Septic Arthritis

2.1. Clinical Relevance

In the elderly, acute joint pain and swelling is much more likely to be caused by crystal-induced arthritis than septic arthritis; the consequences of joint infection, however, are so serious that the possibility of infection requires a targeted diagnostic evaluation to exclude septic arthritis in most patients with acute joint pain. Immediate antibiotic treatment is necessary—loss of cartilage and erosion of subchondral bone from invasion of the cartilage by bacteria and inflammatory cells, increased intra-articular pressure, and release of proteases and cytokines from chondrocytes, begins within days of infection (13). The vast majority of cases of septic arthritis is caused by bacteria and is nearly always hematogenous in origin; nongonococcal arthritis most often involves previously damaged or arthritic joints. The type of infecting organism, to a large degree, also determines the extent of damage: Neisseria gonorrhoeae, for example, causes limited infiltration by leukocytes and results in minimal joint destruction.

2.2. Clinical Manifestations

Sudden onset of articular or periarticular pain, or a change in the pattern of chronic joint pain suggests the possibility of septic arthritis. The presence of pain (100%) and limitation of motion of a joint (90-100%) in a previously normal joint are the most consistent findings in elderly patients with septic arthritis caused by bacteria (13). Most often a single joint is affected, but polyarticular septic arthritis is seen in approximately 10% of cases—patients with rheumatoid arthritis are particularly susceptible (13). In elderly patients with septic (bacterial) arthritis, the knee joint is most frequently involved (35-65%), followed by the hip joint (5-15%) (14). Fever may or may not be present. A complete examination of all joints should be performed even when monoarticular arthritis appears to be present; special attention should be devoted to examining the eyes, skin, mucous membrane, and heart as part of the differential diagnosis. Infection of the hip, shoulder, or sacroiliac joint may not be clinically detectable.

2.3. Diagnostic Tests

Diagnosis of septic arthritis ultimately depends on the results of examination of synovial fluid for leukocytes, crystals, and the presence of bacteria by Gram stain and culture. Although synovial fluid leukocytosis, (white blood cell count exceeding 10,000/mm3) with more than 85% neutrophils, is almost always present, it does not distinguish between septic and crystal-induced arthritis. Normal synovial fluid has <180 white blood cells/mm3 and they are predominantly lymphocytes. In acute bacterial synovitis, white blood cell counts exceed 25,000-50,000 cells/mm3, and typically reach 100,000 cells/mm3, with more than 75-90% neutrophils. Noninfectious inflammatory arthritides and mycobacterial and fungal infections usually have fewer than 50,000 cells/mm3, and more of a lymphocytic response.

When positive, a Gram stain can guide empiric therapy, but it is often negative for bacteria (15,16). Synovial fluid cultures should always be obtained, even when crystals are seen by polarized light examination of a wet mount. If available, polymerase chain reaction (PCR) techniques can detect minute quantities of bacterial DNA within synovial fluid and tissue, but the assay is susceptible to false positives from contamination and cannot distinguish between live and dead organisms. Blood cultures may be positive, especially in S. aureus septic arthritis. As in osteomyelitis, the ESR and CRP are usually positive, and may be helpful in monitoring response to treatment. In fact, a normal ESR and/or CRP should make the diagnosis of bacterial arthritis highly suspect (14,17).

Radiographs are of limited utility. MRI scans and, to a lesser extent, CT cans, may be useful for evaluating periarticular soft tissues for abscesses or cellulitis. As with osteomyelitis, radionuclide bone scans are very sensitive early indicators of septic arthritis. Scans with technetium (99mTc), gallium (67Ga), or indium (111In) localizes inflammation and increased bone metabolism, but cannot reliably distinguish between infection-induced and crystal-induced arthritis. For most patients from whom synovial fluid can be obtained, however, MRI scan or radionuclide imaging adds little to the diagnostic process.

Traditionally, bacterial arthritis is divided into nongonococcal and gonococcal etiologies, the latter of which has a better prognosis. Although a bacterial pathogen can be identified in most cases of nongonococcal septic arthritis, the source of infection is less commonly identified. When a source is found, most are cutaneous, respiratory, and genitourinary infections (18).

