Diaphragmatic paralysis is a loss of movement in the diaphragm, the dome-shaped muscle that separates the chest from the abdomen and does most of the work of breathing. It happens when the phrenic nerve, which signals the diaphragm to contract, is damaged, compressed, or diseased. The condition can affect one side (unilateral) or both sides (bilateral), and the difference between the two matters enormously for how sick a person feels. Causes range from cardiac surgery and spinal cord injury to viral infections and neurodegenerative diseases, and in a surprisingly large share of cases, no cause is ever identified.
How the Diaphragm Normally Works
Breathing feels automatic, and for good reason. A network of neurons in the brainstem generates rhythmic signals that travel down the spinal cord and exit through the phrenic nerves, which originate from the third through fifth cervical vertebrae in the neck. Those nerves deliver the signal to diaphragm muscle fibers, causing them to contract and flatten downward. That downward pull creates negative pressure inside the chest, drawing air into the lungs. When the diaphragm relaxes, air flows back out. This cycle repeats roughly 12 to 20 times per minute without any conscious effort.1PubMed Central. Breathing: Motor Control of Diaphragm Muscle
Because the phrenic nerve has such a long path from the neck to the diaphragm, it is vulnerable at many points along the way. A tumor pressing on it in the chest, cold injury during heart surgery, a stretch injury at birth, or inflammation from a virus can all interrupt the signal. When that signal stops reaching part or all of the diaphragm, the muscle goes slack instead of contracting, and the person’s ability to breathe is compromised to varying degrees.
Unilateral Versus Bilateral Paralysis
The distinction between one-sided and two-sided paralysis is the single most important factor in how much trouble a person has. When only one hemidiaphragm is paralyzed, many people have surprisingly few symptoms at rest. Most patients with unilateral paralysis experience unexplained shortness of breath during exertion and reduced inspiratory muscle capacity.2PubMed Central. Unilateral diaphragm paralysis: a dysfunction restricted not just to one hemidiaphragm The healthy side can compensate enough for everyday activities, though complaints tend to surface when other conditions are present, such as obesity or existing lung or heart disease.3European Respiratory Review. Diaphragm dysfunction: how to diagnose and how to treat?
Bilateral paralysis is a different situation entirely. When both sides are affected, people frequently cannot tolerate lying flat because gravity allows the abdominal organs to push the flaccid diaphragm upward into the chest, further compressing the lungs. They become heavily dependent on accessory breathing muscles in the neck and between the ribs, and even activities that engage those muscles, like raising the arms overhead or lifting objects, can leave them gasping. In severe cases, breathlessness occurs at rest, and mechanical ventilation may be required.3European Respiratory Review. Diaphragm dysfunction: how to diagnose and how to treat?
Cardiac Surgery and Cold Injury
One of the best-documented causes of diaphragmatic paralysis is phrenic nerve damage during open-heart surgery. Surgeons routinely use cold solutions and ice slush to cool the heart and protect it during the procedure. The problem is that the phrenic nerves run right along the pericardium, the sac surrounding the heart, and prolonged contact with ice can injure them. In a prospective study of 133 patients who had open-heart surgery with topical ice-slush cooling, phrenic nerve damage occurred in roughly three out of four patients whose nerves were not shielded from direct ice exposure.4PubMed. The effect of pericardial insulation on hypothermic phrenic nerve injury during open-heart surgery That rate is startlingly high, and it pushed surgical teams to develop insulating pads and other shielding techniques.
Most post-surgical phrenic injuries affect one side and resolve as the nerve recovers from cold-related demyelination over weeks to months. But bilateral injury is the nightmare scenario. A case report documented a 30-year-old woman who developed fatal bilateral phrenic nerve paralysis after mitral valve surgery that used both systemic hypothermia and topical iced saline, and she could never be weaned from the ventilator.5PubMed. Fatal bilateral phrenic nerve injury following hypothermic open heart surgery Cases like that one, though rare, drove home the severity of the risk and motivated changes in intraoperative cooling techniques.
