Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily attacks the spine and the joints where the spine meets the pelvis. Unlike ordinary back pain, which comes from muscle strain or disc problems, AS is driven by an immune system that mistakenly targets the places where ligaments and tendons attach to bone. Over time, this inflammation can cause new bone to form between vertebrae, gradually fusing sections of the spine together and reducing flexibility. It affects men three to four times more often than women, and symptoms typically begin in the twenties or thirties.
How AS Differs From Regular Back Pain
Most back pain gets better with rest and worse with activity. AS does the opposite. The hallmark of inflammatory back pain is stiffness and discomfort that worsens during periods of inactivity, particularly at night and first thing in the morning. Getting up and moving around brings relief. This pattern is one of the clearest early signals that something beyond a pulled muscle or herniated disc is going on.
International rheumatology guidelines use a specific set of features to distinguish inflammatory back pain from the mechanical kind: pain lasting three months or longer, an insidious onset before age 40, improvement with exercise, no improvement with rest, and nighttime pain that eases once you get out of bed. Mechanical back pain, by contrast, can start at any age, often has a sudden onset tied to an injury, tends to worsen with movement, and generally improves when you lie down. If your back pain follows the inflammatory pattern, it’s worth bringing that up with a doctor, because early identification changes the course of the disease.
What Causes It
The core problem in AS is a process called enthesitis, where the immune system sends inflammatory cells to the entheses (the points where tendons and ligaments anchor into bone). These immune cells release signaling molecules that drive a cycle of inflammation, tissue scarring, and eventually new bone formation at the affected sites. The sacroiliac joints at the base of the spine are almost always the first to be involved, but inflammation can spread to the rest of the spine and to peripheral joints like hips, shoulders, and knees.
Genetics play a major role. A gene called HLA-B27 is present in 80 to 95 percent of people with AS, compared to roughly 8 percent of the general population. Carrying this gene doesn’t guarantee you’ll develop the disease, but it significantly raises the risk. Researchers believe HLA-B27 affects how the immune system processes certain proteins, though the exact mechanism is still being worked out. Environmental triggers, possibly gut bacteria or mechanical stress on joints, appear to interact with genetic susceptibility to set the disease in motion.
Symptoms Beyond the Spine
AS is often thought of as a back condition, but it can affect organs well beyond the skeleton. The most common extra-skeletal problem is uveitis, an inflammation of the eye that causes redness, pain, light sensitivity, and blurred vision. Between 20 and 30 percent of people with AS develop uveitis at some point, and the risk increases the longer you have the disease. About 90 percent of these cases involve the front part of the eye (acute anterior uveitis), which is treatable but requires prompt attention to prevent vision damage.
Inflammatory bowel disease, including Crohn’s disease and ulcerative colitis, also shares a strong link with AS. Many people with AS have subclinical gut inflammation, meaning intestinal tissue shows signs of immune activity even without obvious digestive symptoms. Skin conditions like psoriasis are part of the same family of related inflammatory diseases. Because AS can reach into so many systems, rheumatologists typically monitor for eye, gut, and skin involvement alongside spinal symptoms.
How Spinal Fusion Happens
Left unchecked, the inflammation of AS triggers a distinctive process of new bone growth. Bony projections called syndesmophytes form at the edges of the vertebrae, growing outward from the fibrous ring that surrounds each spinal disc. These growths typically appear first along the back and sides of the vertebral rim, usually sprouting from the upper edge of one vertebra before a matching growth descends from the vertebra above.
Over time, syndesmophytes from opposing vertebrae meet and fuse, creating a bony bridge across the disc space. Additional bridges form around other portions of the rim, eventually encircling the disc entirely. When this happens across multiple levels of the spine, the result is a rigid, fused column sometimes called “bamboo spine” on X-rays. This fusion is what produces the characteristic loss of spinal mobility and, in advanced cases, a fixed forward-stooped posture. Modern treatment aims to slow or prevent this process by controlling inflammation early.
Getting a Diagnosis
Diagnosing AS can take years, partly because back pain is so common and partly because early damage doesn’t always show up on standard X-rays. Traditional diagnostic criteria required visible changes to the sacroiliac joints on plain radiographs, which often meant the disease had been progressing for years before it could be confirmed. Newer classification criteria developed in 2009 allow doctors to identify the condition at earlier stages using MRI, which detects active inflammation before permanent bone changes occur.
MRI is substantially more sensitive than X-ray for picking up early sacroiliac joint damage. In head-to-head comparisons using CT scans as a reference standard, MRI correctly identified erosions 79 percent of the time compared to 42 percent for X-rays. For overall detection of structural changes, MRI scored 85 percent versus 48 percent for radiographs. The one area where X-rays performed comparably was detecting sclerosis (bone hardening). If your doctor suspects AS based on your symptoms, blood work for HLA-B27 and inflammatory markers like C-reactive protein, combined with MRI of the sacroiliac joints, forms the backbone of the diagnostic workup.
Treatment Options
Anti-inflammatory medications are the first line of treatment. NSAIDs (drugs like ibuprofen and naproxen) are recommended on an as-needed basis rather than taken continuously, and many people find they provide meaningful relief of pain and stiffness. For those whose symptoms don’t respond adequately, biologic medications that target specific immune signaling molecules are the next step. Current guidelines favor TNF inhibitors as the first biologic to try. These drugs block one of the key inflammatory signals driving enthesitis and can slow or halt the progression of spinal damage.
If a TNF inhibitor doesn’t work, medications that target a different immune pathway (specifically, interleukin-17) are preferred over switching to a second TNF inhibitor. A newer class of oral medications called JAK inhibitors is also available, though biologics are generally favored over them. Older drugs like sulfasalazine may be considered for people who can’t take biologics or don’t have access to them, particularly when peripheral joints (hips, knees, ankles) are the main problem. Methotrexate has limited evidence in AS but is sometimes used for peripheral joint symptoms as well.
Why Exercise Matters
Physical activity isn’t just helpful for AS. It’s one of the most important parts of managing the disease. Regular exercise helps maintain spinal and chest wall mobility, counteracts the stiffness that worsens with inactivity, and supports the cardiovascular fitness that chronic inflammation can erode. The most commonly recommended types are strengthening exercises (used in about 64 percent of studied programs), flexibility and stretching work (57 percent), and breathing exercises (50 percent) designed to preserve rib cage expansion.
Programs for AS typically focus on back stretching, posture control, and spinal extension movements that counteract the forward-curving tendency of the disease. Aerobic exercise, including swimming, cycling, and walking, is increasingly emphasized alongside the traditional flexibility work. Consistency matters more than intensity. Even on days when stiffness is significant, gentle movement tends to produce better outcomes than rest. Many people with AS find that working with a physical therapist to develop a tailored routine gives them a framework they can maintain independently over the long term.