Neurally Mediated Syncope

What is Neurally Mediated Syncope and Why Do Patients Pass Out?

NMS is a condition that occurs in a wide range of people, many of whom do not have any other associated cardiovascular problems. These patients have a history of repeated syncope (passing out spells) over the course of several years. Some of these spells may be preceded by a hot sensation or nausea but often occur without any warning. The patient may fall to the ground and recover consciousness quickly and without any further problems. The episodes of syncope may occur in the grocery store, while a blood specimen is being drawn, or while sitting quietly at home. Surprisingly, the majority of the patients never sustain serious injuries and the diagnosis may go undetected for decades. Patients get used to having "faints". The availability of the Tilt Test, its wide success in establishing the diagnosis of NMS, and effective treatment for NMS has resulted in the well deserved popularity of the test.

To understand this phenomenon, we must first examine how the nervous system of the body functions. During our daily activities, our heart rate (HR) and blood pressure (BP) constantly rise and fall to meet the needs of the body. The HR and BP are lower at rest, while the HR picks up and BP is elevated during emotional stress and exercise. The control of the HR and BP are under the domain of the sympathetic and parasympathetic nervous systems. The former increases the HR and BP while the latter reduces them. To illustrate this concept, imagine that the body is an automobile. Pressing the accelerator pedal makes the car work harder and go faster. In contrast, pressing the brake pedal slows the car down. The sympathetic nervous system behaves like an accelerator for the body while the parasympathetic system functions like a braking system. Normally, the sympathetic and parasympathetic nervous systems work together in a very efficient and cooperative manner. Just like a good driver, the accelerator and brake pedals are used efficiently (one being slowly pressed while the other is smoothly released). The car slows down and speeds up so smoothly that one does not recognize the change. Similarly, our HR and BP go up and down without us being aware of it. From time to time, this system breaks down in people with NMS. It is almost as if the accelerator pedal remains in the control of the car driver while the braking system is occasionally turned over to an extremely nervous "back seat driver." Now, if the car accelerates too rapidly (HR and BP goes up), the back seat driver panics and SLAMS the brake. The HR and BP (either singly or together) drop suddenly and severely. This reduces blood supply to the brain and the patient passes out. The back seat driver then steps off the brake pedal, HR and BP increase, blood flow is restored and the patient awakens.

The condition may be diagnosed by performing a tilt test. Effective treatment is available for patients with a positive test.