Beyond cold injury, the phrenic nerve can also be damaged by direct stretching during chest surgery or by compression from retractors. Phrenic nerve conduction studies have proven useful for sorting out whether the mechanism is cold-related demyelination or a primary stretch injury.6PubMed. Phrenic nerve conduction study in demyelinating neuropathies and open-heart surgery
Spinal Cord Injury
Because the phrenic nerves originate from the upper cervical spinal cord, injuries at or above that level can knock out the diaphragm. High cervical spinal cord injuries are among the most devastating causes of diaphragmatic paralysis, often leaving people dependent on a mechanical ventilator for life.7PubMed Central. Anesthetic Management and Considerations of Diaphragmatic Pacemaker Placement for Cervical Spine Injury: A Case Report In one study of ventilator-dependent patients with complete injuries between C1 and C4, about a third had confirmed diaphragmatic paralysis at the time of their respiratory failure.8PubMed. Delayed diaphragm recovery in 12 patients after high cervical spinal cord injury
The biology here is straightforward in principle but complex in detail. A cervical spinal cord lesion interrupts the descending drive from the brainstem to the phrenic motor neurons. Animal research has confirmed that a unilateral hemisection at the C2 level eliminates inspiratory diaphragm activity on the injured side, though the opposite side continues working normally.9PubMed Central. Size-dependent alterations in phrenic motor neuron perineuronal nets following cervical spinal hemisection Recovery depends on how much of the spinal cord is damaged, whether any crossed neural pathways can pick up the slack, and how early rehabilitation and respiratory support begin.
Infections and Inflammatory Nerve Conditions
Viral infections are an underappreciated trigger. The link between phrenic nerve palsy and viral illness is well documented in case reports spanning several decades, and both SARS-CoV-2 and influenza A have been implicated in recent years.10PubMed Central. COVID-19 Associated Unilateral Diaphragm Paralysis: A Case Report The proposed mechanisms include direct viral damage to the phrenic nerve, inflammation-driven swelling that compresses the nerve, and ischemia from microvascular spasm cutting off blood supply to the nerve during severe illness.11PubMed Central. Diaphragmatic Eventration Caused by SARS-CoV-2 and Influenza A Virus Infection: Two Case Reports Persistent post-infection coughing may also contribute by fatiguing an already compromised diaphragm.
A condition that often gets overlooked in this context is Parsonage-Turner syndrome, also called neuralgic amyotrophy. It typically begins with intense, self-limiting pain in the shoulder and upper arm, followed days or weeks later by weakness or paralysis in the affected muscles. The brachial plexus (the nerve bundle serving the arm) is the classic target, but the phrenic nerve can be involved too, causing diaphragm paralysis on one or both sides.12PubMed Central. Orthopnea secondary to brachial plexitis with bilateral diaphragmatic paralysis In one analysis at a major medical center, more than half of patients originally labeled as having idiopathic phrenic nerve paralysis turned out to have clinical features of Parsonage-Turner syndrome once the right questions were asked.13The Annals of Thoracic Surgery. Phrenic Nerve Palsy Due to Parsonage-Turner Syndrome Researchers have argued that the condition is significantly under-diagnosed as a cause of phrenic nerve palsy because clinicians do not always think to ask about preceding shoulder and arm pain.14PubMed. Phrenic Nerve Palsy Secondary to Parsonage-Turner Syndrome: A Diagnosis Commonly Overlooked
Neurodegenerative Disease and Other Causes
Motor neuron diseases like amyotrophic lateral sclerosis (ALS) progressively destroy the nerve cells that control voluntary muscles, and the diaphragm is no exception. Most ALS patients develop respiratory muscle weakness as the disease advances, but in rare cases, diaphragm paralysis is the very first sign. One reported case involved a 54-year-old woman who presented with acute respiratory failure from unilateral diaphragm paralysis before any other ALS symptoms appeared.15PubMed Central. A Rare Case of Amyotrophic Lateral Sclerosis With Asymmetrical Phrenic Nerve Lesion Presenting With Acute Respiratory Failure as an Initial Manifestation This matters clinically because unexplained diaphragm paralysis in someone without an obvious surgical or traumatic cause should prompt consideration of an underlying neurodegenerative process.
