Ankylosing spondylitis (AS - Spondyloarthritis)
AS (less commonly known as Bechterew disease and Marie Strümpell disease) is a chronic systemic inflammatory disease affecting the sacroiliac joints, spine, and, not infrequently, peripheral joints. The name is derived from the Greek roots ankylos, meaning “bent” (ankylosis means joint fusion), and spondylos, meaning “vertebra.” The diagnosis is “primary AS” if no associated disorder is present and “secondary AS” if associated with reactive arthritis, psoriasis, ulcerative colitis, or Crohn’s disease.
Sacroiliitis and enthesitis are the hallmarks of ankylosing spondylitis (AS).
Rheumatoid factor (RF) and antinuclear antibody (ANA) are negative (seronegative spondyloarthropathy).
Human leukocyte antigen (HLA)-B27 increases the risk of developing AS 50 to 100 times.
Only 2% (1 out of 50) of HLA-B27-positive individuals develop AS during their lifetime.
Tumor necrosis factor (TNF)-α blocking agents are very effective in AS for both spinal and peripheral joints.
Radiographic sacroiliitis is the hallmark of AS. However, it takes an average of 4 to 9 years from onset of inflammatory back pain to the development of definite radiographic sacroiliitis. Because the majority (70%) of patients in early disease will not have definite radiographic changes at the time of diagnosis, the ASAS (Assessment of SpondyloArthritis International Society) classification criteria were developed for patients with back pain greater than 3 months and age of onset less than 45 years. These criteria have a sensitivity of 83% and specificity of 84% for a patient having an axial SpA
ASAS Classification Criteria for Axial SpA:
SpA features: inflammatory back pain, arthritis, enthesitis (heel), uveitis, dactylitis, psoriasis, Crohn’s disease/ulcerative colitis, good response to nonsteroidal antiinflammatory drugs (NSAIDs), family history for SpA, HLA-B27, elevated C-reactive protein (CRP).
Sacroiliitis on imaging: active (acute) inflammation on MRI showing sacroiliitis or definite radiographic sacroiliitis.
Sacroiliitis on imaging
≥1 SpA feature
≥2 other SpA features
Most commonly peripheral joints involved in AS:
Approximately 30% of patients with AS develop a peripheral arthritis. The hips and shoulders (girdle joints) are most commonly involved. Notably, hip involvement in AS is associated with a poor prognosis. Rarely, arthritis of the sternoclavicular, temporomandibular, cricoarytenoid, or symphysis pubis occurs. Involvement of the thoracic costovertebral, sternocostal, and manubriosternal joints may cause chest pain worsened by coughing or sneezing.
Extraskeletal manifestations of AS:
A—aortic insufficiency (3% to 10%), ascending aortitis, and other cardiac manifestations, such as conduction abnormalities (3% to 9%), diastolic dysfunction, pericarditis, and ischemic heart disease.
N—neurologic: atlantoaxial (C1 to C2) subluxation (2%), cauda equina syndrome from spinal arachnoiditis, traumatic spinal fractures with myelopathy (C5 to C6, C6 to C7 most commonly), ossification of the posterior longitudinal ligament with spinal stenosis.
K—kidney: secondary amyloidosis, immunoglobulin A (IgA) nephropathy, chronic prostatitis.
S—spine: cervical fracture, spinal stenosis, significant spinal osteoporosis.
P—pulmonary: upper lobe fibrosis, restrictive changes.
O—ocular: acute anterior uveitis (25% to 30% of patients).
N—nephropathy (IgA), nephrolithiasis
D—discitis or spondylodiscitis (Andersson lesions).
In addition, 30% to 60% of patients have asymptomatic microscopic colitis or Crohn’s-like lesions in their terminal ileum and colon. AS patients with peripheral arthritis are more likely to have colitis lesions.
Typical radiographic features of AS:
The radiographic changes of AS are predominantly seen in the axial skeleton (sacroiliac, apophyseal, discovertebral, and costovertebral) as well as at sites of enthesopathy (“whiskering” of the iliac crest, greater tuberosities of the humerus, ischial tuberosities, femoral trochanters, calcaneus, and vertebral spinous processes). Sacroiliitis is usually bilateral and symmetric. Initially it involves the synovial-lined lower two thirds of the sacroiliac joint. The earliest radiographic change is erosion of the iliac side of the sacroiliac joint, where the cartilage is thinner and has clefts. Progression of the erosive process results in an initial “pseudo-widening” of the sacroiliac joint space with bony sclerosis eventually followed by complete bony ankylosis or fusion of the joint (grade 4). In cases of early sacroiliitis where plain radiographs may be normal, a noncontrast MRI will demonstrate inflammation and edema in the majority of cases.
Natural course of AS:
Although the course is variable, the first 10 years predicts the subsequent course of the disease. The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI, disease activity), Bath Ankylosing Spondylitis Metrology Index (BASMI, spinal mobility), and Bath Ankylosing Spondylitis Functional Index (BASFI) are standardized instruments used by some clinicians to measure disease progression and response to therapy. Early factors that predict a poor prognosis include early hip joint involvement, ESR >30 mm/h or persistently high CRP, poor response to NSAID therapy, and early development of syndesmophytes. Extraarticular manifestations such as uveitis, cardiovascular involvement, and pulmonary fibrosis portend a poor outcome. It is likely that patients with mild AS have a normal life expectancy and will maintain their ability to work. However, patients with poor prognostic signs have a three times increased risk of withdrawing from the workforce and a 1.5 times increased risk of dying. Causes of death include cardiovascular disease, gastrointestinal disease, and spinal fractures.