Ankylosing Spondylitis Facts and Statistics: What You Need to Know

Ankylosing spondylitis, a type of chronic inflammatory arthritis primarily affecting the spine, is an unpredictable and variable disease. There is no cure for it, and its cause is unknown. Fortunately, it is treatable and manageable, and most people who get an early diagnosis and start treatment right away can have full and productive lives.

To help you better understand ankylosing spondylitis, this article will highlight key facts and statistics about the condition, including who is at risk, its effects on mortality, and screening and early detection.

Ankylosing Spondylitis Overview

AS is a form of axial spondyloarthritis that affects the joints of the spine, chest, and pelvis. It is also an autoimmune disease where the immune system malfunctions and attacks healthy tissues. The other type of axial spondylarthritis is non-radiographic axial spondyloarthritis (nr-AxSpA). Nr-AxSpA can later progress into AS.

Ankylosing spondylitis mainly affects the back by causing inflammation of the spine. Spinal inflammation will lead to back, neck, and rib cage stiffness and pain. AS can also cause pain and stiffness in other areas, including the hips, shoulders, knees, and feet.

Researchers do not yet know what causes AS. While AS has a strong genetic basis and can affect multiple family members, the condition isn't always passed down from parent to child.

How Common Is Ankylosing Spondylitis?

In the United States, AS prevalence is estimated to be between 0.2% and 1%. AS affects more males than females.

One 2016 study published in BMC Rheumatology found that around 47% of people with AS were female. That study also found diagnostic rates increased from 0.04% to 0.09% from 2006 to 2016 for both males and females.

The gene variant HLA-B27 is found in 90% of people with AS. HLA-B27 occurs in only about 8% of the general population. This gene mutation increases the risk for AS, but not everyone with the gene will develop AS. HLA-B27 positivity in AS is associated with earlier disease and higher risk due to family history.

AS prevalence may be increasing, but researchers are unsure why. They speculate this might be due mainly to increased disease awareness, among other factors.

Ankylosing Spondylitis by Ethnicity 

In the United States, AS is more common in White people. Research shows it is three times more common in White people even though Black people seem to have a higher disease burden (severity and effects on life). The higher numbers for White people are mainly due to the lower frequency of HLA-B27 in Black Americans and most (but not all) African ethnic groups.

Like Black people, Latinx people in the United States will have fewer AS incidences because of the lower frequency of HLA-B27. However, one 2017 study noted Mexican Americans have similar AS numbers as White Americans.

The analysis also notes that HLA-B27 positivity in White people was associated with earlier disease onset, higher rates of anterior uveitis (inflammation of the middle layer of the eye), and shorter delay to diagnosis. For both White and Black Americans, HLA-B27 was linked to the increased familial incidence of AS. 

Racial Bias

Racial inequalities might make it harder to know the actual prevalence numbers for AS among people of color in the United States. Researchers have raised concerns about a detection bias in Black Americans and other minority groups in the United States.

For example, healthcare providers believe that White people have a higher risk for AS. That means they will suspect AS in this population and ignore symptoms in people of color.

Other researchers have noted that reduced access to care and specialists may affect the number of people of color diagnosed with rheumatic diseases (inflammatory conditions of the joints).

Studies also show Black Americans are less likely to have health insurance and access to primary care providers, making it harder to receive referrals to rheumatologists, the doctors who specialize in diagnosing and treating rheumatic diseases like AS.

Ankylosing Spondylitis by Age & Gender 

Ankylosing spondylitis can affect anyone regardless of age or gender. Specific patterns might exist based on age or gender.

Age

The symptoms of AS most frequently appear in young males between ages 16 and 35. While the condition can also affect females (adults and children), symptoms tend to be milder in females, which makes AS harder to diagnose.

The average onset age for AS is between 17 and 45, and most people will experience early symptoms of AS in their 20s. But 18% of AS cases develop earlier and are classified as juvenile-onset AS (before age 16). Only a small percentage of people begin to experience symptoms after age 40; these cases are considered late-onset.

When AS starts in childhood, symptoms will affect the hips, knees, heels of the foot, and big toes. Spine involvement from AS occurs with the progression of the condition. Young adults will experience back pain and morning stiffness when symptoms of AS first start.

Gender Prevalence 

While some studies suggest AS affects more males than females, newer studies report that gender prevalence gaps might not be as significant as previously thought. For example, one study published in 2021 found no statistically significant differences in gender prevalence rates when analyzing military medical records.

In this study, researchers aimed to better understand the increasing AS rates among females. They started by estimating the incidence of AS in working-age U.S. military service members from March 2014 to June 2017 who had undergone screening for chronic low back pain. Their findings showed the incidence of AS was similar for people of any sex.

Disease Burden 

Ankylosing spondylitis disease burden will differ between the genders. According to a 2018 Current Rheumatology Reports article, men with AS have more severe spinal and hip damage, while women experience higher disease activity (measured by symptoms and blood tests for inflammation).

Studies show that men will have severe radiological progression, including the development of syndesmophytes (bony growths on the spine) and hip involvement. Cervical spine (neck) damage was more common in females, while damage to the lumbar spine (lower back) affects more males.

While men might have worse radiographic progression (changes detected by X-ray), women with AS will have a higher disease burden due to long delays to diagnosis, higher disease activity, and significantly less responsiveness to biological treatments.

Females with AS will also experience several extra-articular manifestations (those appearing in places other than the joints), including enthesitis (inflammation of the entheses—the connective tissue between tendon or ligament and bone) and inflammatory bowel disease.

