Seronegative Spondyloarthropathy (SNSA)

Figure 1: AP radiograph of the lumbosacral spine shows symmetric sclerosis and fusion (ankylosis) of the sacroiliac joints (red arrowheads). There is ossification of the outer fibers annulus fibrosus (arrows).
Figure 2: Lateral view of the spine of the same patient demonstrates ossification of the outer fibers of annulus fibrosus (arrows), squaring of anterior vertebral margin, and "shiny corner" sign (arrowhead).

Facts

• SNSA is a group of diseases that general pathology consists of negative rheumatoid factor (RF) and elevated erythrocyte sedimentation rate (ESR)
• Ankylosing spondylitis, reactive spondyloarthropathy, psoriatic spondyloarthropathy, spondyloarthropathy associated with inflammatory bowel disease, undifferentiated spondyloarthropathy
• Strong association with HLA-B27 haplotype
• Most common clinical presentation is low back pain (greater in the morning) and pathologic fracture

Imaging Findings

• First involves sacroiliac joints (erosion of synovial portion, then fusion in late stage), then thoracolumar junction and may involve whole spine
• Corner erosions of vertebral bodies produce "squaring" and reactive sclerosis produces "shiny corner" sign
• "Bamboo spine" caused by thin ossification of the outer fibers of annulus fibrosus at vertebral margin, common in ankylosing spondylitis

Key Differentiating Imaging Features

• Involvement of SI joints - rare in rheumatoid arthritis, normal in DISH, retinoid therapy
• Infection of SI joint typically unilateral

Our case: ankylosing spondylitis

Reference:

Ross, Brant-Zawadzki, Moore, et al. Diagnostic Imaging: Spine, 2004