How is phrenic nerve injury assessed?

Assessment of Phrenic Nerve Injury

The most effective assessment of phrenic nerve injury involves a combination of clinical examination for paradoxical breathing patterns, imaging studies, and specialized tests including phrenic nerve stimulation techniques that provide objective measurement of diaphragmatic function.

Clinical Examination

• Look for paradoxical abdominal motion during inspiration (Phren's sign) - when the paralyzed diaphragm is pulled upward during inspiration causing the abdomen to move inward instead of outward 1
• This sign is most evident when the patient is in a supine position, as gravity enhances the paradoxical movement of the weakened diaphragm 1
• Assess for respiratory distress with increased work of breathing and difficulty weaning from mechanical ventilation 2
• Evaluate for hemidiaphragmatic paralysis presenting as immobility or elevation of the diaphragm 2

Diagnostic Imaging

• Chest X-ray to identify elevated hemidiaphragm on the affected side 2
• Diaphragmatic ultrasound at bedside to assess diaphragmatic function, particularly during weaning from mechanical ventilation 2
  • Allows real-time visualization of diaphragm movement and thickness during respiration
  • Highly sensitive and specific for the diagnosis of phrenic nerve injuries 3

Specialized Testing

Phrenic Nerve Stimulation

• Phrenic nerve stimulation provides objective assessment of diaphragm function independent of patient effort 2, 4
• Two main techniques are used:
  • Transcutaneous electrical phrenic nerve stimulation (ES) 3
  • Magnetic phrenic nerve stimulation (MS) - technically easier and less uncomfortable for patients 3
• Stimulation should be performed with the patient relaxed at functional residual capacity (FRC) 3
• Posture has minimal effect on measurements, making this useful even in ICU settings 3

Pressure Measurements

• Twitch transdiaphragmatic pressure (Pdi,tw) following phrenic nerve stimulation provides a specific index of diaphragm strength 3, 2
• Measurement of the response to a single supramaximal stimulation of the phrenic nerve is an appealing method of monitoring changes in diaphragmatic contractility 3
• Negative gastric pressure to transdiaphragmatic pressure ratio can indicate severe diaphragmatic dysfunction or paralysis 2

Electrophysiological Testing

• Phrenic nerve conduction studies measure:
  • Conduction time (normal: approximately 8.40 ± 0.78 msec) 5
  • Amplitude of diaphragmatic muscle action potential 5
• These studies have shown 95% sensitivity and 87.5% specificity for diagnosing unilateral diaphragmatic paralysis 6
• Diaphragmatic electromyography via an esophageal electrode can quantify muscle activity and examine precise timing of activation 3

Assessment Algorithm

1. Initial evaluation:

   • Clinical examination for paradoxical breathing pattern 1
   • Chest X-ray to identify elevated hemidiaphragm 2

2. If diaphragmatic dysfunction is suspected:

   • Perform diaphragmatic ultrasound to visualize movement 2
   • Conduct phrenic nerve stimulation tests with measurement of twitch transdiaphragmatic pressure 3

3. For quantitative assessment:

   • Phrenic nerve conduction studies to determine nerve integrity 6, 5
   • Evaluate for potential recovery (most phrenic nerve injuries recover within 3-6 months if the nerve is not transected) 2

Clinical Pitfalls

• Phren's sign may be difficult to observe in obese patients or those with significant abdominal distension 1
• False positives can occur in patients with severe COPD who have flattened diaphragms and rely on accessory muscles for breathing 1
• When the diaphragm is paralyzed, there is a coordinated reorganization of non-diaphragmatic respiratory muscles as compensation that might be obscured by high levels of pressure support ventilation 7
• Phrenic nerve injury is a recognized complication following cardiac intervention or surgery, with the right phrenic nerve at risk during procedures involving the superior vena cava and right superior pulmonary vein 8