What are the differential diagnoses and management options for a patient presenting with lower extremity monoplegia?

Lower Extremity Monoplegia: Differential Diagnoses and Management

Lower extremity monoplegia requires immediate assessment for acute limb ischemia (ALI) as the most time-critical diagnosis, followed by systematic evaluation for neurologic causes including spinal cord lesions, peripheral nerve injuries, and central nervous system pathology.

Immediate Life-and-Limb-Threatening Differential: Acute Limb Ischemia

Evaluate for the "6 Ps" of acute limb ischemia immediately: pain, paralysis, paresthesias, pulselessness, pallor, and poikilothermia (cold extremity). 1, 2

Clinical Assessment

• Perform handheld continuous-wave Doppler examination immediately to assess arterial signals—loss of Doppler arterial signal indicates a threatened limb requiring emergent intervention. 3
• Classify limb viability using the Rutherford classification: 1, 2
  • Category I (Viable): No sensory loss, no muscle weakness, audible arterial and venous Doppler
  • Category IIa (Marginally threatened): Mild sensory or motor loss, inaudible arterial Doppler, audible venous Doppler
  • Category IIb (Immediately threatened): Moderate sensory or motor loss beyond the toes—requires intervention within 6 hours
  • Category III (Irreversible): Profound sensory loss, paralysis with rigor, inaudible arterial and venous Doppler

Management Algorithm for ALI

• Start systemic anticoagulation with intravenous unfractionated heparin immediately to prevent thrombus propagation, even before imaging. 2, 3
• Obtain CT angiography (CTA) of the entire lower extremity emergently as the preferred diagnostic test—it provides rapid anatomic detail including level of occlusion, degree of atherosclerotic disease, and below-knee vessel patency essential for revascularization planning. 2
• Consult vascular surgery immediately, even before imaging is complete, as Category IIb and III limbs require revascularization within 6 hours since skeletal muscle tolerates ischemia for only 4-6 hours before permanent damage occurs. 2
• Patients with salvageable extremities (Categories I, IIa, IIb) should undergo emergent endovascular or surgical revascularization. 1

Neurologic Differentials

Spinal Cord Pathology

Spinal cord infarction can present as acute monoplegia, particularly affecting the anterior cord territory. 4

• Look for: Back pain preceding weakness, sensory level, bowel/bladder dysfunction, and T2 hyperintensity with diffusion restriction on MRI. 4
• Fibrocartilaginous embolism from disc herniation or Schmorl's nodes can cause spinal cord stroke, especially in young athletic patients with recent heavy physical activity. 4
• Traumatic intraparenchymal contusions near the vertex affecting the motor cortex can cause isolated lower extremity monoplegia—consider coronal CT reconstructions if axial imaging is unrevealing. 5

Peripheral Nerve Lesions

Electrodiagnostic evaluation with nerve conduction studies and needle electromyography is essential for suspected mononeuropathies. 6

• Sciatic nerve injury: Affects both knee flexion and all ankle/foot movements
• Femoral nerve injury: Causes knee extension weakness with preserved ankle/foot function
• Obturator nerve injury: Produces hip adduction weakness
• Peroneal nerve injury: Results in foot drop but preserved plantar flexion
• Tibial nerve injury: Causes loss of plantar flexion with preserved dorsiflexion 6

Central Nervous System Causes

• Cerebral infarction or hemorrhage affecting the motor cortex near the vertex can produce isolated lower extremity monoplegia. 5
• Complications of neurovascular procedures: Coil herniation following intra-arterial treatment can cause acute monoplegia. 7

Risk Factors to Assess

For Vascular Causes

• Atrial fibrillation (thromboembolism risk) 2
• Age ≥65 years or ≥50 years with smoking or diabetes 1
• Known peripheral artery disease with intermittent claudication 2
• Chronic kidney disease, neuropathy, or infection (increased risk for critical limb ischemia) 1

For Neurologic Causes

• Recent trauma (spinal cord or cortical contusion) 5
• Heavy athletic activity (fibrocartilaginous embolism) 4
• Recent neurovascular procedures (iatrogenic complications) 7

Diagnostic Algorithm

1. Assess limb perfusion immediately: Check pulses, temperature, color, capillary refill, and Doppler signals. 2, 3
2. If vascular compromise suspected: Start heparin, obtain CTA, and consult vascular surgery emergently. 2
3. If neurologic pattern evident: Determine level (cortical vs. spinal vs. peripheral) based on associated findings:
   • Cortical: Contralateral face/arm involvement, aphasia, visual field defects
   • Spinal: Sensory level, bowel/bladder dysfunction, bilateral findings
   • Peripheral: Dermatomal sensory loss, specific muscle group pattern 6, 4, 5
4. Obtain appropriate imaging: Brain/spine MRI for central causes, nerve conduction studies for peripheral causes. 6, 4

Critical Pitfalls

• Do not delay treatment of ALI for extensive testing—time from symptom onset to revascularization is the major determinant of limb salvage and mortality. 3
• Do not rely on ankle-brachial index (ABI) alone in acute presentations—ABI only confirms arterial occlusion but provides no anatomic information needed for revascularization planning. 2
• Do not miss vertex contusions on standard axial CT—obtain coronal reconstructions when isolated lower extremity monoplegia follows head trauma. 5
• Patients with both coronary artery disease and peripheral artery disease are at extremely high risk for major adverse cardiovascular events, mortality, and major amputation. 2