Syncope Overview
Syncope, commonly known as fainting, refers to a sudden loss of consciousness, followed by a rapid and complete recovery. If you have symptoms of dizziness or lightheadedness, without loss of consciousness, this is often called presyncope, but these symptoms may be due to conditions that are unrelated to syncope.
Syncope should not be confused with sudden cardiac arrest. A person with sudden cardiac arrest also loses consciousness suddenly but will die without immediate medical attention. A person with syncope recovers quickly, almost always without treatment. However, injuries can occur during a syncopal episode, and recurrent episodes can be frightening. Moreover, in patients with heart problems, syncope may be a warning sign of more severe future heart arrhythmias and warrant thorough evaluation. It is important to determine the cause of syncope so that it can be prevented or treated in the future.
There are other reasons that a person may pass out, such as low blood sugar, hyperventilation, or seizures. These are not syncope but are important to assess and prevent.
Frequency — Syncope is surprisingly common in the population, but individual patients, if they have syncope, rarely experience more than one episode in a lifetime Approximately one-third of people have a syncopal episode at some point in their life. In most cases, syncope is not a sign of a life-threatening problem, although some people with syncope have a serious underlying medical condition. In non-elderly people, over 75 percent of cases of syncope are not associated with an underlying medical problem.
Risks — A person who suddenly and unexpectedly loses consciousness can be injured. Up to 35 percent of people who have syncope injure themselves; elderly people are more likely to be injured during a syncopal attack.
Because of the risk of injury and the potential for serious underlying disease, any person who has a syncopal episode should seek medical attention.
Syncope Causes
To remain conscious, a supply of oxygen-rich blood must be pumped to the brain without interruption. If the brain is deprived of this blood supply, even for a brief period, loss of consciousness (passing out) will occur.
A number of medical conditions can cause syncope. Some of the most common are listed here.
Vasovagal syncope — One of the most common types of syncope is called vasovagal syncope or neurocardiogenic syncope. A variety of conditions can trigger vasovagal syncope, including physical or psychological stress, dehydration, bleeding, or pain. The heart rate slows dramatically and the blood vessels in the body expand, causing blood to pool in the legs, resulting in low blood pressure (hypotension). This causes a decrease in blood flow to the brain.
In some cases, vasovagal syncope is triggered by an emotional response to a stimulus, such as fear of injury, heat exposure, the sight of blood, or extreme pain. In other cases, it is caused by abnormal nervous system responses to activities such as urinating, having a bowel movement, coughing, or swallowing. In still other cases, no trigger can be identified.
In most cases of vasovagal syncope, you have some warning that you are near fainting. These signs include dizziness, feeling hot or cold, nausea, pale skin, “tunnel-like” vision, and profuse sweating. After the episode, symptoms may continue because of continued low blood pressure. Some people feel extremely tired.
Heart rhythm problems — A number of disturbances in the rate and/or rhythm of the heart can cause syncope. These disturbances are called arrhythmias.
The heart includes an area of specialized cells in the upper right chamber of the heart (right atrium) called the sinus node (figure 1). These cells send a series of regular electrical impulses to the atria that regulate the heart’s rhythm and pace. These impulses travel in an organized way along conduction tissues within the heart muscle and then spread along smaller fibers that go to each muscle cell of the heart. The impulses cause the heart muscle cells to contract in an organized and regular way, generating an effective pumping of blood to all areas of the body.
Syncope can occur because of problems at several places in this system. The problems may be due to primary heart rhythm problems, underlying heart disease, use of a medication, or a transient abnormal communication between the heart and the nervous system The following are common rhythm problems that cause syncope.
Sinus bradycardia — Bradycardia means a slow heart rate. In sinus bradycardia, the heart rate is slower than normal. A dramatically slowed heart rate can decrease the blood supply to the brain by reducing the amount of blood that the heart can pump per minute.
