Constipation is medically defined as having fewer than three bowel movements a week, or passing stools that are hard, dry, and difficult to pass. Although rare, severe constipation can cause a person to pass out, or experience syncope. This fainting is a temporary loss of consciousness that occurs when the brain does not receive enough oxygenated blood, resulting from the intense physical strain associated with trying to pass a difficult stool.
The Direct Answer: How Constipation Can Cause Syncope
The physiological mechanism linking severe constipation to fainting is known as defecation syncope, triggered by the Valsalva maneuver. This maneuver involves forcefully trying to exhale against a closed airway, which occurs when a person strains to push out a hard stool. The straining momentarily increases intrathoracic pressure, pushing blood out of the heart and down the limbs.
This sudden pressure change significantly reduces the venous return of blood back to the heart, causing the heart to pump less blood. The body attempts to correct this rapid drop in blood flow and pressure by stimulating the vagus nerve, resulting in a vasovagal response. The vagus nerve is the main nerve of the parasympathetic nervous system, and its stimulation causes the heart rate to slow down and blood vessels to widen.
When the straining stops, the intrathoracic pressure drops suddenly, but the heart rate and blood pressure remain low for a moment due to the overreaction of the nervous system. This combination results in a rapid, temporary decrease in blood flow to the brain, a state called temporary cerebral hypoxia. This brief deprivation of oxygen causes the person to lose consciousness and faint.
Other Severe Symptoms Linked to Constipation
While true syncope is uncommon, the intense straining and underlying conditions of severe constipation often produce other symptoms that make a person feel extremely unwell. Before or without a fainting episode, a person may experience lightheadedness, dizziness, or a cold, clammy sweat. These pre-syncopal symptoms indicate that blood pressure is dropping and the body is struggling to maintain adequate blood flow to the brain.
Severe constipation can also lead to systemic symptoms like nausea, vomiting, weakness, and fatigue. The buildup of stool and gas causes significant abdominal pain and bloating, contributing to overall discomfort. Low blood pressure (hypotension) can be exacerbated by dehydration, a common factor in constipation, making the body more susceptible to a severe vasovagal reaction.
Immediate Response and Emergency Warning Signs
If a person feels lightheaded, dizzy, or nauseous while straining, they should stop pushing immediately. They should quickly move to a safe position by lying down and elevating their legs to encourage blood flow back toward the heart and brain. If lying down is not possible, sitting with the head lowered between the knees can help prevent a full loss of consciousness.
Certain accompanying symptoms with constipation are warning signs of a medical emergency requiring immediate attention. These include severe, sudden abdominal pain, vomiting (especially if green or resembling coffee grounds), and the inability to pass gas or stool for several days. Other serious signs are fever, a rapid heart rate, significant rectal bleeding, or black, tarry stools, as these can indicate a bowel obstruction or other serious underlying condition.
Preventing Severe Constipation
The most effective way to prevent defecation syncope is to avoid the severe straining that triggers the vagal response. Prevention begins with dietary changes, specifically increasing the daily intake of fiber to the recommended range of 25 to 34 grams for adults. Fiber adds bulk to the stool, making it softer and easier to pass without excessive force.
It is important to increase fluid intake alongside fiber, as water helps plump the fiber and soften the stool. Adults should aim for eight to ten glasses of caffeine-free and alcohol-free fluids daily to maintain hydration. Regular physical activity, such as walking or running, also promotes better bowel motility, helping food move more quickly through the colon.
If dietary and lifestyle changes are insufficient, over-the-counter options like bulk-forming laxatives or stool softeners can be used for short-term relief. However, consult with a doctor before using any laxative regularly, as overuse can lead to dependency or mask a more serious issue. Maintaining a consistent toilet routine and avoiding the urge to delay a bowel movement also helps keep stools from becoming hard and dry.