Fast heart rates, tachycardias, can also cause syncope/nearsyncope because the heart is unable to fill adequately with blood
to pump to the brain. Orthostatic hypotension (blood
vessels cannot constrict when standing up, therefore causing pooling of blood in the legs-->not enough blood to the brain).
Another cause can be a seizure, in which you lose all muscle tone. This is usually due to too much electrical activity in a certain area in the brain.