Inflammatory Arthropathy – Ankylosing Spondylitis

Introduction Ankylosing spondylitis (AS), also known as Pierre-Marie Strumpell’s disease and, more commonly, as Bechterew’s disease, is nowadays a well-recognized inflammatory disease entity with a prevalence about 0.5%. AS primarily involves the sacroiliac joints (SIJ) and the spine, but also peripheral joints and entheses. The SIJ and spinal involvement cause chronic inflammatory back pain and progressive stiffness due to joint destruction which may result in ankylosis. The disease onset is often in early adulthood and it can over time cause severe disability. Males predominate and there is a genetic disposition with a frequent association with the human leukocyte antigen (HLA) B27 (1). Conventional radiography of the SIJ and the spine has previously been the hallmark for the diagnosis of AS although it is insensitive to early bone and joint damage. However, until recently the absence of effective treatment limited the need for more sensitive imaging techniques. This changed after the introduction of the anti-tumor necrosis factor alpha (anti-TNFα) agents that have proven promising for alleviating inflammatory symptoms of AS and possibly preventing structural damage (2). Magnetic resonance imaging (MRI) has proven effective in depicting disease activity in the SIJ and the spine in AS. MRI has also been shown to be valid for assessing change of disease activity over time in patients treated with anti-TNFα agents (3;4). However, according to the international accepted modified New York criteria a definite AS diagnosis still demands the presence of manifest sacroiliitis by radiography in addition to typical clinical findings (low back pain lasting for three months, limitation of lumbar spine motion, decreased chest expansion) (5). The weakness of the modified New York criteria is the limited ability to diagnose the early stages of AS due to the obligatory demand for manifest sacroiliitis by radiography. This may delay the AS diagnosis with 811 years (6). MRI is the most sensitive method for diagnosing sacroiliitis and can detect signs of activity early in the disease, even before the occurrence of erosion (7). However, the occurrence of active inflammation at the SIJ does not always imply AS because other forms of seronegative arthritides involving the axial skeleton can cause joint inflammation. The concept spondyloarthritis (SpA) was introduced in 1976 (8) and in 1991 the European Spondylarthropathy Study Group (ESSG) classified the disorders into five entities: idiopathic AS, psoriatic arthritis, reactive arthritis, arthritis associated with inflammatory bowel disease (Crohn’s disease or ulcerative colitis) and unclassified SpA (9). The ESSG classification is mainly based on clinical findings, but also includes radiographic evidence of sacroiliitis. It has therefore not solved the problem with a delayed diagnosis due to a low sensitivity of radiography early in the disease. Extensive international collaborative work has recently resulted in new ASAS (Assessment of SpondyloArthritis international Society) classification criteria for SpA encompassing MR signs as the imaging confirmation of sacroiliitis (10). A positive MRI for sacroiliitis was defined mainly based on signs of active inflammation (11). The ASAS criteria will facilitate the early diagnosis of SpA, but it is most important to predict changes as part of AS and thereby valuable with regard to therapy, prognosis and working capacity. However, a certain early diagnosis of AS sacroiliitis by MRI cannot be based on signs of disease activity only because the other forms of arthritides involving the SIJ also cause joint inflammation. Some of the SpA disorders diagnosed with the ASAS criteria (e.g. reactive arthritis) may have a time limited course and do not imply a progressive course as seen in AS. It is therefore important that the early AS diagnosis established by MRI is not only based on active inflammatory SIJ changes. There should also be some degree of detectable chronic structural changes corresponding to the radiographic changes included in the New York criteria for AS. The recent finding of a significant correlation between radiographic erosions and erosions detectable by MRI using both T1 and T1FS sequences (12) is important in this aspect. With the inclusion of erosions by MRI it is possible to establish a correct diagnosis of AS sacroiliitis by MRI in relative