Limp In Children: MCCQE1 Preparation Guide

Introduction

Evaluating a limping child is a common and high-stakes clinical scenario in Canadian pediatric practice. For the MCCQE1, candidates must demonstrate the ability to differentiate between benign, self-limiting conditions (like transient synovitis) and limb-threatening or life-threatening emergencies (like septic arthritis or malignancy).

This guide focuses on the CanMEDS Medical Expert role, emphasizing the diagnostic reasoning required to navigate the broad differential diagnosis based on the child’s age, history, and physical examination.

🇨🇦 Canadian Context: Epidemiology

In Canada, trauma is the leading cause of limp. However, infectious etiologies like Lyme disease are becoming increasingly relevant in specific provinces (e.g., Nova Scotia, Ontario, Manitoba). Always consider geographical history within Canada when evaluating a child with a limp and joint effusion.

Differential Diagnosis by Age Group

The etiology of a limp is heavily age-dependent. A structured approach based on age is crucial for the MCCQE1.

Toddlers (1-3 yrs)

Common Causes:

Infection: Septic arthritis, Osteomyelitis
Trauma: Toddler’s fracture (spiral fracture of distal tibia), Soft tissue injury
Developmental: Developmental Dysplasia of the Hip (DDH) - usually presents as painless limp
Neoplasm: Leukemia, Neuroblastoma (metastases)

Clinical Evaluation

History

A thorough history is the cornerstone of diagnosis. Focus on the following acronym for pain characterization:


OPQRST:
O - Onset (Acute vs. Chronic)
P - Provoking/Palliating factors (Worse in morning? Worse after activity?)
Q - Quality (Sharp, dull, aching)
R - Radiation (Knee pain can be referred from the hip!)
S - Severity (Ability to bear weight)
T - Timing (Night pain is a red flag for malignancy)

Key Historical Questions:

Systemic symptoms: Fever, weight loss, night sweats (Red flags for infection or malignancy).
Recent illness: Recent URI often precedes Transient Synovitis.
Trauma: Witnessed or unwitnessed?
Morning stiffness: Suggestive of rheumatologic etiology (JIA).

Physical Examination

Observe the gait. Is it antalgic (shortened stance phase on affected side) or Trendelenburg (weak abductors)?

💡

MCCQE1 Pearl: In children, knee pain is hip pathology until proven otherwise. Always examine the hip (internal/external rotation) in a child presenting with knee pain.

The “GALS” Screening Exam

For a quick musculoskeletal assessment:

Gait
Arms
Legs
Spine

Diagnostic Approach

Follow this step-by-step approach to investigate a limping child, aligning with Canadian Choosing Wisely recommendations to avoid unnecessary radiation.

Step 1: Rule out “Red Flags”

Assess for fever, systemic toxicity, or inability to bear weight. If present, proceed immediately to workup for septic arthritis/osteomyelitis.

Step 2: Physical Exam & Localization

Localize the pain. If trauma is suspected and focal bone tenderness is present, X-ray the specific area.

Step 3: Plain Radiography

If no focal trauma or if history is concerning (e.g., night pain, chronic limp):

AP and Frog-leg Lateral views of the pelvis (Crucial for SCFE and Perthes).
AP and Lateral views of the symptomatic limb (tibia/fibula/femur).

Step 4: Laboratory Investigations

If infection or inflammation is suspected:

CBC with differential
ESR and CRP (Inflammatory markers)
Blood cultures (if febrile)
Lyme serology (if in endemic area)

Step 5: Advanced Imaging

Ultrasound: Best for detecting hip effusion (sensitive but not specific for cause).
MRI: Gold standard for osteomyelitis, early Perthes, or stress fractures not seen on X-ray.
Bone Scan: Rarely used now but can be helpful if MRI is unavailable or for multifocal pathology.

High-Yield Conditions for MCCQE1

1. Septic Arthritis vs. Transient Synovitis

Differentiating these two is a classic exam competency. Use the Kocher Criteria to estimate the probability of septic arthritis.

The Kocher Criteria

Non-weight bearing on affected side
Sedimentation rate (ESR) > 40 mm/hr
Fever > 38.5°C
WBC count > 12,000/mm³

Probability of Septic Arthritis: 1 predictor = 3%; 2 predictors = 40%; 3 predictors = 93%; 4 predictors = 99%.

