Background-The risk for aortic dissection is increased among relatively young women with Turner syndrome (TS). It is unknown whether aortic dilatation precedes acute aortic dissection in TS and, if so, what specific diameter predicts impending deterioration. Methods and Results-Study subjects included 166 adult volunteers with TS (average age, 36.2 years) who were not selected for cardiovascular disease and 26 healthy female control subjects. Ascending and descending aortic diameters were measured by magnetic resonance imaging at the right pulmonary artery. TS women were on average 20 cm shorter, yet average aortic diameters were identical in the 2 groups. Ascending aortic diameters normalized to body surface area (aortic size index) were significantly greater in TS, and Ϸ32% of TS women had values greater than the 95th percentile of 2.0 cm/m 2 . Ascending diameter/descending diameter ratios also were significantly greater in the TS group. During Ϸ3 years of follow-up, aortic dissections occurred in 3 women with TS, for an annualized rate of 618 cases/100 000 woman-years. These 3 subjects had ascending aortic diameters of 3.7 to 4.8 cm and aortic size indices Ͼ2.5 cm/m 2 . Conclusions-The risk for aortic dissection is greatly increased in young women with TS. Because of their small stature, ascending aortic diameters of Ͻ5 cm may represent significant dilatation; thus, the use of aortic size index is preferred. Individuals with a dilated ascending aorta defined as aortic size index Ͼ2.0 cm/m 2 require close cardiovascular surveillance. Those with aortic size index Ն2.5 cm/m 2 are at highest risk for aortic dissection.

Girls and women with TS need focused screening of the aortic valve and root to identify the many asymptomatic individuals with abnormal valvular structure and/or aortic root dilation.