What is Phren's sign?

Phren's Sign in Respiratory Medicine

Phren's sign refers to the paradoxical abdominal motion during inspiration observed in patients with diaphragmatic paralysis or dysfunction, where the abdomen moves inward during inspiration rather than outward as seen in normal breathing patterns. 1

Pathophysiology and Clinical Significance

• Phren's sign occurs when the diaphragm is paralyzed or severely weakened, causing paradoxical movement during the respiratory cycle 1
• During normal inspiration, the diaphragm contracts and descends, pushing abdominal contents downward and causing the abdomen to move outward 2
• In diaphragmatic paralysis, the paralyzed diaphragm is pulled upward during inspiration by negative intrathoracic pressure, causing the abdomen to move inward (paradoxical motion) 1
• This sign is particularly important in assessing diaphragmatic function in patients with suspected phrenic nerve injury 1

Diagnostic Context

• Phren's sign is a clinical observation that can be made during physical examination of patients with suspected diaphragmatic dysfunction 1
• It is often associated with other signs of respiratory distress including increased work of breathing and difficulty weaning from mechanical ventilation 1
• The sign is most evident when the patient is in a supine position, as gravity enhances the paradoxical movement of the weakened diaphragm 2
• Observation should be made during quiet breathing, as forced respiratory efforts may mask the sign 2

Clinical Applications

• Phren's sign serves as an important bedside indicator of possible diaphragmatic paralysis in post-surgical patients, particularly after cardiac surgery where phrenic nerve injury is a known complication 1
• It can help identify patients who may benefit from further diagnostic evaluation with more specialized tests 2
• The presence of this sign should prompt consideration of additional diagnostic studies such as:
  • Chest X-ray (may show elevated hemidiaphragm) 1
  • Diaphragmatic ultrasound (to assess diaphragmatic excursion and thickness) 1
  • Phrenic nerve stimulation tests (electrical or magnetic) 2
  • Measurement of transdiaphragmatic pressure 2

Confirmatory Testing

• While Phren's sign is clinically useful, it should be confirmed with objective measures of diaphragmatic function 2
• Phrenic nerve stimulation provides specific assessment of diaphragm function independent of patient effort 2
• Twitch transdiaphragmatic pressure (Pdi,tw) following phrenic nerve stimulation can quantify diaphragm strength 2
• Electrical phrenic nerve stimulation (ES) or magnetic stimulation (MS) can be used to objectively evaluate diaphragmatic function 2

Clinical Implications

• Recognition of Phren's sign should prompt early intervention to prevent respiratory complications 1
• Patients showing this sign may require respiratory support, including non-invasive ventilation 1
• Most phrenic nerve injuries causing this sign will recover within 3-6 months if the nerve is not transected 1
• Management strategies include respiratory muscle training, physiotherapy, and in some cases, nocturnal non-invasive ventilation 1

Pitfalls and Caveats

• Phren's sign may be difficult to observe in obese patients or those with significant abdominal distension 2
• The sign may be absent during sleep or when the patient is under sedation 2
• False positives can occur in patients with severe chronic obstructive pulmonary disease who have flattened diaphragms and rely on accessory muscles for breathing 2
• Assessment should always be performed in conjunction with other clinical and diagnostic findings rather than in isolation 2, 1