Blogation Numero Cuatro

The pleura covers the surface of the lung, and is composed of a double-layered membrane. The outer of the two layers of the pleura, the parietal pleura, attaches to the chest while. The inner layer, the visceral pleura, is intimately associated with the surface of the lung and cannot be separated from it. The space between the two membranes is the fluid-filled pleural cavity, which helps the lung reach optimal function (inflation) during respiration. When the diaphragm contracts, there is a resulting negative pressure in the pleural cavity, which causes the lungs to expand in passive inhalation.

 A penetrating injury to the parietal pleura can result in the entry of air into the pleural cavity, which is called a pneumothorax. Broken ribs can also produce a pneumothorax, which result in a collapsed lung. Additional substances can fill the pleural space upon injury, including significant amounts of water (hydrothorax) and blood (hemothorax). The procedure used to remove the unwanted air, water or blood is called Thoracentesis. In this procedure, the clinician will utilize a hypodermic needle and pass it through the intercostal space into the pleural cavity to evacuate the substance or to obtain a sample.

The mediastinum is the center compartment of the thoracic cavity, and contains extremely vital tissue. It ranges from the diaphragm inferiorly to the thoracic inlet superiorly. Important structures of note in the mediastinum are the heart, the roots of the great vessles, the pericardium, vagus nerve, phrenic nerve, lymph nodes, esophagus and thoracic duct, among others. It is divided into the anterior, middle, and posterior mediastinum based on its relationship to the pericardium.

Widening of the mediastinum (seen in the picture on the right) on chest radiographs may be an indicator to clinicians that a problem in present in the tissues/structures within the mediastinum. The causes of the widening can vary from aortic aneurysm to a mediastinal mass to a rupture of the esophagus. An accident can also stimulate the widening observed on a scan when one of the greats vessels is lacerated and blood accumulates. Another cause of mediastinal widening could be a malignant lymphoma which causes the mediastinal lymph nodes to grow very large and expand the mediastinal space. The appearance of a widened mediastinum upon scan should indicate that several diagnostic tests need to be done to discover and attempt to resolve the problem.

The vagus nerve (cranial nerve ten) gives rise to the recurrent (inferior) laryngeal nerve in the anterior thorax. The recurrent laryngeal nerves (right and left) innervate all of the intrinsic muscles of the larynx (voice box) except for one. The right and left recurrent laryngeal nerves take very different pathways and ultimately join fibers at the larynx (as seen in the photo below). The right recurrent laryngeal nerve raps around the right subclavian artery, while the left recurrent laryngeal nerve loops around the arch of the aorta. They both travel from the brain (via the vagus) in an endoneurial sheath.

Damage to these recurrent laryngeal nerves can occur by disease process or by procedure (most commonly seen in thyroid procedures). Damage to the nerves on one side (unilateral damage) can lead to hoarseness, while damage on both (bilateral) can actually lead to difficulty breathing and loss of voice. The pathological damage to these nerves can be caused by various mechanisms, including carcinoma of the lower respiratory tract, aortic arch aneurysm, or enlargement of the mediastinal lymph nodes. The left recurrent laryngeal nerve takes a path that leads it in close proximity to all of these pathologies, which could lead to compression of the nerve, and a hoarseness.

References
Moore et al. "Clinically Oriented Anatomy" (2010) 6th ed
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