What Is Ankylosing Spondylitis?

Reviewed by: HU Medical Review Board | Last reviewed: February 2019 | Last updated: February 2023

Ankylosing spondylitis (AS) is a type of arthritis that mainly affects the joints in the spine (between the vertebrae). It can also affect other joints in the body. It’s pronounced “ankle-oh-sing spohn-dih-lie-tiss.” AS causes chronic inflammation in the joints which can lead to joint damage over time.1 “Ankylosing” refers to abnormal stiffness and difficulty moving a joint due to joining, or fusion, of the bones. “Spondylitis” means inflammation of the spine.

The parts of the spine

AS is a form of spondyloarthritis, a term that encompasses several chronic arthritis conditions that cause joint inflammation.2

Symptoms of ankylosing spondylitis

AS causes severe, chronic pain and stiffness in the joints affected, most often the spine and hips. Often the earliest symptoms are felt in the sacroiliac joints, which are where the hip bones (or pelvic bones) meet the base of the spine. Some people also have pain and inflammation in other joints, like the shoulders, ribs, hands or feet. The back and joint pain typically are worse after rest and feel better with activity. It can cause morning stiffness lasting longer than 30 minutes and buttock pain. As the condition progresses, it can cause joints in the spine to become fixed or fused, which can lead to a loss of mobility and/or a curved posture.1,3

Loss of curvature in the spine

illustration of curvature loss in spine

The SI joint

Approximately 40% of people with AS also experience inflammation in their eyes, called uveitis or iritis. This can cause eye pain and sensitivity to light, called photophobia.1

Who gets ankylosing spondylitis?

AS typically begins in adolescents or younger adults, under the age of 45. Approximately 10-20% of people with AS begin having symptoms before age 16. Most begin to have symptoms in between the ages of 20 and 30.1,4

The exact number of how many people are affected by AS isn’t well defined. A study published in 2012 with data from the U.S. National Health and Nutrition Examination Survey (NHANES) estimated that up to 1% of U.S. adults may have spondyloarthritis, which includes AS as well as other forms of arthritis that affect the spine.2 Other organizations have estimated the incidence of AS to be between 1.1 million to 2.7 million Americans.1,4

AS is more common – and more severe – in men than in women, and it is estimated that it occurs in 2-3 men for every woman with AS.4

AS occurs more frequently in Caucasian adults than in adults of African American descent. However, African Americans tend to have more severe disease and greater levels of impairment.5

What causes ankylosing spondylitis?

The exact cause of AS remains unknown, but researchers believe it is a combination of genetic and environmental factors that triggers the disease. Several genetic mutations have been identified as having an association with AS, primarily the HLA-B27 gene. However, not everyone with the identified genetic mutations goes on to develop AS, and not everyone with AS has the same (or any) of the known genetic mutations.1,3

One known environmental risk factor is frequent gastrointestinal (GI) infections. Infections in the GI tract can alter the microbiome. This refers to the microorganisms such as bacteria, viruses, and fungi that normally live in the human body. A healthy microbiome is diverse and is known to play an important role in the immune system. Frequent GI infections have been linked to an increased risk of developing AS, although exactly how a change in the microbiome causes immune conditions isn’t clear.6

How is ankylosing spondylitis diagnosed?

Diagnosis may involve several tests. This is because AS can cause different symptoms in each person. Usually, a rheumatologist makes the diagnosis. Rheumatologists are specialists in disorders that affect the joints. Tests used to diagnose AS may include a physical exam, medical history, family history, x-rays, magnetic resonance imaging (MRI), and blood work.7,8

How is ankylosing spondylitis treated?

There is currently no cure for AS. Treatment usually involves a combination of medications, physical therapy, and exercise. In some cases, surgery may be recommended to relieve severe joint damage. Many people with AS also find complementary methods to be helpful in relieving symptoms, such as using heat and cold or dietary changes.

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