Sometimes, sinus bradycardia occurs because of an abnormality in the sinus node itself. This is called sick sinus syndrome. In other cases, the slowed firing of the sinus node is due to medications. In still others, problems with the nerves that lead to the heart muscle and regulate the sinus node rate are to blame. For syncope to occur due to this problem, the heart usually stops for six or more seconds. This is known as asystole.
Heart block — Sometimes, part of the conduction system between the sinus node and the rest of the heart becomes disrupted due to heart disease. Most often, this occurs in the middle of the heart where a special set of fibers conduct the electrical impulse to the pumping chambers (the ventricles), preventing the normal flow of electrical impulses. If the electrical signal from the sinus node fails to get through the entire conduction pathway, the heartbeat can be interrupted. If the interruption is significant and the heart rate is too slow, it can impair blood flow to the brain.
Ventricular tachycardia — Tachycardia is a fast heart rate. The ventricles are the heart’s main pumping chambers (figure 2). Ventricular tachycardia (VT) occurs when muscle in the ventricles send out their own rapid electrical impulses, taking over the rhythm normally controlled by the sinus node. The heartbeat that results from these abnormal impulses is also abnormal, and often fails to pump blood in an adequate way. The heart is racing but does not pump effectively, so blood flow to the brain may be decreased.
Most people with syncope due to VT have underlying heart disease, most commonly coronary heart disease. Treatment of people with syncope caused by VT usually includes an implantable cardioverter-defibrillator.
Supraventricular tachycardia — Rapid heartbeats can originate above the ventricles (supraventricular tachyarrhythmias). This is not commonly associated with syncope, unless the heart rate is very rapid.
Blockage of blood flow from the heart — Any problem with the structure of the heart that interferes with the flow of blood can cause syncope. The two most common causes of outflow obstruction are hypertrophic cardiomyopathy and aortic stenosis.
●Hypertrophic cardiomyopathy – Hypertrophic cardiomyopathy is an inherited condition in which the areas of the left ventricular muscle walls are thickened. In some cases, the condition can interfere with blood flow out from the left ventricle and can cause syncope. More information about hypertrophic cardiomyopathy, including treatment recommendations, is available separately.
●Mitral stenosis or aortic stenosis – In the normal circulation, blood flows through the mitral and aortic valves and into the body’s largest artery, the aorta, to supply blood to the body (figure 2). Disease causing narrowing or obstruction of these valves will reduce the amount of blood being pumped and may predispose to syncope.
When severe, mitral or aortic stenosis can reduce blood flow to the brain and the rest of the body. Treatment of syncope caused by mitral or aortic stenosis often includes valve replacement surgery.
Orthostatic hypotension — Hypotension means low blood pressure. Orthostatic hypotension refers to low blood pressure that occurs when a person stands or sits up. Due to the effects of gravity, this causes an inadequate amount of blood to the brain, leading to syncope.
Causes of orthostatic hypotension include the following:
●Blood or fluid loss – Sometimes there is not enough blood in the body to ensure adequate flow to the brain when sitting or standing up. Low blood volume can be caused by blood loss or severe dehydration.
●Medications – Certain medications can interfere with the normal mechanisms that maintain blood pressure. Examples include some antidepressants, certain blood pressure or heart medicines, or medicines containing opiates, such as morphine.
●Illnesses that affect the nervous system – A number of illnesses can affect the specialized branch of the nervous system that helps maintain blood pressure (the autonomic nervous system). Examples are Parkinson disease, diabetes mellitus, the Shy-Drager syndrome, and amyloidosis.
●Alcohol – Drinking alcohol can cause blood vessels to expand, causing blood pressure to fall and syncope to occur.
●Carotid sinus hypersensitivity – Carotid sinus hypersensitivity is a condition in which reflexes lead to a slow heart rate and/or enlargement of blood vessels. This may be triggered by pressure on the carotid arteries (the main artery in the neck), and can lead to low blood pressure and syncope.
●Neurologic disease – (eg, Parkinsonism) that prevents blood vessels from constricting in compensation for upright posture.
Other causes — Less common causes of syncope include a heart attack, cardiac tumor, or blood clot in the arteries supplying the lungs.
Source: UpToDate
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