How Psoriatic Arthritis and Rheumatoid Arthritis Differ

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Psoriatic arthritis and rheumatoid arthritis are both autoimmune diseases that affect the joints. Both are inflammatory and progressive, causing joint stiffness, pain, and swelling as well as chronic fatigue. Both occur in flares and can be treated with many of the same medications and therapies.

So, while it may seem reasonable to assume that they are basically the same disease, they are not. In fact, the differences in symptoms and causes are so distinctive as to require different approaches to diagnosis and treatment.

Interestingly, until the 1950s, psoriatic arthritis was simply regarded as psoriasis that co-occurred with rheumatoid arthritis. Over time, as scientists gained a greater understanding of the mechanisms of autoimmune diseases, the two diseases were gradually seen as being clinically distinct.

It was only in 1964 that psoriatic arthritis was finally classified as a unique disease entity by the American Association of Rheumatism (now the American College of Rheumatology).

Symptoms

One of the main differences between psoriatic arthritis (PsA) and rheumatoid arthritis (RA) is the distribution of affected joints. Both diseases can cause destruction of the small joints in the hands and feet as well as the larger joints of the knees, hips, shoulders, and spine. 

However, with PsA, the pattern of joint involvement is most often asymmetrical, meaning that they occur sporadically and are not mirrored on the other side of the body. With RA, the pattern is characteristically symmetrical, meaning that the same joints on both sides of the body are affected.

(This is not always the case, of course. In fact, as many of 15 percent of people with PsA will have symmetrical arthritis, a condition considered more advanced and severe than asymmetrical arthritis. It also makes the differentiation of PsA and RA all the more difficult.)

Another notable difference between PsA and RA is the involvement of the spine. PsA will often manifest with arthritis in the axial spine of the trunk, while RA will mostly be limited to the cervical spine of the neck.

It is for this reason that PsA is included in the body of disorders called spondyloarthropathies and RA is not.

Bone Damage

Of the two diseases, RA is arguably more severe. Bone erosion is a central feature of RA, causing localized and irreversible bone loss (osteolysis) as well as joint disfigurement and the loss of joint function.

The same can occur with PsA, but the effects tend to be far less profound. Much of the bone loss is limited to the distal phalanges (the bones nearest the fingernails or toenails). It is only when an uncommon form of the disease (called arthritis mutilans) occurs that joint disfigurement can develop rapidly and severely.

Fingers, Toes, and Skin

Another telling clue is the presentation of the disease on the fingers and toes. With PsA, the distal joints (those nearest the nails) will be the focus of pain, swelling, and stiffness. By contrast, RA primarily involves the proximal joints (those situated just above the knuckles).

With severe PsA, the fingers can also take on a sausage-like appearance (called dactylitis), making it difficult to ball your fist. While this can occur with RA, it is not the hallmark that it is with PsA.

Around 85 percent of people of PsA with also have psoriasis (characterized by dry, flaky skin plaques). Moreover, half will have nail psoriasis at the time of their diagnosis. Neither of these occurs with RA.

Causes

Autoimmune diseases are those in which the immune system mistakenly attacks normal cells and tissues. It does so by producing immune proteins, called antibodies, that target receptors, called antigens, on the surface of cells. If the antibodies are "misprogrammed," they can target normal rather than abnormal cells. These are referred to as autoantibodies.

Although PsA and RA both affect the joints, the actual targets of the immune assault differ considerably.

Rheumatoid Arthritis

With RA, the primary target of the autoimmune assault is the joints, most specifically the cells in the lining of the joint called synoviocytes. The ensuing inflammation will cause synoviocytes to proliferate abnormally, resulting in a cascade of events, including;

  • The thickening of the joint lining (synovial hyperplasia)
  • The infiltration of inflammatory proteins (called cytokines) into the joints
  • The progressive destruction of joint cartilage, bone, and tendons

Psoriatic Arthritis

With PsA, the inflammatory assault is indirect. Instead of synoviocytes, the immune system will target skin cells called keratinocytes. When this occurs, the cells will proliferate at an accelerated rate, leading to the development of psoriasis in most (but not all) cases.

Over time, the persistent inflammation will start to affect other organ systems, including the nails, eyes, brain, kidneys, and pancreas. When it affects the joints, PsA can occur.

Although synovial hyperplasia is also characteristic of PsA, it tends to be less severe than RA. This is likely due to the "indirect" inflammatory assault on the joints, as opposed to RA in which the assault is intense and direct.

While this may suggest that PSA is simply the consequence of psoriasis, there are some who believe that they are two distinct diseases with different genetic or environmental causes. Others argue that PSA and psoriasis are, in fact, one disease better classified under the unified title psoriatic disease.

Diagnosis

Comparatively speaking, doctors have the tests, tools, and diagnostic criteria needed to render a definitive diagnosis of RA. The same cannot be said of PsA.

Rheumatoid Arthritis

If RA is suspected, the doctor will order the following tests see if the results meet the diagnostic criteria established by the American College of Rheumatology (ACR):

  • Autoantibody blood tests, including rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) autoantibodies found in the majority of people with RA
  • Inflammatory blood markers, including C-reactive protein (CRP) and erythrocyte sedimentation (ESR) which measure inflammation
  • Imaging tests, like X-rays and magnetic resonance imaging (MRI), which look for bone erosion and the narrowing of the joint space

The results of the tests—as well as the duration, location, and severity of symptoms—are then scored on the ACR classification system. A cumulative score of 6 to 10 offers a high degree of confidence that RA is the cause of your symptoms.

Psoriatic Arthritis

Unlike RA, PsA is mainly diagnosed with a physical exam and a review of your medical history. There are no blood tests or imaging studies that can definitively diagnose the disease. Instead, the doctor will look for clues that are strongly indicative of PsA, including:

  • Asymmetrical joint involvement
  • Skin involvement
  • Nail involvement
  • A family history of PsA and/or psoriasis
  • Instigating factors known to trigger the disease, including strep infections, certain medications, and cold, dry weather

An X-ray or MRI may be able to detect a so-called "pencil-in-a-cup" deformity, in which the tip fo the finger looks like a sharpened pencil and the adjacent bone is worn down to a cup-like shape. However, the deformity only affects around 5 percent to 15 percent of people with PsA and mostly in the more advanced stages of the disease.

If the skin is affected, a tissue biopsy can provide strong evidence of PsA. Under the microscope, psoriatic skin cells will appear acanthotic (compressed) unlike eczema, cancer, or other skin diseases.

Other lab and imaging tests are used mainly to exclude other possible causes rather than to confirm PsA. This process of elimination, known as a differential diagnosis, may include the investigation of similar arthritic diseases, including:

Treatment

PsA and RA are often treated with the same medications and therapies, albeit to varying degrees of success. Exercise, weight loss, and smoking cessation are standardly addressed as facets of treatment. Mild to moderate symptoms are commonly treated with nonsteroidal anti-inflammatory drugs (NSAIDs), both over-the-counter and prescription.

Where treatment approaches diverge in four specific areas:

Corticosteroids

Corticosteroids are a type of drug used temper inflammation. Prednisone is the most commonly used corticosteroid, taken either in pill form or injected into a joint to provide short-term relief. Their use in treatment varies by disease:

  • With PsA, corticosteroids are sometimes used during acute flares when symptoms are severe. However, they are used with caution as they can trigger a severe form of psoriasis known as Von Zumbusch pustular psoriasis.
  • With RA, low-dose corticosteroids are often prescribed in combination with other drugs. They are intended for short-term use to avoid side effects. Corticosteroids can also be injected into a joint to treat acute pain.

Disease-Modifying Antirheumatic Drugs (DMARDs)

Disease-modifying antirheumatic drugs (DMARDs) like methotrexate and Arava (leflunomide) are effective in managing both RA and PsA. Although there is a wealth of evidence supporting their use in treating RA, their effectiveness in people with PsA is far less conclusive.

As a result, methotrexate (considered the first-line DMARD for many autoimmune disorders) is approved for the treatment of psoriasis but not psoriatic arthritis. With that being said, it is frequently used off-label for such purpose.

TNF Inhibitors

TNF inhibitors are biologic drugs that block a type of cytokine known as tumor necrosis factor (TNF). While TNF plays a role in both PsA and RA, it is more central to the damage caused by PsA. As a result, TNF inhibitors tend to work better in people with PsA than RA.

According to a 2011 study from Denmark, 60 percent of people with PsA achieved sustained remission while on TNF inhibitors compared to only 44 percent of those with RA.

TNF inhibitors commonly used in the treatment of PsA and RA are Enbrel (etanercept), Humira (adalimumab), Remicade (infliximab), and Orencia (abatacept).

Staging of Treatment

Generally speaking, RA is treated at the time of diagnosis. This is to prevent irreversible bone erosion and osteolysis that can develop within a span of two years. Early aggressive treatment is especially important for those who are likely to develop severe RA based on the outcomes of testing.

PsA, unlike RA, may only need to be treated when symptoms arise. When the symptoms subside or in remission, it may be possible to stop treatment if no other symptoms occur. However, if PsA is accompanied by moderate to severe psoriasis, ongoing treatment (including methotrexate, biologics, or a combination of therapies) may be prescribed to benefit both conditions.

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