Mediastinum And Pleura Question And Answers

Mediastinum

Question 1. Define mediastinum and give its boundaries and major contents. 
Answer:

Mediastinum:

It is a central compartment/region of the thoracic cavity, sandwiched between two lungs enclosed in the pleural sacs. It contains all the thoracic viscera and great vessels except lungs. The contents of the mediastinum are packed in the loose areolar tissue and limited on each side by mediastinal pleura.

Hence, the contents of the mediastinum form a kind of bulky mobile septum between the two pleural sacs. Mediastinum is elongated during inspiration and displaced to one or the other side of the pressure in the two pleural cavities is not the same.

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Mediastinum Boundaries:

• Anterior: Sternum
• Posterior: Thoracic vertebral column.
• Superior: Thoracic inlet.
• Inferior: Diaphragm.
• On each side: Mediastinal pleura.

Mediastinum Contents:

The major contents of the mediastinum are

• Heart (enclosed in the pericardium)
• Thoracic aorta
• Pulmonary trunk
• Trachea
• Esophagus
• Thoracic duct
• Lymph nodes
• Thymus
• Three pairs of the neural structures (i.e., sympathetic trunks, vagus nerves, and phrenic nerves)

Question 2. What are the subdivisions of the mediastinum? 
Answer:

The mediastinum is broadly divided into superior mediastinum and inferior mediastinum by an imaginary horizontal plane (sternal plane) passing from the sternal angle to the lower border of the 4th thoracic vertebra.

The inferior mediastinum is further subdivided into the middle part (middle mediastinum) containing the heart enclosed in the pericardial sac, the anterior part (anterior mediastinum) between the pericardium, and the sternum and posterior part (posterior mediastinum) between the pericardium and the vertebral column.

This is depicted as a Subdivision of the mediastinum:

Note: The anterior mediastinum is the narrowest, the middle mediastinum is the widest, and posterior mediastinum is the longest.

Question 3. Write a short note on the superior mediastinum. 
Answer:

It is an area of the median region of the thoracic cavity above the sternal plane.

Superior mediastinum Boundaries:

• Anterior: Manubrium sterni
• Posterior: Upper four thoracic vertebrae.
• Superior: Thoracic inlet.
• Inferior: Sternal plane.
• Lateral (on each side): Mediastinal pleura.

Superior mediastinum Contents:

Superior mediastinum Retrosternal structures

• Sternohyoid and sternothyroid muscles.
• Superior vena cava and its tributaries, viz. right and left brachiocephalic veins, azygos vein (terminal part).
• Thymus (remnant).

Superior mediastinum Intermediate structures

• Aorta and its branches (i.e., brachiocephalic trunk, left common carotid artery, and left subclavian artery).
• Two sets of nerves, viz. right and left vagus, and right and left phrenic.

Superior mediastinum Prevertebral structures

• Trachea
• Esophagus
• Thoracic duct
• Left recurrent laryngeal nerve
• Lymph nodes (tracheal and tracheobronchial)
• Longus colli muscles
• Sympathetic chains

Superior mediastinum Applied anatomy:

Mediastinal syndrome:

The compression of structures in the superior mediastinum by space-occupying lesion, viz. enlarged lymph nodes, produces diverse signs and symptoms called mediastinal syndrome.

Clinically, it presents as:

• Dyspnea: (difficulty in breathing), due to compression of trachea.
• Dysphagia: (difficulty in swallowing), due to compression of the esophagus.
• Dysphonia: (hoarseness of voice), due to compression of left recurrent laryngeal nerve.
• Horner’s syndrome: Due to compression of the sympathetic chain.
• Engorgement of veins: In the upper half of the body, due to obstruction of the superior vena cava.

Mediastinitis:

It is inflammation of loose connective tissue of the mediastinum. Generally, it occurs when deep infections of the neck track downward along the fascial continuity between the neck and superior mediastinum.

Question 4. Write a short note on the anterior mediastinum. 
Answer:

It is a narrow part of the inferior mediastinum in front of the pericardium. It is continuous above with pre tracheal space, through the superior mediastinum.

Anterior mediastinum Boundaries:

• Superior: Sternal plane
• Inferior: Diaphragm
• Anterior: Sternum
• Posterior: Pericardium

Anterior mediastinum Contents:

• Thymus
• Superior and inferior sternopericardial ligaments
• Retrosternal lymph nodes
• Mediastinal branches of internal mammary arteries
• Loose areolar tissue

Question 5. Write a short note on the middle mediastinum. 
Answer:

It is the widest middle part of the mediastinum occupied by the pericardium enclosing the heart and great blood vessels.

Middle mediastinum Boundaries:

• Superior: Sternal plane
• Inferior: Diaphragm
• Anterior: Anterior mediastinum
• Posterior: Posterior mediastinum

Middle mediastinum Contents:

• Heart enclosed in the pericardium
• Ascending aorta, pulmonary trunk, and pulmonary arteries
• Superior vena cava (lower part), inferior vena cava (upper part), pulmonary veins, and azygos vein (terminal part)
• Bifurcation of trachea and right and left principal bronchi
• Inferior tracheobronchial lymph nodes
• Pericardiophrenic vessels
• Phrenic nerve and deep cardiac plexus

Question 6. Write a short note on the posterior mediastinum.
Answer:

It is the longest posterior part of the inferior mediastinum.

