2. AS is a type of spondyloarthritis
Ankylosing spondylitis falls under the umbrella term of “spondyloarthritis” or “spondyloarthropathies,” which is a group of types of inflammatory arthritis that affect the spine. AS is the prototypical disease, while related types of spondyloarthritis include psoriatic arthritis, reactive arthritis (associated with an infection), and enteropathic arthritis (associated with inflammatory bowel disease). “There’s no strict line to differentiate one from the other,” says Bharat Kumar, MD, associate rheumatology fellowship program director in the division of immunology at the University of Iowa Hospitals and Clinics, “but ankylosing spondylitis has a certain set of signs and symptoms that make it more likely to be ankylosing spondylitis than other causes of inflammatory back pain.”
3. Back pain in AS has some unique traits
For one thing, AS back pain gets worse when you sit still or rest, particularly when you sleep, and particularly in the second half of the night. Patients commonly report getting out of bed in the second half of the night in pain. AS also causes something people refer to as “alternating buttock pain,” which stems from the lower back into the rear, often striking one side at a time.
4. AS symptoms vary from person to person
With so much overlap between ankylosing spondylitis and other spondyloarthropathies, there’s a wide-ranging checklist of symptoms that doctors look for when diagnosing AS. A classic case would start from the bottom of the back, where the spine meets the pelvis, and work its way up, but not everyone will present that way. Women in particular are more likely to see the pain start from higher up on the back, near the bra line, says Dr. Norton. Other people with AS might have peripheral spondyloarthritis, which means that the inflammation spreads to joints beyond the spine, such as the knees and hips.
While chronic lower back pain that gets better with movement and worsens with rest and that starts at a younger age (before age 45) is trademark symptom of AS, people are diagnosed with AS without having back pain as their main complaint.