There are several conditions that fall under the SpA category, including:
- Enthesitis-related arthritis
- Juvenile ankylosing spondylitis (JAS)
- Reactive arthritis
- Arthritis associated with inflammatory bowel disease (IBD) and psoriasis (see table 1)
Since they differ in important ways, each will be considered separately.
Enthesitis-Related Arthritis (ERA)
In many children and adolescents, ERA begins as arthritis in the large joints of the lower extremities, particularly the hips and knees. It is also common to have pain and tenderness due to inflammation where tendons or ligaments attach to bones. This is called “enthesitis.” Common sites for this inflammation are at the heel, the top and bottom of the kneecap (patella), the ball of the foot and bottom of the foot at the heel (plantar fascia). Also, in some patients with ERA, inflammation in the joints of the foot, particularly in the mid-foot region (tarsitis), is seen. It is uncommon for arthritis to occur in the elbows and wrists, but it does occur in the shoulders. Sometimes patients with ERA will have inflammation in other parts of their body. Eye inflammation (anterior uveitis) occurs in approximately half of patients. Unlike the inflammation in the eyes seen in other types of arthritis, the uveitis seen in ERA almost always causes pain and redness.
Juvenile Ankylosing Spondylitis (JAS)
JAS is a more fully developed form of SpA in children that requires the child to have developed arthritis in the lower back in either the spinal joints or the sacroiliac joints (sacroiliitis).
Diagnosing JAS can be difficult if the spine and/or lower back are not involved when other symptoms begin. Since the back involvement may not occur for many years after other symptoms have begun, in this situation the child is said to have ERA. In studies looking at children who eventually developed JAS but initially had arthritis that looked just like ERA, it was found that they were more likely to have had lower extremity arthritis and pain and tenderness where tendons or ligaments attach to bones during the first year of their illness, and frequent involvement of the foot (tarsitis).
Thus when many of the symptoms are present but the child has yet to develop back pain (sacroiliitis), we typically call it ERA. Some studies suggest that as many as 50 percent of these children will develop complete JAS with time either in late adolescence or as an adult. However, this depends on genetic factors, and it is not yet possible to predict with certainty who with ERA will or will not develop JAS.
Reactive Arthritis (ReA)
Reactive arthritis (ReA) usually occurs two to four weeks after an infection in some other part of the body like the gastrointestinal tract, urinary tract or the genitals. Gastrointestinal infections are usually accompanied by diarrhea, abdominal pain and cramping, and are caused by bacteria like salmonella. Infection in the urinary track or genitals may not be associated with specific symptoms (especially in females). The inflamed joints do not contain these bacteria, but are still the site where many inflammatory cells accumulate and cause symptoms for reasons that are unknown. Like other forms of SpA, development of ReA is much more common in people with the genetic marker HLA-B27. This is a gene that is present in about 70 percent to 90 percent of patients with arthritis that affects the spine. ReA in children can be short-lived with complete resolution of symptoms, or may continue with a more chronic course. Some patients may go on to develop complete JAS over several years, but again this is difficult to predict.
Arthritis Associated with Inflammatory Bowel Disease (IBD) or Psoriasis
Arthritis can occur as a manifestation of either type of IBD − Crohn's disease or ulcerative colitis. The arthritis can involve joints in the arms or legs and/or spinal joints including the sacroiliac (SI). When the spine or SI joints are involved it is usually associated with the HLA-B27 gene. Arthritis is also seen in some but not all people who have the skin disease psoriasis.