Syncope results when the cerebral blood flow is reduced. The possible cause of this reduction is hypotension or the sudden drop in blood pressure. There are certain situations that may trigger syncope, e.g. coughing, deglutition, laughter, and micturition.
Micturition syncope is a form of syncope triggered by the act of emptying the bladder (urination). A temporarily loss of consciousness ensues during or immediately after urinating. Unlike the other triggers of syncope, this is a rather common occurrence and adult males are mostly affected.1
Those who had episodes of micturition syncope reported experiencing prodromal signs such as feeling light-headed or dizzy. These signs are felt when they abruptly rise up to urinate after a reclining position or when lying down. This eventuates to the actual fainting during or immediately after urination when the vagus nerve is stimulated by the urine flow. The heartbeat slows down (brachycardia) and the blood pressure drops (hypotension).
Word origin: micturition: Latin micturire (to urinate), syncope: Greek syn- (together) +-koptein (to cut)
1 Wedro B. Micturition Syncope (Fainting During Urination or Bowel Movements). Link