Introduction
The phrenic nerve is a mixed (contains both the sensory and motor fibers) bilateral nerve. It is considered an important nerve due to its involvement in respiration. It gives the primary motor supply to the major respiratory muscle, the diaphragm. It causes the diaphragm to contract and expand as one inhales and exhales. An injury to the nerve can cause paralysis or dysfunction of the diaphragm. This can cause breathing difficulties while sleeping.
What Is the Phrenic Nerve?
The phrenic nerve is a peripheral nerve (a part of the nervous system outside the brain and the spinal cord). It is thought to be derived from the neural crest cells (a transient group of cells that gives rise to many other types of cells) and the neural plate (the progenitor of the nervous system). The neural plate starts differentiation from the third week following fertilization.
The phrenic nerve controls the diaphragm, which is the largest muscle that lies between the chest and abdominal cavities. The dome-shaped muscle is important in breathing. The signals from the phrenic nerves cause the diaphragm to contract (to become flatter and thicker). This provides the lungs space to expand and fill in air. This process is termed ‘inhalation’. Following this, the signals from the nerve become weak, causing the diaphragm to relax. As the space reduces, the lungs recoil and push the air out, becoming smaller. This is termed ‘exhalation’.
What Function Does the Phrenic Nerve Serve?
The phrenic nerve plays a major role in respiration. It is the sole nerve responsible for the motor supply to the diaphragm. Motor nerves supply the voluntary muscles (like muscles of the arms or legs) of the body. It supplies the ipsilateral side of the diaphragm (the same side of the diaphragm as the nerve). It signals the diaphragm to contract and expand. As a result, air moves in and out of the lungs. The sensory functions of the nerve can include:
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Provide touch and pain information to the diaphragmatic pleura (the portion covering the upper surface of the diaphragm) and diaphragm.
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Provide pain and touch information to the thin tissue covering the space in between the lungs, called the mediastinal pleura.
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Pain and touch signals to the pericardium (the fibrous sac containing the heart).
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Touch and pain signal to the peritoneum (the serous membrane that surrounds the abdominal cavity).
Where Does the Phrenic Nerve Originate?
The phrenic nerve primarily originates from the fourth cervical nerve (C4). (Cervical nerves are those nerves that arise from the cervical or the neck part of the spinal cord). However, the third (C3) and fifth (C5) cervical nerves also contribute to the origin. In short, it can be remembered as ‘C3, 4, and 5 keep the diaphragm alive’.
The nerve is at the level of the C3 vertebra, and it helps one to bend and rotate the neck. It then connects the C4 and C5 vertebrae. It reaches the diaphragm after traveling through the neck and the chest, passing through the heart and the lungs. The phrenic nerve is a paired nerve, right and left phrenic nerves. Both nerves perform the same function, though they are on different sides.
What Is the Clinical Relevance of the Phrenic Nerve?
The phrenic nerve provides the motor supply to the diaphragm. Hence, an injury to the nerve can cause weakness or paralysis of the diaphragm. This condition is also termed phrenic nerve palsy. Paralysis of the diaphragm can cause difficulty in gaseous exchange. The paralysis can be unilateral (affecting one side) or bilateral (affecting both sides), depending on the severity and extent of damage. In severe cases, a mechanical ventilator (a machine that helps one to breathe or breathes for them when they are unable to breathe on their own) might be required. The paralyses of the phrenic might be caused by:
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Autoimmune Disorders and Inflammatory Conditions: It can occur due to conditions like Guillain-Barré syndrome (a rare autoimmune condition that affects the nerves) and neuralgic amyotrophy (a disorder of the peripheral nervous system).
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Birth defects.
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Injuries to the baby while giving birth.
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Cervical spondylosis (a condition that occurs due to the wear and tear of the vertebra).
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Complications of surgeries done in the neck or chest, like coronary artery bypass surgery.
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Injuries to the spinal cord due to trauma like a motor vehicle accident falls from heights, etc.
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Neurologic conditions like multiple sclerosis (a condition where the immune system affects the protective covering of the nerves) or amyotrophic lateral sclerosis (a condition affecting the nervous system and the muscles).
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Tumors in the spinal cord.
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Lung cancer.
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Metastasis (spread of the cancerous cells from the place of origin to other distant places) from other cancers.
What Are the Signs of Phrenic Nerve Paralysis?
Unilateral paralysis of the phrenic nerve might not have specific symptoms. Occasionally, the patient might have pain in the shoulder or the arm on the affected side. However, signs of bilateral paralysis can include:
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Frequent episodes of pneumonia (inflammation of the air sacs in the lungs).
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Shortness of breath, which increases while lying down or being immersed, like in a swimming pool.
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Sleep apnea (a condition where one finds it difficult to breathe during sleep).
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In a child or baby, it can be seen as weak or faint cries.
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Frequent episodes of headaches, especially in the morning.
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Fatigue.
Does Hiccup Have a Link With Phrenic Nerve?
Sometimes, an irritated phrenic nerve can cause persistent hiccups. These hiccups may last for many days, months, or even years. The irritation could be due to some surgical procedures, tumors, or other issues. It can be uncomfortable and makes life difficult. It may hinder a person’s ability to talk, sleep, or eat. Oral medicines like Chlorpromazine (an antipsychotic medication) or nerve blocks can be used to manage the hiccups caused by an irritated phrenic nerve. Severe cases might require surgical interventions like phrenicotomy (surgical removal of the irritated part of the phrenic nerve). However, this might be very rare.
Conclusion
The phrenic nerve is a paired mixed nerve that plays a vital role in respiration. It originates from the third, fourth, and fifth cervical nerves. It provides the motor and sensory supply to the diaphragm. An injury to this nerve can cause paralysis, which can result in breathing difficulties. An irritation to this nerve can cause persistent hiccups. However, this is rare.
