The diaphragm is the major muscle of respiration and separates the thoracic and abdominal cavities. The body is dependent on the diaphragm for normal respiratory function. Contraction of the diaphragm has following functions: (1) decreasing intrapleural pressure, (2) expanding the rib cage through its zone of apposition by generating positive intra-abdominal pressure, and (3) expanding the rib cage using the abdomen as a fulcrum.  Therefore, understanding how different disease processes and disorders result in diaphragm dysfunction is important.
A concomitant respiratory dysfunction exists any time a decrease in diaphragmatic function is present. The body possesses inherent mechanisms of compensation for decreased diaphragmatic function, but none of these processes can successfully prevent respiratory compromise if excursion of the diaphragm is moderately diminished or simply absent.
The easiest approach to diaphragmatic problems is to observe both the neurologic and anatomic processes that result in decreased function. Neurologic problems of the diaphragm occur when a traumatic injury or disease process decreases or terminates the impulse of respiratory stimuli originating in the brain. Anatomic disorders decrease the integrity of the musculature of the diaphragm, thus decreasing its excursion. Both anatomic and neurologic problems related to the diaphragm ultimately result in the inability of the diaphragm to provide adequate negative intrathoracic pressure, thereby decreasing ventilation and the amount of oxygen provided to the alveoli.
Anatomy of the diaphragm
The diaphragm is a modified half-dome of musculofibrous tissue that separates the thorax from the abdomen. The 4 embryologic components that make up the formation of the diaphragm are the septum transversum, 2 pleuroperitoneal folds, cervical myotomes, and the dorsal mesentery. Development begins during the third week of gestation and is complete by the eighth week. Failure in the development of the pleuroperitoneal folds and subsequent muscle migration results in congenital defects (see Disorders of anatomy in Pathophysiology).
The muscular origin of the diaphragm is from the lower 6 ribs bilaterally, the posterior xiphoid process, and from the external and internal arcuate ligaments. A number of different structures traverse the diaphragm, but 3 distinct apertures allow the passage of the aorta, esophagus, and vena cava. The aortic aperture is the lowest and most posterior of the openings, lying at the level of the 12th thoracic vertebra. The aortic opening also transmits the thoracic duct and, sometimes, the azygous and hemiazygous veins. The esophageal aperture is surrounded by diaphragmatic muscle and lies at the level of the 10th thoracic vertebra. The vena caval aperture is the highest of the 3 openings and lies level to the disk space between the eighth and ninth thoracic vertebrae.
Arterial supply to the diaphragm comes from the right and left phrenic arteries, the intercostal arteries, and the musculophrenic branches of the internal thoracic arteries. Some arterial blood is provided from small branches of the pericardiophrenic arteries that run with the phrenic nerve, mainly where the nerves penetrate the diaphragm. Venous drainage occurs via the inferior vena cava and azygous vein on the right and the adrenal/renal and hemizygous veins on the left.
The diaphragm receives its sole neurologic impulse from the phrenic nerve, which originates primarily from the fourth cervical ramus but also has contributions from the third and fifth rami. Originating around the level of the scalenus anterior muscle, the phrenic nerve courses inferiorly through the neck and thorax before reaching its end point, the diaphragm. Thus, the phrenic nerve has a relatively long course before reaching its final destination. Any process that disrupts the transmission of neurologic impulses through the phrenic nerve directly affects the diaphragm.
Medscape Drugs & Diseases article Diaphragm Paralysis may also be of interest.
Disorders of innervation
During normal respiration, the brain stem sends action potentials to the third through fifth cervical spine levels, which then give off dorsal rami that convalesce to form the phrenic nerves bilaterally. The phrenic nerves then traverse the neck and thorax and innervate the diaphragm. The successful impulse of respiratory stimulus from the brain to the diaphragm can be compromised by an interruption of the phrenic nerve at any point along this course.
Traumatic injury to the head or brain stem prevents nerve signals from reaching the phrenic nerve. Generally, injuries that affect the brain and brain stem are catastrophic, with the chance of survival being poor.
