The wolf in sheep’s clothing: vasovagal syncope in acute aortic dissection – International Journal of Emergency Medicine

Mechanisms of syncope in the setting of AAD may vary from case to case but are commonly thought to be due to cardiac tamponade, coronary involvement or cerebral ischemia secondary to precerebral vessel involvement [9, 10]. In one study however, approximately half of patients presenting with an AAD and a syncope were not found to have any of the classical mechanical complications explaining the syncope [6]. Increasing vagal tone is another possible mechanism explaining syncope and bradyarrythmia in the setting of AAD [6, 10] and our case is an illustration of this mechanism.

Vasovagal syncope (or reflex syncope) is a common neurologically mediated form of syncope typically associated with a precipitating factor such as pain or stressors. The physiopathology of vasovagal syncope has not yet been entirely elucidated but involves autonomic pathways: hypotension through vasodepression due to sympathetic system inhibition, and bradycardia through parasympathetic-mediated cardioinhibition [11]. Vasovagal mechanisms have been suggested to be involved in type A AAD related syncope in the past [6, 10]. Dissection tear can stimulate mechanoreceptors located in the vascular wall of the aortic arch, triggering the aortic arch baroreflex via an afferent branch of the vagal nerve called the aortic depressor nerve, which provides sensory innervation of the aortic arch [12, 13]. Vagal-mediated efferences can then induce hypotension, as well as cardioinhibition slowing the sinus and AV node conduction. This can result in sinus bradycardia, sinus arrest, functional AV block, and sometimes asystole which can be reverted by atropine [14].

In our patient, the dissection tear showed a unique limited extension with precerebral and coronary vessel sparing. The absence of tamponade physiology or valvular dysfunction, the documentation of a functional atrio-ventricular block, the adequate response to atropine, and the negative syncope work up support the hypothesis of a vagally mediated syncope, through activation of the aortic arch baroreflex. The other symptoms of the patient such as borderline low blood pressure, slight decrease of consciousness and nausea can also be explained via persistently increased vagal tone [12].

Syncope is a non-specific symptom that can be due to multiple causes, ranging from the benign to the life-threatening. Risk stratification is an important part of the evaluation of syncope in the ED. International guidelines recommend managing patients with presumptive vasovagal syncope in an outpatient setting, as this category of syncope is generally regarded as low-risk and distinct from syncope of cardiac origin [15, 16]. Scores, such as the San Francisco rule, provide guidance for syncope risk stratification but have not been shown to be better than clinical judgment in identifying patients at higher risk of adverse outcomes [17]. A more recent externally validated syncope score, the Canadian Syncope risk score (CSRS), would categorize this patient as low risk if cardiac syncope was not suspected [18]. Considering a recent normal echocardiography, a benign EKG in the ED and no history of coronary heart disease, cardiac syncope was unlikely. However, some features of this case were concerning. A new onset syncope in an elderly patient, a fall, a non-specified prehospital rhythm disturbance and borderline blood pressure in the ED were considered enough arguments for inpatient rhythm and clinical monitoring, as a cardiac cause of syncope could not be formally ruled out.

This raises the question if syncope of unclear origin should trigger further testing in the ED, such as biomarkers. There is a debate surrounding biomarker testing to rule out pulmonary embolism in patients presenting to the ED with syncope of unclear origin. It has recently been shown that the prevalence of pulmonary embolism in patients presenting to the ED with syncope is very low [19, 20] and therefore indiscriminate biomarker testing should be discouraged because this strategy would result in low imaging yield and high costs [21]. Similarly, D-dimer levels may assist in the diagnosis of AAD [22]. The prevalence of AAD in patients presenting to the ED with syncope is not well known but is likely extremely low as suggested by a recent observational study including patients presenting with potential AAD symptoms [23]. By analogy to pulmonary embolism, syncope, which is an uncommon presentation of AAD, should not prompt indiscriminate D-dimer testing in the ED [24]. The balance between rapidly identifying life-threatening conditions and preventing harm from over-investigation [25] remains a delicate challenge in the fast-paced, time-critical environment of the emergency department.

In conclusion, this case serves as a reminder that AAD should be considered in the differential diagnosis of patients presenting to the emergency department with syncope. It also demonstrates that AAD can manifest specifically as vasovagal syncope. To the best of our knowledge, isolated painless vasovagal syncope in the setting of AAD has never been reported and is probably a rare presentation. Finally, despite many clinical tools, risk stratification of syncope in the ED remains challenging and still relies on clinical gestalt.