Cough syncope, or fainting spells triggered by coughing, is a perplexing condition that can stem from a variety of underlying physiological processes. Understanding the mechanisms behind cough syncope is crucial for accurate diagnosis and management. This article delves into the complex pathophysiology of cough syncope, exploring the multiple factors that can contribute to this condition.
The Mechanics of Cough and its Systemic Effects
Coughing is a protective reflex designed to clear irritants and secretions from the respiratory tract. It begins with a deep breath, increasing lung volume and decreasing pressure within the chest cavity (intrathoracic pressure). This is followed by forceful contraction of chest and abdominal muscles against a closed glottis – a maneuver known as Valsalva. This action dramatically increases intrathoracic pressure. Finally, the glottis opens, resulting in a rapid expulsion of air and a sharp decrease in intrathoracic pressure.
This forceful mechanical process of coughing can initiate a cascade of physiological events that, in susceptible individuals, can lead to syncope. Several distinct pathophysiological mechanisms have been identified in Cough Syncope Diagnosis.
Pathophysiological Mechanisms of Cough Syncope
Valsalva-Induced Reduced Cardiac Output
A forceful cough acts much like a Valsalva maneuver, significantly raising intrathoracic pressure – sometimes up to 300 mm Hg. This increased pressure impedes the return of blood to the heart (venous return). Reduced venous return leads to a decrease in stroke volume, which is the amount of blood the heart pumps with each beat. Consequently, blood pressure drops, reducing blood flow to the brain (cerebral perfusion). This temporary reduction in cerebral blood flow can result in syncope. Furthermore, the intermittent nature of coughing can trigger vasodilation following the cough bursts, potentially due to baroreceptor stimulation from arterial pressure spikes during coughing, leading to reduced peripheral vascular resistance and further contributing to blood pressure drops.
Increased Intracranial Pressure
The elevated intrathoracic pressure during a cough is also transmitted to the cerebrospinal fluid, resulting in a rapid spike in intracranial pressure. This sudden pressure increase within the skull can have effects similar to a concussion. It may also force blood out of the cranial cavity, further diminishing cerebral blood flow and potentially causing transient circulatory arrest in the brain.
Cardiac Arrhythmias
Coughing can sometimes induce vagally mediated cardiac arrhythmias. The vagus nerve, when stimulated, can slow down the heart rate. In some individuals, coughing can trigger excessive vagal nerve stimulation, leading to conditions like sinus arrest (where the heart’s natural pacemaker pauses) or atrioventricular conduction block (disruption in the electrical signals that coordinate heartbeats). In susceptible patients, particularly those with heightened vagal tone due to certain medications (like acetylcholinesterase inhibitors), low oxygen levels (hypoxia), high carbon dioxide levels (hypercapnia), or procedures like tracheal suctioning during respiratory infections, vagal stimulation can even cause ventricular asystole (complete cessation of heart contractions).
Hypersensitive Carotid Sinus Stimulation
Coughing can trigger syncope in individuals with a hypersensitive carotid sinus. The carotid sinus is a region in the carotid artery in the neck that senses blood pressure changes. Coughing may mechanically stimulate this hypersensitive area, perhaps due to pressure from neck muscles during coughing or through blood pressure fluctuations. This stimulation can lead to reflex hypotension (sudden drop in blood pressure) or vagally mediated atrioventricular block, both of which can cause syncope.
Cough-Triggered Neural Reflex-Mediated Hypotension-Bradycardia
Many patients experiencing cough syncope exhibit prolonged hypotension after coughing or performing a Valsalva maneuver. In some cases, this prolonged drop in blood pressure seems to be mediated by a neural reflex mechanism, contributing to the fainting episode.
Laryngospasm
Severe gastroesophageal reflux disease (GERD) can lead to laryngospasm, a spasm of the vocal cords, and syncope. This is more likely to occur during or after a respiratory infection, which can exacerbate both coughing and reflux, increasing the amount of refluxate and triggering laryngospasm.
Tracheal Compression
In rare instances, tracheal compression can be a cause of cough syncope. One reported case involved a retrosternal goiter (enlargement of the thyroid gland behind the breastbone) causing tracheal compression and cough syncope. However, this case was complicated by a pulmonary embolism. The patient’s symptoms improved with anticoagulation and further improved after thyroid surgery.
Augmentation of Left Ventricular Outflow Obstruction
Coughing can worsen left ventricular outflow obstruction in patients with hypertrophic cardiomyopathy (idiopathic hypertrophic subaortic stenosis). This is partly due to reflex sympathetic nervous system stimulation triggered by the cough.
Impaired Left Ventricular Diastolic Filling
In individuals with constrictive pericarditis (a condition where the sac around the heart is thickened and stiff), coughing can exacerbate impaired diastolic filling (the heart’s ability to relax and fill with blood between beats). This can lead to a significant reduction in cardiac output and potentially syncope.
Impaction of Brainstem Herniation (Chiari Malformation)
In patients with Chiari malformation, a condition where brain tissue extends into the spinal canal, coughing or sneezing can cause impaction of brainstem herniation in the foramen magnum (the opening at the base of the skull). This impaction can compress vital cardiovascular pathways in the brainstem, disrupt the midbrain reticular activating system (involved in consciousness), cause pressure imbalances within the craniospinal system, increase intracranial pressure, and compromise blood flow in the vertebrobasilar arteries supplying the brainstem. This complex cascade can lead to syncope and other symptoms like pre-syncope, visual disturbances, vertigo, drop attacks, and headaches.
Reduced Cerebral Blood Flow in Compromised Circulation
Coughing can further impair cerebral circulation in individuals who already have compromised cerebral blood flow due to narrowed or blocked arteries in the brain or neck (extracranial or intracranial arterial stenosis or occlusion). In these cases, several factors can act synergistically, including reduced cardiac output, increased cerebrospinal fluid pressure, and hypocapnia (low carbon dioxide levels) secondary to coughing, which can cause cerebral arterial vasoconstriction (narrowing of blood vessels in the brain).
Internal Jugular Vein Valve Insufficiency
Cough syncope can occur due to incompetent valves in the internal jugular veins. These valves normally prevent backflow of blood. If they are insufficient, the increased pressure in the chest cavity during coughing can be transmitted to the internal jugular veins and subsequently to the intracranial space, leading to increased intracranial pressure and reduced cerebral blood flow.
Seizure (Rare)
In rare instances, cough and its associated physiological changes might trigger a seizure, leading to loss of consciousness followed by a postictal state (the period after a seizure). However, documented cases of cough-induced seizures confirmed by electroencephalography (EEG) are lacking.
Conclusion
Cough syncope is a multifaceted condition with a range of potential underlying mechanisms. Accurate cough syncope diagnosis requires a comprehensive understanding of these various pathophysiological processes. Recognizing the diverse ways in which a simple cough can lead to fainting is essential for healthcare professionals to effectively evaluate and manage patients presenting with this concerning symptom. Further research is continuously refining our understanding of cough syncope and improving diagnostic approaches.
