Lyme Arthritis vs. Septic Arthritis in Children: Can MRI Help?

Symptoms can overlap closely
Clinical and laboratory findings may be similar
MRI findings may help avoid unnecessary surgery

Lyme arthritis and septic arthritis can look remarkably similar in children, particularly when the knee is swollen. Both conditions may cause fever, difficulty bearing weight, elevated inflammatory markers, and a high white blood cell count in the joint fluid.

However, distinguishing Lyme arthritis from septic arthritis is critical. Bacterial septic arthritis can rapidly damage the cartilage and typically requires emergency treatment, including surgical drainage and antibiotics. Lyme arthritis usually does not cause the same rapid joint destruction and often can be treated without surgery.

In a study published in JBJS Open Access, investigators assessed whether MRI findings, combined with clinical and laboratory information, could help distinguish Lyme arthritis from septic arthritis in children presenting with acute knee swelling.

Why Lyme Arthritis and Septic Arthritis Are Difficult to Distinguish

Children with Lyme arthritis may exhibit findings commonly associated with septic arthritis, including:

• Fever
• Inability to bear weight
• Elevated C-reactive protein (CRP)
• Elevated erythrocyte sedimentation rate (ESR)
• A high synovial fluid white blood cell count
• A large knee effusion

Because septic arthritis is a medical and surgical emergency, clinicians may proceed with operative drainage before Lyme disease test results are available. This cautious approach can protect children with true septic arthritis, but it may also result in unnecessary surgery when the underlying condition is Lyme arthritis.

Study Examines MRI Findings in Children With Knee Effusions

The investigators retrospectively reviewed the medical records and MRI scans of children and adolescents who presented between 2009 and 2019 with an acute knee effusion and were ultimately diagnosed with Lyme arthritis or bacterial septic arthritis.

The study included:

• 87 children with confirmed Lyme arthritis
• 9 children with confirmed septic arthritis

The two groups had substantial clinical overlap in their ability to bear weight, presence of fever, and joint aspirate white blood cell counts.

Among the 87 children with Lyme arthritis, 14 patients—approximately 16%—were initially presumed to have septic arthritis and underwent operative irrigation and debridement.

All of the children diagnosed with Lyme arthritis received one month of antibiotic treatment.

Does Lyme Arthritis Show Up on MRI?

MRI does not directly diagnose Lyme disease. It cannot identify Borrelia burgdorferi or replace appropriate Lyme disease testing. However, certain MRI findings may help clinicians determine whether an inflamed knee is more consistent with Lyme arthritis or septic arthritis.

The researchers found significant differences between the two groups in several MRI characteristics. Findings that favored Lyme arthritis included:

• Myositis involving multiple muscles, including the vastus lateralis
• Lymphadenopathy near the affected joint
• Absence of subcutaneous edema

MRI can be useful when evaluating musculoskeletal infections. However, it can be costly and may require sedation in some young children.

Findings That Favored Lyme Arthritis

In the study’s multivariable analysis, the following combination of clinical, laboratory, and MRI findings predicted Lyme arthritis:

• Ability to bear weight
• CRP below 3 mg/L
• Absence of subcutaneous edema
• Myositis involving multiple muscles, including the vastus lateralis
• Lymphadenopathy

The authors proposed that children with these findings could potentially be treated with oral antibiotics for suspected Lyme arthritis while clinicians awaited confirmatory serologic results.

These findings should not be interpreted as a stand-alone diagnostic rule. A CRP below 3 mg/L and the listed MRI features were components of this study’s prediction model and should be considered alongside the child’s complete clinical presentation.

Could MRI Help Children Avoid Unnecessary Surgery?

The authors emphasized an important difference between these two conditions. Lyme arthritis caused by the B. burgdorferi spirochete does not typically produce the rapid cartilage destruction associated with bacterial septic arthritis. Therefore, Lyme arthritis usually does not require emergency surgical drainage.

Fourteen children with Lyme arthritis underwent an operative procedure because septic arthritis was initially suspected. The investigators suggested that if their proposed clinical and MRI criteria had been applied, all 14 of these children might have avoided surgery.

