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Ankylosing Spondylitis

Updated: Nov 26, 2020

Ankylosing spondylitis is a type of arthritis, which means it causes inflammation that affects joints. It typically affects the joints of the spine, leading to back pain and stiffness, but it can affect other joints, too, such as the knee and shoulder.


Symptoms of ankylosing spondylitis vary from person to person. One person may experience occasional flares of neck pain and fatigue while another experiences severe chronic back pain, painful eye symptoms, and even spine deformities. Many people report pain with stiffness that appears over many months. Pain most often occurs in the:

  • Low back

  • Alternating buttocks

  • Hip(s)

  • Thigh(s)—this is radiating pain that typically does not go past the knee

This pain results from inflammation of the lower spine and/or sacroiliac joints. The sacroilioac joints, sometimes called the iliosacral joints, are where the sacral bone—a triangle shaped bone located at the bottom of the spine—attaches to the ilium bones of the pelvis.

The name ankylosing spondylitis is derived from the symptoms doctors initially observed in patients, who often had a hunched appearance:

  • Ankylos, derived from the Greek root ankylosis, refers to a joint that is stiff, fused, bent, and/or crooked

  • Spondylo refers to vertebra

  • itis refers to inflammation

Because of today’s treatments, most ankylosing spondylitis patients do not progress to the point where posture is visibly affected. However, partial fusion of one or just a few segments of the spine can lead to back pain and restricted movement. Early diagnosis and treatment can reduce pain and limit the loss of joints’ range of motion.

When to Seek Medical Attention

People are advised to see their doctor if they have:

  1. Experienced pain in the low back or buttocks (particularly alternating buttock pain) for 3 months or longer, especially if they are males under the age of 45. Ankylosing spondylitis pain is worse during rest and when getting out of bed. The pain typically gets better with activity and exercise.

  2. Eye pain and redness, significant sensitivity to light, or blurred vision—in addition to or instead of back pain.

People experiencing eye symptoms are encouraged to make an appointment with their primary care doctor or an ophthalmologist as soon as possible, as permanent eye damage can occur without treatment.


In a normal spine, the vertebrae are spaced out, separated by intervertebral discs and connected by ligaments. Ankylosing spondylitis causes changes in the spine, resulting in two or more vertebrae to grow closer together and possibly even fuse. How does this happen?

  1. Inflammation occurs at the spinal joints, where vertebra, intervertebral discs, ligaments, and tendons are located. (Ligaments attach bones to other bones. Tendons attach muscles to bones.)

  2. The inflammation causes pain, stiffness, and restricted range of motion in the spine.

  3. As the inflammation subsides, the body’s natural healing process causes scar tissue to form where the inflammation took place. When normal tissue is replaced with scar tissue it is called fibrosis.

  4. Over time and perhaps repeated periods of inflammation, the scar tissue calcifies, causing it to turn to bone tissue.

  5. The new bone tissue is rigid, and the affected joints become less flexible.

  6. As more bone tissue is created, the affected bones can grow together, causing the joints of the spine and/or the sacroiliac joint to become fused. When fusion occurs joint flexibility is lost.

Losing flexibility in a joint can be frustrating and painful. Preserving flexibility through exercise and treatment can help control pain.

Genes and Family History

Experts do not understand exactly why some people get ankylosing spondylitis, but they do know many factors play a role.

The clearest risk factors involve inherited genes and family history:

  • A gene called HLA-B27 is found in 85% to 95% of people with ankylosing spondylitis, though some estimates are lower. Experts estimate only about 8% of people around the world have this gene.

  • Not everyone who has HLA-B27 will get AS. Experts estimate people who carry HLA-B27 have a 2% to 10% chance of developing spondyloarthritis.

  • A family history of ankylosing spondylitis is an additional risk factor for developing the disease. For example, a person who inherits HLA-B27 and has a parent with ankylosing spondylitis has about 20% chance of developing AS.

  • Black people who have ankylosing spondylitis are less likely to have the HLA-B27 gene than white people who have the disease.

  • In addition to HLA-B27, genetic variations in genes ERAP1, IL1A, and IL23R are also linked to ankylosing spondylitis.

Exactly how genes and family history affect the risk of ankylosing spondylitis is not known.

Other Risk Factors

In addition to genes and family history, factors that can affect risk include:

Sex. Men are two to three times more likely to get ankylosing spondylitis than women. Men also tend to have more severe symptoms. Because AS is more common in men, diagnosis in women is often overlooked or missed, especially because women tend to have pain in the neck, hips, and/or peripheral joints instead of the low back.

Age. Ankylosing spondylitis is usually diagnosed in people in their teens, 20s and 30s. Ninety-five percent of patients are diagnosed before the age of 46. It is possible for children to be diagnosed: 15% of people with ankylosing spondylitis are diagnosed before age 15.


