Illustrated Encyclopedia of Human Anatomic Variation: Opus IV: Organ Systems: Endocrine System

Thymus

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The size and shape of the thymus is variable. It may be located behind the brachiocephalic vein. It occasionally contains aberrant parathyroid tissue.

According to Jaretzki, III, et al., (1977) eighteen of twenty-two patients (82%) had surgically important variations in thymic anatomy in the neck, in the mediastinum, or in both. The authors provided the following table:

The authors explain that 10 patients (45%) had one or more surgically important variations in the neck. Seven had superior pole abnormalities, 5 without thymic tissue cephalad to the innominate vein and 2 with a discontinuous superior pole. The discontinuous superior poles were located caudal to the thyroid in both cases. In 1, the discontinuous superior pole was attached to the main gland by a fibrous courd. Three patients had accessory thymic tissue below the level of the thyroid, 2 medial to the lobe and 1 lateral to it. The discontinuous superior poles and accessory tissue were difficult to distinguish from normal fat, parathyroid glands, and thyroid macroscopically. In 1 patient a bulky left superior pole passed behind the thyroid gland and terminated at the angle of the jaw.

In the mediastinum, 16 patients (73%) had one or more variations. These were, in many instances, large, and in some cases multiple, masses of thymic tissue. The authors found 17 accessory lobes lying adjacent to or distant from the main lobes in 10 patients. In some they lay posterior to the phrenic nerves, in the aortopulmonary window, or at the level of the diaphragm. The accessory lobes were either well encapsulated or had distinct borders and in some cases they had indistinct borders and some were indistinguishable from mediastinal fat. In 7 cases one or both lobes had thin, feathery lateral margins often extending well beyond the phrenic nerves. In 3 patients, one or both lobes lay behind the innominate vein and in 2 patients a microscopical focus of thymus was found in fat lying distant from gross thymic tissue.

Accessory Thymus

Thymic Remnants

References

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Jaretzki, III., A., Bethea, M., Wolff, M., Olarte, M.R., Lovelace, R.E., Penn, A.S. and L. Rowland. (1977) A rational approach to total thymectomy in the treatment of Myasthenia Gravis. Ann. Thoracic Surgery 24(2):120-130.

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Martin, K.W. and W.H. McAlister. (1987) Case Report. Intratracheal thymus: A rare cause of airway obstruction. Am. J. Roentgenol. 149(6):1217-1218.

Saade, M., Whitten, D.M., Necheles, T.F., Leape, L. and D. Darling. (1976) Posterior mediastinal accessory thymus. J. Pediatrics 88:7172.

Shackelford, G.D. and W.H. McAlister. (1937) The aberrantly positioned thymus. A cause of mediastinal or neck masses in children. Am. J. Roentgenol. 120:291-296.

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