Many forms of syncope are preceded by a prodromal state that often includes dizziness and loss of vision ("blackout") (temporary), loss of hearing (temporary), loss of pain and feeling (temporary), nausea and abdominal discomfort, weakness, sweating, a feeling of heat, palpitations and other phenomena, which, if they do not progress to loss of consciousness and postural tone are often denoted "presyncope".^{[citation needed]} Abdominal discomfort prior to loss of consciousness may be indicative of seizure which should be considered different than syncope.^[1]

There are two broad categories of syncope, cardiogenic or reflex, which underlie most forms of syncope. Cardiogenic forms are more likely to produce serious morbidity or mortality and require prompt or even immediate treatment. Although cardiogenic syncope is much more common in older patients, an effort to rule out arrhythmic, obstructive, ischemic, or cardiomyopathic causes of syncope and circulatory inadequacy is mandatory in each patient.

Variants of reflex syncope often have characteristic histories, including precipitants and time course which are made evident by skilled history taking. Thus, the clinical history is the foremost tool used in the differential diagnosis of syncope. Physical examination, and electrocardiogram are part of the initial evaluation of syncope and other more specific tools such as loop recorders may be necessary in clinically uncertain cases.

Syncope does not occur with hypoxia which can lead to death by suffocation and does not fulfill the definition of syncope above. Syncope needs to be distinguished from coma or cerebrovascular accident which can include persistent states of loss of consciousness.

Although syncope may cause physical injury such as head trauma, it is specifically not directly caused by head trauma (concussion) or by a seizure disorder which may also produce short-lived unconsciousness unless these are also associated with globally reduced brain blood flow. Syncope is extraordinarily common, occurring for the most part in two age ranges: the teen age years, and during older age. Estimates of lifetime incidence of at least one syncopal episode include 40-50% of the general populace.