The diagnosis of anaphylaxis can be very easy or very difficult depending on the clinical manifestations and situation. Acute syncope and hypotension can be due to vasovagal syncope, especially in a medical setting. It may be distinguished by bradycardia (unusual in the face of hypotension) and recovery with recumbency. In a small percentage of cases, bradycardia may accompany anaphylaxis. All other causes of shock must be considered in the context of the clinical setting and the presenting signs and symptoms (hypovolemic, cardiogenic, toxic). The presence of generalized urticaria along with the other signs may facilitate the diagnosis and treatment, but does not clarify the cause. Non-allergic causes include systemic mastocytosis (onset can be in childhood or as an adult; manifestations can be quite variable) and scombroid fish poisoning which is due to high levels of histamine in poorly preserved fish. Although other foods can contain histamine and other vasoactive amines, these limited quantities cause symptoms usually not as severe as anaphylaxis.