Definition and classification of transient loss of consciousness

Consciousness is defined as the ability to maintain awareness of self and of the environment. Unconsciousness is a condition in which this ability is lost and there is a marked reduced responsiveness to environmental stimuli (Adams et al. Principles of neurology, 6th edn. McGraw-Hill, New York, 1997, pp 344\ue2\u20ac\u201d345). Loss of consciousness (LOC) can last briefly and resolve with no clinical intervention, be prolonged until a specific cause is treated and then be followed by a complete recovery or neurological symptoms, or sustain indefinitely. The National Institute for Health and Clinical Excellence defines transient loss of consciousness (T-LOC) as a brief and spontaneous loss of consciousness with complete recovery (Cooper et al., Ann Intern Med 155:543\ue2\u20ac\u201d549, 2001). The purpose of this chapter is to describe briefly the causes of transient loss of consciousness and then more thoroughly the possible aetiologies of syncope