eComment: Acute aortic dissection in the young - distinguishing precipitating from predisposing factors

We read with interest the report of Uchida and colleagues [1]. As reported, acute aortic dissection (AAD) is a rare entity in the young. In most cases of AAD, the pathogenesis is a result of the interplay of three factors – a predisposition provided by an abnormality or weakening of the aortic media, an agent of intimal injury or tear resulting in the intimomedial flap, and hemodynamic factors that propagate the dissection once it has been initiated. Congenital cardiovascular disorder (coarctation, bicuspid aortic valve) remains the most important predisposing factor in young patients whereas long-standing hypertension is the commonest predisposition in adults. As shown by Uchida's group [1], AAD can occur without any predisposing factors in young children or adolescents, an indication that the predisposition theory provides only part of the pathogenetic mechanisms responsible for AAD. We conceptualize that the normal aorta presents a dissection threshold that is commonly not reached by the usual hemodynamic stresses encountered during the cardiac cycle. The tensile strength and elasticity of the aortic wall resides primarily in the media. Predisposing factors render the aorta more vulnerable to dissection by weakening the media. The aorta so weakened presents a lower than normal threshold to dissection. Therefore, the factors that precipitate the onset of AAD need clarification. Several precipitating factors of AAD have been identified by Hatzaras and colleagues [2]. These factors are not necessarily identical with the predisposing factors but they act on the basis of a transient severe hypertensive reaction to precipitate AAD. In their report [2], strenuous physical activity and severe emotional stress were identified as clear precipitating factors of the acute onset of thoracic pain of AAD in 67% of patients. Strenuous physical activities included lifting heavy weights, swimming, snow shovelling, and vigorous sex. Severe emotional stresses included very upsetting news, large financial losses, and extremely stressful business trips. We believe that AAD occurs when the precipitating factors (on the basis of a transient, severe hypertensive reaction) are potent enough to overcome the dissection threshold of the aortic wall. Clearly, a far more potent inciting event is required to cause AAD in the otherwise normal aorta than in one rendered vulnerable by a congenital or acquired weakness of the aortic wall. Swimming has been reported to precipitate AAD [2] and triple vessel coronary artery dissection (without concomitant AAD) in the absence of any predisposing factors [3]. Possibly, swimming constitutes a potent inciting event for arterial dissections in certain individuals.