Early Spondyloarthritis in an HLA-B27-positive Monozygotic Twin Pair: A Highly Concordant Onset, Sites of Involvement, and Disease Course

Abstract: Susceptibility to ankylosing spondylitis (AS) and the related spondyloarthropathies (SpA) is determined by a strong polygenetic predisposition interacting with as-yet unknown environmental factors. HLA-B27 is one of the major disease susceptibility genes, and studies of HLA-B27-positive twins have shown a concordance rate of 63% among monozygotic versus 24% in dizygotic twin pairs1-5. Making an early diagnosis of AS and related SpA is often challenging, and there are no validated diagnostic criteria6-10. Magnetic resonance imaging (MRI) is the imaging method of choice for early detection and assessment of inflammatory lesions in the axial skeleton prior to the development of structural damage seen on standard radiographs11. We describe HLA-B27-positive monozygotic Caucasian twin brothers who had experienced the onset of undifferentiated spondyloarthritis at 23 years of age only 10 months apart, and with almost identical clinical manifestations: stiffness of the neck and dactylitis of a toe in the left foot, followed by right-side sacroiliitis. The twins were living far apart in 2 different cities. TwinA developed neck stiffness (that resolved within a few weeks) and dactylitis of the fourth toe of his left foot (that lasted about 6 months), fol-Figure 1. Conventional MRI (STIR sequence; performed 9 months after onset of relapsing back pain; top) and standard radiograph (performed 12 months after onset of back pain; bottom) of the pelvis in Twin A. MRI displays acute inflammatory changes of the right sacroiliac joint, and the radiograph shows advanced chronic inflammatory lesions of the right sacroiliac joint.