What is the primary cause of syncope?

The primary cause of syncope, or fainting, is a decrease in cerebral blood flow. When the brain does not receive adequate blood supply, it can lead to a temporary loss of consciousness. This reduction in blood flow can occur for a variety of reasons, including sudden changes in posture, dehydration, emotional stress, or cardiac issues, which can impair the heart's ability to pump blood effectively. A critical factor in syncope is that the brain relies on a steady supply of oxygenated blood, and any disruption can lead to symptoms of dizziness or loss of consciousness.

Other causes, like dehydration, while they can contribute to syncope by leading to decreased blood volume, are more indirect. They do not directly address the immediate reason for the lack of cerebral perfusion. Also, excessive blood flow is not a typical cause of syncope, as more blood flow would generally ensure adequate cerebral perfusion, and increased lung capacity relates to respiratory function rather than blood flow to the brain. Therefore, the most direct and clinically relevant reason for syncope is indeed decreased cerebral blood flow.