This is an important observation, but it should be interpreted cautiously. The study included only nine children with septic arthritis and was a retrospective, single-center investigation. Larger prospective studies are needed before the model can be used as a definitive diagnostic pathway.

Septic Arthritis Must Still Be Considered an Emergency

The possibility of avoiding unnecessary surgery is important, but clinicians must also avoid delaying treatment for true septic arthritis. Untreated bacterial septic arthritis can rapidly damage a joint and lead to serious systemic complications.

A child with acute joint swelling, fever, severe pain, inability to bear weight, or systemic illness requires prompt medical evaluation. MRI findings alone cannot safely exclude septic arthritis.

Evaluation may include a physical examination, blood tests, Lyme disease serology, blood cultures, arthrocentesis with synovial fluid analysis and culture, and imaging when appropriate.

Why Lyme Arthritis Should Be Considered in Children

Lyme arthritis should be considered when a child from or visiting a Lyme-endemic region develops a swollen knee, even when there is no known tick bite or recalled erythema migrans rash.

The authors concluded that Lyme arthritis should be strongly suspected in endemic areas of the United States when children present with a knee effusion. They also reported that adding MRI criteria to clinical and laboratory findings significantly improved the predictive value for identifying Lyme arthritis.

This diagnostic challenge is one reason musculoskeletal symptoms deserve careful consideration as part of the broader range of pediatric Lyme disease presentations.

Frequently Asked Questions

Can Lyme arthritis look like septic arthritis?

Yes. Lyme arthritis may resemble septic arthritis because both conditions can cause a swollen joint, fever, difficulty bearing weight, elevated inflammatory markers, and a high synovial fluid white blood cell count.

Does Lyme arthritis show up on MRI?

MRI cannot directly diagnose Lyme disease. However, findings such as lymphadenopathy, myositis involving multiple muscles, and the absence of subcutaneous edema may support Lyme arthritis when combined with clinical findings and laboratory testing.

How can doctors distinguish Lyme arthritis from septic arthritis?

Doctors consider the child’s symptoms, physical examination, geographic exposure, inflammatory markers, Lyme disease testing, synovial fluid analysis, cultures, and sometimes MRI findings. No single finding reliably distinguishes every case.

Does Lyme arthritis require surgery?

Lyme arthritis usually does not require surgical drainage and is generally treated with antibiotics. Surgery may be performed when clinicians cannot safely exclude bacterial septic arthritis or when another complication is suspected.

Is septic arthritis an emergency?

Yes. Bacterial septic arthritis can rapidly damage joint cartilage and requires urgent evaluation and treatment. A child with an acutely swollen or painful joint, fever, inability to walk, or systemic illness should receive prompt medical care.

Clinical Takeaway

Lyme arthritis and septic arthritis can be difficult to distinguish in children because their clinical and laboratory findings may overlap considerably. MRI does not diagnose Lyme disease or independently exclude septic arthritis, but selected MRI findings may improve diagnostic accuracy when combined with the child’s clinical presentation, laboratory results, and Lyme disease testing.

The small number of septic arthritis cases in this study means that its proposed prediction model requires additional validation. Clinicians must continue to treat suspected septic arthritis urgently while considering Lyme arthritis in children with knee effusions in endemic areas.

Careful integration of clinical, laboratory, and MRI findings may help distinguish Lyme arthritis from septic arthritis while reducing unnecessary surgery without overlooking a potentially destructive joint infection.

Related Articles

Lyme arthritis symptoms in a child years after a tick bite
Diagnosing Lyme arthritis of the hip in children
The high cost of treating Lyme arthritis in children with surgery

References

1. Yen, Y. M., Sanborn, R. M., Donohue, K., Miller, P. E., Milewski, M. D., & Ecklund, K. Lyme arthritis in the pediatric knee: Clinical and magnetic resonance imaging differentiators. JBJS Open Access. 2022;7(4):e22.00067.

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Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention