Ankylosing spondylitis is a chronic inflammatory condition that primarily affects the spine, causing pain, stiffness, and in some cases, a gradual loss of flexibility. Although there is no cure, understanding this condition and working closely with healthcare providers can help people manage symptoms and maintain a good quality of life.

Understanding How Common Ankylosing Spondylitis Is

Ankylosing spondylitis, often called AS, is not as rare as many people might think. In the United States, approximately 0.2 to 0.5 percent of the population lives with this condition^[6]. To put this in perspective, that means about 1 in every 200 to 500 people may be affected. When we look at how many new cases appear each year, studies from multiple countries show that between 0.4 and 14 people out of every 100,000 are newly diagnosed^[6].

The condition shows clear patterns in who it affects most often. Men are about two to three times more likely to develop ankylosing spondylitis compared to women^[6][14]. Age is another important factor. Most people who develop AS experience their first symptoms before they turn 40 years old. In fact, about 80 percent of individuals with this condition notice something is wrong before they reach age 30, and the peak time for symptoms to appear is typically in a person’s second and third decades of life^[4][6]. Only about 5 percent of people with ankylosing spondylitis develop symptoms after age 45^[6].

There is also a strong connection between AS and a specific genetic marker. Among people who carry a gene variant called HLA-B27 (human leukocyte antigen-B27), the prevalence of ankylosing spondylitis rises to approximately 5 to 6 percent^[4][6]. The presence of this gene varies by ethnic background. A 2009 survey in the United States found that about 7.5 percent of non-Hispanic white individuals carry HLA-B27, compared to 4.6 percent of Mexican-Americans and 1.1 percent of non-Hispanic Black individuals^[4].

What Causes Ankylosing Spondylitis

The exact cause of ankylosing spondylitis remains largely a mystery to medical researchers. What is known, however, is that AS is an autoimmune disease. This means the body’s immune system, which normally protects us from germs and illness, mistakenly attacks the body’s own tissues^[3]. In the case of AS, the immune system targets joints in the spine and other areas, leading to inflammation.

Scientists have discovered a strong genetic component to this condition. More than 60 different gene mutations have been linked to ankylosing spondylitis^[3]. The most well-known is the HLA-B27 gene variant. Remarkably, more than 90 percent of white people who have AS also carry this mutated gene^[3]. However, it is important to understand that having the gene does not guarantee someone will develop the disease. Many people carry HLA-B27 and never develop AS. Similarly, some people who do not have this gene variant still develop ankylosing spondylitis^[8].

Beyond HLA-B27, other genetic factors play a role. Genes called IL23R, ERAP1, and IL1R2, among others, have been identified through large genetic studies as potential contributors to the development of AS^[6]. Additional genes such as HLA-B60 and HLA-DR1 may also have minor roles^[6]. Heritability is frequently cited as a significant contributor to the condition, with major histocompatibility alleles (specific variations in immune system genes) accounting for up to one-third of the genetic effect^[6].

Who Is at Greater Risk

While anyone can develop ankylosing spondylitis, certain groups face a higher likelihood. Young adults are particularly at risk, with the condition most commonly appearing before age 40^[3]. Males have about twice the risk compared to females^[3]. Family history matters greatly. If you have a close biological relative with AS, especially a parent, your risk increases^[3].

Certain medical conditions are associated with a higher chance of developing ankylosing spondylitis. People who have Crohn’s disease, a chronic inflammatory bowel condition, are at increased risk^[3]. The same applies to individuals with ulcerative colitis, another form of inflammatory bowel disease, and those with psoriasis, a skin condition that causes scaly, itchy patches^[3].

Recognizing the Symptoms

The symptoms of ankylosing spondylitis can vary greatly from one person to another, but there are some common experiences. The hallmark symptom is back pain and stiffness, particularly in the lower back and hips^[1]. This pain typically has a specific character: it tends to be worse in the morning or after periods of rest, and it often improves with movement and exercise^[1][6].

Pain frequently affects the sacroiliac joints, which are located where the base of the spine meets the pelvis. This can cause discomfort in the buttocks that may alternate between the left and right sides^[14]. The pain can spread to other areas, including the hips, neck, abdomen, and even the shoulders^[3].

Morning stiffness is particularly troublesome for many people with AS. This stiffness can disturb sleep, causing people to wake up during the early morning hours^[6]. The discomfort and reduced flexibility are often most noticeable first thing in the morning and after prolonged periods of sitting or lying in one position^[3].

Beyond joint symptoms, people with ankylosing spondylitis may experience a range of other issues. Fatigue is very common, leaving people feeling tired much of the time^[3]. Some individuals have shortness of breath, lose their appetite, or experience unexplained weight loss^[3]. Digestive problems such as diarrhea and stomach pain can occur, along with skin rashes^[3].

