The growth hormone and prolactin responses to the head-up tilt test in patients with vasovagal syncope

Fainting, or syncope, is a sudden loss of consciousness that happens when the brain does not receive enough blood flow for a brief moment. While the exact reasons why this occurs vary, the most common type is known as vasovagal syncope. This specific form involves a complex reaction where the body's automatic control system, which regulates heart rate and blood pressure, suddenly overreacts to a stressor like standing up too quickly or feeling emotional distress. The result is a drop in blood pressure and a slowing of the heart, leading to a collapse. Scientists have long studied the electrical and mechanical parts of this event, but the chemical signals that travel through the blood to coordinate these changes remain less understood. Among these chemical messengers are hormones, which act as the body's internal communication system, telling organs how to respond to stress. Two such hormones, growth hormone and prolactin, have been suspected of playing a role in this process, yet their behavior during a fainting episode has been a mystery waiting to be solved.

To uncover these hidden chemical shifts, researchers gathered a group of seventy-nine people who had a history of fainting. These individuals, ranging in age from nineteen to seventy-three, were brought into a controlled setting to undergo a specific test designed to mimic the conditions that trigger a faint. The procedure involved lying the participants flat and then slowly tilting them upright, a method known as a head-up tilt test. This position places a natural stress on the body's circulatory system, forcing it to work harder to keep blood flowing to the brain. During the test, the medical team measured levels of prolactin and growth hormone in the participants' blood at the start and again at the end of the procedure. The goal was to see if these hormone levels changed differently in those who actually fainted during the test compared to those who remained conscious.

The results revealed a clear split in how the body reacted. Fifty of the participants experienced a fainting episode during the test, while the others did not. In the group that fainted, the level of prolactin rose significantly by the end of the test. In contrast, the people who stayed conscious showed a decrease in prolactin levels. The researchers found no such difference with growth hormone; its levels remained largely unchanged in both groups, regardless of whether the person fainted or not. This suggests that growth hormone is not a primary driver of the chemical changes seen during this type of fainting, whereas prolactin appears to be intimately linked to the event.

The study also looked at how these hormone levels related to the physical state of the participants. The researchers observed that higher levels of prolactin measured after the test were connected to lower blood pressure readings. Specifically, as the prolactin levels went up, both the top and bottom numbers of the blood pressure reading tended to go down. This connection was strong enough that the team identified a specific threshold: if a person's prolactin level rose above 5.8 micrograms per liter after the test, they were more likely to be the type of person who would faint during the procedure. These findings point toward a specific pathway in the brain involving serotonin, a chemical that influences mood and body control, suggesting that this system may be the trigger for the hormonal surge.

While the study does not claim to have solved the entire puzzle of why people faint, it offers a new way to look at the problem. The data suggests that the body's chemical response, particularly the surge in prolactin, is a real and measurable part of the fainting process. This insight could eventually help doctors identify different types of fainting based on a patient's chemical profile. If a patient shows this specific hormonal reaction, it might open the door to personalized treatments, such as medications that target the serotonin system, rather than relying on a one-size-fits-all approach. For now, the work stands as a clear demonstration that the chemistry of the blood changes in a distinct way when the body loses its balance, offering a tangible clue to the hidden mechanics of vasovagal syncope.