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Key Points

  • Disease summary:

    • The spondyloarthropathies (SpA) are a group of disorders causing inflammatory arthritis that typically involve the sacroiliac joints of the pelvis and usually the spine, and in which inflammation occurs at the insertion point of tendons into bone (enthesitis). They also share genetic associations notably with HLA-B27. Ankylosing spondylitis (AS) is the archetypal spondyloarthropathy; other members of this group include reactive arthritis (ReA), psoriatic arthritis (PsA), arthritis of inflammatory bowel disease (IBD), enteropathic arthritis, and axial spondyloarthritis (axial SpA). Where only peripheral joints are involved, the condition is termed peripheral SpA.

    • The main symptom of SpA is inflammatory back pain (IBP). This is chronic back pain that improves with activity, worsens with rest, and can cause patients to wake with back pain at night.

    • Inflammation in the sacroiliac joints (sacroiliitis) is universal in axial SpA.

    • Inflammation occurs where tendons join to bone (enthesitis). Commonly affected sites include the Achilles tendon, plantar fascia, and costal joints in SpA.

    • Peripheral arthritis occurs in axial SpA and usually involves large joints such as the knee, often asymmetrically.

    • Ankylosis, or bony fusion, of the sacroiliac joints and spinal vertebrae are the pathognomic feature of AS and other SpA.

    • SpA-associated conditions include skin psoriasis, IBD, and iritis.

  • Differential diagnosis:

    • The most common alternate diagnoses for chronic back pain are degenerative lumbar disc disease (spondylosis) or facet joint osteoarthritis, and lumbar disc prolapse or tear. Diffuse idiopathic skeletal hyperostosis (DISH) often resembles AS radiographically but usually does not cause marked symptoms. The differential diagnoses of sacroiliitis include DISH, osteitis condensans ilii, degenerative arthritis, and septic arthritis.

  • Monogenic forms:

    • No monogenic forms of SpA exist.

  • Family history:

    • There is often a family history of one or more of the following: spondyloarthropathy, iritis, psoriasis, or IBD.

  • Twin studies:

    • Twin studies in AS have shown that the environmental contribution to disease etiology is small; the identical twin concordance rate for the disease is greater than or equal to 60%. There is concordance among related individuals for SpA depending on the type of SpA, HLA-B27 status, and genetic relationship to the affected person (see Screening and Counseling section).

  • Environmental factors:

    • ReA is caused by an immune reaction to microbial infection. Microbial infections are also thought to underlie the development of other forms of SpA, but thus far no specific organism has been convincingly implicated. Cigarette smoking increases the severity of AS and may also increase the risk of developing the disease.

  • Genome-wide associations:

    • There have been three genome-wide association studies (GWASs) to date, one of which studied only nonsynonymous single-nucleotide polymorphisms (SNPs). These studies and other more targeted association studies have described 13 non-MHC confirmed associations. Along with the long known HLA-B association the results include SNPs in IL23R, ERAP1, CARD9, IL12B, TBKBP1, TNFR1, PTGER4, RUNX3, intergenic loci (2p15, 21q22), IL1R2, ANTXR2, and KIF21B. Some of these SNPs have also been identified in other autoimmune diseases like psoriasis and IBD.

  • Pharmacogenomics:

    • There is currently limited evidence for the role ...

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