presyncope
Presyncope is the feeling of being about to faint without actually losing consciousness — the gray-out before the blackout. People describe lightheadedness, dimming or tunneling vision, ringing ears, sweating, weakness, or a sense that the floor is rising. It is essentially syncope that stops just short of completion, usually because the person sits or lies down, or the body corrects itself, in time.
The underlying mechanism mirrors that of fainting: a transient fall in blood flow or blood pressure to the brain that is enough to disturb function but not enough to switch it off completely. The same families of causes apply — reflex triggers, standing up quickly, dehydration or medication, and cardiac problems such as arrhythmias or outflow obstruction.
From a clinical standpoint, presyncope is taken about as seriously as full syncope, because it can share the same dangerous causes; the difference is largely a matter of degree and luck. A careful history of what brought it on, what symptoms came with it, and whether the heart rhythm was abnormal guides whether it is reassuring or a signal that needs deeper investigation.
On standing up quickly from a couch, an older man on blood-pressure pills feels his vision briefly dim and grabs the wall, steadying himself without falling — a presyncopal spell from a drop in standing blood pressure.
A brief gray-out on standing, corrected by steadying oneself, is a common orthostatic presyncope.
Because presyncope and syncope can share the same causes, repeated near-fainting episodes — especially during exertion or with palpitations — warrant the same workup as actual fainting.