Axial spondyloarthritis/ankylosing spondylitis

Axial spondyloarthritis (Axial SpA) is a spectrum of aseptic inflammatory disease of the joints and entheses of the spine. When there is clear radiographic change at the sacroiliac joints and the modified New York criteria are met, the term ankylosing spondylitis (AS) is used, whilst for individuals in whom inflammatory changes can only be demonstrated by magnetic resonance imaging (MRI), the term non‐radiographic axial SpA (nr‐axSpA) is applied. Most current data are derived from studies of AS.

Ankylosing spondylitis occurs in 0.2% of the general population, in 2% of the B27‐positive population, and in 20% of B27‐positive individuals with an affected family member. Males predominate with a male/female ratio ranging from 2.5:1 to 5:1 whereas in nr‐axSpA the sex ratio is equal. AS typically begins in young adulthood, but symptoms may arise in adolescence or earlier. Up to 15% of children with juvenile idiopathic arthritis are classified as having juvenile‐onset spondyloarthritis (JoSpA). Such children present with pauciarticular peripheral arthritis with a predilection for the tarsal joints; axial complaints, with the development of radiographic sacroiliitis, tend only to develop in late teenage or later.

The first symptom of AS is usually inflammatory back pain (Box 14.1) – the insidious onset of low back pain and/or buttock pain which persists more than 3 months, awakens the patient from sleep, is accompanied by early morning stiffness and is typically improved by exercise. Fatigue often accompanies inflammatory back pain but may also be present in fibromyalgia and other conditions. Persistent uncontrolled disease leads to persistent stiffness and progressive loss of spinal mobility.

The diagnosis of axial SpA is based on the ASAS classification criteria (Figure 14.2) which require characteristic SpA features combined with evidence of sacroiliitis demonstrated either by MRI scanning or X‐ray. The modified New York criteria (Box 14.2), requiring radiographic sacroiliitis, describe AS: classic radiographic changes in the sacroiliac joints include erosions in the joint line, pseudo‐widening, subchondral sclerosis and finally ankylosis, reflected as obliteration of the sacroiliac joint. Radiographic and MRI changes of sacroiliitis are shown in Figure 14.3. Radiographs of the spine may reveal squaring and ‘shiny corners’ of the vertebral bodies (Figure 14.4a) and, later, syndesmophytes and facet joint fusion (Figure 14.4b). Not all patients with nr‐axSpA will develop radiographic change so MRI is able to identify both patients who have not yet developed radiographic change and those who never will. HLA‐B27 is also of diagnostic value when there is a high index of suspicion.