A type of arthritis, spondylitis or SpA is commonly known as osteoarthritis. It is caused due to slow wear and tear of the spine. This slow wearing and tearing are also known as degenerative changes. These changes in the spine are caused by bone spurs and degenerating intervertebral discs located in the back.

Most of the time, spondylitis does not have any symptoms. It is mostly related to age. It is estimated that nearly 85 percent of adults older than 60 years have some form of spondylitis. According to the Spondylitis Association of America, there are two classification systems of spondylitis—traditional SpA classification system and newer SpA classification system.

The traditional SpA classification system
This recognizes six diseases or types of spondylitis. These include ankylosing spondylitis, enteropathic arthritis, reactive arthritis, undifferentiated spondyloarthritis, and juvenile spondyloarthritis.

• Ankylosing spondylitis : This type of spondylitis is caused by the inflammation of the spine and pelvis. Before the age of 45, many people experience inflammatory back pain. Usually, the pain starts slowly. It improves with physical activities. However, being on rest does not help. In fact, it causes stiffness. People with ankylosing spondylitis generally experience stiffness when they wake up in the morning. This stiffness lasts for about 30 minutes. Over time, as the intensity of inflammation increases, ankylosing develops. Ankylosing means the formation of new bone in the spine. This causes sections of the spine to fuse together leading to a fixed immobile position. This type of spondylitis can also cause inflammation, pain, as well as stiffness in other parts of the body including hips, shoulders, heels, ribs, and other joints.
• Enteropathic arthritis : This type of spondylitis is arthritis related to inflammatory bowel disease. Crohn’s disease and ulcerative colitis are two common IBDs associated with enteropathic arthritis. It has been observed that nearly one in five people with ulcerative colitis or Crohn’s disease will have this type of spondylitis. It causes inflammation of the back along with joint pain. A predominant feature of enteropathic arthritis is inflammation of the intestine including the bowel. The symptoms associated with this type of spondylitis include abdominal pain, chronic diarrhea, blood in the stool, and weight loss. Also, the peripheral joints and the spine are affected.
• Reactive arthritis : This type of spondylitis causes pain and inflammation in the mucous membranes, joints, skin, eyes, genitals, and bladder. Unlike most other forms of spondylitis, reactive arthritis does not affect the spine and the joints in the pelvic area. This form of spondylitis is usually caused due to infection in the urinary tract or intestine. It is a reaction to an infection, mostly in the gastrointestinal tract. It has quite a limited course; the symptoms last for about 3 to 12 months. However, reactive arthritis can recur. Often people with reactive arthritis have a chronic form of arthritis.
• Psoriatic arthritis : It is estimated that about 30 percent of people who have psoriasis have psoriatic arthritis. Generally, psoriasis precedes arthritis by several years. When psoriatic arthritis begins, there is inflammation and pain in joints of the fingers. It can also affect the joints of the wrists, knees, and ankles. Fingernails and toenails may also be affected; small pits may develop in the nails. This may progress to complete crumbling of the nail bed. A toe or finger may also start swelling around the joints or between the joints. Nearly 20 percent of the people with psoriatic arthritis experience inflammation, stiffness, and pain in the spine as well. The effect can be similar to ankylosing spondylitis—there is a complete fusion of sections of the spine. It can also affect the neck or lower back.
• Undifferentiated spondyloarthritis : In this type of spondylitis, the symptoms and effects are common to both spondylitis and arthritis. However, the symptoms cannot be classified into any of the known forms of spondylitis. Hence, the name, undifferentiated spondyloarthritis (USpA). Thus, a person with this type of spondylitis may have some of the common symptoms of the ailment such as iritis, heel pain, and knee swelling. However, they might not have back pain, psoriasis, intestinal symptoms, or any other infection that may give a definite diagnosis of spondylitis. Often people with USpA are given an incorrect diagnosis of fibromyalgia or even told that they have anxiety or depression that are causing these symptoms. Over time, USpA may progress to a more well-defined type of spondylitis, such as ankylosing spondylitis.
• Juvenile spondyloarthritis : The symptoms of this form of spondylitis start during childhood or teenage years. The symptoms include enthesitis and inflammation where ligaments and tendons meet the bone. Also, there can be peripheral arthritis where joints in the lower extremities are affected.

The newer SpA classification system
This classification system includes two broad categories—axial spondyloarthritis and peripheral spondyloarthritis. In axial spondyloarthritis (AxSpA), there is inflammation in the spine, pelvis, and the joints between the lower back and the pelvis. AxSpA can be further categorized into radiographic and nonradiographic. In radiographic AxSpA, the changes in the bones can be seen on X-ray, which is not true for nonradiographic AxSpA.

In peripheral spondyloarthritis, inflammation is observed in the tendons and joints outside the spinal and pelvic areas. Joints of the feet, ankles, knees, shoulders, elbows, wrists, and hands are affected.