Septic arthritis and transient synovitis (pediatrics): Clinical sciences

Septic arthritis is a bacterial infection of the synovium and joint space, while transient synovitis, also called toxic synovitis, refers to a non-infectious synovial inflammation. While transient synovitis is a benign and self-limited illness, it’s crucial to identify and treat septic arthritis promptly to prevent permanent bone and joint damage.

When a pediatric patient presents with a chief concern suggesting septic arthritis or transient synovitis, first perform an ABCDE assessment to determine if the patient is unstable or stable. If unstable, stabilize their airway, breathing, and circulation. Next, obtain IV access, consider starting IV fluids, and put your patient on continuous vital sign monitoring, including respiratory rate, pulse oximetry, and cardiac monitoring. Finally, if needed, don’t forget to provide supplemental oxygen and administer broad-spectrum intravenous antibiotics.

Now, let’s go back to the ABCDE assessment and take a look at stable patients. In this case, obtain a focused history and physical exam; and order labs, including CBC, CRP, and ESR.

First, let’s focus on septic arthritis! Affected children typically experience a rapid onset of monoarticular joint pain and swelling that most commonly involves a large joint in a lower extremity, such as the knee or hip. Caregivers also often report systemic symptoms, like fever, malaise, or decreased appetite. Some ambulatory children may develop a limp, but most will refuse to bear weight on the affected limb. Finally, infants and non-ambulatory children may display “pseudoparalysis,” which is an inability to move the affected joint due to pain.

On physical exam, patients are usually ill-appearing, with signs of inflammation, including erythema, warmth of the skin overlying the affected joint, and joint effusion. Patients classically display limited range of motion in the affected joint, and those with hip involvement typically prefer to keep their hips flexed, abducted, and externally rotated.

Finally, labs will reveal an elevated white blood cell count with neutrophilia, CRP greater than 1 milligram per deciliter, and ESR greater than 40 millimeters per hour.

The presence of these findings should make you suspect septic arthritis, so your next step is to perform an arthrocentesis. This procedure can be both diagnostic and therapeutic, since removing infected synovial fluid can relieve pressure within the joint space. Next, send synovial fluid for analysis, which includes a Gram stain and culture, as well as a cell count and differential. Additionally, consider ordering a blood culture if your patient is ill-appearing or if they are an infant.

Once you’ve performed arthrocentesis, don’t wait for results; instead, start empiric IV antibiotics immediately!

Now, here’s a high-yield fact to keep in mind! When selecting an empiric antibiotic, be sure to consider your patient’s age and any risk factors to cover the most likely causative pathogens. Septic arthritis in children is most commonly caused by Staphylococcus aureus, followed by Streptococcus pyogenes. For unimmunized children or those younger than 5, also consider Kingella kingae, Streptococcus pneumoniae, and Haemophilus influenzae type b. Neonates with suspected septic arthritis should receive antibiotics that cover Group B Strep, Staphylococcus aureus, and gram-negative bacilli. Finally, patients with sickle cell disease should receive coverage for Salmonella, while sexually active adolescents may require coverage for Neisseria gonorrhoeae.

Now, let’s review the lab results. The synovial fluid analysis will reveal a positive Gram stain and culture, with synovial fluid that appears cloudy and purulent. Additionally, the cell count will be greater than 50,000 white blood cells per cubic millimeter. In some cases, a blood culture might be positive. These findings confirm the diagnosis of septic arthritis.

Here’s a clinical pearl to keep in mind! Although you could use imaging studies to support the diagnosis of septic arthritis, you should not delay arthrocentesis, since synovial fluid analysis is the gold standard for diagnosis.