Tumors within the chest can also cause diaphragmatic paralysis by directly invading or compressing the phrenic nerve. Lung cancer is the most common malignant culprit, but lymphoma, thymoma, and metastatic disease from cancers elsewhere in the body can all do the same thing.16PubMed Central. Thoracic complications and emergencies in oncologic patients When a new diaphragm elevation shows up on a chest X-ray in a patient with a known cancer, phrenic nerve involvement from the tumor is high on the list of explanations.
In the intensive care unit, a different form of diaphragm dysfunction can develop that blurs the line between weakness and true paralysis. Prolonged mechanical ventilation puts the diaphragm on standby. Without regular contractions, the muscle atrophies quickly, a process accelerated by the systemic inflammation that comes with sepsis and critical illness. The combination of disuse and inflammation-driven damage can leave the diaphragm too weak to sustain breathing once the ventilator is turned down, trapping patients in a cycle of failed weaning attempts.17PubMed Central. Diaphragm Dysfunction in Critical Illness
How Diaphragmatic Paralysis Is Diagnosed
The condition is frequently discovered by accident. A routine chest X-ray shows one hemidiaphragm sitting higher than expected, and the workup begins. The classic confirmatory test has been fluoroscopy during a “sniff” maneuver: the patient sniffs sharply while the radiologist watches the diaphragm in real time. A paralyzed hemidiaphragm moves paradoxically upward during the sniff instead of descending.
Ultrasound is increasingly replacing fluoroscopy for this purpose. It can be done at the bedside, involves no radiation, and is easy to repeat over time. A prospective study comparing the two found that ultrasound has acceptable correlation with fluoroscopy for evaluating diaphragm excursion and can serve as the primary tool for the assessment.18PubMed Central. Replacement of fluoroscopy by ultrasonography in the evaluation of hemidiaphragm function, an exploratory prospective study M-mode ultrasound during the sniff maneuver has proven particularly useful for confirming the diagnosis at the bedside.19CHEST. Unilateral Diaphragmatic Paralysis Diagnosed by M-Mode Ultrasound
Lung function tests provide another clue. Spirometry typically shows reduced lung volumes, and a distinctive finding is that the numbers drop further when the patient lies down compared to sitting up.20PubMed Central. Diaphragmatic Palsy This sitting-to-supine fall in vital capacity reflects the loss of diaphragm support against gravity, though the exact threshold that best identifies dysfunction remains a matter of debate.21Respiratory Physiology & Neurobiology. Predictive value of positional change in vital capacity to identify diaphragm dysfunction In practice, a drop of more than about 20 to 25 percent is considered suspicious, and larger drops strongly suggest significant diaphragm weakness. The test is simple enough that it can be a useful screening tool before more advanced imaging is ordered.
Treatment Options
What happens next depends on the cause, whether one or both sides are affected, and how much the paralysis interferes with daily life. For unilateral paralysis with mild symptoms, watchful waiting is often reasonable because a substantial share of cases recover on their own. One long-term follow-up study found that functional recovery occurred in about 43 percent of patients within 12 months and 52 percent within two years, regardless of the cause of the paralysis.22Respiratory Medicine. Functional recovery of diaphragm paralysis: A long-term follow-up study That recovery rate is encouraging but still means that nearly half of patients are left with a persistently paralyzed hemidiaphragm.
For those who remain symptomatic, diaphragm plication is the most established surgical option. The procedure tightens the slack, paralyzed diaphragm by suturing it into a flattened position, which prevents it from ballooning upward into the chest and crowding the lung. Plication increases the thoracic space, relieves compression on collapsed lung tissue, and improves the mechanical advantage of the remaining respiratory muscles. Studies have confirmed that it reduces symptoms and improves quality of life in patients with persistent paralysis.23PubMed Central. Quality-of-life impact of diaphragm plication in patients with diaphragmatic paralysis: A retrospective study The surgery can often be done minimally invasively through a thoracoscopic approach, which reduces recovery time.
