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The thymus gland is the most common structure in the anterior mediastinum that requires surgical extirpation. The indication for thymectomy is typically either for thymic neoplasm or for the treatment of the autoimmune disorder myasthenia gravis (MG). Thymus removal is usually a safe, straightforward procedure, but this depends on knowledge of the anatomic and embryologic characteristics of thymic development.

The thymus lies in the anterior mediastinum. It is a lymphoepithelial organ that is derived embryologically from the third pharyngeal pouches bilaterally and descends caudally and medially into the mediastinum during gestation, fusing into a bilobed gland. However, fusion and descent are often variable, and islands of tissue may be found throughout the neck and mediastinum. In addition to being embedded within the thyroid gland or associated with the parathyroid glands, aberrant thymic rests can occur independently along the entire path of thymic descent (Fig. 161-1).1 The gland weighs 10 to 35 g at birth, attains its greatest mass in puberty (20–50 g), but involutes and is replaced by fat in adulthood. It occupies the anterior mediastinum, with the superior horns often extending into the neck, lying deep to the sternothyroid. At the completion of its development, the thymus is separated from the sternum by a thin film of loose connective tissue lying anterior to the pericardium and great vessels and is in especially close contact with the left brachiocephalic (innominate) vein. The gland, in continuity with the mediastinal fat, usually extends laterally to the phrenic nerves and is partially covered on either side by the pleural reflections. The arterial supply to the thymus is principally from the internal thoracic arteries, with contributions from the inferior thyroid and pericardiophrenic arteries. The veins from both lobes ascend between the lobes posteriorly and usually drain into the left brachiocephalic vein or, rarely, directly into the superior vena cava. The phrenic nerves are in immediate proximity to the gland, as are the vagus and recurrent laryngeal nerves. Careful attention to the anatomy is essential for avoiding injury to these vital nerves (Fig. 161-2).

Figure 161-1

The locations of the thymus gland and ectopic extracapsular tissue are shown. There is a high incidence of thymic tissue lateral to the phrenic nerves.

Figure 161-2

The relationships of the thymus gland to critical surrounding structures are shown. The gland and associated tissue may be in close proximity to the phrenic and recurrent laryngeal nerves. A. The thymus gland lies immediately anterior to the great vessels of the middle mediastinum, with venous drainage directly into the innominate vein. B. The thymus gland will typically be in close approximation to the phrenic nerves as they course along the pericardium.

The thymus gland is a central lymphoid organ that performs the important ...

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