Chapter 29: Descending Thoracic Aorta

INTRODUCTION

Acute aortic syndrome presents challenging diagnostic and therapeutic disease. Intramural hematoma (IMH), penetrating aortic ulcer, aortic dissection (AD), and aneurysm can all have a similar clinical presentation with advanced disease. Many of these syndromes have undergone significant revision in terms of classification and therapeutic strategy in the last 30 years. The increasing sophistication and speed of computed tomographic (CT) scan has raised it to a prominent role in the evaluation of this disease. The exact role endovascular intervention will play in the treatment algorithms remains to be determined. Despite the benefit of modern imaging and this collective experience, it remains a highly morbid malady.

EMBRYOLOGY

The embryonic vascular system begins formation in the third week of gestation. From the primitive aortic sac arise six ventral paired aortic arches. These pass laterally around the primitive gut to terminate in the paired dorsal aortae. Eventually, there is fusion of the paired dorsal aortae. The first pair of aortic arches contributes to formation of maxillary and external carotid arteries. The second pair contributes to formation of the stapedial arteries. The third pair contributes to formation of the common and internal carotid arteries. The left fourth aortic arches contribute to form the aortic arch and the right fourth arch contributes to formation of the right subclavian artery. The fifth pair of aortic arches usually has no anatomic contribution. An association between persistent fifth aortic arch and chromosome 22q11.2 deletion has been described.¹ The left sixth aortic arch contributes to formation of the left pulmonary artery and the ductus arteriosus. The right sixth aortic arch contributes to formation of the right pulmonary artery.

ANATOMY

The descending thoracic aorta arises from the aortic arch just after the origin of the left subclavian artery, at the inferior border of the fourth thoracic vertebrae. This point of transition is termed the aortic isthmus. In adults, the average diameter of the descending thoracic aorta is 2.8 cm in men and 2.6 cm in women.² This narrows as it descends into the abdomen. It terminates as it enters the abdomen via the diaphragmatic aortic hiatus, at the 12th intercostal space. The thoracic aorta descends in the posterior mediastinum to the left of the vertebral column and gradually shifts to the midline at the aortic hiatus. It is surrounded by the thoracic aortic plexus. Anteriorly, the left pulmonary hilum crosses with the left main bronchus and left pulmonary artery being closely associated. Continuing inferiorly, the esophagus, pericardium, and diaphragm are also situated at the anterior border of the thoracic aorta. As the thoracic aorta descends, the esophagus crosses anteriorly and then laterally at the diaphragm. Posteriorly, the hemiazygous vein and anterior vertebral column are associated. Laterally, it is closely applied to the inferior lobe of the left lung. Medially, the esophagus, thoracic duct, and azygous vein are closely associated. There are bronchial, esophageal, intercostal, ...