How Common Is Ankylosing Spondylitis?

Ankylosing spondylitis (AS) is an autoimmune disease that is somewhat rare. It causes pain and stiffness in your spine and other joints. While this illness is one of more than 100 types of arthritis, many people have never heard of it because it affects so few.

Studies show that this disease tends to run in families. Most people who have this illness have a genetic marker called HLA-B27. However, having this marker doesn't mean that you will develop the disease.

Finding out all you can about the onset and impact of this illness can help you know what to expect if you have it or are at risk. Most people who have this disease can lead normal lives with the right treatment. The impact of symptoms like swelling in the back and disease flares on daily life differs for each person.

This article describes the most important findings about this disease and who it affects.

Prevalence of Ankylosing Spondylitis

The prevalence of ankylosing spondylitis in the general population is about 0.2% to 0.5%. However, this varies further among demographic groups and the presence of certain risk factors.

Recent research indicates an increase in the prevalence of the disease. This may be attributed to healthcare providers' improved recognition of symptoms or confidence in making the diagnosis. Increased awareness of the impact of inflammatory back pain and the value of early treatment in outcomes may affect these factors.

It is difficult to determine how many people have this illness because few prevalence studies have been done. In addition, researchers report little consistency regarding the population of subjects studied and diagnostic criteria in the existing research.

AS in the United States

The prevalence of ankylosing spondylitis in the United States is 0.5% to 0.6%, according to data from the National Health and Nutrition Examination Survey (NHANES). The number of Americans with ankylosing spondylitis is estimated to be about 300,000.

According to the Centers for Disease Control and Prevention (CDC), at least 1.7 million U.S. adults have ankylosing spondylitis, and about 3.2 million U.S. adults have spondyloarthritis, a disease category that includes ankylosing spondylitis and related diseases.

Results of a 2021 study investigating the diagnostic prevalence of ankylosing spondylitis and axial spondyloarthritis in the United States considered data retrieved using the Medicare and MarketScan patient claims databases. Data indicate that the number of people diagnosed with both diseases in the United States is increasing.

(Note that when health authorities or research is cited, the terms for sex or gender from the sources are used.)

AS Globally

Research indicates that the global prevalence of ankylosing spondylitis varies by continent. A review of 36 eligible studies found the following mean prevalence rates of ankylosing spondylitis:

• Europe: 23.8 per 10,000
• Asia: 16.7 per 10,000
• North America: 31.9 per 10,000
• Latin America: 10.2 per 10,000
• Africa: 7.4 per 10,000

In this study, researchers found sufficient evidence to estimate the presence of between 1.30 million and 1.56 million cases in Europe and between 4.63 million and 4.98 million cases in Asia.

In another study, researchers sought to identify the number of people worldwide with spondyloarthritis, a broader disease category. Data also indicated a wide variety of global prevalence.

Researchers found geographic clustering of the disease, likely due to the degree of HLA-B27 positivity in the population, which runs in families. Prevalence rates for spondyloarthritis ranged from 0.20% in Southeast Asian populations to a high of 1.61% in Northern Arctic communities.

Influencing Factors in AS Statistics

There are likely several influencing factors in ankylosing spondylitis statistics. This has resulted in varying diagnostic and population prevalence findings for the disease. These factors make it challenging to define the real prevalence of ankylosing spondylitis, especially since relatively few studies of this nature have been performed.

Following are some of the most significant factors that may play a role in the few research studies that exist.

Delayed Referrals

Research shows that it is relatively common for people to wait a decade or longer to get an accurate diagnosis of ankylosing spondylitis. This delay in referral to a rheumatologist or other specialist means that these people remain unaccounted for in studies that tally the number of people affected by the disease.

The delay between the onset of symptoms and the diagnosis of ankylosing spondylitis is up to 13 years in the United States and about eight to 11 years in Europe. The delay is likely related to the fact that back pain is a relatively common occurrence in the general population. It is also a potential symptom of many other conditions, ranging from acute injury to various forms of arthritis.

Diagnostic Criteria

The type of diagnostic criteria used can affect research that attempts to report on the prevalence of ankylosing spondylitis. There are no standard criteria or laboratory tests that definitively indicate the diagnosis of this disease.

As more information is learned about the disease, diagnostic criteria have changed over the years. This can make it challenging to combine or compare data from different studies. In 1963, researchers established clinical criteria known as the Rome criteria. This was followed by the New York criteria in 1966. A modified New York version was established in 1984.