In rare cases, people with severe ankylosing spondylitis may develop serious complications, including:

A hunched posture. A curled forward, chin-to-chest stance can occur if the spine fuses together in a hunched forward position. People who develop this deformity have a permanent downward gaze. The heart, lungs, and other organs can be affected.

Bone fractures. Bones may lose mineral density (osteopenia and osteoporosis) making them brittle and prone to fractures.

Cauda equina syndrome. This rare condition involves extreme pressure and swelling of the nerves at the end of the spinal cord. The condition can cause weakness, tingling, or numbness in the legs, and/or feet on one or both sides of the body. It also causes bowel or bladder dysfunction. This condition is considered a medical emergency.

Spondylodiscitis. In this condition, one or more intervertebral discs or disc spaces become infected. One study estimates that 8% of people with ankylosing spondylitis develop spondylodiscitis.

Bamboo spine.The spine is a series of joints that provide a high degree of movement and flexibility in all directions. In severe, advanced cases of ankylosing spondylitis there is a complete fusion of the bones of the spine, turning the spinal column into one long bone, which some people say resembles a bamboo stalk. It is quite rare for complete spinal fusion to occur in patients receiving treatment.


Diagnosing ankylosing spondylitis can be somewhat difficult because:

  • Low back pain is a common problem and usually caused by sore muscles or other conditions, such as osteoarthritis

  • X-rays are often normal, particularly in the early stages of the disease.

During diagnosis, doctors will review a patient’s medical history, perform a physical exam, and—when necessary—order diagnostic tests, such as x-rays and blood tests.

Medical Imaging

X-ray evidence of sacroiliitis—inflammation of the sacroiliac joint at the base of the spine—is one of the most telling signs of ankylosing spondylitis. However, a patient might feel sacroiliitis or other back pain years before changes in the spine’s anatomy can be seen on x-rays.

Magnetic resonance imaging (MRI scans), CAT scans (CT scans), and ultrasound can provide more accurate and detailed images, helping doctors identify earlier changes in the joints. These medical imaging techniques are more expensive and time consuming than x-rays, so they are only done if needed.

Blood Tests

There is no single lab test that can be used to diagnose ankylosing spondylitis. Instead, a variety of blood tests can be used to help rule out or provide evidence for an ankylosing spondylitis diagnosis. In addition to a routine complete blood count (CBC), a doctor may order testing for:

C-Reactive protein (CRP). When there is inflammation in the body, the liver produces a protein called CRP. A high CRP level indicates an inflammatory condition, including infection. For this reason, a high CRP level does not automatically mean a patient has ankylosing spondylitis. In addition to using this test for diagnosis, doctors often monitor CRP levels to gauge a patients’ response to treatment.

Erythrocyte sedimentation rate (ESR). Similar to CRP, this test also screens for inflammation. Because inflammation is a factor in many conditions, this test is not specific for ankylosing spondylitis and by itself does not diagnose a patient, but it can support the diagnosis. It is also used to see how active the condition is.

HLA-B27 gene. The majority of people with ankylosing spondylitis carry a gene called HLA-B27. A positive test for this gene can help confirm a suspected case of ankylosing spondylitis, but this test is not required for diagnosis when a patient has several obvious symptoms, or when sacroiliitis can be seen on an x-ray.


Ankylosing spondylitis is a chronic disease that cannot be cured but it can be treated. Exercise, changing daily routines, and taking medication can decrease pain and the risk of future complications.

Many people who have ankylosing spondylitis take medications to help reduce the inflammation that causes pain and stiffness. What type of medication a doctor recommends will depend on the person’s symptoms and their severity.

There are four types of medications commonly used to treat ankylosing spondylitis: Nonsteroidal anti-inflammatory drugs (NSAIDs), biologics, steroids, and DMARDs. Like all medications, these drugs can cause side effects or interact with other medications. Patients are advised to discuss any drug therapy plan with their health care professional and report side effects.

Most newly-diagnosed patients are started on NSAID medications. If symptoms are not alleviated within a reasonable window of time, or if the case is severe, the doctor may recommend using another type of medication, usually a biologic. Over months or years, a medication may become less effective or cause unacceptable side effects, and a new medication can be prescribed.

Role of Surgery

The majority of people who have ankylosing spondylitis may never require surgery; however, surgery may be an option for patients who have spinal deformities or severe joint problems. What surgery is recommended depends on the patient’s clinical situation and symptoms.

A patient may benefit from spine surgery if he or she has:

  • Severe, unremitting back or neck pain that has not responded to nonsurgical treatment

  • Nerve damage caused by spinal deformity

  • An unstable spine, meaning that a bone(s) has fractured

  • Decreased ability to hold the head up and see horizontally

  • Difficulty completing everyday activities like eating and drinking because of spinal deformity.