Chemoreceptor organs are sensitive barometers of changes in the carbon dioxide and oxygen content and pH of the blood and aid in the regulation of respiration and circulation. Although chemoreceptor tissue appears to be widely distributed in the body, tumors (chemodectomas) develop principally in the aortic (more frequent in animals) and carotid (more frequent in people) bodies. These tumors are found primarily in dogs and rarely in cats and cattle. Brachycephalic breeds of dogs, such as the Boxer and Boston Terrier, are predisposed to tumors of the aortic and carotid bodies.
Aortic body tumors appear most frequently as single masses or as multiple nodules within the pericardial sac near the base of the heart. They vary considerably in size (0.5–12.5 cm), with carcinomas generally larger than adenomas. Solitary, small adenomas either are attached to the adventitia of the pulmonary artery and ascending aorta or are embedded in the adipose connective tissue between these major vascular trunks. Larger adenomas may indent the atria or displace the trachea, are multilobular, and partially surround the major arterial trunks at the base of the heart.
In dogs, malignant aortic body tumors occur less frequently than adenomas. Carcinomas may infiltrate the wall of the pulmonary artery to form papillary projections into the lumen or invade through the wall into the lumen of the atria. Although tumor cells often invade blood vessels, metastases to the lungs and liver are infrequent in dogs with aortic body carcinomas. Nonetheless, the local and physiologic effects are important, including those of adenomas.
Aortic body tumors in animals are not functional (ie, they do not secrete excess hormone into the circulation) but, as space-occupying lesions, may result in various functional disturbances. These include manifestations of cardiac decompensation due to pressure on the atria or vena cava (or both) associated with larger aortic body adenomas and carcinomas. Aortic body tumors tend to be more benign than carotid body tumors. They grow slowly by expansion and exert pressure on the vena cava and atria. Aortic body carcinomas may invade locally into the atria, pericardium, and adjacent large, thin-walled vessels.
Carotid body tumors arise near the bifurcation of the common carotid artery, usually as a unilateral, slow-growing mass. Adenomas are usually 1–4 cm in diameter. The bifurcation of the carotid artery is incorporated in the mass, and tumor cells are firmly adherent to the tunica adventitia. Complete excision or biopsy often is difficult because of the high degree of vascularity and intimate relationship with major arterial trunks in the neck.
Malignant carotid body tumors are larger and more coarsely multinodular than adenomas. Although carcinomas appear to be encapsulated, tumor cells invade the capsule and penetrate into the walls of adjacent vessels and lymphatics. The external jugular vein and several cranial nerves may be incorporated by the neoplasm. Metastases of carotid body tumors occur in ~30% of cases and have been found in the lung, bronchial and mediastinal lymph nodes, liver, pancreas, and kidneys. Multicentric neoplastic transformation of chemoreceptor tissue occurs frequently in brachycephalic breeds of dogs.
The histologic characteristics of chemodectomas are essentially similar whether derived from the carotid or aortic body.
Although the etiology of carotid and aortic body tumors is unknown, it has been suggested that a genetic predisposition aggravated by chronic hypoxia may account for the higher risk in certain brachycephalic breeds. Carotid bodies of several mammalian species, including dogs, have undergone hyperplasia when subjected to chronic hypoxia by living in a high-altitude environment.