Infectious Arthritis

Pathogenesis: Joints become infected by hematogenous seeding (the most common route), by spread from a contiguous site of infection, or by direct inoculation (e.g., during trauma or surgery). Acute bacterial infection can rapidly destroy articular cartilage as a result of increased intraarticular pressure and the elicited host immune response.
Microbiology: The predominant etiologic agents differ with the pt's age; S. aureus is the most common nongonococcal isolate in adults of all ages.
- - In children <5 years old, S. aureus, S. pyogenes, and Kingella kingae predominate.
- - In young adults, Neisseria gonorrhoeae is the most common etiology.
- - In adults, S. aureus predominates, but gram-negative bacilli, pneumococci, and β-hemolytic streptococci are involved in one-third of cases in older adults.
- - Other causes of septic arthritis include Borrelia burgdorferi (Lyme disease), tuberculosis and other mycobacterial infections, fungal infections (e.g., coccidioidomycosis, histoplasmosis), and viral infections (e.g., rubella, mumps, hepatitis B, parvovirus infection).
Epidemiology and clinical manifestations: The risk factors and presentation differ depending on whether N. gonorrhoeae is the cause.
- - Nongonococcal bacterial arthritis: Risk is increased in pts with rheumatoid arthritis, diabetes mellitus, glucocorticoid therapy, hemodialysis, malignancy, and IV drug use.
  - In 90% of pts, one joint is involved—most often the knee, which is followed in frequency by the hip, shoulder, wrist, and elbow; IV drug users often have spinal, sacroiliac, or sternoclavicular joint involvement.
  - Pts have moderate to severe pain, effusion, decreased range of motion, and fever.
- - Gonococcal arthritis: Women are 2-3 times more likely than men to develop disseminated gonococcal infection (DGI) and arthritis, particularly during menses and during pregnancy (see Chap. 83. Sexually Transmitted and Reproductive Tract Infections).
  - DGI presents as fever, chills, rash, and articular symptoms (migratory arthritis). The cutaneous and articular findings result from an immune reaction to circulating gonococci and immune-complex deposition; thus synovial fluid cultures are consistently negative.
  - In true gonococcal arthritis (which always follows DGI), a single joint (hip, knee, ankle, or wrist) is usually involved.
- - Prosthetic joint infections: complicate 1-4% of joint replacements and are usually acquired intra- or perioperatively
  - Acute presentations are seen in infections caused by S. aureus, pyogenic streptococci, and enteric bacilli.
  - Indolent presentations are seen in infections caused by coagulase-negative staphylococci and diphtheroids.
- - Reactive arthritis: follows ~1% of cases of nongonococcal urethritis and 2% of enteric infections (e.g., Yersinia enterocolitica, Shigella flexneri, Campylobacter jejuni, Salmonella spp.). Only a minority of pts have the other classic findings associated with reactive arthritis, including urethritis, conjunctivitis, uveitis, oral ulcers, and rash.
Diagnosis: If there is concern about joint infection, examination of synovial fluid from the affected joint is essential. There is considerable overlap in the cell counts due to different etiologies, but synovial fluid culture and examination for crystals (to rule out gout and pseudogout) can help narrow the diagnosis.
- - Normal synovial fluid contains <180 cells (mostly mononuclear)/µL. Acute bacterial infection of joints results in synovial fluid cell counts averaging 100,000/µL (range, 25,000-250,000/µL), with >90% PMNs. Synovial fluid in gonococcal arthritis contains >50,000 cells/µL, but results of Gram's staining are usually negative, and cultures of synovial fluid are positive in <40% of cases. Other mucosal sites should be cultured to diagnose gonorrhea. Pts with septic arthritis due to mycobacteria or fungi can have 10,000-30,000 cells/µL in synovial fluid, with 50-70% PMNs. Synovial fluid cell counts in noninfectious inflammatory arthritides are typically <30,000-50,000/µL.
- - Gram's staining of synovial fluid should be performed. Injection directly into blood culture bottles can increase the yield of synovial fluid cultures.
- - Blood cultures are positive in 50-70% of cases due to S. aureus but are less commonly positive with other organisms.
- - Plain radiographs show soft tissue swelling, joint space widening, and displacement of tissue planes by distended capsule. Narrowing of the joint space and bony erosions suggest advanced disease.

Treatment: Infectious Arthritis

Drainage of pus and necrotic debris is needed to cure infection and to prevent destruction of cartilage, postinfectious degenerative arthritis, and joint deformity or instability.
A third-generation cephalosporin (cefotaxime, 1 g IV q8h; or ceftriaxone, 1-2 g IV q24h) provides adequate empirical coverage for most community-acquired infections in adults when smears demonstrate no organisms. Vancomycin (1 g IV q12h) should be used to cover the possibility of MRSA when there are gram-positive cocci on the smear.
- - In IV drug users and other susceptible pts, treatment for gram-negative organisms such as P. aeruginosa should be considered.
- - If a pathogen is identified by culture, treatment should be adjusted according to the specific bacterial organism and its antibiotic susceptibility.
Treatment for S. aureus should be given for 4 weeks, that for enteric gram-negative bacilli for 3-4 weeks, and that for pneumococci or streptococci for 2 weeks. Treatment for gonococcal arthritis should commence with ceftriaxone (1 g/d) until improvement; depending on the susceptibilities of the isolate, the 7-day course can be completed with an oral fluoroquinolone (e.g., ciprofloxacin, 500 mg bid) or amoxicillin (500 mg tid).
Prosthetic joint infections should be treated with surgery and high-dose IV antibiotics for 4-6 weeks. The prosthesis often has to be removed; to avoid joint removal, antibiotic suppression of infection may be tried. A 3- to 6-month course of ciprofloxacin and rifampin has been successful in S. aureus prosthetic-joint infections of relatively short duration, although prospective trials confirming the efficacy of this regimen are still needed.

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