Pφ

The overwhelming majority of these cases are caused by staphylococci and streptococci as a result of hematogenous dissemination. Acute joint infection can also be the result of trauma to the joint or may have spread from other localized areas where there may be an infection, such as bursitis, overlying cellulitis, or a nearby abscess or osteomyelitis. Rarely, a patient can develop a septic joint as a complication of a diagnostic or therapeutic arthrocentesis via direct inoculation or following use of contaminated tissue allografts. Risk factors for the development of a nongonococcal septic arthritis include rheumatoid arthritis (RA), immunodeficient states, immunosuppressive therapy, diabetes, preexisting joint damage, age, injection drug use, prosthetic joints, indwelling catheters, tissue allograft surgery, and malignancy.

TP

Patients present with pain and limited movement in one joint. The joint is usually red, warm, and swollen, and most patients have accompanying constitutional symptoms such as fever and malaise. The knee is the most commonly affected joint, followed by the hip and ankle.

Dx

The gold standard for diagnosis is the isolation of the causative organism in a synovial fluid culture. Arthrocentesis fluid is sent for cell count and differential, Gram stain, and culture. The white blood cell (WBC) count in the fluid is usually >50,000 cells/μL and is mostly made up of neutrophils. A Gram stain is positive in half of the patients at presentation but results depend on the causative organism. Cultures will grow in 80% to 90% of patients and blood cultures are positive in 50% to 70% of patients. Peripheral blood may show an elevated WBC count, erythrocyte sedimentation rate (ESR), or C-reactive protein (CRP). Radiographs are not generally helpful but may show soft-tissue swelling.

Pφ

Neisseria gonorrhoeae is the most common cause of septic arthritis in the United States, especially in young, sexually active people. Joint infection occurs in half of patients with disseminated gonococcal infection (DGI), and it always follows a mucosal infection by the organism. An important host factor that may increase risk of DGI is the presence of terminal complement component deficiency (C5–C8).

TP

This form of septic arthritis is more common in women and in younger patients. The disease has two distinct clinical presentations. The first is a dermatitis–arthritis syndrome that includes rash, fever, bacteremia, and polyarthralgias that are usually asymmetric. There is often a tenosynovitis, usually of the hands and fingers. The rash can be maculopapular, vesicular, pustular, or necrotic on an erythematous base, and it can occur anywhere on the body. The second presentation is that of a frank purulent arthritis usually in a single joint (or less often in a few joints) in the absence of any skin manifestations.

Dx

Arthrocentesis with synovial culture is positive in only 50% of monoarticular cases and in <30% of disseminated cases. Polymerase chain reaction is somewhat helpful to distinguish the causative agent (sensitivity of 80% in patients with DGI). Rectal, pharyngeal, and genitourinary tract cultures can also be helpful in identifying the organism.

Pφ

In patients with a uric acid level > 6.7 mg/dL, monosodium urate supersaturates in the serum and deposits in tissues. Over years this may lead to the development of arthropathy and tophi.

TP

Gout presents as intermittent attacks of joint pain. There is usually redness, swelling, warmth, and significant pain in the joint, sometimes accompanied by fever and chills. Gout commonly presents as a monoarthropathy in the lower extremities. Half of all first episodes of gout occur in the first metatarsophalangeal joint. This presentation is also referred to as podagra.

Dx

Definitive diagnosis of gout requires the identification of monosodium urate crystals from a joint or tophus aspirate. The crystals are needle-shaped and can be seen as negatively birefringent with polarized light microscopy. The serum uric acid level often does not correlate with disease and is not useful in diagnosis.

Pφ

CPPD crystals form in patients with alterations of inorganic pyrophosphate metabolism and abnormalities in calcium metabolism. CPPD crystals precipitate in cartilage and provoke an inflammatory response. Other conditions commonly associated with CPPD arthritis include hypophosphatemia, hypomagnesemia, hypothyroidism, hemochromatosis, and hyperparathyroidism.

TP

Many patients are asymptomatic, but when the disease presents as an inflammatory arthropathy it mimics the presentation of gout—thus the nickname pseudogout. These attacks are often seen in postsurgical patients or following an illness. Knees and wrists are the most common joints involved.

Dx

Diagnosis depends on the presence of weakly positive birefringent CPPD crystals in a joint aspirate. Inflammatory synovial fluid is characteristic of pseudogout. Patients may also have chondrocalcinosis on radiography.

Pφ

The most common viral joint pathogens are parvovirus B19, rubella, and hepatitis B and C viruses.

TP

Viral arthritis typically presents as a mild to moderate polyarthritis. Parvovirus B19 usually involves the small joints of the hands and feet and the knees, and may mimic the pattern of RA. It is usually self-limited and sometimes associated with a rash. Acute rubella has a concomitant rash, fever, and lymphadenopathy. Patients can develop a polyarthritis after a rubella vaccination, usually about 2 weeks after immunization.

Dx

Parvovirus B19 is diagnosed by the presence of specific serum IgM antibodies. These patients may also develop autoantibodies such as rheumatoid factor and antinuclear antibodies that can persist for years.