Posterior mediastinum Boundaries:

• Superior: Sternal plane
• Inferior: Diaphragm
• Anterior: From above downward
  • Bifurcation of trachea
  • Pulmonary vessels
  • Pericardium
  • The posterior surface of the diaphragm
• Posterior: Lower 8 thoracic vertebrae and intervening intervertebral discs.
• Lateral (on each side): Mediastinal pleura.

Posterior mediastinum Contents:

Longitudinal structures:

• Esophagus
• Descending thoracic aorta
• Thoracic duct
• Azygos vein
• Vagus nerves
• Splanchnic nerves

Horizontal structures:

• Hemiazygos and accessory hemiazygos veins
• Posterior intercostal arteries and veins

Others: Posterior mediastinal lymph nodes.

Posterior mediastinum  Applied anatomy:

Extension of pus/fluid from the neck into posterior mediastinum:

• The posterior mediastinum through the superior mediastinum is continuous with the spaces in the neck between paratracheal and prevertebral fascia (i.e., retropharyngeal space, space on either side of esophagus and trachea) through superior mediastinum.
• Therefore fluid/pus from these spaces may spread into the posterior mediastinum.

Pleura

Question 7. Write a short note on the pleura. 
Answer:

It is a serous membrane lined by mesothelium (flattened epithelium) which encloses the lung on each side of the mediastinum.

Layers of pleura:

• This membrane is in the form of a closed sac, which is invaginated by lungs from its medial side.
• As a result, it becomes a double-layered sac, forming an outer parietal layer and an inner visceral layer.
• Visceral layer (pulmonary pleura):
• It invests the lung and adheres to it.
• At the hilum of the lung, it may be traced as a tubular extension toward the mediastinum, where it is continuous with the parietal pleura.

Parietal layer (parietal pleura):

It lines the internal surface of the thoracic cavity made up of parieties i.e., thoracic wall, diaphragm, and mediastinum, and bounds to these structures by loose areolar tissue – endothoracic fascia.

Question 8. What are the subdivisions of the parietal pleura?
Answer:

According to the region where it lies or the surface which it covers.

It is divided into 4 parts as follows:

1. Cervical pleura:
   • It extends into the root of the neck [1 to 1.5 inches (2.5 to 5 cm) above the middle third of the clavicle] and covers the apex of the lung.
   • This part lines the inner surface of the subpleural membrane or Sibson’s fascia.
2. Costal pleura: Lines the inner surface of the thoracic wall (i.e., lining the ribs, costal cartilages, intercostal spaces, and back of the sternum).
3. Diaphragmatic pleura: Lines the upper thoracic surface of the diaphragm.
4. Mediastinal pleura: Lines the side of the mediastinum and thus forms the lateral boundary of the mediastinum.

Question 9. Enumerate the sites where the pleura extends beyond the thoracic cage. 
Answer:

The pleura extends beyond the thoracic cage at the following 5 sites:

• On either side in the root of the neck
• In the right xiphocostal angle
• On either side in the costovertebral angle

Question 10. What are the differences between visceral and parietal pleurae? Answer:

The differences between parietal and visceral pleurae are given in the box belo

Question 11. Write a short note on the pulmonary ligament and give its functional significance.
Answer:

Pulmonary ligament:

It is a double-layered triangular fold of cuffofpleura, which hangs downward below the root of the lung as far as the diaphragm.

Note: The parietal and visceral layers of the pleura become continuous with one another by means of a cuffofpleura that surrounds the root of the lung.

Pulmonary ligament Functions:

• Provides the dead space for the expansion of pulmonary veins during the increased venous return.
• Allows the descent of the root of the lung (along with the descent of the diaphragm) during inspiration to create a dead space between the apex of the lung and the subpleural membrane, which helps in the expansion of the apex of the lung.

Question 12. Define pleural cavity and pleural recesses and discuss their applied anatomy.
Answer:

Pleural cavity:

• It is a potential space between the parietal and visceral layers of the pleura.
• Normally, it contains 5 to 10 ml of clear fluid, which lubricates the opposing surfaces of parietal and visceral pleurae during respiratory movements i.e. there is no free fluid.

Pleural recesses:

• These are parts of pleural cavities, which are not occupied by lungs during quiet respiration.
• They provide reserve spaces to be occupied by the lungs during deep inspiration.

The various pleural recesses are:

Costodiaphragmatic recesses:

• These are two slit-like spaces one on each side in the thoracic cavity, between the costal and diaphragmatic pleurae.
• They are the most dependent parts of pleural cavities and are situated in the costophrenic angles.
• Vertically it measures about 5 cm in the mid-axillary line from the 8th to 10th rib.

Costomediastinal recesses:

• These are slit-like spaces one on each side between the costal and mediastinal pleurae.
• They are situated along the anterior margins of the lungs.

Pleural cavity and Pleural recesses  Applied anatomy:

Pleurisy/Pleuritis:

• It is the inflammation of the pleura. Due to inflammation, the pleural surfaces become coated with inflammatory exudates, causing their roughening.
• This roughening produces friction, and the pleural rub can be heard with the stethoscope during inspiration and expiration.
• Often, the exudates become invaded by fibroblasts, which lay down collagen fibers, leading to the formation of fibrous bands.

Pleural effusion:

• It is the abnormal accumulation of fluid in the pleural cavity.
• Being the most dependent part, the fluid first collects in the costodiaphragmatic recess.

Clinically, it presents as:

• Decreased lung expansion on the side of effusion.
• Decreased breath sounds
• Dullness on percussion over the area of effusion