Septic arthritis (infection within a joint space) is often difficult to diagnose early in its course. It is frequently not diagnosed on the first visit. There must be a high index of suspicion to make the correct diagnosis. It is essential to recognize septic arthritis early because a delay in the diagnosis and treatment can lead to serious complications. Septic arthritis is a condition that urgently requires intravenous antibiotics and sometimes open drainage. Serious complications include joint destruction and long term disability.

Septic arthritis is more common in children than in adults. Most of the cases occur in children less than 5 years old. The highest incidence occurs in children between 6 months and 24 months of age. Males outnumber females 2:1.

Predisposing factors include significant trauma (in 30% to 45% of cases), preceding upper respiratory infection, and otitis media. Less frequently there could be an associated skin or soft tissue infection. Often there are no preceding factors identified.

There are three mechanisms by which the organism can enter the joint space. The most common mechanism in children is hematogenous spread. Rarely there could be direct inoculation or extension from a contiguous site of infection. With the bacteria in the joint space there is a subsequent inflammatory response. There is neutrophil infiltration and the purulent material accumulates within the joint space. The direct pressure as well as proteolytic enzymes cause the symptoms of tenderness, swelling, and erythema.

The organism most frequently found in septic arthritis varies with age. In the neonatal period (first 2 months of life), the most common organisms are group B Streptococcus and Staphylococcus aureus. There also may be gram negative enteric organisms. Beyond the neonatal period up to 2 years of age the most common organisms have been Hemophilus influenzae and Staph aureus. The frequency of H. influenzae has decreased because of the ubiquitous use of H. flu vaccine. In the 2 to 5 year old age range the most common organisms have been H. influenzae, S. aureus, group A Streptococcus, and Streptococcus pneumoniae. In children older than 5 years old the most frequent organism is S. aureus. In sexually active adolescents, Neisseria gonorrhea has also been implicated as an etiology of septic arthritis. Pseudomonas aeruginosa is associated with puncture wounds. In patients with sickle cell anemia, Salmonella species may be recovered in the culture. Frequently there is no organism recovered in patients diagnosed with septic arthritis (up to 30%).

The most common joint involved in children is the hip, followed by the knee and then the ankle joint. Less frequently the site is a joint in the upper extremity elbow, shoulder, or wrist. Since a joint in the lower extremity is the more common site, the patient often presents with a limp or a refusal to walk. Occasionally more than one joint is involved (6-11%).

Common symptoms include erythema, swelling, tenderness and lack of active motion at the affected joint. The signs are often subtle, especially in the neonates. Fever of greater than 38.3 is found in about 75% of the patients.

The WBC count in septic arthritis is variable. In most patients there is a leukocytosis with a shift to the left, but about 50% of patients will have a WBC count less than 15,000/cu mm. A normal white count does not rule-out septic arthritis. The ESR and CRP are more frequently abnormal in patients with septic arthritis. The ESR is elevated in about 90% of the cases. Although this test is more sensitive than the WBC count, it is nonspecific. The ESR may be elevated in many other conditions. Blood cultures are positive in about 40% of the cases, more frequently if the organism is H. influenzae. Radiographs are recommended although they are usually normal in early septic arthritis. Late findings may include joint widening and bony destruction. There is no laboratory test or X-ray that can rule-out septic arthritis. The joint fluid must be analyzed if there is any suspicion of septic arthritis.

The joint fluid should be sent for WBC count, gram stain and culture, and glucose. The WBC count is usually greater than 50,000 cells/cu mm, with a predominance of neutrophils (>75%). There have been cases of septic arthritis with the WBC count as low as 5,000 cells/cu mm. The glucose is usually less than 40 mg/dL. The joint fluid culture is positive in about 60% of cases. If an organism is recovered it is most frequently found in the joint fluid, therefore an arthrocentesis is very important.