2.4. Specific Types of Septic Arthritis 2.4.1. Gonococcal Septic Arthritis

Although less common than in younger patients, disseminated gonococcal infection (DGI) must be considered in all sexually active older patients with septic arthritis (13,19). In a review of gonococcal arthritis in young women, the knee was most often affected, followed by hand and wrist synovitis (20). Skin manifestations—small numbers of papules that progress to hemorrhagic pustules on the trunk and the extensor surfaces of the distal extremities—and/or migratory polyarthralgias were present in about half of patients. Joint fluid may be difficult to obtain, and cultures are consistently negative in DGI. Blood cultures were positive in only 13% of cases (20). Although antibiotic resistance of gonococci is an international problem, resistance to ceftriaxone or ciprofloxacin have not been a problem in DGI in the U.S. An appropriate initial regimen is ceftriaxone (1 g parenterally once daily); alternate regimens include ceftizoxime (1 g intravenously (I.V.) every 8 h) or cefotaxime (1 g I.V. every 8 h) until 24-48 h after resolution of symptoms) (21). Persons allergic to ^-lactam drugs may be prescribed ciprofloxacin (500 mg I.V. every 12 h), ofloxacin (400 mg I.V. every 12 h), or spectinomycin (2 g intramuscularly (I.M.) every 12 h). Subsequent oral therapy should be with a ciprofloxacin (500 mg twice daily) or ofloxacin (400 mg twice daily), or cefixime (400 mg twice daily), plus empiric treatment for Chlamydia trachomatis infection with oral doxycycline (unless a week of ofloxacin is used for followup therapy) (21).

2.4.2. Staphylococcal and Streptococcal Septic Arthritis

S. aureus accounts for the great majority of cases of septic arthritis in older adults (14), and for more than 80% of cases in rheumatoid arthritis or hemodialysis patients (22,23). As a result, all empiric regimens for patients with nongonococcal septic arthritis should include coverage for S. aureus, and the probability of bacteremia with possible endovascular infection considered. There is now an increasing prevalence of methicillin-resistant S. aureus (MRSA) and S. epidermidis, especially in patients with prosthetic joint septic arthritis. Suspected staphylococcal joint infection should be treated initially with vancomycin (1 g I.V. every 12 h), until methicillin resistance can be excluded, with follow-up therapy with a cephalosporin for the remainder of the 4-6 wk course.

The streptococci (nongroup A ^-hemolytic streptococci—groups B, C, and G, and pneumococci) and enterococci are the second most common causes of nongonococcal septic arthritis. Group B streptococcal infection may be particularly virulent in diabetic patients. Enterococci can infect native or prosthetic joints. If the Gram stain shows Gram-positive cocci, empiric therapy with vancomycin provides sufficient coverage, including activity against penicillin-resistant S. pneumoniae.

2.4.3. Gram-Negative Bacilli

Up to 20% of cases of septic arthritis cases at teaching hospitals are caused by a variety of Gram-negative bacilli, especially among immunosuppressed patients (18,24). In addition, predispositions typical of elderly nursing facility patients—prior antibiotic use, urinary tract infections, and pressure ulcers—are associated with Gram-negative bacilli as causes of septic arthritis. If Gram-negative bacilli are seen on Gram's stain, empiric therapy should include a fluoroquinolone such as ciprofloxacin (500 mg twice daily) or levofloxacin (500 mg daily), or a third-generation cephalosporin such as ceftazidime (2 g twice daily), or imipenem (500 mg every 6 h).

2.4.4. Unusual Causes

Anaerobic joint infections are rare, but should be considered if abdominal abscess or anaerobic soft-tissue infection, such as that associated with pressure ulcers, is present.

Pasteurella multocida, a Gram-negative coccobacillus that is sensitive to penicillin, may cause septic arthritis following an animal bite. Quite rarely, Mycobacterium marinum, from exposure to freshwater, saltwater, swimming pools, or fish tanks, may cause mildly painful septic arthritis, without systemic symptoms. Other rare causes that are included in a broad differential diagnosis are M. tuberculosis and other myco-bacterial species (chronic monoarticular arthritis), Ureaplasma urealyticum, Brucella spp. (ingestion of unpasteurized dairy products), oral flora such as Eikenella corrodens (human bites), and Treponema pallidum. Other pathogens to consider in relevant circumstances include Borrelia burgdorferi (Lyme disease), Coccidiodes immitis (endemic areas), Sporothrix schenkii (thorns or splinters), and Streptobacillus moniliformis (rat bites). Several viruses, such as mumps, rubella, and parvovirus, may cause infectious arthritis, but are not likely to be encountered in the elderly.

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