The most recent diagnostic criteria used for AS is the Assessment of SpondyloArthritis International Society (ASAS) classification criteria. The modified NY version is typically used in clinical trials and tends to miss early disease.

A review of 36 eligible studies on the global prevalence of ankylosing spondylitis found that studies used either clinical diagnosis or the New York/modified New York validated criteria to identify ankylosing spondylitis. Estimates using clinical diagnosis were consistently lower than those using validated criteria, making a comparison between studies difficult.

Misconceptions

Misconceptions about ankylosing spondylitis can affect both people with the condition and healthcare providers by interfering with an early and accurate diagnosis of the disease. They can also prevent someone from seeking care for early symptoms.

To the extent that misconceptions interfere with a diagnosis, they affect the report of statistics related to the prevalence of this disease.

Common misconceptions that can affect statistics on disease prevalence include:

• Ankylosing spondylitis primarily affects men: Traditionally, there has been a higher incidence of ankylosing spondylitis among men. However, new research indicates that the gap between sexes may not be so wide. Prior research suggests up to a 3-to-1 ratio of males versus females for ankylosing spondylitis. However, a 2022 study indicates the incidence of ankylosing spondylitis to be relatively equal between the sexes in the United States.
• Everyone with ankylosing spondylitis has back pain: Ankylosing spondylitis affects everyone differently. While most people have back pain, some people have pain in peripheral joints like the hip, ankle, knee, elbow, shoulder, or heel. This can make ankylosing spondylitis resemble some other forms of arthritis. These symptoms can occur more frequently in juveniles and women who have ankylosing spondylitis.
• If you don't have X-ray evidence of joint involvement, you don't have axial spondyloarthritis that might progress to ankylosing spondylitis: Non-radiographic axial spondyloarthritis does not show X-ray evidence of joint damage. However, it may progress to ankylosing spondylitis, and might be regarded as an early stage of the condition. Treatment is available at this stage.

AS Risk Factors

While the cause of ankylosing spondylitis is unknown, research indicates that having certain factors can make you more likely to develop ankylosing spondylitis. The following characteristics are considered to increase the likelihood of who develops this condition.

Genetics

Genetics seems to have a significant impact on who gets ankylosing spondylitis. One of the most important risk factors for the disease is having the HLA-B27 gene. HLA-B27 is present in over 85% of people with ankylosing spondylitis.

However, 5% to 15% of the general population has this gene, and only about 5% of people with the gene develop ankylosing spondylitis. The risk of having the disease increases if you are HLA-B27 positive and have a parent with ankylosing spondylitis.

Race

In the United States, a 2017 study of disease between three ethnic groups reported that ankylosing is three times more commonly diagnosed in White people.

While Black and Latinx Americans are less likely to be diagnosed with ankylosing spondylitis, they are more likely to have more severe disease activity. These two groups also have a lower frequency of the HLA-B27 gene mutation, a risk factor for the disease, which may relate to their lower incidence of the disease.

However, disparities in diagnosis may also relate to access to health care and healthcare provider bias, especially in a condition in which there are noted delays in diagnosis.

Age

Ankylosing spondylitis is usually diagnosed in people younger than 40 years old. About 80% of people experience their first symptoms before age 30, with fewer than 5% of people having their first symptoms at age 45 or older.

Frequent Gastrointestinal Inflammation

About 10% to 20% of people with inflammatory bowel disease (IBD) develop arthritis that manifests in the sacroiliac joint. About 3% to 4% of people have IBD at the time of ankylosing spondylitis diagnosis, while 10% have IBD 20 years after diagnosis. Up to 60% of people with ankylosing spondylitis have evidence of microscopic inflammation in the gut when a colonoscopy is done.

Summary

Ankylosing spondylitis is a fairly rare form of arthritis. It causes pain and stiffness mainly in the spine.

The prevalence of this illness varies around the world. The highest disease rates occur in North America and Europe. It is diagnosed more often in White people, which may account for geographic contrasts.

Research shows that having the genetic marker called HLA-B27 raises your risk of this disease. Having a close family member with this disease raises your risk of getting it.

Little research has been done on this disease, so much remains unknown about its causes. Factors like inconsistent diagnostic criteria, delays in diagnosis, and disease misconceptions have made it hard for researchers to compile and compare data.

Prevalence statistics can help you and those close to you understand your risk of getting ankylosing spondylitis. A healthcare provider can help you put this information into perspective regarding your condition and personal characteristics.