A severe form of chronic inflammatory back pain is called ankylosing spondylitis (AS), where damage to joints separating vertebrae can be seen on X-rays. Less severe cases are called non-radiographic back pain, because no damage can be seen on X-rays, despite inflammation and pain which is very similar to AS. Inflammation intensity tends to increase with age, so some patients' non-radiographic back pain will become radiologically-confirmed AS. The most severe cases are called “bamboo spine”, where joints between each vertebrae have been completely destroyed, and the vertebrae have fused together.
Spondyloarthropathies
Individuals with AS-type chronic inflammatory back pain have a markedly increased risk of suffering from a variety of seemingly unrelated peripheral symptoms, including Crohn’s disease and psoriasis.
Historically, Crohn’s disease, psoriasis, ulcerative colitis, spondylitis, sacroiliitis, seronegative arthritis, acute anterior uveitis (AAU), chronic prostatitis (men), non-specific urethritis (men) and chronic cervicitis (women), were all considered separate diseases, and were studied by doctors in different medical specialties. Epidemiologists have known since the 1950s that these diseases run in families, and that having any one increases the risk of having any other. This meant these diseases must have a common genetic basis.
Some symptoms tend to occur together, and common combinations are given clinical names such as ankylosing spondylitis (AS), reactive arthritis (ReA), psoriatic arthritis (PsA) and enteropathic arthritis (EnA). However, many patients do not fit well into these categories, and are given the label “undifferentiated” spondyloarthropathies. It is now realized that all symptom combinations are possible, and each symptom listed above is likely a different manifestation of the same disease process occurring in different organs.
Spondyloarthropathies are caused by T cells
All spondyloarthropathy symptoms are mediated by T cells which are recognizing one or more antigenic proteins in each affected organ, causing chronic inflammation. Why these T cells are activating to cause this inflammation, and which proteins they are targeting, are two long-open questions in medicine.
Some researchers think these T cells are fighting an infection, while others think these T cells have gone haywire and are attacking healthy human cells for no good reason. After more than sixty years of research, neither camp has been able to prove that they are correct.
NB: This post is based on Laurence et al 2018 (simplified for clarity). While many cases of back pain are the results of spondyloarthropathies, others are due to physical trauma, spinal disc herniation, etc.
Animated back pain video (intro): https://youtu.be/S-VhDiqwBnk
Animated back pain video (full): https://youtu.be/e2vpMqmLYQo
