Ankylosing spondylitis (AS) is a type of chronic (long-term) arthritis that affects the bones, muscles and ligaments of the spine. Anyone can develop ankylosing spondylitis at any time, although it is five times more common in men than it is in women. It usually occurs between 15 and 35 years of age.

The spine

The spine consists of a column of interlocking bones called vertebrae that run from the coccyx (tailbone) to the top of the neck, where it joins the skull. There are flexible ‘discs’ of tissue between each vertebral body, which help to cushion the load that is carried by the spine.

The vertebrae are roughly circular and between each vertebra is a ‘disc’. The discs are made of strong rubber-like tissue which allows the spine to be fairly flexible. Strong ligaments also attach between adjacent vertebrae to give extra support and strength to the spine. Muscles and ligaments (tissue that connect bones) and help to control the movements (flexion and rotation) of the spine support the vertebrae. The surrounding tissues help to strengthen the spine and hold it in place.

There are also small facet joints that help to attach vertebrae to each other. The sacrum is formed from five fused vertebrae that are joined together. They form a triangular shaped structure at the bottom of the spine. The two sacro-iliac joints are the large long joints that join the sacrum to the ilium (the main bone of the pelvis).

The bones behind each vertebral body form a protective ‘canal’ for the spinal cord. They also make joints above and below the vertebra, allowing the spine to bend and twist. Behind the spinal canal there are vertebral spines, which can be felt as a series of bumps along the spinal column.

The spine carries the weight of the head, arms, chest, and abdomen. The sacroiliac joints connect the spine to the pelvis. The purpose of the sacroiliac joints is to transmit the weight of the upper body to the pelvis.

How does ankylosing spondylitis occur?

In ankylosing spondylitis, the spinal joints and ligaments and the sacroiliac joints become inflamed. Inflammation in the spine can cause pain and stiffness in the neck and back. Sacroiliitis (inflammation of the sacroiliac joints) causes pain in the lower back and buttocks. If not treated over many years, the inflammation causes the neck and back to become rigid. This process is called ankylosis. In some people who have severe, long-standing ankylosis, their rib cage (chest) can also become stiff and inflexible.

Ankylosing spondylitis can also lead to arthritis developing in the large joints, such as the hips and knees.

Symptoms of ankylosing spondylitis 

The symptoms of ankylosing spondylitis (AS) vary greatly from person to person, but they will take a long time to develop. You may also find that the symptoms of ankylosing spondylitis come and go, becoming better or worse over many years.

The main symptoms of ankylosing spondylitis are:

• back pain and stiffness,
• buttock pain,
• joint inflammation (arthritis),
• enthesitis (painful inflammation where tendons or ligaments attach to bone), and
• fatigue.

Back pain and stiffness

Back pain and stiffness are usually the main symptoms of ankylosing spondylitis. The pain tends to differ from other types of backache. If you have ankylosing spondylitis, you may find that:

• your pain gets better with exercise, but not with rest,
• your back is particularly stiff in the morning, lasting for more than 30 minutes after you start to move around,
• you wake up in the second half of the night with pain and stiffness, and
• you have pain in your buttocks, sometimes on one side, and sometimes on the other.

As well as causing symptoms in your back and spine, ankylosing spondylitis can also cause arthritis in your hip, knee and other joints.

However, it is unlikely that you will develop symptoms in all of these areas. The main symptom that you will experience is likely to be back pain.

Arthritis

The main symptoms that are associated with arthritis are:

• pain on moving the joint,
• tenderness when the joint is examined,
• swelling, and
• warmth in the affected area.

Enthesitis

Enthesitis is painful inflammation where a tendon or ligament joins a bone. Common sites for enthesitis are at the top of the shin bone, behind the heel, under the heel, and at the ends of the ribs.

If your ribs are inflamed, you will have chest pain and you may find it difficult to expand your chest when breathing deeply.

Fatigue

Fatigue is a common symptom of untreated ankylosing spondylitis. The condition makes you feel tired and lacking in energy.

Causes of ankylosing spondylitis 

The cause of ankylosing spondylitis (AS) is not fully understood. However, certain factors have been identified which may increase your risk of developing the condition.

Genetic markers (HLA-B27)

Research has shown that most people who have ankylosing spondylitis carry a particular genetic marker, known as human leukocyte antigen B27 (HLA-B27). This means that people who have this antigen are more likely to develop ankylosing spondylitis than those who do not.

However, as most people with the HLA-B27 gene never develop ankylosing spondylitis, genetics cannot be the only cause of the condition.

Also, most people who have the HLA-B27 gene do not develop ankylosing spondylitis, so testing for the marker is not a very reliable method of diagnosis.

Family history

Ankylosing spondylitis can run in families and the HLA-B27 antigen can be inherited (passed on) from one member of a family to another.

If you have a close relative, such as a parent, or sibling, who has ankylosing spondylitis, you are three times more likely to develop it compared with someone who does not have a relative with the condition.

However, as ankylosing spondylitis is a relatively rare condition, your overall likelihood of developing it is very small.

Diagnosing ankylosing spondylitis 

See your doctor if you think that you may have ankylosing spondylitis (AS).

Your doctor will ask you about your symptoms and they may do some blood tests. These may include the following:

• full blood count (FBC) to check for a reduced number of red blood cells (anaemia),
• erythrocyte sedimentation rate (ESR) to measure the rate at which red blood cells settle through a column of liquid.
• C-reactive protein (CRP) to see if your blood contains a particular protein that is produced by your liver.

These blood tests can help to confirm whether or not there is inflammation in your spine and joints, which is one of the main symptoms of ankylosing spondylitis.

If your doctor thinks that you may have ankylosing spondylitis, they will refer you to a rheumatologist (a specialist in conditions affecting the muscles and joints) for further tests.

Further tests

Your rheumatologist will carry out some imaging tests in order to examine the appearance of your spine and pelvis. You may have X-rays, a magnetic resonance imaging (MRI) scan or an ultrasound scan.

X-rays

An X-ray uses short bursts of high-energy radiation to create images of hard substances in your body, such as bones.

X-rays of your lower back can show severe signs of ankylosing spondylitis, such as damage to the joints in your pelvis, and new bone forming between the vertebrae in your spine.

MRI scan

A magnetic resonance imaging (MRI) scan creates an image of the inside of your body using strong magnetic and radio waves.

A MRI scan may be able to highlight changes in the joints of your pelvis that may not show up on an X-ray.

Ultrasound scan

An ultrasound scan uses sound waves to examine the inside of your body, in the same way that they are used to view a baby inside the womb.

An ultrasound scan can pick up inflammation of the tissues (tendons and ligaments) that are attached to your bones.

Diagnosing ankylosing spondylitis

The imaging procedures described above can show the extent of any spinal inflammation and ankylosis (fusing of the spine) that you may have.

However, as ankylosing spondylitis often takes a long time to develop, it will not be possible to see damage to your spine that is yet to occur. For this reason, the condition is often difficult to diagnose and, in many cases, confirming a diagnosis is a long process.

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