Seronegative spondyloarthropathies (SNSA) affect genetically predisposed individuals and are triggered by environmental factors (Khan 2002). They consist of a group of closely related skeletal disorders characterized by the concurrence of arthritis and spondylitis, in which serological rheumatoid factor is absent, hence seronegative. SNSA include ankylosing spondylitis, Reiter's syndrome, psoriatic arthritis, arthritis associated with inflammatory bowel disease, and other rare forms of arthritides. The great majority of patients with one of these disorders demonstrate a positive test for HLA-B27, although in psoriatic arthritis the positiv-ity rate is low. Two clinical features have been shown to be useful in distinguishing SNSA from rheumatoid arthritis. One is oligoar-thropathy that is asymmetrical with a predilection for the peripheral joints and the other is enthesopathy (the enthesis is the bone insertion of ligament and tendon). In addition and fundamentally, SNSA can be differentiated from rheumatoid arthritis on genetic, immun-ological, pathological and radiological bases as well as by symptoms and signs. Radiography is helpful and 99mTc-MDP bone scintigraphy is highly sensitive but not specific (Hays and Green 1972; Desaulniers et al. 1974). Fortunately, however, a recent study by Kim et al. (1999) has shown that pinhole scintigraphy can provide specific information on SNSA, Reiter's syndrome in particular.
SNSA are ubiquitous, occurring worldwide in both sexes, and often present in a subclinical form, defying diagnosis. However, with a high index of suspicion and appropriately performed 99mTc-MDP bone scanning, the correct diagnosis can be reached without much diffi culty. Actually, the whole-body bone scan portrays the characteristic distribution pattern of the affected spine and joints, and subsequently performed pinhole scan confirms the specific features of the individual arthritis and entheso-pathy. In particular, plantar fasciitis and Achilles tendinitis in Reiter's syndrome, which are difficult to diagnose radiographically, can be sensitively detected by noting subtle tracer uptake in the characteristic sites (Kim et al. 1999). The pinhole scintigraphic features in the established cases closely correlate with those of radiography. In the following, the combined scintigraphic and radiographic manifestations of SNSA are discussed.
Pathology The basic pathological process that underlies the radiographic and scinti-graphic manifestations of SNSA is enthesopa-thy. In addition, nonspecific inflammation and granulation attack the synovium and articular cartilage and bone. SNSA is expressed by fibro-sitis, ankylosis, ossification, and periarticular "whiskering". All these changes are relatively more mild in severity than in rheumatoid arthritis. Of particular interest in ankylosing spondylitis is the peculiar chondral type of ossification of fibrosed tissues, which have been transformed into cartilage, causing the "bamboo spine" deformity and the obliteration of the sacroiliac joints.
Radiographic Manifestations Radiographic manifestations may vary somewhat from disease to disease and also according to the stage of disease. On the whole, as opposed to rheumatoid arthritis, the periarticular osteopenia and subchondral erosions with articular nar rowing are not conspicuous features or may even be absent in SNSA. Instead, diffuse fibrosis, articular closure without narrowing, osteophytosis, whiskering, and spinal syndes-mophytosis and ankylosis characterize the radiographic manifestations. Subchondral erosion may be a key feature in occasional sacroi-liitis. Three radiographic features more or less common to ankylosing spondylitis, Reiter's syndrome, and psoriatic arthritis have been described. First, the numbers of joints affected in the appendicular skeleton are not many and asymmetrical with a predilection for the smaller limb joints (the lower limbs in ankylosing spondylitis and Reiter's syndrome and the distal phalanges in psoriatic arthritis). Second, the spine and sacroiliac joints are involved, especially in ankylosing spondylitis. Third, spinal syndesmophytosis ensues in ankylosis and the "bamboo" deformity and the enthesopathies in the pelvis and appendicular joints produce the "whiskering" sign.
The articulations regularly involved include the diskovertebral joints, sacroiliac joints, sym-physis pubis, and parasternal joints and the en-theses frequently affected include the tendon and ligament insertion sites of the pelvis, calca-neus, femoral trochanters, humeral tuberosi-ties, and patella. The distribution pattern of affected joints in the individual SNSA is fairly typical. Thus, in ankylosing spondylitis the synovial joints and cartilaginous diskovertebral junctions and entheses of the spine are mainly affected. The median cartilaginous joints such as the manubriosternal joint and the symphysis pubis are also frequently involved (Bluestone 1979). The appendicular bones are less consistently and less severely affected. On the other hand, in Reiter's syndrome the lower limbs are more regularly involved, showing asymmetrical, spotty foci and calcaneal spur. Psoriasis affects many appendicular joints, including the distal interphalangeal joints.
Bone Scintigraphic Manifestations Bone scintigraphy reinforced with pinhole scanning is extremely useful in diagnosing the changes in the axial skeleton and peripheral joints in ankylosing spondylitis, Reiter's syndrome, and psoriatic arthritis. Whole-body scanning is ideal in providing a panoramic display of asymmetrical tracer uptake in the joints, bones, and entheses in the spine, sacroiliac joints, pelvis, and lower limbs (Fig. 11.1). Subsequent pinhole scanning of the region of interest greatly facilitates the in-depth analysis of the findings disclosed by whole-body scintigraphy, leading to specific diagnosis. As mentioned already, pinhole scintigraphy frequently helps detect preradiographic changes such as subtle and seemingly trivial tracer uptake in radiographi-cally normal yet metabolically abnormal bone; for example, in the plantar aspect of the calcaneus in Reiter's syndrome (Fig. 11.2). In this situation, pinhole scintigraphy is clearly more sensitive and specific than any other diagnostic method. It is, however, beyond doubt that collateral radiography significantly enhances diagnostic accuracy and efficacy.
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