What is the differential diagnosis for a 2-year-old child presenting with sudden weakness of the lower limb?

Differential Diagnosis of Sudden Lower Limb Weakness in a 2-Year-Old

In a 2-year-old with sudden lower limb weakness, septic arthritis is the most critical diagnosis to exclude immediately, as it can cause permanent joint damage within hours if untreated, followed by toddler's fracture (spiral tibial fracture) as the most common non-infectious cause in this age group. 1, 2

Immediate Life-Threatening Conditions to Rule Out First

Septic Arthritis (Orthopedic Emergency)

• Fever >101.3°F (38.5°C) combined with refusal to bear weight or move a joint is the hallmark presentation requiring urgent intervention within hours. 1, 2
• Check for systemic toxicity signs: lethargy, irritability, poor feeding, which indicate serious infection requiring urgent evaluation. 2
• Laboratory markers supporting septic arthritis include: ESR >40 mm/hour, WBC >12,000/mm³, or CRP >20 mg/L. 3
• Immediate hip ultrasound is mandatory if symptoms localize to the hip to assess for joint effusion, followed by ultrasound-guided aspiration for cell count, Gram stain, and culture. 1, 2

Osteomyelitis

• Often coexists with septic arthritis and requires MRI for definitive diagnosis. 1
• Presents with fever, focal bone tenderness, and elevated inflammatory markers. 3
• MRI of the affected area with and without IV contrast is the imaging modality of choice. 2

Common Non-Infectious Causes in This Age Group

Toddler's Fracture (Most Common Occult Fracture)

• Spiral tibial fracture is the most common cause of nonlocalized limp in children under 4 years, even without clear trauma history. 4
• Critical pitfall: 10-41% of toddler's fractures appear normal on initial radiographs and only become visible on follow-up films obtained 7-10 days later. 1, 2, 4
• Initial imaging should be bilateral tibia/fibula radiographs (anteroposterior and lateral views). 1, 4

Other Fractures

• Femur, foot, or other lower extremity fractures from unwitnessed trauma. 5
• If no focal findings on exam, obtain radiographs of both entire lower extremities rather than just the painful area. 1

Neurological Causes of True Weakness (vs. Antalgic Gait)

Guillain-Barré Syndrome

• Presents with ascending limb weakness and paralysis that may progress to respiratory failure. 6
• Diagnosis is clinical with supportive laboratory testing (elevated CSF protein with normal cell count). 6
• Requires urgent recognition as it can rapidly progress to life-threatening respiratory compromise. 6

Cerebral Palsy (Acute Presentation of Chronic Condition)

• May present acutely if previously undiagnosed, though typically has gradual onset. 5
• Spasticity is most common motor type (85-91%), with bilateral lower limb involvement (diplegia) in 37% of cases. 5
• MRI brain has 86-89% sensitivity for detecting cerebral palsy after 5 months corrected age. 5

Spinal Cord Pathology

• Transverse myelitis, spinal cord compression, or epidural abscess can present with acute lower limb weakness. 3
• Look for bowel/bladder dysfunction, sensory level, and upper motor neuron signs. 3

Systematic Physical Examination Approach

Critical Examination Components

• Examine systematically from hip to foot, as 2-year-olds cannot verbalize or accurately localize symptoms, and hip pathology refers pain to thigh, knee, or buttock in up to 30% of cases. 4
• Assess the "three As" (anxiety, agitation, analgesic requirement) which indicate serious pathology when verbal communication is limited. 1
• Palpate entire lower extremity for focal tenderness, swelling, erythema, and warmth. 4
• Evaluate weight-bearing ability: complete refusal raises concern for fracture or septic arthritis. 4
• Perform neurological assessment including muscle tone, strength, reflexes, and sensation to distinguish true weakness from pain-related pseudoparalysis. 1

Diagnostic Algorithm Based on Initial Findings

If Fever Present (Temperature >101.3°F):

1. Obtain hip ultrasound immediately if symptoms localize to hip. 2
2. If symptoms localize to other lower extremity area, obtain MRI of that area without and with IV contrast. 2
3. If symptoms are nonlocalized, obtain MRI of entire lower extremity without and with IV contrast. 2
4. Check CBC, ESR, CRP, and blood cultures. 1, 3
5. Arrange urgent orthopedic consultation for possible joint aspiration. 1

If Afebrile with Localized Symptoms:

1. Obtain anteroposterior and lateral radiographs of the involved site. 3
2. If radiographs normal but symptoms persist, provide short-course NSAIDs and arrange follow-up in 7-10 days for repeat radiographs. 1, 2
3. If symptoms worsen or persist beyond 7-10 days despite normal repeat radiographs, obtain MRI to detect occult fractures, stress fractures, or early inflammatory changes. 2, 4

If Afebrile with Nonlocalized Symptoms:

1. Obtain bilateral tibia/fibula radiographs as initial imaging, since spiral tibial fractures are most common in this age group. 4
2. Consider hip ultrasound to evaluate for joint effusion, as hip pathology frequently presents with referred pain. 4
3. If initial workup negative and symptoms persist, obtain MRI of entire lower extremity. 2

If True Weakness (Not Pain-Related):

1. Perform comprehensive neurological examination to localize lesion (upper vs. lower motor neuron). 5
2. Consider urgent MRI of brain and/or spine depending on examination findings. 5
3. Check for ascending pattern suggesting Guillain-Barré syndrome and assess respiratory function. 6
4. Obtain lumbar puncture if Guillain-Barré suspected (after imaging excludes mass lesion). 6

Less Common but Important Diagnoses

Transient Synovitis

• Most common diagnosis overall in limping children, but typically presents with limp rather than true weakness. 3
• Self-limited inflammatory condition of the hip following viral illness. 3
• Diagnosis of exclusion after ruling out septic arthritis. 3

Legg-Calvé-Perthes Disease

• Avascular necrosis of femoral head, though more common in 4-8 year age range. 1
• Presents with insidious onset hip pain and limp. 1

Acute Compartment Syndrome

• Consider in children with trauma showing anxiety, agitation, and high analgesic requirements despite treatment. 1
• Requires urgent surgical fasciotomy to prevent permanent damage. 1

Metabolic/Endocrine Causes

• Hyperthyroidism can rarely present with isolated proximal lower limb weakness, though uncommon in this age group. 7

Critical Management Pitfalls to Avoid

• Never assume normal radiographs exclude fracture in a 2-year-old with persistent symptoms. 1, 2
• Never focus only on the site of reported pain; young children frequently mislocalize pain. 1
• Never delay septic arthritis workup in a febrile child refusing to bear weight, as joint damage occurs within hours. 1, 2
• Do not order whole-body bone scan as initial imaging; it lacks soft-tissue detail and anatomic specificity needed in this age group. 5
• Do not rely on ultrasound alone for fracture detection; it has only 73% sensitivity compared to radiography. 5