Vision problems are another important symptom to watch for. Eye inflammation, called uveitis or anterior uveitis, affects about 25 to 35 percent of people with AS^[4]. This can cause red, irritated eyes and sometimes trouble seeing^[7]. Some people may also develop pain at sites where tendons attach to bones, a condition called enthesitis, or swelling of the fingers and toes known as dactylitis^[4][6].

As the disease progresses over time, inflammation can lead to new bone formation in the spine. When new bone gradually bridges the gaps between vertebrae (the individual bones of the spine), sections of the spine can fuse together^[1]. This process can reduce flexibility and may lead to a hunched-forward posture^[1][7]. Some people develop limited chest expansion if the joints connecting the ribs are affected, which can make taking deep breaths more difficult^[1].

Steps Toward Prevention

Because the exact cause of ankylosing spondylitis is not fully understood and involves genetic factors that cannot be changed, there are no proven methods to prevent the condition from developing in the first place. However, for people who already have AS or are at risk, there are steps that can help prevent the disease from getting worse and reduce the severity of symptoms.

Quitting smoking is one of the most important actions a person can take. Smoking can make AS symptoms worse, speed up joint damage, and make treatments less effective^[19]. For people with ankylosing spondylitis, the inflammation can affect the joints that allow the rib cage to expand, so breathing problems caused by smoking become even more serious^[19].

Staying physically active is essential, not just for general health but specifically for managing AS. Exercise helps preserve posture and spine flexibility, and it can prevent the spine from becoming increasingly stiff^[19]. It is, in fact, one of the only ways to maintain flexibility and strength and avoid abnormal posture^[6]. Even though it may seem counterintuitive when experiencing pain and stiffness, movement often makes these symptoms better, while prolonged inactivity tends to make them worse.

Maintaining good posture throughout daily activities is another preventive measure. Standing straight, sitting properly, and avoiding slouched positions can help prevent the spine from settling into a bent or curved position over time. Some simple exercises, like standing against a wall with heels, bottom, shoulders, and head touching the wall, can help reinforce good posture habits^[19].

Early diagnosis and treatment are crucial for preventing complications. The sooner AS is identified and managed, the better the chances of slowing disease progression and maintaining function. Anyone experiencing persistent lower back pain, especially if it worsens at night and improves with activity, should seek medical evaluation^[1].

How the Body Changes with Ankylosing Spondylitis

To understand what happens in the body with ankylosing spondylitis, it helps to know that this is fundamentally a disease of chronic inflammation. Inflammation is the body’s natural response to injury or infection, causing redness, warmth, swelling, and pain. In AS, this inflammatory process occurs inappropriately in the joints and surrounding tissues of the spine and pelvis.

The disease primarily targets the axial spine, meaning the central column of vertebrae that runs from the base of the skull to the tailbone^[4]. The sacroiliac joints, where the spine connects to the pelvis, are characteristically involved and often the first area affected^[2][6]. When inflammation affects these joints, the condition is called sacroiliitis, which causes the lower back and buttock pain that is the hallmark of AS^[3].

The inflammatory process causes the body to respond by trying to heal itself, but in doing so, it may create new bone^[1]. Over time, these new bone formations called syndesmophytes can bridge adjacent vertebrae^[6]. When enough of this new bone forms, it leads to ankylosis, which means complete fusion or joining together of the bones^[2]. This fusion severely restricts movement in affected areas. The process classically begins at the sacroiliac joints and can progress upward through the lumbar spine toward the cervical spine in the neck^[6]. In severe cases, the spine may develop what doctors call a “bamboo spine” appearance on X-rays because of extensive fusion^[6].

The natural progression of this bone formation tends to be slow, but when it advances, it ultimately leads to impaired spinal mobility^[6]. The flattening of the spine’s natural curves can result in a fixed, hunched posture^[1]. The fused vertebrae no longer move independently, which limits a person’s ability to bend, twist, or turn their back and neck.

Beyond the spine, AS can cause inflammation in other parts of the body. Up to 50 percent of people with AS experience inflammatory bowel disease^[4]. Inflammation can affect the eyes, causing acute anterior uveitis in 25 to 35 percent of cases^[4]. About 10 percent of individuals develop psoriasis^[4]. The heart and blood vessels can also be involved, with AS increasing the risk of cardiovascular disease due to the systemic inflammation present throughout the body^[4].

The lungs can be affected as well. Diminished chest wall expansion and limited spinal mobility can predispose individuals to a restrictive pattern of breathing^[4]. In rare cases, people may develop cauda equina syndrome, a serious condition affecting the nerves at the base of the spine^[4]. There is also an increased risk of atlantoaxial subluxation, where the top vertebrae in the neck become unstable, and spinal cord injury^[4].

Peripheral joints beyond the spine can also be involved in AS. The shoulders, hips, and knees may develop arthritis, causing pain, swelling, and limited range of motion^[3][6]. The hip joints are particularly vulnerable, with about one-third of patients experiencing hip symptoms^[12]. Both hips may be affected, and hip involvement often presents more serious damage than other peripheral joints^[12].