Is It Infectious Arthritis or Lyme Disease?

Diagnosing young patients presenting with swollen knees can get dicey. Does the child have infectious arthritis of the knee or Lyme disease? A new study in the May 4 issue of The Journal of Bone and Joint Surgery identifies four symptoms that are predictive of septic arthritis in children.

According to the May 11, 2016 news release, “Septic arthritis is considered a medical emergency requiring surgical irrigation (flushing) and drainage to prevent rapid and irreversible destruction of the cartilage surrounding the knee. Lyme disease, an infection common in the northwest and northeast parts of the U.S. and caused by bacteria normally found in deer and transmitted to humans through ticks, is treated with antibiotics. In addition to knee swelling and pain, both conditions may cause a fever, swelling of other joints, irritability and an inability to bear weight on the affected extremity. Common laboratory tests to measure inflammation, as well as white blood cell counts, also may be elevated.”

“In the new study, researchers reviewed the records of 189 patients under age 18 who presented to Children’s Hospital of Philadelphia (CHOP) with excess fluid in the knee between 2005 and 2013. Of these patients, 23 had culture-positive septic arthritis; 26, culture-negative septic arthritis; and 140, Lyme disease.”

“The study authors found four characteristics that are predictive factors for septic arthritis: Knee pain with a short range of motion; A C-reactive protein (CRP test; used to measure inflammation) of >4.0 mg/L; Fever; Age younger than 2.”

“The probability of septic arthritis with any one factor present was 18% compared to 100% will all four factors present, ” said study author Wudbhav N. Sankar, M.D., an orthopedic surgeon at CHOP. “Our study offers a useful prediction algorithm to help distinguish septic arthritis from a knee effusion caused by Lyme disease in children.”

Dr. Sankar told OTW, “In this era of precise and detailed laboratory testing, we were surprised at how important the physical exam component (i.e., pain with short motion) was in distinguishing Lyme disease from septic arthritis.”

“Our predictive model demonstrates clear direction for patients on either end of the model: Those with all four risk factors (pain with short arc knee motion, CRP >4, history of fever, and age <2) should clearly be treated as septic arthritis with urgent surgery while those with zero risk factors can be safely observed. In between, the risk of having septic arthritis increased with each additional risk factor.”