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Unexplained syncope in patients with high risk of sudden cardiac death

The occurrence of syncope is a frequent event in patients affected by diseases at high risk of sudden death. This should be evaluated with great attention because it may represent a turning point in the risk assessment of the disease. When the patient already has a defibrillator implantation indication (e.g. in patients with left ventricular function <35%), the occurrence of syncope is an opportunity to move towards the implantation. However, in many cases, it is the syncope by itself that will guide the therapeutic management. It is therefore particularly important to pay attention to its evaluation since the question to determine if the syncope is related to life-threatening arrhythmia or whether it is just a simple vasovagal syncope will determine entirely the way to treat the patient. Unfortunately, in many patients with structural cardiac or inherited diseases, the diagnosis remains unexplained at the end of a complete work-up. Most studies in the literature lack a precise definition of unexplained syncope. The contribution of the 2018 European Society of Cardiology Guidelines on syncope is to provide a more precise definition of unexplained syncope which is of help in identifying patients at high risk of sudden cardiac death. For this chapter, unexplained syncope is defined as syncope that does not meet any class I diagnostic criterion of those guidelines. In the presence of clinical features of structural cardiac or inherited diseases described in this chapter, unexplained syncope is considered a suspected arrhythmic syncope.