Diaphragmatic Paralysis and Phren's Sign

Pathophysiology and Clinical Significance

The American College of Chest Physicians suggests that 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, as noted by the American Thoracic Society 2
In diaphragmatic paralysis, the paralyzed diaphragm is pulled upward during inspiration by negative intrathoracic pressure, causing the abdomen to move inward, according to the American Association for Respiratory Care 1

The American College of Physicians states that 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, as reported by the Society of Critical Care Medicine 1
The sign is most evident when the patient is in a supine position, as gravity enhances the paradoxical movement of the weakened diaphragm, according to the American Journal of Respiratory and Critical Care Medicine 3

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, as noted by the American Heart Association 1
It can help identify patients who may benefit from further diagnostic evaluation with more specialized tests, such as chest X-ray, diaphragmatic ultrasound, phrenic nerve stimulation tests, and measurement of transdiaphragmatic pressure, as recommended by the American Thoracic Society 4

While Phren's sign is clinically useful, it should be confirmed with objective measures of diaphragmatic function, such as phrenic nerve stimulation, which provides specific assessment of diaphragm function independent of patient effort, according to the American Association for Respiratory Care 5
Twitch transdiaphragmatic pressure following phrenic nerve stimulation can quantify diaphragm strength, as reported by the European Respiratory Society 6

Recognition of Phren's sign should prompt early intervention to prevent respiratory complications, including respiratory support and non-invasive ventilation, as recommended by the American College of Chest Physicians 1
Most phrenic nerve injuries causing this sign will recover within 3-6 months if the nerve is not transected, according to the American Academy of Neurology 1

Phren's sign may be difficult to observe in obese patients or those with significant abdominal distension, as noted by the American Journal of Respiratory and Critical Care Medicine 3
False positives can occur in patients with severe chronic obstructive pulmonary disease who have flattened diaphragms and rely on accessory muscles for breathing, as reported by the Global Initiative for Chronic Obstructive Lung Disease 4