Ankylosing spondylitis (AS) is a chronic inflammatory disorder of unknown etiology that primarily affects the spine, axial skeleton, and large proximal joints of the body. Distinctive features of the disease are the striking tendency towards ossification and ankylosis of the spine and involvement of entheses. There is a spectrum of clinical severity, ranging from asymptomatic sacroiliitis to immobilizing spinal encasement. The most common manifestations of the disease are back pain and stiffness. Genetic and environmental factors play key roles in the pathogenesis of all the spondyloarthropathies, including AS.
Classification criteria have been developed for AS (Table 39-1). These are employed in epidemiologic studies in an attempt to standardize an AS diagnosis.
Table 39-1. Proposed classification criteria for ankylosing spondylitis
Definite AS is present if one of the radiologic criteria is associated with at least one clinical criterion.
Probable AS is present if 3 clinical criteria are present or if one of the radiologic criteria is present without any signs or symptoms satisfying the clinical criteria.
From van der Linden S, et al. Evaluation of diagnostic criteria for ankylosing spondylitis. A proposal for modification of the New York criteria. Arthritis Rheum 1984; 27:366
The diagnosis of AS requires a combination of clinical features, laboratory tests, and radiographical evidence of sacroiliitis. Clinical suspicion prompting testing should be aroused when patients complain of back pain that suggests inflammation (i.e. worse in the morning and better as the day goes on ).
The course of AS varies. In some patients, the disease progresses relentlessly (often despite therapy), with fusion of the spine and peripheral joints. In others, bony ankylosis may develop gradually with little pain or discomfort. In still others, skeletal involvement may be limited to only mild sacroiliitis and never progress to serious disease.