Phrenic Nerve — Atlas of electromyography

Phrenic Nerve

The phrenic nerve arises chiefly from the third and fourth cervical nerves, with a communicating branch from the fifth (Gray’s Anatomy, 1977). It descends to the root of the neck beneath the sternocleidomastoid, the posterior belly of the omohyoid, and the transversalis colli muscles. It passes over the first part of the subclavian artery, lying between it and the subclavian vein. Within the chest the phrenic nerve descends almost vertically in front of the root of the lung and by the side of the pericardium, between it and the mediastinal portion of the pleura. It reaches the diaphragm, where it divides into branches that separately pierce the muscle and are distributed to its undersurface.
The two phrenic nerves differ in their length and in their anatomical relations in the upper thorax. The right nerve is shorter, more deeply situated, and more vertical in its descent than the left. It lies on the outer side of the right brachiocephalic vein and superior vena cave. The left nerve is longer due to the inclination of the heart to the left and from the diaphragm being lower on this side. It enters the thorax behind the left brachiocephalic vein and crosses in front of the vagus nerve, aortic arch, and root of the lung. In the thorax, each phrenic nerve is accompanied by a branch of the internal mammary artery.

Phrenic Nerve Lesion


Trauma to the side of the neck causes direct nerve injury.
latrogenic causes include surgery in the neck, compression by retractors or other instruments, and subclavian vein catheterization (Drachler et al., 1976).
Neuralgic amyotrophy is causative.
Malignant neoplasms, including bronchogenic carcinoma and other lesions within the chest, can cause a phrenic nerve lesion.
Abnormalities may occur in C3-C5 radiculopathy, spinal cord injury or other pathological conditions affecting the anterior horn cells at the C3-C5 segments, motor neuron disease, and polyneuropathy.

General Comments

Lower motor neuron damage to the phrenic nerve may occur anywhere along its course, from the anterior horn cells at the C3-C5 segments to the terminal innervation of the diaphragm (Bolton, 1993).

Clinical Features

Dyspnea occurs. If there is incomplete injury to the high cervical cord, all respiratory movements will be weak and breathing will be rapid and shallow.
Weakness of diaphragm typically causes paradoxic respiration.

Electrodiagnostic Strategy

Perform phrenic nerve conduction studies (Bolton, 1993). In axonal loss lesions affecting the phrenic nerve, compound muscle action potentials from the diaphragm will be reduced or absent. In demyelinating lesions, phrenic nerve conduction latencies may be markedly prolonged.
Demonstrate neurogenic EMG changes in the diaphragm.
Evaluate for C3, C4, or C5 radiculopathy by performing EMG in other muscles that have C3-C5 innervation (cervical paraspinals, stemocleidomastoid, trapezius, levator scapulae, and rhomboids).
Perform nerve conduction studies to assess for cervical or brachial plexus lesions.

Anatomical Illustrations




Innervation is via the anterior rami of C3, C4, and C5 via the cervical plexus («C3, C4, C5 keeps the diaphragm alive»).


The diaphragm originates at the circumference of the thoracic cavity. The diaphragm separates the thorax from the abdomen.


Anteriorly and laterally: Insertion is to the inner surface of the xiphoid cartilage and cartilages and bony portions of the six or seven inferior ribs. Posteriorly: Insertion is to the aponeurotic arches and crura that attach to the lumbar vertebrae.

Activation Maneuver

There is activation of the diaphragm with each inspiration. Motor unit potentials MUPs) in the diaphragm are of shorter duration and smaller amplitude, but are more numerous than MUPs from chest wall muscles (the needle will first pass through the external oblique or rectus abdominus muscles and the external and internal intercostal muscles before entering the diaphragm). With quiet respiration, the chest wall muscles do not fire or only recruit a few MUPs (Bolton, 1993).


EMG Needle Insertion

Introduce the needle at a right angle to the chest wall in any one of several interspaces usually the seventh, eighth, or ninth rib interspace) between the anterior axillary and midclavicular lines. The needle should be inserted just above the costal margin, where there is an approximately 1.5 cm distance between the pleura reflection and the lower costal cartilage upon which the diaphragm inserts see illustrations). Thus, the needle does not traverse either the pleural space or the lung.


If the needle is inserted too deeply, it will enter the peritoneum. Entry of the needle into the peritoneum produces pain and loss of muscle insertional activity.
If the needle is inserted below the costal margin or at a more proximal interspace, it may penetrate the neurovascular bundle intercostal nerve, artery, and vein) and continue into the pleura and lung.

Clinical Comments

The technique is reported to be safe, causes little discomfort, and provides good recordings of diaphragm activity (Bolton et al., 1992; Bolton, 1993). Neurogenic changes, including abnormal spontaneous activity, can be recorded from chest wall muscles and diaphragm Bolton, 1993).
If there is total denervation of the diaphragm, MUPs will not fire with attempted inspiration. Hence, an important sign that the needle is in the diaphragm will be lost.
Pneumothorax is a rare complication of this technique, particularly in patients with chronic obstructive pulmonary disease. The phrenic nerve may be unilaterally damaged, as in radiculopathy, tumor, trauma, or surgery.
In high cervical spinal cord injury, denervation will usually be present in one or both diaphragms; MUPs will recruit in a neurogenic pattern or not at all if denervation is complete.
Look for neurogenic changes in other C3–C5 innervated muscles to support the diagnosis of radiculopathy or high cervical spinal cord injury. Muscles to be examined include cervical paraspinals, trapezius, levator scapulae, and rhomboids.