Comparison Table

Feature	Transient Synovitis	Septic Arthritis
Age	3–10 years	Any (peak < 3 years)
Onset	Acute/Subacute	Acute/Fulminant
Systemic Signs	Afebrile or low-grade fever	High fever, toxic appearance
Pain	Mild to moderate	Severe, pain with any ROM
Labs	Normal/Mildly elevated ESR/CRP	Elevated WBC, ESR, CRP
X-ray	Normal	Normal or joint space widening
Management	NSAIDs, rest, observation	Emergency: Surgical drainage + IV Antibiotics

2. Slipped Capital Femoral Epiphysis (SCFE)

Demographics: Obese adolescents (11-16 years), Male > Female.
Pathophysiology: Displacement of the femoral neck relative to the femoral head (growth plate failure).
Presentation: Pain in groin, thigh, or knee. Limp. Leg held in external rotation. Limited internal rotation.
Imaging: “Ice cream falling off the cone” appearance on X-ray. Klein’s Line does not intersect the femoral head.
Management: Non-weight bearing immediately and surgical pinning.

3. Legg-Calvé-Perthes Disease (LCPD)

Demographics: 4-10 years, Male > Female.
Pathophysiology: Idiopathic avascular necrosis of the femoral head.
Presentation: Painless or mild painful limp. Insidious onset.
Management: Containment (bracing/surgery) to maintain spherical femoral head.

Canadian Guidelines & Choosing Wisely

Choosing Wisely Canada (Pediatrics): Do not order a screening hip ultrasound for developmental dysplasia of the hip (DDH) in infants with a normal physical exam (no instability) and no risk factors.
Lyme Disease: In endemic Canadian regions, the Canadian Paediatric Society recommends considering Lyme arthritis in children with acute monoarthritis. Two-tier testing (ELISA followed by Western Blot) is the standard.
Wait Times: Recognize that MRI access varies across Canada. In a rural setting, X-ray and Ultrasound are the primary modalities.

Key Points to Remember for MCCQE1

Toddler’s Fracture: Spiral fracture of distal tibia. May not be visible on initial X-ray. Mechanism: twisting injury.
SCFE: Always consider in an obese adolescent with knee pain. Missed diagnosis leads to avascular necrosis.
Septic Arthritis: Surgical emergency. The hip is a “closed space”; pus destroys cartilage rapidly.
DDH: Risk factors include Breech presentation, Female sex, Family history (The “3 F’s”: Female, First-born, Feet first/Breech).
Malignancy: Persistent pain, night pain, weight loss, or pain out of proportion to injury requires ruling out Osteosarcoma or Ewing Sarcoma.

Sample Question

Clinical Scenario

A 13-year-old male presents to the emergency department with a 3-week history of a dull ache in his left knee and a limp. He denies any specific trauma. He has no fever or systemic symptoms. His BMI is 31 kg/m². On physical examination, the left knee has full range of motion with no erythema or effusion. However, when the patient lies supine and the left hip is flexed, the leg obligatorily goes into external rotation. Internal rotation of the left hip is limited and painful.

Question

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

A. MRI of the left knee
B. Reassurance and NSAIDs for Osgood-Schlatter disease
C. X-rays of the pelvis (AP and frog-leg lateral)
D. Aspiration of the left knee joint
E. Bone scan of the lower extremities

Explanation

The correct answer is:

C. X-rays of the pelvis (AP and frog-leg lateral)

Detailed Explanation: This clinical vignette is classic for Slipped Capital Femoral Epiphysis (SCFE).

Demographics: The patient is an adolescent male and is obese (BMI 31), which are the strongest risk factors for SCFE.
Presentation: While he complains of knee pain, the physical exam reveals the pathology is in the hip (referred pain). The obligatory external rotation upon hip flexion is a hallmark sign of SCFE.
Management: The diagnosis must be confirmed immediately with X-rays of the hips. The frog-leg lateral view is the most sensitive view for detecting early SCFE. Once diagnosed, the patient must be made non-weight bearing immediately to prevent further slippage and catastrophic outcomes (avascular necrosis), followed by surgical pinning.

Why other options are incorrect:

A. MRI of the left knee: The knee exam is normal. The pain is referred from the obturator nerve. Imaging the knee would expose the patient to unnecessary cost/time and miss the diagnosis.
B. Reassurance and NSAIDs: This would be dangerous. Delay in treatment of SCFE increases the risk of femoral head necrosis and permanent disability. Osgood-Schlatter presents with tibial tubercle tenderness, not hip rotation issues.
D. Aspiration of the left knee: There is no effusion or sign of infection in the knee.
E. Bone scan: Not the first-line imaging modality. X-rays are diagnostic for SCFE.

References

Canadian Paediatric Society. (2024). Evaluation of the limping child. Ottawa, ON.
Choosing Wisely Canada. (2023). Pediatrics: Five Things Physicians and Patients Should Question.
Medical Council of Canada. (2024). MCCQE Part I Objectives: Musculoskeletal System.
Kocher, M. S., et al. (1999). Validation of a clinical prediction rule for the differentiation between septic arthritis and transient synovitis of the hip in children. Journal of Bone and Joint Surgery.
Kliegman, R. M., et al. (2020). Nelson Textbook of Pediatrics (21st ed.). Elsevier.