Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily attacks the spine, gradually stiffening the joints where vertebrae connect and, in some cases, fusing them together over years. It affects roughly 40 to 90 per 100,000 people in the United States, and it typically begins before age 40. Unlike the common back pain most people experience from muscle strain or disc problems, AS starts with inflammation at the points where ligaments and tendons attach to bone, and it can eventually reshape the spine itself.
What Happens in the Spine
The core problem in AS is inflammation at structures called entheses, the spots where ligaments and tendons anchor into bone. This inflammation, called enthesitis, is the primary trigger of the disease. Everything else that follows is secondary to it.
The process unfolds in stages. First, immune cells infiltrate the area where ligaments meet bone, creating small erosions at the corners of the vertebrae. The body tries to repair this damage, but the repair process goes too far. Fibrous tissue fills the eroded areas, then gradually converts to cartilage, and finally hardens into new bone. These bony bridges between vertebrae are called syndesmophytes. When enough of them form along the spine, they can lock vertebrae together in what’s known as a “bamboo spine” on X-rays. In most patients, it takes years from the first symptoms before this bone formation becomes visible on imaging. One study tracking patients over six years found spinal damage scores increased at a steady rate of about 1.3 units every two years, and 58% of participants already had at least one syndesmophyte at the start of the study.
Symptoms That Set It Apart
AS produces a distinctive pattern of back pain that behaves differently from the mechanical back pain caused by lifting something heavy or sitting at a desk too long. The key differences are what rheumatologists use to identify it early:
- It starts gradually, not after a specific injury
- It begins before age 40, often in the late teens or twenties
- It improves with movement and gets worse with rest
- Morning stiffness is prominent, sometimes lasting 30 minutes or more
- Pain at night that improves once you get up and start moving
- It lasts at least 3 months without letting up
The pain often starts in the lower back and buttocks, sometimes alternating sides. That improvement-with-exercise pattern is the biggest clue. Most back pain feels better after rest and worse after activity. In AS, the opposite is true. Sitting still or sleeping for hours makes the stiffness worse, while getting up and moving loosens things.
The Genetic Connection
A gene called HLA-B27 plays a major role. About 85% of people with AS carry this gene, and the association between HLA-B27 and AS is one of the strongest genetic links in any complex human disease, with an odds ratio of 60. That said, carrying the gene doesn’t guarantee you’ll develop AS. Most people with HLA-B27 never do. Other genes inside and outside the same region of the immune system also contribute, which is why the disease runs in families but doesn’t follow a simple inheritance pattern.
AS is more common in men, who historically have been diagnosed at roughly two to three times the rate of women, though this gap may partly reflect underdiagnosis in women whose disease presents differently.
Problems Beyond the Spine
AS doesn’t stay confined to the back. The same inflammatory process can affect other parts of the body. The most common complication outside the joints is uveitis, a painful inflammation of the eye that causes redness, light sensitivity, and blurred vision. In one study of spondyloarthritis patients followed for two years, uveitis was the most frequent new diagnosis, occurring primarily in those with AS, with a cumulative incidence of 3.1%. Psoriasis and inflammatory bowel disease also occur at higher rates.
Cardiovascular risk is a serious long-term concern. A large Canadian study of over 21,000 AS patients found they had a 43% higher risk of vascular disease, 60% higher risk of cerebrovascular disease, and 35% higher cardiovascular mortality compared to the general population. The chronic inflammation driving AS also accelerates artery damage, reduces protective cholesterol levels, and increases rates of metabolic syndrome. Adding to the complexity, the long-term use of anti-inflammatory painkillers that many patients rely on for symptom control carries its own heart risks.
AS can also directly affect the heart. It’s associated with inflammation of the aorta, abnormal electrical conduction in the heart, and an elevated risk of atrial fibrillation.
How It’s Diagnosed
There’s no single blood test that confirms AS. Diagnosis relies on a combination of symptoms, imaging, and sometimes genetic testing. The current classification criteria require at least three months of continuous back pain that started before age 45 as a starting point.
From there, doctors follow either an imaging path or a clinical path. On the imaging side, they look for signs of sacroiliitis, inflammation of the joints connecting the spine to the pelvis. MRI can detect active inflammation as bone marrow swelling adjacent to the joint surface before any permanent damage shows on X-rays. For an MRI to be considered positive, the pattern and location of inflammation must be clearly suggestive of AS and not explained by other causes. Standard X-rays can show structural damage like erosions and fusion, but these changes take years to develop, which is why MRI has become essential for earlier diagnosis.
The diagnostic delay for AS has historically been long, sometimes 7 to 10 years from symptom onset to diagnosis, largely because early symptoms mimic common back pain and X-ray changes appear late.
Treatment Approach
Medication and exercise are considered the two cornerstones of managing AS. For most people, anti-inflammatory painkillers are the first step, providing symptom relief and reducing inflammation. If these aren’t enough after an adequate trial, biologic medications that target specific parts of the immune system become the next option.
The first-line biologics block a protein called TNF-alpha, which drives inflammation. If those don’t produce meaningful improvement within three to six months, guidelines recommend switching to a different type that targets another inflammatory pathway involving a protein called IL-17. Both approaches have proven effective in clinical trials and real-world use, and the choice between them often depends on individual response and whether there are coexisting conditions like psoriasis or bowel disease that might respond better to one class.
The Role of Exercise
Exercise programs for AS focus on maintaining spinal and chest mobility, improving posture, strengthening muscles around the spine, and preserving lung capacity (since rib joint stiffness can restrict breathing). Only 10% to 20% of people with AS in the United States receive formal physical therapy, despite exercise being recommended as a treatment equal in importance to medication.
The evidence for exercise improving spinal mobility specifically is surprisingly uncertain. A Cochrane review of multiple trials found that exercise produced modest improvements in spinal mobility scores compared to no intervention, around a 7% absolute reduction in stiffness measures, but the quality of evidence was rated very low. This doesn’t mean exercise is unhelpful. It likely prevents deconditioning, supports cardiovascular fitness (especially important given AS’s heart risks), and helps manage pain and stiffness day to day. The uncertainty is more about the size and consistency of the measured benefit than about whether staying active matters.
Living With AS Long-Term
The course of AS varies widely. Some people experience mild, intermittent symptoms that respond well to anti-inflammatory medications and regular movement. Others develop progressive spinal fusion that significantly limits flexibility. Spinal progression tends to be gradual and linear, meaning the rate at which new bone forms stays relatively constant over time for a given person rather than accelerating suddenly.
Factors associated with faster progression include already having syndesmophytes (new bone bridges tend to form near existing ones), higher levels of systemic inflammation, and smoking. The cardiovascular risks that come with chronic inflammation make managing traditional heart risk factors like blood pressure, cholesterol, and physical activity especially important for people with AS, not just as general health advice but as a direct response to the disease’s effects on blood vessels and heart tissue.