and how they differ from other forms of arthritis—is essential to rendering an accurate diagnosis.
Joint Distribution
Joint pain and stiffness are often the only outward signs of psoriatic arthritis. For some people, these may be the only symptoms they ever develop. Others may present with more “classic” forms of the disease involving the hands, feet, or spine. Fatigue, swelling, joint deformity, and the restriction of joint function is also common.
Unlike some forms of arthritis in which symptoms develop abruptly (e.g., gout, enteropathic arthritis), the symptoms of psoriatic arthritis tend to develop gradually and worsen over time.
With that being said, as psoriatic arthritis progresses, it can sometimes become symmetrical and manifest with severe symptoms (including, in rare cases, a potentially disfiguring condition known as arthritis mutilans).
Bone Damage
Psoriatic arthritis affects bones differently than other types of arthritis. With psoriatic arthritis, cortical bone (the outer protective surface) will begin to thin and narrow, especially on the fingers and toes. At the same time, new bone will begin to form near the margins of a joint.
The bone changes can cause a “pencil-in-cup” deformity on X-ray in which the tip of the finger is narrowed as the adjoining bone develops a cup-like shape. This is a classic symptom of severe psoriatic arthritis as well as scleroderma.
In contrast, ankylosing spondylitis will cause the excessive formation of new cortical bone, while rheumatoid arthritis will manifest with the erosion of cortical bone and the narrowing of the joint space.
Hands and Feet
One characteristic feature of psoriatic arthritis is dactylitis, the sausage-like swelling of the fingers and toes caused by chronic inflammation. Dactylitis only affects a small proportion of people with psoriatic arthritis but is considered a classic presentation of the disease.
Psoriatic arthritis also tends to affect the distal joints (those nearest the nails) of the fingers and toes. Rheumatoid arthritis tends to affect the proximal (middle) joints, while osteoarthritis can affect any joint in the body.
Psoriatic arthritis can sometimes cause the “opera-glass hand” deformity in which the fingers telescope backward and bend irregularly. It mainly occurs with severe psoriatic arthritis and less commonly with rheumatoid arthritis.
Skin, Nails, and Eyes
Psoriatic arthritis is inextricably linked to the autoimmune skin disease psoriasis. In fact, psoriasis will precede the onset of psoriatic arthritis in roughly 30% of cases, oftentimes as early as 10 years prior. Occasionally, arthritis and psoriasis will appear simultaneously.
Nail abnormalities can sometimes occur with inflammatory or non-inflammatory arthritis. But, with psoriatic arthritis, the signs tend to be more distinctive and include:
“Oil drops” (reddish-yellow spots beneath the nail plate)Spotted lunula (redness in the white arch just above the cuticle)Splinter hemorrhages (vertical black lines under the nail where capillaries have burst)
Spine Involvement
Spondyloarthropathies like psoriatic arthritis can be differentiated from rheumatoid arthritis and gout in that the spine is frequently affected. In fact, the prefix spondylos is derived from the Greek for “spine” or “vertebra.”
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Although rheumatoid arthritis can affect the cervical spine of the neck, spondyloarthropathies can involve the axial spine, spanning the torso to the tailbone.
The condition, referred to as psoriatic spondylitis, can affect up to 35% of people with psoriatic arthritis, according to a 2018 review in Current Rheumatology Reports.
Blood Tests
There are no blood tests that can definitively diagnose psoriatic arthritis. Nevertheless, such tests can help support the diagnosis, characterize the disease, and differentiate it from other forms of arthritis.
One factor associated with spinal spondyloarthropathy is a genetic mutation of the human leukocyte antigen B27 (HLA-B27) gene. Of all people with psoriatic arthritis, 60% to 70% will have the HLA-B27 mutation. More specifically, around 90% of white people with ankylosing spondylitis will have the mutation.
The same is not true for antibody tests used to diagnose rheumatoid arthritis. Rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) are two antibodies commonly used in the diagnosis of rheumatoid arthritis. While the antibodies are sometimes detected in people with psoriatic arthritis, they are almost invariably low and inconsequential.
As inflammatory diseases, blood tests done on those with psoriatic arthritis, rheumatoid arthritis, or gout will reveal elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). These inflammatory markers will not be elevated with osteoarthritis since the disease is not inflammatory.
Differentiation
Because there are no blood or imaging tests that can definitively diagnose psoriatic arthritis, a differential diagnosis may be used to rule out other possible causes. Chief among the investigation are the various forms of arthritis which share similar symptoms.
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