A 4-year-old boy is brought to the emergency department by his parents because he has been… — USMLE Step 2 CK

A 4-year-old boy is brought to the emergency department by his parents because he has been refusing to walk since he woke up this morning. The parents note that he complains of pain in his right anterior thigh and knee. He had a mild upper respiratory tract infection two weeks ago but has otherwise been healthy. His temperature is $38.7^\circ\text{C}$ ( $101.7^\circ\text{F}$ ), blood pressure is $90/60\text{ mm Hg}$ , pulse is $118/\text{min}$ , and respirations are $22/\text{min}$ . On physical examination, the child appears uncomfortable and refuses to stand or bear any weight on his right leg. The right hip is held in slight flexion, abduction, and external rotation. There is no visible erythema or swelling of the hip, thigh, or knee. Range of motion of the right knee is full and painless, but any attempt to internally rotate the right hip elicits severe pain and crying. Laboratory studies show:

Laboratory Study	Result	Reference Range
Leukocyte count	$10,500/\text{mm}^3$	$4,000 - 12,000/\text{mm}^3$
Erythrocyte sedimentation rate	$28\text{ mm/hr}$	$0 - 20\text{ mm/hr}$
C-reactive protein	$1.8\text{ mg/dL}$	$< 2.0\text{ mg/dL}$

Which of the following is the most appropriate next step in the management of this patient?

A
Magnetic resonance imaging of the right femur and pelvis
B
Ultrasound-guided arthrocentesis of the right hip
Bilateral ultrasonography of the hipsAnswer
D
Intravenous vancomycin and ceftriaxone
E
Oral ibuprofen and discharge with outpatient follow-up

Answer

Bilateral ultrasonography of the hips is the most appropriate next step for an intermediate-risk patient to evaluate for a joint effusion before attempting arthrocentesis.

The correct answer is bilateral ultrasonography of the hips. This patient presents with an acute limp, fever, and right hip pain (with referred pain to the thigh and knee), which raises strong clinical suspicion for septic arthritis. The Kocher criteria are utilized to differentiate septic arthritis from transient synovitis. The four criteria include: non-weight-bearing on the affected side, temperature

>38.5^\circ\text{C}

(

101.3^\circ\text{F}

), erythrocyte sedimentation rate (ESR)

>40\text{ mm/hr}

, and leukocyte count

>12,000/\text{mm}^3

. This patient fulfills exactly two criteria (temperature of

38.7^\circ\text{C}

and refusal to bear weight). Patients meeting two criteria have an intermediate probability (approximately 40%) of having septic arthritis. According to established treatment algorithms, the most appropriate next step for intermediate-risk patients is ultrasonography of the hips (often performed bilaterally to compare with the asymptomatic side) to evaluate for a joint effusion. If an effusion is identified, ultrasound-guided arthrocentesis is then performed to definitively diagnose or rule out joint infection.

Step-by-Step Solution

Identify the clinical presentation and physical exam findings.

A 4-year-old boy presents with fever, non-weight-bearing status, and severe pain with internal rotation of the right hip. The pain is referred to the anterior thigh and knee, a classic pediatric presentation.

To recognize the classic presentation of a pediatric hip joint pathology and rule out isolated knee pathology.

Evaluate the patient using the Kocher criteria for septic arthritis.

The patient has a temperature

>38.5^\circ\text{C}

and is non-weight-bearing, fulfilling 2 criteria. His ESR (

28\text{ mm/hr}

) and WBC (

10,500/\text{mm}^3

) are below the thresholds of

40\text{ mm/hr}

and

12,000/\text{mm}^3

, respectively.

To objectively stratify the risk of septic arthritis versus transient synovitis.

Determine the appropriate algorithmic management based on the risk stratification.

With exactly 2 criteria, the patient has an intermediate probability (approximately 40%) of septic arthritis. The indicated next step is an ultrasound of the hips.

To confirm the presence of a joint effusion non-invasively before proceeding to an invasive diagnostic arthrocentesis.

Key Concept

Risk stratification and stepwise management of suspected pediatric septic arthritis using the Kocher criteria.

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