• Micturation syncope is a relatively rare phenomenon (2.4 to 8.4% of fainting episodes) which most commonly affects males, and can often be diagnosed by simply taking a thorough history.
• Straining to urinate triggers the vagus nerve which results in hypotension and bradycardia; in turn, cardiac output and brain perfusion is decreased, often resulting in diaphoresis, pallor, and weakness, followed by syncope or fainting. 
• This process is transient and vital signs as well as consciousness typically return to normal rapidly.
• When evaluating a patient for syncope, pay close attention for the presence of the following factors in order to make the diagnosis:

             -- occurs during or immediately following urination, often when bladder is full.

             -- occurs at night or after standing from the recumbent position of a deep sleep to urinate.

             -- risk factors: enlarged prostate, alpha blocker therapy, dehydration, alcohol, fatigue.

• Sometimes defecation, coughing, or severe vomiting can also result in syncope.

References

• Faintness and syncope. In: Ropper AH, et al. Adams & Victor's Principles of Neurology. 9th ed. New York, N.Y.: The McGraw-Hill Companies; 2009.
• Seizures and syncope. In: Aminoff MJ, et al. Clinical Neurology. 7th ed. New York, N.Y.: The McGraw-Hill Companies; 2009.