The early diagnosis of Axial Spondyloarthritis (Axial SpA) has been the focus of the Rheumatological community, with the Assessment SpondyloArthritis international Society (ASAS) recently publishing classification criteria to assist in this regard.
The criterion presented below was published by Rudweleit and his associates in 2009 (Rudweleit et al.Annals of Rheumatic disease2009; 68: 777-783)
The entry criteria require back pain to be present for at least 3 months and the onset of the symptoms begin before the age of 45. These have been determined because back pain is very common in the community, most commonly related to a structural cause such as an injury to the intervertebral disc. However, back pain of this type resolves in the majority of those affected within the first 3 months. In addition, the young age of onset reflects the fact that SpA often affects patients in the 3rd decade of life, with the average age of onset commonly reported to be in the mid 20s.
Of the SpA features, inflammatory back pain (IBP) is most prominent. However, the definition of (IBP) has itself been often been debated with a number of criteria being put forward to identify this symptom most accurately. Most recently ASAS also put forward a criterion to define back pain as inflammatory in nature. The clinical features of back pain that are suggestive of inflammation include:
- Age at onset <40 years
- Insidious onset
- Improvement with exercise
- No improvement with rest
- Pain at night with improvement upon getting out of bed
They found that when 4 of these 5 features are present the pain can be confidently categorised as being inflammatory back pain. It should be noted however that the presence of a single feature is not sufficient to accurately discriminate back pain as either due to inflammation or a mechanical cause.
The ability to visualise inflammation involving the spine has been a major advance in the diagnosis of SpA.
Magnetic Resonance Imaging (MRI) has revolutionised the approach to assessing patients with IBP, given the sensitivity this modality has demonstrated in the identification of inflammation present at involved joints, particularly at the sacroiliac joints (which are most commonly affected by the disease). However, MRI imaging does not identify inflammation in all patients with SpA (being negative in about 20% of those affected) and so a negative MRI scan does not rule out the disease.
Therefore, ASAS has suggested HLA B27 can be helpful in the diagnosis of SpA. HLA B27 is a protein that is present on the surface of certain cells, where it has an important role in the action of the immune system. HLA B27 is strongly associated with Ankylosing Spondylitis and to a lesser extent the other Spondyloarthritides. This marker is present in 90-95% of those with AS, which is higher compared to those with Psoriatic spondyloarthritis, Enteropathic spondyloarthritis, or Reactive arthritis where HLA B27 is present in 50%, 60%, and 75% respectively. However, HLA B27 is present in 8-12% of the ‘normal’ Australian population and as such, on its own does not define SpA. Consequently, for the presence of HLA B27 to be considered relevant, two or more clinical features of SpA need to be present.
Assessing for the presence of inflammation throughout the body via blood tests, by measuring the level of C-Reactive Protein (CRP) and/or Erythrocyte Sedimentation Rate (ESR) can be useful in determining the severity of disease, even though these tests have limited utility in the diagnosis of the disease. They are elevated in only a proportion of patients with SpA, but when raised, suggest that the condition is more likely to develop damage and result in functional impairment as well as predict a shorter survival in those that develop AS.
As a result, certain treatment options that have recently become available are only subsidized by Medicare Australia in those in whom the inflammatory markers mentioned above are raised.
Finally and interestingly a rapid response to the use of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) is also considered as a diagnostic test for SpA. NSAIDs are an important component in the management of SpA as patients generally demonstrate a significant improvement in their symptoms, usually within the first two days of use. However, a lack of response to an NSAID certainly does not exclude the diagnosis of SpA.
As described above, none of these diagnostic tests are individually able to rule the diagnosis either in or out, but rather it is the combination of positive results to these tests that allow the diagnosis of SpA to be made with confidence early in the course of the disease. This is of value since chronic back pain is a common problem in primary care, for which it has estimated that SpA is the cause in 5%.