Females with AS also experience a significantly lower quality of life than males. This is consistent with other quality-of-life studies for rheumatoid arthritis and other autoimmune diseases.

Studies on juvenile spondyloarthritis (JoSpA) suggest children with JoSpA have poorer outcomes than children with other types of juvenile idiopathic arthritis (JIA), especially regarding remission (little or no disease activity).

Studies focusing on HLA-B27 with all JIA conditions have found that HLA-B27 positivity is higher in boys. Boys who are positive for HLA-B27 are at an increased risk for tenosynovitis (inflammation of the synovial tendon lining), sacroiliitis (inflammation of one or both sacroiliac joints), and decreased spinal mobility more often than girls. Like adult females, girls might experience reduced responsiveness to biologic drug therapy treatments.

Causes of Ankylosing Spondylitis and Risk Factors 

While researchers do not know exactly what causes AS, they have some theories. They believe that a combination of genetic, environmental, and immune system factors might lead the condition to develop.

Genetics

It is estimated that 90% to 95% of White people with AS are HLA-B27 positive. Not everyone with an HLA-B27 genetic marker will develop AS, and others develop the condition without this genetic marker.

HLA-B27 positivity and AS varies based on ethnic groups, but White people have the highest rates of HLA-B27 and AS. Researchers have also identified additional genetic markers they suspect might be linked to AS, but those are not understood.

Environment and Immune System Causes

Additional factors in the environment combined with genetic factors can lead to the development of AS. Smoking is a major environmental risk factor for AS and worsens disease severity.

Some environmental factors will affect certain parts of the immune system, changing how it functions. For example, changes in the microbiome (the organisms that live inside the gut and aid in digestion) can lead to dysfunction in the immune system.

Risk Factors

AS is also linked to factors that might increase your risk for the condition. These include:

• Genders: Males seem to have a higher risk for AS, although researchers see these prevalence numbers between the genders narrowing.
• Age: AS is most common in teens and younger adults, and most people will begin to experience symptoms between ages 20 and 30.
• HLA-B27 positivity: Having this genetic marker increases your risk for AS.
• Family history: A family history of AS can increase your risk for the condition.
• Gastrointestinal (GI) infections: Frequent GI infections have been found to increase AS risk.
  

What Are the Mortality Rates for Ankylosing Spondylitis?

AS mainly targets the spine and other joints throughout the body. Severe or untreated AS can lead to serious complications, including those that affect the heart and lungs.

While AS does not necessarily shorten a person's life or cause early death, some of its most serious complications can affect a person's longevity and lead to premature death.

According to a 2015 population study, people with AS have:

• A 43% increased risk of death from vascular problems (affecting the blood vessels)
• A 60% increased risk of death from cerebrovascular issues (affecting the blood vessels in the brain)
• A 35% increased risk of death from heart disease

That study also found a difference in vascular death rates between men and women, with men having a 50% greater risk than men without AS and women at 34% greater risk compared to women without the condition. With each year of advancing disease, the risk for vascular mortality from AS increased by 12%.

Screening and Early Detection of Ankylosing Spondylitis

The earliest signs of AS are pain and stiffness of the lower back and hips, especially in the morning, and after inactivity, AS can also cause neck pain and fatigue. AS symptoms will worsen over time but can improve with treatment.

The goal of treatment is to relieve pain and stiffness and to prevent or delay disease complications. AS treatment can be successful if AS is diagnosed and treatment starts immediately.

There are no specific lab tests that confirm AS. However, blood tests can be done to look for inflammation markers and test for the HLA-B27 gene. A healthcare provider will also rely on your symptoms and medical history. They will also want to know if AS and other autoimmune diseases run in your family.

A healthcare provider will also examine your low back, spine, and other joints to look for stiffness and pain. They will look for tender spots and check your breathing. AS can affect how you breathe when the upper body curves forward and the chest wall becomes stiff.

Imaging, including X-rays, can allow a healthcare provider to check for changes in your bones and joints. Magnetic resonance imaging (MRI) can offer more detailed images of the bones and soft tissues and is helpful early in the disease process.

It is well-established that the longer the delay in diagnosis of AS and treatment, the more likely bone, joint, and spine damage will occur. Most studies report that 10% to 40% of people with non-radiographic axial spondylitis (a precursor to AS) will progress to AS in two to 10 years.

Delays in diagnosis also lead to delays in treatment, and untreated AS can lead to severe complications—many of which increase the risk of early death.

You should reach out to a healthcare provider as soon as you start experiencing signs of AS, including stiffness and pain in your spine and low back. The sooner you get a diagnosis, the sooner you can start treating the condition.

Summary

Ankylosing spondylitis affects around 1% of Americans. It is a lifelong inflammatory disease of the spine that often is seen in young adults. It affects more males than females, although researchers think the gender gap for AS is narrowing.

AS can affect most ethnic groups, but it seems to affect more White people. Most people with AS have a gene mutation called HLA-B27. Up to 95% of White people with the condition are HLA-B27 positive.

If AS is appropriately managed and treated, morality risk is low. But severe or untreated AS can lead to serious complications, including joint damage, disability, impaired quality of life, and death. The risk for mortality is further increased for people with vascular, cerebrovascular, and cardiovascular disease complications.

An early diagnosis of AS is crucial so that treatment can start. You should reach out to a healthcare provider if you begin to experience symptoms of the condition, especially if AS or other autoimmune diseases run in your family.