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Published 2/1/2011
Maureen Leahy

Distinguishing Lyme arthritis from septic arthritis in children

Knowing the difference has significant impact on treatment

Caused by the Borrelia burgdorferi spirochete, Lyme disease is transmitted to humans through the bite of infected ticks. Clinical manifestations can range from an initial skin rash at the site of the bite to neurologic and cardiac complications. A common late-stage manifestation of the disease is Lyme arthritis, which can mimic bacterial septic arthritis, especially in children.

What does this mean for orthopaedic surgeons who practice in areas where Lyme disease is prevalent? To find out, AAOS Now spoke with Brian G. Smith, MD, lead author of “Lyme Disease and the Orthopaedic Implications of Lyme Arthritis,” which appears in the February issue of the Journal of the AAOS (JAAOS).

AAOS Now: Why is it important to distinguish between Lyme arthritis and septic arthritis?

Dr. Smith: In endemic areas, it’s important to be able to distinguish Lyme arthritis from septic arthritis—particularly in children—because the two conditions and their treatments are very different. The two diseases, however, have similar symptoms—fever, swollen joints, elevated white blood cell count, and a limp or refusal to bear weight—making the differential diagnosis a challenge.

Septic arthritis in children can be a devastating disorder. If it’s not detected early, it can lead to loss of the joint’s articular cartilage, or chondrolysis, which can have lifelong implications for disability. In the hip of a young child, septic arthritis can potentially lead to osteonecrosis of the femoral head. Septic arthritis requires emergent treatment including surgical drainage and irrigation of the joint to preserve the articular cartilage.

Lyme arthritis, on the other hand, does not appear to cause long-term problems and is effectively treated in most cases with a one-month course of antibiotics.

AAOS Now: What joints are most affected by Lyme arthritis?

Dr. Smith: Our retrospective review of 400 children (average age: 8 years) who had a joint aspirated either in the emergency department (ED) or hospital between 1992 and 2009 found the knee was the number one joint involved. Of the children who had joint effusion in the knee, 45 percent had Lyme arthritis and 10 percent had septic arthritis. The hip is the second most common joint for Lyme arthritis, and it is also common in septic arthritis.

Knowing that Lyme arthritis has a predilection for the knee can be helpful for physicians. For example, if a child in an endemic area refuses to bear weight on a warm, swollen knee and has an elevated white blood cell count or other suggestive symptoms of either a septic joint or a Lyme joint, there is almost a 1 in 2 chance that it is Lyme arthritis and not septic arthritis.

AAOS Now: Will every patient with Lyme disease develop Lyme arthritis?

Dr. Smith: No. Lyme disease is a multisystem disorder that begins with a dermatologic manifestation—erythema migrans rash—and is often followed by cardiac, neurologic, and ultimately orthopaedic manifestations. Lyme arthritis, therefore, is a late-stage or tertiary form of the disease.

The presentation of Lyme disease and Lyme arthritis is really quite variable and, especially in endemic areas, physicians must always have a high index of suspicion when treating a patient of any age with a swollen joint, particularly the knee joint. Although it is common among children aged 6 to 10 years, Lyme arthritis has a bimodal age distribution and can also affect adults in their 40s and 50s.

AAOS Now: What is the long-term prognosis for patients with Lyme arthritis?

Dr. Smith: The long-term prognosis is really quite good and depends on early diagnosis and appropriate early treatment. Pediatricians and family physicians are more aware of Lyme arthritis now than they used to be, and testing for the disease is done more expeditiously now. Anecdotally, we know that arthroscopy of the knee joint for synovectomy in Lyme arthritis is being performed less today than a decade ago. That is likely due to better understanding of the incidences of Lyme disease, better diagnostic testing, and earlier treatment.

AAOS Now: What are the positive predictors for Lyme arthritis versus septic arthritis that orthopaedic surgeons know?

Dr. Smith: Our efforts to distinguish Lyme arthritis from septic arthritis were based on the criteria used in the guidelines developed by Mininder S. Kocher, MD, for managing septic arthritis in children—fever, white blood cell count and erythrocyte sedimentation rate, and weight-bearing status. In our series of patients, we found that erythrocyte sedimentation rate, as well as C-reactive protein values, were not helpful in separating the Lyme patients from septic patients.

We did find, however, that Lyme arthritis is more likely to involve the knee, and that children with Lyme arthritis are also more likely to have low-grade fever, a lower peripheral white blood cell count, a lower white blood cell count on joint-aspirated fluid, and the ability to tolerate limited weight bearing compared to children with septic arthritis.

Dr. Smith’s coauthors include Aristides I. Cruz Jr, MD; Matthew D. Milewski, MD; and Eugene D. Shapiro, MD.

Disclosure information: Dr. Smith—Stryker; Section on Orthopaedics, American Academy of Pediatrics; Dr. Shapiro—American Board of Pediatrics; Subboard on Infectious Diseases; American Lyme Disease Foundation; Drs. Cruz and Milewski—no conflicts.

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org