The pathophysiology of septic arthritis exemplifies the seriousness of the condition and the risks of long-term sequelae. Synovial edema and hyperemia accompany increased secretion of synovial fluid, which may be serosanguinous early on, cloudy, or suppurative with polymorphonuclear (PMN) leukocyte counts ranging from 5000 to 200,000 and exceeding 50,000/pL3 after the earliest stages. Synovial fluid glucose concentration is decreased, and the protein content is elevated. The mucin string is poor to very poor. Within days, the synovial fluid becomes frankly purulent, if it was not so initially. The hyaline articular cartilages degenerate initially at points of contact between opposing articular surfaces. The synovium itself is eventually replaced by granulation tissue, and the infection invades surrounding bone, particularly epiphyseal and metaphyseal bone. Adhesions are created within the joint and restrict motion. Subluxation or dislocation may occur in the setting of marked distention of the damaged joint capsule. In the hip joint, avascular necrosis of the femoral head ensues. Eventually, the untreated infection leads to ankylosis or total destruction of the joint.
Although systemic symptoms can be subtle in the newborn, the diagnosis is not usually in doubt in the older infant or toddler. Symptoms are acute in onset and predominantly involve pain in the affected joint. The child with a lower extremity septic arthritis walks with a severely antalgic limp or, more typically, cannot bear weight at all. Profound signs of constitutional and systemic illness are the rule, with high fever [40 to 40.5°C (104 to 105°f)], apprehension, irritability, anorexia, and prostration manifestly apparent. Examination of the affected joint demonstrates warmth, soft tissue swelling, and exquisite palpation tenderness. The child maintains an infected hip in 30° to 60°of flexion, with milder degrees of abduction and external rotation.
Radiographic imaging studies are helpful but nondiagnostic. They are important in order to ensure the absence of changes that would indicate adjacent osteomyelitis, fracture, or other processes. The expected findings are those of joint effusion and distention ( Fig 132-7). Comparison films of the contralateral joint in question are often helpful.
The differential diagnosis includes osteomyelitis, ARF, acute pauciarticular juvenile rheumatoid arthritis (JRA), transient tenosynovitis, cellulitis and suppurative bursitis, hemarthrosis, Legg-Calve-Perthes disease, and SCFE (when the involved joint is the hip).
Distinguishing suppurative arthritis from osteomyelitis involving an adjacent metaphysis is probably the most difficult problem in the differential diagnosis. Gentle examination may enable the physician to ascertain the area of greatest tenderness. Osteomyelitis is most tender over the metaphysis, while septic arthritis manifests greatest tenderness directly over the joint line. Motion of the joint is much more painful and more restricted in septic arthritis compared to osteomyelitis. Osteomyelitis creates more swelling over the limb as a whole. Arthrocentesis is necessary to establish the diagnosis of suppurative arthritis. Care is necessary during the arthrocentesis to avoid contaminating a noninfected joint by introducing organisms from an infected metaphysis, cellulitis, or bursa. A sympathetic effusion from an area of metaphyseal osteomyelitis usually is serous and contains only a few thousand PMNs.
The other differential diagnostic considerations usually pose fewer problems to the clinician. Transient tenosynovitis does not manifest symptoms and signs of systemic toxicity. The peripheral WBC and ESR are normal or only minimally elevated. Occasionally, arthrocentesis is necessary to adequately exclude the diagnosis of suppurative arthritis, however. JRA of the pauciarticular form may present with an isolated inflamed joint but is usually an illness of gradual onset, and the child does not appear toxically ill. The affected joint is not as tender as in septic arthritis and has better range of motion. Although the WBC is elevated and the mucin string is poor in both conditions, the Gram stain of synovial fluid in JRA is negative and its culture is sterile.
Rheumatic fever is characterized by the fleeting, migratory nature of the arthritis and may be associated with other stigmata, such as carditis, as illustrated by Jones'
revised criteria for the diagnosis of ARF (see Table 1.32-5). There is a remarkable responsiveness of the arthritis of ARF to salicylates. The response of the arthritis to salicylates should be used as a diagnostic tool only after the diagnosis of suppurative arthritis has been excluded by arthrocentesis, with Gram stain and culture of the synovial fluid.
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