The Diaphragm

Embryology

The development of the diaphragm begins in the seventh week of gestation and is complete by the tenth week. It is derived from four embryologic precursors: the septum transversum, the right and left pleuroperitoneal membranes, and the dorsal mesentery of the esophagus (Fig. 1). The septum transversum is an anterior structure that becomes the central tendon and fuses with three dorsal structures to form the primitive diaphragm. The dorsal mesentery, containing the primitive aorta, inferior vena

Anatomy

The Greek derivation of the words dia (in between) and phragma (fence) aptly describes this organ. The diaphragm is a musculofibrous dome-shaped membrane that separates the thoracic from the abdominal cavity. It has a muscular portion peripherally, and a fibrous portion centrally (Fig. 2). It has three major muscle groups: sternal, costal, and lumbar and a large fibrinous central tendon composed of three leaflets: right, left, and middle. Major structures pass through three openings: the caval

Blood supply

The major arterial blood supply to the diaphragm comes from the left and right phrenic arteries (see Fig. 2). These paired arteries arise directly from the abdominal aorta near the aortic hiatus. They bifurcate posteriorly and give off a large anterior branch, which courses along the anterior and superior portions of the muscle, merging with the pericardiophrenic artery. Additional arterial blood is supplied to the diaphragm from branches off of the right and left internal mammary arteries.

The

Innervation

The diaphragm is innervated exclusively from the right and left phrenic nerves that originate from C3–4–5. These paired nerves provide both sensory and motor function to the diaphragm. The phrenic nerve traverses the thoracic cavity posteriorly and moves anterior over the pericardium. The right phrenic nerve pierces the diaphragm just lateral to the caval hiatus, and the left pierces just lateral to the left heart border. Each nerve divides into four trunks: an anterolateral, posterolateral,

Function

The principle function of the diaphragm is breathing: inhalation and exhalation. During inhalation the diaphragm contracts. With the aid of the external intercostal muscles, the thoracic cavity expands, reducing intrathoracic pressure and allowing air to rush into the lungs. When the diaphragm relaxes, the elastic recoil allows air to be drawn out passively from the lungs. For optimum respiratory function, both hemidiaphragms must be intact. Injury to one phrenic nerve results in an elevated

Surgical considerations

Preservation of at least one phrenic nerve is critical during any surgical procedure. The nerve courses posteriorly in the lateral compartment of the neck and can be inadvertently injured during posterior neck dissections. As it courses through the thoracic cavity, it transitions from a posterior to anterior position and can be seen on the anterior surface of the pericardium before it pierces the diaphragm. Care must be taken to identify the course of the nerve before opening the pericardium.

Imaging and testing

Many pathologic conditions affect the diaphragm, both anatomically and functionally. Precise localization and characterization of tumors and diaphragmatic injury may be necessary, and identifying functional abnormalities before and after surgery is often helpful. Chest radiography is a useful screening tool for diaphragmatic abnormalities.⁷ Hernia defects and functional defects can be seen easily. Cross-sectional imaging with ultrasound,⁸ high-resolution CT, and MRI can illustrate intrinsic

Eventration of the diaphragm

The broader definition of eventration is the abnormal elevation of the hemidiaphragm. It can be classified into congenital and acquired forms. The acquired form is usually caused by phrenic nerve injury and is reviewed in the next section. In the strictest sense, however, eventration refers to the congenital abnormality that occurs from failure of the fetal diaphragm to muscularize.²⁸ A narrow rim of muscle is present peripherally that contracts with electrical stimulation, resulting in a thin,

Diaphragmatic paralysis

Diaphragmatic paralysis can result from direct injury to the diaphragm or injury to the phrenic nerve. In infants, direct injury is most common during cardiac surgery and can result in life-threatening respiratory insufficiency. This condition has been attributed to three factors: weak intercostal muscles preventing a significant increase in intrathoracic dimensions, a mobile mediastinum shifting away from the paralyzed side during inspiration limiting lung expansion, and a tendency toward

Diaphragmatic pacing

There are few indications for diaphragmatic pacing. Patients with central alveolar hypoventilation (Ondine’s curse) and those with high cervical cord injuries (quadriplegics) may benefit.³⁵ Although it has been used in patients with chronic obstructive pulmonary disease, intractable hiccups, and phrenic nerve injury, it has no proven benefits in these conditions.

Central alveolar hypoventilation is caused by a lack of response to hypercarbia and hypoxia during sleep by the receptors in the

Diaphragmatic tumors

Primary diaphragmatic tumors are rare and most are benign. Benign lesions include lipomas, fibromas, schwannomas, neurofibromas, leiomyomas, and bronchial, mesothelial, echinococcal, or teratogenous cysts. Malignant diaphragmatic tumors include solitary fibrous tumors and fibrosarcomas, which are the most common. Much more common than primary diaphragmatic tumors is direct invasion of the diaphragm by a tumor arising in an adjacent structure, such as a lower lobe bronchogenic carcinoma,

Summary

This article discusses the diaphragm from a surgical perspective. Although it is a relatively simple organ compared with other structures, the diaphragm serves its host well by providing the mechanics necessary for vital respiratory function. It is replaceable in part, but as a whole it is as important as the heart and lungs, and without it the body would lack the ability to draw the wind into our sails and set our ships on course for greater adventures.