Who Does AS Affect?

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

The exact number of how many people are affected by ankylosing spondylitis (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.1 Other organizations have estimated the number of people who have AS to be between 1.1 million to 2.7 million Americans.2,3

At what age does AS occur?

AS typically begins in teens 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 between the ages of 20 and 30.2,3 An onset of AS in people older than 50 is rare. But, because diagnosis can take time or some people may have very mild symptoms for many years, some people may be diagnosed at older ages.4

How does AS affect men and women?

AS is more common in men than in women. Earlier estimates suggested AS occurred in 3 men for every 1 woman, but more recent estimates suggest it occurs in 2-3 men for every woman with AS.3 However, there is some concern that women may be underdiagnosed. Women with AS may present with different symptoms than men, and there may be a bias among healthcare professionals that AS is a disease that mainly occurs in men. Studies have found that men with AS have more evidence of disease on x-rays than women, and men also have higher levels of inflammatory markers than women with AS. Women with AS may have symptoms that impact their day-to-day living more so than men with AS.5,6

How does AS affect different ethnicities?

AS occurs more frequently in Caucasian adults than in adults of African American or Latino descent. However, African Americans tend to have more severe disease and their day-to-day lives are affected more negatively than Caucasians or Latinos.7,8

Where does AS occur in the body?

AS primarily affects the spine, although other parts of the body can also be affected. The sacroiliac joints (also called the SI joints), located where the pelvis meets the base of the spine, are often the first area where symptoms of pain and stiffness are felt. Many other areas of the body can also be affected, including:2,9

  • An estimated 75% of people with AS have symptoms in their neck.
  • Approximately 40% of people with AS experience inflammation in their eyes, called uveitis or iritis. This can cause eye pain and sensitivity to light, called photophobia.
  • Approximately 30% of people with AS experience hip or shoulder involvement. When AS affects the hips, it may cause referred pain, a confusing symptom that causes pain in the front of the thigh or in the knees.
  • About 30% of people with AS have symptoms in their heels and about 5% have symptoms in the joints in the toes.
  • Approximately 70% of people have inflammation from AS where the ribs attach to the spine and 20% experience AS in the front of the ribs, where the ribs attach to the breastbone.
  • Only about 20% of people with AS have symptoms in their knees.
  • While rare, about 15% of people with AS experience jaw involvement, which can make it difficult to open the mouth and may impair eating.
  • The wrist or joints in the fingers are affected in approximately 5% of people with AS.

What are the genetic causes of AS?

While the exact cause of AS isn't fully understood, researchers believe it is a combination of genetic and environmental factors that trigger the disease. Several genetic mutations have been identified as having an association with AS. The most common is a mutation that produces a protein called HLA-B27. However, not everyone with AS tests positive for HLA-B27, and some people that do test positive for HLA-B27 do not develop AS. Researchers estimate that HLA-B27 accounts for approximately 30% of the risk of developing AS.2 HLA-B27 is seen more commonly in Caucasian populations, which corresponds with the higher incidence of the condition in this ethnic group.8

Variations in several additional genes, including ERAP1, IL1A, and IL23R, have also been associated with ankylosing spondylitis.

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