Is it just a sore back — or is it something else?

Back pain is a top medical complaint. It’s also a leading cause of missed work. According to the National Institute of Neurological Disorders and Stroke, virtually all adults will seek attention for back pain at some point in their lives. The American Chiropractic Association reports that Americans spend about $50 billion a year on treating back pain.

There are many possible causes of low back pain. Usually it’s caused by trauma from a sudden strain on the spine. But you should be aware that back pain can also signal a more serious condition called ankylosing spondylitis.

Unlike ordinary back pain, ankylosing spondylitis (AS) is not caused by physical trauma to the spine. Rather, it’s a chronic condition caused by inflammation in the vertebrae (the bones of the spine). AS is a form of spinal arthritis.

The most common symptoms are intermittent flare-ups of spinal pain and stiffness. However, the disease can also affect other joints, as well as the eyes and the intestines. In advanced AS, abnormal bone growth in the vertebrae may cause joints to fuse. This can severely reduce mobility. People with AS may also experience vision problems, or inflammation in other joints, such as the knees and ankles.

Sign #1: You have unexplained pain in the lower back.

Typical back pain often feels better after rest. AS is the opposite. Pain and stiffness are usually worse upon waking. While exercise may make ordinary back pain worse, AS symptoms may actually feel better after exercise.

Lower back pain for no apparent reason is not typical in young people. Teens and young adults who complain of stiffness or pain in the lower back or hips should be evaluated for AS by a doctor. Pain is often located in the sacroiliac joints, where the pelvis and spine meet.

Sign #2: You have a family history of AS.

People with certain genetic markers are susceptible to AS. But not all people who have the genes develop the disease, for reasons that remain unclear. If you have a relative with either AS, psoriatic arthritis, or arthritis related to inflammatory bowel disease, you may have inherited genes that put you at greater risk for AS.

Sign #3: You’re young, and you have unexplained pain in the heel(s), joints, or chest.

Instead of back pain, some AS patients first experience pain in the heel, or pain and stiffness in the joints of the wrists, ankles, or other joints. Some patient’s rib bones are affected, at the point where they meet the spine. This can cause tightness in the chest that makes it hard to breathe. Talk to your doctor if any of these conditions occur or persist.

Sign #4: Your pain may come and go, but it’s gradually moving up your spine. And it’s getting worse.

AS is a chronic, progressive disease. Although exercise or pain medications may help temporarily, the disease may gradually worsen. Symptoms may come and go, but they won’t stop completely. Often the pain and inflammation spread from the low back up the spine. If left untreated, vertebrae may fuse together, causing a forward curvature of the spine, or humpbacked appearance (kyphosis).

Sign #5: You get relief from your symptoms by taking NSAIDs.

At first, people with AS will get symptomatic relief from common over-the-counter anti-inflammatory drugs, such as ibuprofen or naproxen. These drugs, called NSAIDs, do not alter the course of the disease, though.

If your doctors think you have AS, they may prescribe more advanced medications. These drugs target specific parts of the immune system. Immune system components called cytokines play a central role in inflammation. Two in particular — tumor necrosis factor alpha and interleukin 10 — are targeted by modern biological therapies. These drugs may actually slow the progression of the disease.

AS is more likely to affect young men, but it can affect both males and females. Initial symptoms usually appear in the late teen to early adult years. AS can develop at any age, however. The tendency to develop the disease is inherited, but not everyone with these marker genes will develop the disease. It’s unclear why some people get AS and others don’t. A majority of Caucasian people with the disease carry a particular gene called HLA-B27, but not all people with the gene develop AS. Up to 30 genes have been identified that may play a role.

There is no single test for AS. Diagnosis involves a detailed patient history and physical exam. Your doctor may also order imaging tests, such as computed tomography (CT), magnetic resonance imaging (MRI), or X-ray. Some experts believe MRI should be used to diagnose AS in the early stages of the disease, before it shows up on X-ray.