What Kind of Arthritis Is Ankylosing Spondylitis?
Fifty million Americans have arthritis and that number is expected to hit 67 million by 2030. According to the U.S. Centers for Disease Control and Prevention, it’s the most common cause of disability in the nation. But not all cases of arthritis are alike. There are more than 100 forms of the disease, including ankylosing spondylitis (AS), which affects the joints in the spine.
As Dimitrios Pappas, MD, a rheumatologist at New York-Presbyterian Hospital/Columbia University Medical Center points out, “many, many diseases present with joint symptoms.” That’s why AS can be tricky to diagnose, and recognizing how it differs from other types of arthritis is key to getting proper treatment.
AS usually develops in men under the age of 40, though it can affect women, children, and older adults. Symptoms typically include pain and stiffness in the lower back since AS attacks sacroiliac joints between the spine and the pelvis.
AS often goes undiagnosed because its symptoms can come and go, and common warning signs might be dismissed as “just an aching back.” Delaying treatment can lead to serious health consequences.
The main complication of AS is a rigid spine, “which can make moving around very difficult,” said Scott Zashin, MD, a rheumatologist in Dallas and a member of the American College of Rheumatology. “AS patients, particularly those with a rigid back, are also susceptible to fractures.”
Psoriatic arthritis can affect the spine and cause back pain. But it can strike any joints in the body, and often occurs in people who have the skin condition psoriasis. Looking for signs of psoriasis, such as dry, red patches of skin on the torso, elbows, and knees, is one way doctors distinguish psoriatic arthritis from conditions like AS.
Osteoarthritis, the most common type of arthritis, can affect joints throughout the body including in the lower back. Like AS, it can lead to the formation of bony outgrowths, known as bone spurs, around joints. As a result, osteoarthritis may resemble AS in x-ray images.
While AS is an inflammatory disease, osteoarthritis is “a wear and tear process that’s degenerative,” said Alpesh Patel, MD, chief of orthopedic spine surgery and the co-director of Northwestern Spine Center. The result is that cartilage that cushions the bones breaks down.
Unlike AS, osteoarthritis is more common among women and older adults. While activity can lessen the pain and stiffness caused by AS, osteoarthritis pain is often greater after exercise.
Rheumatoid arthritis (RA) is a chronic autoimmune disorder that damages joint tissues. As with AS, its symptoms tend to be worst in the morning and improve with activity. A key differentiator, however, is the location of the pain: RA usually begins in the smaller joints of the hands and feet before spreading to other parts of the body. AS has “a more specific pain site than rheumatoid arthritis,” said Dr. Patel.
RA can affect joints in the spine, according to Patel, making them unstable and causing vertebrae to slip, as opposed to AS which makes the spine stiffer.
Besides a patient’s medical history and x-rays, there are several blood tests that can help doctors identify AS and other specific types of arthritis:
- The HLA-B27 test looks for the presence of a protein that’s found in most – but not all – people who have AS.
- A high erythrocyte sedimentation rate (ESR) or “sed rate” may point to an autoimmune disorder like RA or AS. However, it can also be a sign of anemia or certain cancers.
- Elevated levels of the rheumatoid factor (RF) antibody is a possible indicator for RA. But people with RA can have low RF, and high levels may be related to other conditions like scleroderma.
While these are useful screening tests, they’re not conclusive and one of several factors that go into making a diagnosis.
AS is “a pretty challenging diagnosis to make upfront,” said Patel.
Video: Ankylosing Spondylitis
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