Injuries or disease processes that affect the respiratory nervous impulse along its long course are widely described. A number of distinct entities, including trauma, spinal cord disorders, syringomyelia, poliomyelitis, and motor neuron diseases, decrease the neural impulses traveling from the central respiratory centers to the diaphragm.
Peripheral phrenic nerve injuries result from damage to the nerve along its path in the cervical area or the thorax. A number of clinical entities can affect the phrenic nerve directly, including trauma, open heart surgery or thoracic surgery, chiropractic cervical spine manipulation, radiation therapy, demyelinating diseases (eg, Guillain-Barré syndrome), neoplasm, uremia, lead neuropathy, postinfectious neuropathies, and other processes.
Disorders of anatomy
Anatomic disorders of the diaphragm are typically classified into 2 broad categories: congenital and acquired. Congenital diaphragmatic hernias occur when the muscular entities of the diaphragm do not develop normally, usually resulting in displacement of abdominal components into the thorax.  The most common cause of acquired diaphragmatic disorders is trauma [3, 4] . However, other important entities should be considered when evaluating anatomic defects of the diaphragm in adults. For example, endothelin-1 (ET1) may be important in the pathobiology of congenital diaphragmatic hernia. Infants with congenital diaphragmatic hernia and poor outcome have higher plasma ET1 levels and worse pulmonary hypertension than infants discharged on room air. The severity of pulmonary hypertension is associated with ET1 levels. 
Bochdalek hernias represent the majority of congenital diaphragmatic hernias. The major defects in Bochdalek hernias are posterolateral defects of the diaphragm, which result in either failure in the development of the pleuroperitoneal folds or improper or absent migration of the diaphragmatic musculature. [6, 7, 8] Animal models suggest that one potential cause of congenital diaphragmatic hernias is abnormalities of the retinoid system that may result from maternal vitamin A deficiency.  Patients with congenital diaphragmatic hernias generally present in the neonatal period and have a mortality rate of 45-50%. The morbidity and mortality associated with congenital diaphragmatic hernias relate mostly to hypoplasia of the lung on the affected side. Thus, timely diagnosis and proper management remain the key ingredients to survival.
Traumatic diaphragmatic rupture can occur secondary to both blunt and penetrating trauma. The prevalence of diaphragmatic rupture is 0.8-1.6% in patients admitted to the hospital for blunt trauma. The major etiologies of diaphragmatic rupture are motor vehicle accidents and penetrating trauma from gunshot and stab wounds. Several theories have been postulated regarding the mechanism of rupture from blunt trauma, including shearing of a stretched membrane, avulsion of the diaphragm from its points of attachment, and sudden force transmission through viscera acting as a viscous fluid. Left-sided rupture is more common than right-sided rupture (68.5% vs 24.2%) because of both hepatic protection and the increased strength of the right hemidiaphragm. However, the greater incidence of posttraumatic left-sided hernias may also result from weaknesses in points of diaphragmatic embryologic fusion.
The exact frequency of many diaphragmatic disorders is not known. The incidence of diaphragmatic dysfunction due to specific etiologies is known. For example, postcardiac surgery diaphragmatic dysfunction is reported in 11% of patients.
Morbidity and mortality resulting from diaphragmatic disorders are associated with the etiology of the dysfunction. Individuals with anatomic defects of the diaphragm are much more likely to survive than individuals with unresolved defective or absent neurologic impulses. Persons with unilateral dysfunction are much more likely to remain asymptomatic compared with individuals with bilateral involvement.
Patients with neurologic involvement generally recover if dysfunction is not due to a neuropathic process. Phrenic nerve injury during coronary artery bypass surgery is a good example. Recovery may take up to 2 years or longer.
Patients with anatomic defects generally do well once the defect is repaired. The outcome of neonates with congenital diaphragmatic hernias generally relates to the pulmonary development after repair of the diaphragmatic defect.
Patients with congenital diaphragmatic hernias generally present in the neonatal period and have a mortality rate of 45-50%. The morbidity and mortality associated with congenital diaphragmatic hernias relate mostly to hypoplasia of the lung on the affected side. Thus, timely diagnosis and proper management remain the key ingredients to survival.