Blackout

Note that the terms ‘syncope’ and ‘loss of consciousness’ are not interchangeable as loss of consciousness can be due to either syncopal or non-syncopal causes. Syncope is a form of loss of consciousness in which hypoperfusion of the brain is the cause (from the Greek syn (together) and kopein (to cut), referring to the fact that the blood flow that joins the brain together with the rest of the body has been cut). Syncopal causes can be subdivided by mechanism as follows: • ‘Reflex’: this is believed to involve activation of a primitive reflex that leads mammals to ‘play dead’ when faced with danger. Their heart rate slows and their blood pressure drops temporarily, reducing cerebral perfusion and leading to syncope. Some people appear to have a low threshold for activating this reflex in specific situations—for example after standing still for a long time, after seeing something frightening (e.g. blood, needles), or when straining (micturition, defecation). • ‘Cardiac’: pathologies causing a reduction in cardiac output (such as arrhythmias or outlet obstruction) can also lead to syncope. • ‘Orthostatic’: orthostatic hypotension basically means low blood pressure on sitting or standing (as opposed to lying flat). When we stand up there is a sudden drop in blood pressure that we compensate for by vasoconstriction, particularly of the ‘capacitance’ veins in the legs. This reduces the intravascular space, enabling us to maintain the pressure. However, this vasoconstriction takes a few seconds, so to prevent a transient fall in blood pressure every time we stand, there is a temporary increase in heart rate. Patients with reduced intravascular volume (e.g. from dehydration) and/or in whom the normal autonomic response (transient tachycardia and peripheral vasoconstriction) to standing is blunted (e.g. due to drugs or autonomic neuropathy) are vulnerable to blackouts. • ‘Cerebrovascular’: these are non-cardiac structural causes of reduced cerebral perfusion, i.e. obstructions to the blood flow between the heart and the brain. They are relatively uncommon. The main causes of a transient loss of consciousness are summarized in Figure 3.1, with the most common in large font. You should also remember that patients may believe they have lost consciousness when in fact they haven’t.