Thursday, May 26, 2016

Septic hip or synovitis? Use ultrasound!

The presentation 
The parents said their 5 year-old boy was limping on his right leg for the past day. The don't think he had hurt himself, and my exam confirmed that his foot, ankle, leg, and knee were atraumatic, but his hip was painful to range. He could still eke out an antalgic gait on the right leg, though. 

He had a slight temperature elevation of 100.2 F (37.9 C), and labs showed a WBC of 11.8, and an ESR of 28.

Was this a septic hip? What was the next test?

"The best decision rule is Kocher’s criteria."
Some folks still recommend using Kocher's criteria to rule-out septic arthritis in the limping child. If the child is able to weight bear, has no fever, and WBC < 12, and ESR < 40, then the risk of septic arthritis was only 0.2% in Kocher's original study.

Problem is, these sorts of results are all tied up with the population being studied and the prevalence of septic hip. Subsequent studies have found that kids with Kocher score of 0 can still have a risk of septic arthritis that ranges from < 0.2% to 17%.
Sultan & Hughes
In my patient's case, the kid's "Kocher score" was zero. And if we had trusted in this score we might have missed a septic hip!  

Don't guess with Kocher - just look at the joint with ultrasound!
First, we looked at the left (contralateral) hip, seeing what normal should look like. Then we looked at the painful right hip. It wasn't subtle!

An anechoic space space in the hip joint > 5 mm is generally considered positive for effusion.

The live clips were even clearer:

IR was contacted, and we performed sedation for the aspiration. The aspirate from his hip was wildly positive; grossly purulent, with > 100,000 WBC/mL. The child was admitted, and did well.

Note: Case was modified slightly for teaching purposes.
Only in 2 respects: His WBC was actually trivially higher, at 12.2, and he couldn't bear any weight at all on the right leg. These are small differences, and further highlight the potential pitfalls in using indirect criteria like Kocher's.

Open-Access References:
Limping Child? Think LIMPSS.
Septic arthritis or transient synovitis of the hip in children: The value of clinical prediction algorithms

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