Diaphragmatic paralysis is severe weakness of the diaphragm, leading to abnormalities of respiration. Paralysis can be unilateral or bilateral. Those with unilateral paralysis may be asymptomatic or have dyspnea on exertion and difficulty exercising. Dyspnea may worsen when patients are supine, especially if there is coexisting obesity, other muscle group weakness, or underlying cardiopulmonary disease. Bilateral diaphragmatic paralysis leads to more severe weakness, often presenting with shortness of breath, worsening while supine, or with respiratory failure. Other symptoms may include fatigue, hypersomnia, headache, depression, anxiety, nocturnal awakenings, and gastrointestinal complaints, often due to sleep-disordered breathing and sleep fragmentation. Patients may also have atelectasis and respiratory infections.

Examination typically reveals tachypnea, respiratory accessory muscle use, and a paradoxical breathing pattern with rostral diaphragmatic movement and inward movement of the abdominal wall during inspiration. There can be absent breath sounds and dullness to percussion over the lower chest of the affected side(s) due to reduced diaphragmatic excursion.

Possible causes include motor neuron disease (such as amyotrophic lateral sclerosis, syringomyelia, spinal muscular atrophies), peripheral neuropathy, myopathy, muscular dystrophy, spinal cord disease, multiple sclerosis, metabolic or inflammatory disorders, mediastinal masses, and trauma or injury to the phrenic nerve. In unilateral diaphragmatic paralysis, the most common cause is tumors causing phrenic nerve compression. In bilateral diaphragmatic paralysis, the most common cause is severe generalized weakness from neuromuscular disease.

The progression and prognosis of disease depends on the etiology; prognosis is often better in unilateral paralysis. Those with surgical or traumatic phrenic nerve injuries can recover spontaneously after several years. In degenerative neuromuscular diseases, dysfunction progressively worsens.

Infants are more severely affected as they are typically supine and their weakened respiratory muscles cannot stabilize their more compliant chests.