Mediastinal Hernia

Abstract

THERE has been little in the American literature of the past few years on the origin and development of mediastinal hernia or pneumatocele. During this time, however, the Italian and French schools have devoted considerable attention to this subject. Besta (1) has recently published a comprehensive review of the Continental literature with considerable original data, but there is little available in English. I, therefore, have thought that it would be of some interest to review the origin of this condition, and to consider the mechanism of its development, presenting several cases illustrating the various types. First, just what do we mean by the term mediastinal hernia? It can be defined as the projection through the mediastinum of one pleural space into the other hemithorax. It was only with the development of artificial pneumothorax therapy coupled with the greater use of the x-ray both for fluoroscopy and plate work, that the possibility of such a condition was appreciated. Artificial pneumothorax is still the most common cause of hernia of the mediastinum, although it does occur in spontaneous pneumothorax and, as we shall see, occasionally in the absence of pneumothorax. The walls of the hernial sack as it protrudes through the mediastinum are formed by the parietal pleura lining each hemithorax. This is seen as a thin line on the film or the fluoroscopic screen, protruding into the normal lung-field. Second, just where does this hernia through the mediastinum occur? Anatomically there are two so-called “weak spots in the mediastinum.” The first and the most important of these lies behind the sternum above and in front of the heart at the site of the atrophied thymus. The second lies below and posteriorly and is a space bounded below by the crura of the diaphragm, posteriorly by the spinal column, and in front by the esophagus as it bends forward to pass through the diaphragm. Once a small wedge has been driven through one of these spaces, however, the margins may be spread until the hernia occupies an area far greater than the original “weak spot.” Third, how does such a herniation of the mediastinum come about? What are the physical factors that lead to its development? It takes but little consideration to realize that the force that causes the hernia is a difference in pressure in the two halves of the thoracic cage and that this difference in pressure is the result of a difference in volume. It is also quite obvious that the mediastinum itself must be considerably more rigid than the “weak spot” otherwise the mediastinum as a whole would be shifted toward the hemithorax with the lowest pressure. It is also evident that if volume plays any part in the development of this condition then there must be a change in the size of the herniation during the different phases of respiration dependent on the changes in the size of the thorax as a whole.