Diagnosing Lyme Arthritis of the Hip in Children

Lyme arthritis of the hip can resemble septic arthritis or transient synovitis
Children with Lyme arthritis are less likely to have fever or refuse to bear weight
No single symptom or laboratory result can safely establish the diagnosis

Diagnosing Lyme arthritis of the hip in children can be challenging. Its presentation may resemble both acute bacterial septic arthritis and transient synovitis, yet these conditions require different treatment approaches.

“The clinical presentation of Lyme arthritis (LA) of the hip can be similar to both acute bacterial septic arthritis (SA) and transient synovitis (TS),” Cruz and colleagues explained.¹

The investigators conducted a systematic review and meta-analysis to identify clinical and laboratory features that might help clinicians distinguish these conditions.

What the Researchers Found

The investigators identified 88 children diagnosed with Lyme arthritis of the hip. Their average age was 7.5 years.

The findings were compared with previously published data involving children with septic arthritis or transient synovitis. Transient synovitis is a temporary inflammation of the hip that frequently follows a viral illness, although its precise cause is not always known.

Fever and Ability to Bear Weight

Children with Lyme arthritis were more likely to continue bearing weight than children with septic arthritis or transient synovitis.

Still, one-third of the children with Lyme arthritis refused to bear weight:

Lyme arthritis: 33.3%
Transient synovitis: 61.1%
Septic arthritis: 62.2%

Fever above 38.5°C (101.3°F) was also less common with Lyme arthritis:

Lyme arthritis: 22.9%
Transient synovitis: 30.5%
Septic arthritis: 53.8%

These differences may help guide the evaluation, but neither the absence of fever nor the ability to bear weight rules out septic arthritis.

Can Blood Tests Distinguish the Conditions?

Peripheral white blood cell counts and erythrocyte sedimentation rates overlapped among the three conditions. Therefore, neither test could reliably distinguish Lyme arthritis from septic arthritis on its own.

The reported 95% confidence intervals for ESR were:

Transient synovitis: 21–33 mm/hr
Lyme arthritis: 37–46 mm/hr
Septic arthritis: 44–64 mm/hr

The authors proposed that an ESR of 40 mm/hr or higher should prompt clinicians to consider hip aspiration and synovial fluid testing for septic arthritis. When the ESR is below 40 mm/hr, Lyme serology and evaluation for transient synovitis may be appropriate, depending on the clinical presentation and geographic exposure.

Synovial Fluid White Blood Cell Counts

Synovial fluid white blood cell counts also overlapped, although substantially higher counts were more suggestive of bacterial septic arthritis.

The reported 95% confidence intervals were:

Transient synovitis: 5,644–15,388 cells/mm³
Lyme arthritis: 47,533–64,242 cells/mm³
Septic arthritis: 105,432–260,214 cells/mm³

The authors proposed a synovial white blood cell count of at least 65,000 cells/mm³ as a threshold that should increase concern for septic arthritis. However, this cutoff cannot establish or exclude either diagnosis by itself.

Clinical Judgment Remains Essential

The study’s diagnostic algorithm incorporated ESR followed by synovial fluid analysis. However, the authors emphasized that the algorithm had not been prospectively validated.

A child with substantial hip pain, marked restriction of movement, fever, systemic illness, or other findings suggestive of septic arthritis requires urgent evaluation. Septic arthritis can damage the joint if treatment is delayed.

As Cruz and colleagues noted, if significant hip irritability or other clinical signs of septic arthritis are present, treatment for septic arthritis should be initiated while the evaluation continues.

Clinical Takeaway

Lyme arthritis should be considered when a child develops an acutely painful or swollen hip, particularly in a Lyme-endemic region. Children with Lyme arthritis may be less likely to have fever or refuse to bear weight than children with septic arthritis, but considerable overlap exists.

No single symptom, ESR value, or synovial fluid white blood cell count can reliably distinguish these conditions. The diagnosis should incorporate the complete clinical presentation, Lyme exposure risk, serologic testing, imaging, and—when indicated—joint aspiration.

This article is for educational purposes and is not a substitute for medical evaluation. A child with acute hip pain, fever, inability to walk, or significant illness should receive prompt medical care.