Prominent morning stiffness, swelling, redness, and warmth of the joints – these are all indicators that you may have inflammatory arthritis. The question is, what kind?
While rheumatoid arthritis (RA) might come to mind first, psoriatic arthritis (PsA) is more common than you might think. Among those living with psoriasis, about 1 in 3 will develop PsA, and it can happen at any age.
Both RA and PsA result in inflammation inside the lining of the joints, and both involve the immune system. A number of key differences, however, can help health care providers distinguish one from the other. Cleveland Clinic rheumatologist Cassandra Calabrese, D.O., discussed the two diseases and how they differ.
Diagnosis and Differentiators
There is currently no blood test for PsA, but in general, the joint pain and swelling for PsA would be asymmetric, involving potentially fewer joints, and typically would involve different joints than RA, says Dr. Calabrese.
“Not every patient with psoriasis and joint pain has psoriatic arthritis, but if there's new joint pain, worsening joint pain, or joint pain that's interfering with daily activities, then that patient should see a rheumatologist,” she recommends.
PsA involves the entheses, which is where tendons connect to bones. This means that areas such as the back of the heel where the Achilles tendon inserts, or above the knee where the patellar tendon inserts, are common sources of pain for those with PsA, says Dr. Calabrese. Other areas more common in PsA, she says, include the lower spine and the distal interphalangeal (DIP) joints, the joints closest to the tips of the fingers and toes. Unique to PsA is dactylitis, or the so-called “sausage digit,” where an individual finger or toe will swell along the full digit without prior physical trauma.
PsA, of course, frequently coincides with psoriasis, although Dr. Calabrese says the presence of psoriatic lesions isn’t necessary for a PsA diagnosis. PsA has additional indicators that don’t involve the joints, she says, including uveitis, or swelling of the eye, and changes in the fingernails and toenails. Typical nail changes are pitting of the nail, discoloration – either yellow or white – or spots underneath the nail, as well as nails that are detached, or lifted from the nail bed. Dr. Calabrese points out that these nail changes can often mimic fungal diseases, so it is important to discuss them with a dermatologist as well.
Diagnosis for RA is often more straightforward. “The majority of cases have positive autoantibodies, so [these are] blood markers called rheumatoid factor and anti-CCP [anti-cyclic citrullinated peptide] antibody that test positive,” says Dr. Calabrese.
Unlike PsA, RA has symmetric joint involvement, typically with small joints such as the fingers, wrists, ankles, and elbows. While RA can affect the spine, it is typically focused in the upper spine. RA rarely involves the lower spine or the DIP joints, which are most common in PsA.
For both types of inflammatory arthritis, “the diagnosis is based on patient history of joint swelling and pain,” says Dr. Calabrese, including any family history of joint disease. “The doctor might order blood tests looking for evidence of inflammation, X-rays to see a specific kind of damage from joint inflammation, and then sometimes an MRI [magnetic resonance imaging] or an ultrasound of the joint is needed to actually show the acute inflammation,” she adds.