[A mortal cause of syncope].

Anaphylaxis is a generalized reaction produced by the massive release of mediators from mast cells, is rapid in onset and may cause death. The diagnosis is clinical, and the initial symptoms often overlap with those of many other disorders such as, vasovagal reactions, anxiety or asthma, and is often underdiagnosed. A high index of suspicion is necessary for the diagnosis. We report the case of a young woman who presented with syncope in a Primary Care Centre. In spite of the absence of skin lesions, we suspected an anaphylaxis because of the refractory low blood pressure. Thus treatment was started she was transferred to the Hospital. In the Emergency Department, the elevated serum tryptase levels supported our diagnosis. The absence of a clear exposure to a known allergen lead us to suspect a hydatid cysts rupture. The abdominal ultrasound and CT scan confirmed this.