Infectious Arthritis

(Septic Arthritis)

Inflammation of the joints due to infection of the synovial tissues with pyogenic bacteria or other infectious agents.

The risk of infectious arthritis increases with age. Patients who are immunocompromised as a result of corticosteroid therapy, malignancy, or diabetes are also more likely to develop infectious arthritis.

In the elderly, as in younger patients, the most common organism is Staphylococcus aureus; however, gram-negative bacteria cause a significant number of cases in elderly patients. Bacterial infection is due to direct inoculation or to bacteremia from a known or unknown source. It most often affects joints with preexisting disease, usually osteoarthritis or rheumatoid arthritis.

Symptoms, Signs, and Diagnosis

The presenting symptom is usually acute febrile illness with monarticular or polyarticular arthritis. Primarily large joints are affected, most commonly the shoulder, elbow, wrist, hip, and knee. Many patients do not look toxemic, particularly elderly persons, who may have a low-grade or no fever and whose peripheral WBC count may be < 14,000/µL.

In febrile patients who cannot give a good history, all of the diarthrodial joints must be examined for a source of infection. Infection is diagnosed by analysis of synovial fluid. A WBC count > 50,000/µL indicates infection unless crystals are present. Infected fluid can have a WBC count of < 50,000/µL, although polymorphonuclear leukocytes predominate in most instances. A synovial fluid glucose level that is 40 mg/dL (2.2 mmol/L) less than the serum glucose level is highly suggestive of infection.

Gram stain and culture identify the infectious organism in up to 50% of cases. Blood cultures should be obtained, because the organism often grows in blood but not in synovial fluid. A specific organism is identified in > 80% of cases when all appropriate sites are cultured.

Other biochemical tests of synovial fluid include lactate level measurement, bacterial antigen detection, and nitroblue tetrazolium test. These tests should be recommended by a rheumatologist, infectious disease specialist, or pathologist.

Treatment

Immediate treatment is required to avoid cartilage destruction and permanent joint damage. Joint fluid should be aspirated repeatedly and as completely as possible. If fever and the signs of arthritis are not substantially reduced in 48 to 72 hours, surgical drainage of the joint may be necessary. Infectious arthritis responds to appropriate systemic antibiotics if the organism is sensitive and the dosage is adequate. Intra-articular antibiotics are not needed.