Spondyloarthritis
Spondyloarthritis (SpA) is a chronic autoinflammatory condition that affects spine structures, entheses and joints. SpA typically affects just the spine and posterior pelvis (sacroiliac joints) in many people - termed 'axial' (as opposed to ‘peripheral’) SpA.
SpA causes inflammatory back pain, often at a young age (it can occur in children and adolescents) but it starts to cause symptoms typically in the 20s and 30s. Inflammatory back pain is notable for its effect - causing stiffness and pain often with immobility rather than movement. So people with SpA can get night-time back pain and stiffness which can be bad first thing in the morning then ease up as they move around.
The advanced form of SpA is termed Ankylosing Spondylitis (AS). AS occurs in about 0.5% of the population (1 in 200 people) and often runs in families. AS is very closely associated with the having the gene HLA B27 (which occurs in about 8-9% of the general UK, European or USA population). It's easy to test for this gene with a blood test.
If you are getting back pain with stiffness at night, first thing in the morning or after sitting still for long time (e.g. after long car journey) then you may have inflammatory back pain and you should see a rheumatologist. SpA is diagnosed through a rheumatologist's assessment, lab tests (e.g. checking inflammatory markers and HLA B27) and a scan of the back/pelvis (MRI).
An important feature of SpA (and indeed psoriatic arthritis) is that musculoskeletal symptoms/features can come and go over many years - particularly as both these conditions can cause pain and stiffness owing to inflammation at entheses (the insertion point of ligaments and tendons at bone edges). Any diagnostician/rheumatologist will need to be aware of the condition potentially appearing and disappaearing (mostly mildly) over a long duration of time - possibly back to childhood. Unfortunately ‘enthesitis’ is poorly understood by the vast majority of people (including doctors) and frequently the condition is missed and not diagnosed for many years (and other ‘wrong’ diagnoses given by many otheres!)
Links:
Patient advocacy charity in The UK formed pre-2009 before the modified ASAS criteria for SpA were published. Historically then geared more to people with AS compared with axial SpA: The National Ankylosing Spondylitis Society (NASS)