The phrenic nerve is an important motor nerve originating in the neck from the cervical spinal nerve roots C3, C4, and C5, with C4 typically providing the largest contribution. This nerve travels downward through the chest cavity before reaching its destination. Though it carries some sensory fibers, its primary purpose is to control the body’s main muscle for breathing. The phrenic nerve is often summarized by the mnemonic, “C3, 4, 5 keep the diaphragm alive.”
Role of the Phrenic Nerve in Respiration
The phrenic nerve provides the sole motor supply to the diaphragm, the dome-shaped musculofibrous structure separating the chest and abdominal cavities. When the nerve activates, it sends electrical impulses causing the diaphragm to contract and flatten, moving downward. This downward movement increases the volume of the thoracic cavity, creating negative pressure that draws air into the lungs for inspiration. During exhalation, the diaphragm relaxes and returns to its dome shape, pushing air out. Because the nerve is the exclusive motor control, the diaphragm performs the majority of the work for normal, effortless breathing. The nerve is a bilateral structure, meaning there are separate left and right nerves, each controlling the corresponding half of the diaphragm.
The Primary Consequence: Diaphragmatic Paralysis
Damage to the phrenic nerve directly results in diaphragmatic paralysis, or paresis if the damage is partial, because the muscle no longer receives the necessary motor signals. When the nerve is compromised, the affected side of the diaphragm remains high in the chest, and its movement becomes paradoxical. Instead of moving downward during inspiration, the paralyzed side is pulled upward into the chest cavity by the negative pressure created by the working breathing muscles.
The symptoms experienced depend on whether the damage is unilateral (one side) or bilateral (both sides). Unilateral paralysis may be asymptomatic at rest, but commonly causes exertional shortness of breath (dyspnea) because the overall lung capacity is reduced. A specific symptom is orthopnea, which is difficulty breathing when lying flat, as abdominal contents push the paralyzed diaphragm further into the chest.
Bilateral phrenic nerve damage is significantly more severe and constitutes a medical emergency. With both sides of the diaphragm unable to contract, the patient experiences severe respiratory failure and often requires mechanical ventilation for survival. This condition severely limits lung capacity and can also lead to sleep-disordered breathing, chronic fatigue, and recurrent pneumonia.
Common Causes of Phrenic Nerve Damage
The phrenic nerve’s long course from the neck to the abdomen makes it vulnerable to injury from various sources. One frequent cause is iatrogenic injury, which occurs unintentionally during medical procedures, particularly cardiac or thoracic surgeries. Traumatic causes include blunt or penetrating injuries to the neck or chest, such as those sustained in motor vehicle accidents or falls. Additionally, the nerve can be compromised by neurological or infectious conditions, including viral infections, inflammatory disorders, and malignancies that compress or infiltrate the nerve tissue.
Diagnosis and Treatment Approaches
Diagnosing phrenic nerve damage requires a high index of suspicion, as symptoms can mimic other respiratory issues. Initial assessment often involves a chest X-ray, which may show the elevated position of the paralyzed hemidiaphragm. A more definitive test is chest fluoroscopy, sometimes called a “sniff test,” which uses moving X-rays to visualize the paradoxical upward motion of the paralyzed diaphragm during a forceful inhalation. Neurophysiological tests, such as electromyography (EMG) or nerve conduction studies, are used to quantify the extent of nerve damage.
Treatment for mild or temporary injury often begins with watchful waiting, physical therapy to strengthen accessory breathing muscles, and non-invasive positive-pressure ventilation at night. For chronic, symptomatic cases, surgical options exist, including diaphragm plication, phrenic nerve reconstruction with nerve grafting, or the implantation of a diaphragmatic pacing system to stimulate the nerve electrically.