The spondyloarthritides are a group of overlapping disorders that share certain clinical features and genetic associations. These disorders include ankylosing spondylitis (AS), reactive arthritis, psoriatic arthritis and spondylitis, enteropathic arthritis and spondylitis, juvenile-onset spondyloarthritis (SpA), and undifferentiated SpA. The similarities in clinical manifestations and genetic predisposition suggest that these disorders share pathogenic mechanisms.
AS is an inflammatory disorder of unknown cause that primarily affects the axial skeleton; peripheral joints and extraarticular structures are also frequently involved. The disease usually begins in the second or third decade; male-to-female prevalence is between 2:1 and 3:1. The term axial spondyloarthritis is coming into common use, supported by criteria formulated in 2009 (Table 384-1). This classification includes both definite AS and early stages that do not yet meet classical criteria for AS, but it probably also includes other conditions with a different natural history.
AS shows a striking correlation with the histocompatibility antigen HLA-B27 and occurs worldwide roughly in proportion to the prevalence of B27 (Chap. 373e). In North American whites, the prevalence of B27 is 7%, whereas it is 90% in patients with AS, independent of disease severity.
In population surveys, AS is present in 1–6% of adults inheriting B27, whereas the prevalence is 10–30% among B27+ adult first-degree relatives of AS probands. Concordance rate in identical twins is about 65%. Susceptibility to AS is determined largely by genetic factors, with B27 comprising less than one-half of the genetic component. Genome-wide single-nucleotide polymorphism (SNP) analysis has identified over 30 additional susceptibility alleles.
Sacroiliitis is often the earliest manifestation of AS. Knowledge of its pathology comes from both biopsy and autopsy studies that cover a range of disease durations. Synovitis and myxoid marrow represent the earliest changes, followed by pannus and subchondral granulation tissue. Marrow edema, enthesitis, and chondroid differentiation are also found. Macrophages, T cells, plasma cells, and osteoclasts are prevalent. Eventually the eroded joint margins are gradually replaced by fibrocartilage regeneration and then by ossification. The joint may become totally obliterated.
In the spine, the specimens studied have either been surgically resected in advanced disease or taken from autopsies. There is inflammatory granulation tissue in the paravertebral connective tissue at the junction of annulus fibrosus and vertebral bone, and in some cases along the entire outer annulus. The outer annular fibers are eroded and eventually replaced by bone, forming the beginning of a syndesmophyte, which then grows by continued endochondral ossification, ultimately bridging the adjacent vertebral bodies. Ascending progression of this process leads to the “bamboo spine.” Other lesions in the spine include diffuse osteoporosis (loss of trabecular bone despite accretion of periosteal bone), erosion of vertebral bodies at the disk margin, “squaring” or “barreling” of vertebrae, and inflammation and destruction of the disk-bone ...