The phrenic nerve is the main nerve that controls your diaphragm, the large dome-shaped muscle responsible for breathing. You have two phrenic nerves, one on each side, and they originate from the neck at spinal levels C3, C4, and C5. Medical students remember this with the phrase “C3, 4, and 5 keep the diaphragm alive,” which captures just how essential these nerves are. Without functioning phrenic nerves, the diaphragm cannot contract, and independent breathing becomes impossible.
Where the Phrenic Nerve Starts and Where It Goes
The phrenic nerve forms from nerve roots in the middle of the neck, primarily from C4 with smaller contributions from C3 and C5. From there, it takes one of the longest paths of any nerve originating in the neck, traveling all the way down through the chest to reach the diaphragm.
In the neck, the nerve runs along the front of a muscle called the anterior scalene (on the side of your neck) before crossing over the collarbone area and entering the chest. Once inside the chest, each phrenic nerve descends along the sides of the pericardial sac, the membrane surrounding the heart. The right phrenic nerve passes alongside the right side of the heart and pierces the diaphragm at roughly the level of the T8 vertebra. The left phrenic nerve travels down in front of the left side of the heart before ending at the central tendon of the diaphragm. This long journey, running right next to the heart and lungs, is part of what makes the nerve vulnerable during chest surgeries.
What the Phrenic Nerve Does
The phrenic nerve’s primary job is motor: it tells the diaphragm to contract. When the nerve fires, the diaphragm pulls downward and flattens, expanding the lungs and drawing air in. When the signal stops, the diaphragm relaxes back into its dome shape, pushing air out. This cycle happens roughly 12 to 20 times per minute during normal breathing.
But the phrenic nerve isn’t purely motor. It also carries sensory information back to the brain from the diaphragm, the pericardium (the sac around the heart), and parts of the pleura (the membrane lining the chest cavity). This is why irritation in these areas can produce pain that feels like it’s coming from the shoulder. The phrenic nerve shares spinal roots with nerves that supply the shoulder, so the brain sometimes misinterprets where the signal originated.
The Phrenic Nerve and Hiccups
Hiccups are essentially a phrenic nerve malfunction. They arise from erratic stimulation of a reflex arc that includes the phrenic nerve, the vagus nerve, and sympathetic nerves from the mid-spine. Signals travel to processing centers in the midbrain, which then send impulses back through the phrenic nerve to the diaphragm, causing it to contract suddenly. The sharp intake of air hits a closing glottis (the flap at the top of your windpipe), producing the characteristic “hic” sound.
For persistent hiccups that won’t resolve, doctors can actually block the phrenic nerve temporarily in the neck to interrupt this reflex loop. The block targets the nerve as it travels past the anterior scalene muscle, disrupting the final signal to the diaphragm and, in some cases, resetting the abnormal pattern entirely.
Unilateral vs. Bilateral Paralysis
When one phrenic nerve is damaged, the opposite side of the diaphragm compensates. Many people with unilateral (one-sided) paralysis have no symptoms at all, or they notice mild shortness of breath during exercise. The working half of the diaphragm handles enough of the breathing load that daily life continues normally, and no treatment is needed in many cases.
Bilateral paralysis, where both phrenic nerves are affected, is a different situation entirely. Without any diaphragm function, the body depends on accessory muscles in the neck and between the ribs to breathe. These muscles fatigue quickly, especially during sleep when muscle tone naturally drops. People with bilateral paralysis often experience a telltale pattern: their oxygen levels are normal while standing (around 99% saturation in one documented case) but drop significantly when lying down (to 93%). Breathing difficulty that worsens in a reclined position is a hallmark symptom. Many of these patients require non-invasive ventilation, particularly at night, to prevent progressive respiratory failure.
How Phrenic Nerve Damage Happens
Cardiac surgery is one of the most common causes of phrenic nerve injury, with reported rates ranging widely from about 1% to as high as 60% depending on the surgical technique. Cold-induced injury is a particular concern. One study found a 24% rate of left phrenic nerve weakness in patients whose bypass surgery involved iced slush placed near the heart, which can freeze the nearby nerve. Stretching the nerve during retraction of the heart is another mechanism.
Shoulder surgery poses a different risk. Interscalene nerve blocks, a common form of regional anesthesia for shoulder procedures, are injected near the brachial plexus nerve roots in the neck. Because the phrenic nerve originates from the same area, temporary paralysis of one side of the diaphragm occurs in up to 100% of patients receiving this block. The effect typically wears off as the anesthetic fades. Persistent paralysis from an interscalene block is exceedingly rare, occurring in roughly 0.06% of cases.
Other causes include tumors pressing on the nerve, viral infections, spinal cord injuries above C3 (which knock out the nerve roots before signals can reach the phrenic nerve), and in some cases, no identifiable cause at all.
Testing Phrenic Nerve Function
The simplest and most commonly used test is the sniff test, also called chest fluoroscopy. You stand in front of an X-ray camera while a technician watches your diaphragm move in real time as you breathe normally, take deep breaths, and sniff quickly. A healthy diaphragm pulls downward and flattens when you inhale, then arches upward when you exhale. If one side stays arched upward during inhalation, or moves in the wrong direction, it suggests that the phrenic nerve on that side isn’t working properly.
Doctors may also use nerve conduction studies, which measure how quickly electrical signals travel through the phrenic nerve, and ultrasound of the diaphragm to assess its thickness and movement without radiation exposure.
Recovery and Treatment Options
Phrenic nerve recovery depends heavily on the type and severity of the injury. After cardiac surgery, recovery is slow and often incomplete. In one study tracking patients with left phrenic nerve injury following coronary bypass, all five patients showed some recovery over 12 months, though only one recovered fully. The remaining four had partial recovery even at 14 months, consistent with the general pace of peripheral nerve regeneration.
For people with permanent bilateral paralysis, particularly those with high spinal cord injuries, diaphragm pacing offers an alternative to lifelong mechanical ventilation. A surgically implanted device delivers rhythmic electrical pulses to the phrenic nerve or directly to the diaphragm muscle, mimicking the brain’s natural breathing signals. Patients with spinal cord injuries above C3 are the strongest candidates because their phrenic nerves remain structurally intact, just disconnected from the brain’s commands. For injuries in the mid-cervical range where the phrenic nerve itself is damaged, a newer approach stimulates the nerve endings directly where they insert into the diaphragm, bypassing the injured segment.