Is Obstructive Sleep Apnoea a Risk Factor for Thoracic Aortic Aneurysm Expansion? A Prospective Cohort Study.

Thoracic aortic aneurysm An aortic aneurysm is defined as a localised dilatation of the aorta, which includes all three layers of the vessel, intima, media and adventitia. The incidence of thoracic aortic aneurysm is estimated to be six to ten cases per 100,000 patient years, most commonly occurring in the sixth and seventh decade of life. Thoracic aortic aneurysms are two to four times more commonly found in males than in females. Sixty percent of thoracic aortic aneurysms involve the ascending aorta, 40% the descending aorta, and 10% involve the thoraco-abdominal aorta. The pathogenesis and natural history as well as the therapy of thoracic aortic aneurysms differ for each of these segments.

Thoracic aneurysms of the ascending aorta are considered to result from cystic degeneration of the media layer, a process associated with weakening of the aortic wall. Cystic medial degeneration occurs with aging, but seems to be increased in some families and with arterial hypertension. Other risk factors which have been suggested to be associated primarily with thoracic aneurysm formation of the descending aorta are the same as those for atherosclerosis (e.g. hypertension, smoking and hypercholesterolemia). Whether atherosclerosis itself is a prerequisite for aortic aneurysm development is a matter of debate. A multifactorial, non-atherosclerotic cause such as a defect in vascular structural proteins and breakdown of extracellular matrix proteins in combination with increased mechanical stress has been postulated as the most likely mechanism for thoracic aneurysm formation.

Transthoracic echocardiography, computed tomography or MR angiography are recommended for serial re-evaluation of a thoracic aortic aneurysm.

Natural history data on thoracic aortic aneurysms is scarce; however, aneurysm diameter has been shown to increase by 1 to 10 mm per year. The rate of expansion is related to the diameter of the aneurysm with larger aneurysms expanding faster; the findings of an early cohort study on the natural course of thoracic aortic aneurysms found a yearly expansion of 7.9mm in aneurysms >50mm, compared to 1.7mm in aneurysms ≤50mm.

In several series of patients, aneurysm rupture occurred in 32% to 68% of medically treated patients. The most important identified risk factor for rupture seems to be the size of the aneurysm; the yearly rate of dissection or rupture ranges from 2%, to 3%, to 7%, for thoracic aortic aneurysms less than 50 mm, 50-59 mm, and ≥60 mm in diameter, respectively. Expansion rate per year, male gender, hypertension, systemic steroid therapy and inflammation are other factors associated with increased risk of aneurysm rupture.