The phrenic nerve is one of a pair of nerves, designated left and right, that sends signals between the brain to the diaphragm. When the diaphragm moves, it pulls air into and pushes it out of the lungs. The nerves receive automatic signals to keep the body breathing, but people can also exert some control over the movement of the diaphragm, allowing them to do things like hold their breath. Damage to the nerves impairs that ability and can sometimes cause very serious medical problems.
The Nerve's Path
The phrenic nerve originates in the brain and initially travels down alongside the main spinal cord that is encased within the backbone. The twin nerves immediately branch away, exiting by the 3rd, 4th, or 5th vertebra in the neck. The right phrenic nerve passes underneath the muscles of the neck and bones of the shoulder to the base of the right lung, where it comes into contact with the heart and the windpipe. The left nerve follows a similar path, passing close to the heart before entering the diaphragm.
The Diaphragm
The diaphragm is a large, dome-shaped muscle beneath the lungs that receives its operating signals via the phrenic nerve. When a person inhales, it contracts to give the lungs more room to fill with oxygen-rich air. When a person exhales, it relaxes and pushes up against the lungs, helping expel waste gases like carbon dioxide. It also exerts pressure on the abdominal cavity to help with other bodily functions, like throwing up or urinating. It is one of the few muscles that can operate both involuntarily and under human control. During sleep, the brain sends regular instructions for the diaphragm to contract and relax, but the brain can override these instructions if a person needs to hold a breath.
Motor and Sensory Signals
Besides sending signals from the brain to the diaphragm, the phrenic nerves also collect sensory signals to send back to the brain. This includes things like reports on whether it is contracting properly, any pain felt, and the temperature from it and the other organs and tissue within the chest. This enables the brain to regulate respiration properly.
Causes of Damage
Illnesses affecting the phrenic nerves specifically are uncommon; diseases of the nerves are usually systemic, affecting the entire body. Physical trauma causes most cases or direct damage to these nerves. This usually happens when a person has a neck injury, or when a surgeon accidentally damages them during an operation in the chest or abdomen.
Symptoms of Irritation and Damage
Irritation of the phrenic nerves can produce some unusual symptoms. It can trigger the hiccup reflex, which is a sudden, involuntary contraction of the diaphragm that causes the lungs to take a small gulp of air. A less common symptom is a pain at the tip of the shoulder blade, a phenomenon known as Kehr’s sign. This type of pain can have other more serious causes as well, and should be evaluated by a healthcare professional.
The telltale symptom of phrenic nerve damage is difficulty with breathing. Since there are two of these nerves, a person will still be able to breathe if one is damaged, but it will be difficult. Damage to both nerves is a medical emergency, since the diaphragm will be paralyzed and a person will be unable to breathe.
Treatment
Breathing difficulty from phrenic nerve damage may resolve on its own with time, since the nerves can potentially regenerate themselves and establish new connections. Anyone with breathing difficulty should be seen by a medical professional though, to rule out other causes. If a person continuously has trouble breathing related to phrenic nerve damage, he or she may be given a breathing pacemaker, which is a surgically implanted, battery operated device that discharges regular electrical pulses that stimulate the diaphragm to contract. A procedure called plication is also sometimes used. This consists of surgically modifying the diaphragm so that it stays down more than usual, which allows the lungs more room to expand.