What is spondyloarthritis?
A group of inflammatory arthritis diseases
The term spondyloarthritis (SpA) describes a group of inflammatory arthritis diseases with common features, including inflammation of the spine, eyes, skin and gastrointestinal tract. This group was also sometimes referred to as spondylitis and spondyloarthropathies. This group is clinically and genetically related but has distinct entities with several genetic, prognostic and therapeutic differences.
Characteristics of spondyloarthritis:
- Spinal arthritis
- The presence of the gene HLA-B27
- Peripheral arthritis that differs from rheumatoid arthritis
- Extra articular manifestations of inflammatory bowel disease (IBD), psoriatic arthritis (PsA), and Uveitis
Two Classification of Spondylitis:
1. Traditional SpA group of diseases includes:
- Ankylosing spondylitis (AS)
- Psoriatic arthritis (PsA)
- Enteropathic arthritis (arthritis associated with inflammatory bowel disease or IBD)
- Reactive arthritis (ReA)
- Juvenile idiopathic arthritis that is enthesitis related arthritis (JIA-ERA)
- Undifferentiated spondyloarthritis (USpA)
2. Newer SpA Classification:
As more is learnt about spondyloarthritis, the terminology is changing. Today, instead of using the names of the diseases above, we break down spondyloarthritis into two subsets:
- Axial spondyloarthritis (AxSpA)
- Peripheral spondyloarthritis (pSpA)
Axial spondyloarthritis (AxSpA) affects the spine and sacroiliac joints (the joints between the pelvis and the sacrum or base of the spine). This term is explained as a subheading in the section on ankylosing spondylitis.
Peripheral spondyloarthritis affects areas away from the spine and sacroiliac joint such as finger, arm, foot and leg joints as well as the insertion of muscles and ligaments on bones (enthesis). It includes psoriatic arthritis, reactive arthritis and enteropathic arthritis or inflammatory bowel disease.
Spondyloarthritis’ Two Other Conditions
Two other conditions fall under spondyloarthritis. The first is enthesitis-related arthritis (ERA), which is a type of juvenile idiopathic arthritis. Juvenile means beginning before age 16. JIA-ERA was known as juvenile spondyloarthritis (JSpA).
The second is undifferentiated spondyloarthritis. Undifferentiated spondyloarthritis describes someone with all the features of spondyloarthritis but without the confirming characteristics.
Although spondyloarthritis is form of inflammatory arthritis, it differs from rheumatoid arthritis (RA) because people with SpA do not have rheumatoid factor antibodies in their blood. They are seronegative whereas people with RA are seropositive.
What causes spondyloarthritis?
We don’t know what causes spondyloarthritis, but we do know that there is a genetic factor. About 90% of people with spondyloarthritis carry the HLA-B27 gene, a gene that is found in about 7% of the white western European population. The involvement of this gene has been known since 1973 but there is no defining mechanism known by which B27 causes SpA.
Confusingly, the other 10% of people with SpA do not have the HLA-B27 gene, so there must be other factors at play. Researchers have now identified at least twenty genes that have been linked to SpA but their role is not well understood. It is known that the genes ERAP 1 and 2 are closely related to B27, and it may be that it is the interaction of these genes with B27 that causes the disease. It is also known that another gene, that codes for the receptor of IL-23 is involved.
A great deal of genetic research is going on worldwide to identify the genes associated with SpA and the susceptibility of someone, with these genes, developing disease.
Who gets spondyloarthritis?
People with a family member with spondyloarthritis are at higher risk of developing spondyloarthritis depending on whether they inherited the HLA-B27 gene. About 90% of people who develop spondyloarthritis will carry the HLA-B27 gene.
Teens and Early Twenties
Spondyloarthritis often starts in the teen years and early twenties. It is unusual for it to start after age 45. It affects both males and females but ankylosing spondylitis is two to three times more common in men than women.
Predominately in White Western European Population
The HLA-B27 gene is found in about 7% of the white western European population with higher and lower rates in other ethnic groups. It tends to be higher in far northern groups such as Inuit and Samis and in certain North American tribes on the Pacific coast and lower in African populations. It is non-existent in Australian Aboriginal populations.
1 in Every 200 People
The frequency of ankylosing spondylitis is about 0.50% of the population (1 in every 200). The frequency of axial spondyloarthritis is about 1.4% of the population.