For patients with bilateral paralysis who are ventilator-dependent, particularly those with high spinal cord injuries, diaphragm pacing offers an alternative to round-the-clock mechanical ventilation. The concept involves implanting electrodes that electrically stimulate the phrenic nerve, causing the diaphragm to contract rhythmically and mimic natural breathing. In one reported case, a quadriplegic patient who was completely dependent on a ventilator was able to breathe independently for up to three hours per day after unilateral phrenic nerve pacing was established.24PubMed Central. Phrenic nerve stimulation for diaphragm pacing in a quadriplegic patient Results are not uniformly positive, though. In a case series of children with acute flaccid myelitis, diaphragm pacing did not show clear improvement over matched controls who did not receive the intervention, suggesting the technique works best when the phrenic nerve itself is intact and the interruption is above the nerve’s origin in the spinal cord.25PubMed Central. Diaphragm Interventions in Acute Flaccid Myelitis: A Single-Center Retrospective Case Series of Diaphragm Pacing Systems and Phrenic Nerve Transfers
Newborns and Birth-Related Injury
Diaphragmatic paralysis in newborns deserves a separate mention because the cause and presentation differ from adults. Birth trauma, particularly lateral hyperextension of the neck during a difficult delivery, can stretch and injure the phrenic nerve. The result is paralysis of the diaphragm on the affected side, which in a tiny infant can produce irregular breathing and respiratory distress that may require continuous positive airway pressure or mechanical ventilation.26PubMed Central. Phrenic nerve palsy: A rare cause of respiratory distress in newborn In cases that do not respond to supportive care, surgical plication of the diaphragm may be needed even in the neonatal period. The condition is rare enough that it can be missed initially, with the diagnosis sometimes delayed until a chest X-ray is obtained for another reason and the elevated hemidiaphragm is noticed.
The Idiopathic Problem
Perhaps the most frustrating aspect of diaphragmatic paralysis for both patients and clinicians is how often no cause is found. After a thorough workup that excludes surgery, trauma, tumor, infection, and neuromuscular disease, a significant proportion of cases remain labeled idiopathic. As noted earlier, some of these may actually represent unrecognized Parsonage-Turner syndrome, and researchers have stressed that careful questioning about prior shoulder or arm pain can unmask this diagnosis in cases that would otherwise go unexplained. Others may result from subclinical viral infections that damaged the phrenic nerve without producing any other symptoms. The practical challenge is that idiopathic does not mean untreatable. Plication, respiratory support, and watchful waiting all remain valid approaches regardless of whether the underlying cause is identified, and the roughly 50 percent spontaneous recovery rate within two years applies across causes, including the idiopathic category.22Respiratory Medicine. Functional recovery of diaphragm paralysis: A long-term follow-up study
Ventilator-Induced Diaphragm Weakness in the ICU
Intensive care medicine has had to confront the paradox that the very machine keeping a critically ill patient alive can weaken the muscle they need to breathe without it. Mechanical ventilation takes over the work of breathing, and a diaphragm that is not contracting begins to atrophy within days. Add in sepsis-driven inflammation, which directly damages muscle fibers, and the diaphragm can lose substantial force-generating capacity in a short time.17PubMed Central. Diaphragm Dysfunction in Critical Illness This is not classical paralysis in the sense of a severed or compressed nerve; rather, it is a rapid, acquired weakness that looks and acts like partial paralysis from a functional standpoint.
ICU teams now try to balance adequate ventilatory support with preserving some diaphragm activity. Modes of ventilation that allow the patient to trigger breaths and do partial respiratory work are preferred over fully controlled modes when feasible. Ultrasound monitoring of diaphragm thickness during an ICU stay can track whether the muscle is thinning, providing an early warning that the patient may struggle to wean. The recognition that the ventilator itself is part of the problem, not just the solution, has reshaped how critical care physicians think about prolonged respiratory support.