Syncope refers to the transient loss of consciousness caused by reversible disturbances in cerebral function. In dental practice, stress is the primary cause of unconsciousness in majority of cases. Other predisposing factors include:
- Impaired physical status: There is increased likelihood of syncope in patients with severe systemic disease that is not incapacitating (ASA 3) or constant threat to life (ASA 4).
- Administration or ingestion of drugs: Most of the drugs used in dentistry (Analgesics and Anti-anxiety) have CNS depressant effect and produces alteration in the level of consciousness eg. sedation or loss of consciousness.
Pathophysiology
Under normal conditions, the brain derives most of its energy from the oxidation of glucose. To maintain this energy source, the brain must receive a continuous supply of glucose and oxygen.
- Without oxygen, some glucose can still be metabolised into lactic acid to provide limited energy, but this source cannot fulfil the brain's requirements for more than a few seconds, leading to rapid loss of consciousness.
- The human brain uses approximately 20% of the total oxygen and 65% of the total glucose the body consumes. To accomplish this, 14% of cardiac output goes to the brain. If the supply of either oxygen or glucose is diminished, brain function is rapidly affected.
- The cerebral blood flow of a normal individual in the supine position is around 750 ml/minute. Thus, at any given time the blood circulating through the brain contains 7 ml of oxygen that can supply the brain's requirement for less than 10 seconds.
Mechanisms
- Inadequate cerebral circulation.
- General or metabolic changes.
- Actions on central nervous system.
- Psychic mechanisms.
Inadequate Cerebral Circulation
The most common mechanism of loss of consciousness is a sudden decrease in the delivery of blood or oxygen to the brain. e.g., vasodepressor syncope, orthostatic hypotension. This may occur due to:
- Dilation of peripheral arterioles.
- Failure of normal peripheral vasoconstrictor activity (orthostatic hypotension).
- Significant drop in cardiac output (from heart disease, dysrhythmias, or decreased blood volume).
- Constriction of cerebral vessels as carbon dioxide is lost through hyperventilation.
- Occlusion or narrowing of the internal carotid or other arteries to the brain.
- Life threatening ventricular dysrhythmias.
General or Metabolic Changes
Changes in the quality of blood perfusion to the brain, caused by chemical or metabolic derangements, may also provoke the loss of consciousness or predispose a patient to its occurrence.
Most frequently encountered situations that lead to syncope through this mechanism include:
- Hyperventilation.
- Hypoglycaemia.
- Administration or ingestion of drugs.
- Acute allergic reaction.
In these situations, consciousness will not be regained until the underlying chemical or metabolic cause is corrected.
Actions on the Central Nervous System
Loss of consciousness can occur due to alterations within the brain itself or through reflex effects on parts of the CNS that regulates consciousness and equilibrium. These manifests clinically as:
- Convulsions (seizures), or,
- Cerebrovascular accident (CVA).
Psychic Mechanisms
- Emotional disturbances, are the most common cause of transient loss of consciousness in the dental practice.
- Vasodepressor syncope and hyperventilation are included in this category.
Types
Neurocardiac/Vasodepressor syncope
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Also known as Vasovagal syncope, Neurocardiogenic syncope or Neurally mediated syncope.
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Usually associated with some type of noxious stimuli such as, pain, fear, sight of blood or anaesthetic needle.
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Pre-syncopal or prodromal symptoms:
- Warmth in the face and neck, lightheadedness and pallor.
- Cold, sweaty palms or forehead, dilated pupil, elevated blood pressure and pulse rate and palpitation.
- Piloerection.
- Dizziness, Vertigo, Nausea, Yawning or Visual changes.
- Inability to sit still, Trembling or Fidgeting.
- Shortness of breath and hyperventilation.
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Syncope: Pallor, Unconsciousness and Weak, slow pulse.
Cardiac Syncope
- Second most common type of syncope.
- Occurs as a result of serious underlying heart disease.
- Arrhythmias (Electrical problems): Tachyarrhythmia (rapid, irregular heartbeat), Bradyarrhythmia (Slow, irregular heartbeat), or pacemaker malfunction.
- Obstructive (Mechanical problems): Left ventricular obstruction (most common), Right ventricular obstruction, or other myocardial diseases.
Non-cardiac Syncope
- Orthostatic hypotension: A 20 mm Hg drop in systolic blood pressure or 10 mm Hg drop in diastolic, when a person moves from supine to an upright position, can cause a syncopal episode. Unlike vasovagal syncope, there are no prodromal symptoms.
- Seizures.
- Hyperventilation: There is disruption in normal levels of oxygen and carbon dioxide in the blood due to excessive breathing. This condition is often stress induced and my produce sufficient vasoconstriction leading to syncope.
- Metabolic Diseases: Conditions such as Hypoglycaemia or Hypoxemia can deprive the brain of essential nutrients and can cause coma or somnolence (sleep).
- Situational Syncope: May result from different circumstances, such as coughing, micturition, defecation, neck stretching, hair grooming, venipuncture, or even swallowing. This occurs due to the production of Valsalva maneuver.
Points to Note
- Syncope is a symptom, that may occur in healthy individuals or may be a manifestation of serious medical disorder.
- Any loss of consciousness, however brief, represents a potentially life-threatening situation that requires prompt recognition and effective management.
- An unconscious person is described as the one who does not respond to sensory stimulation (eg. shake and shout or peripheral pain), has lost protective reflexes (eg. swallowing or coughing) and is unable to maintain a patent airway.
- In an unconscious patient, hypopharyngeal obstruction by the base of the relaxed tongue occurs when the head is flexed or is maintained in mid-position. Until the obstruction is removed, the patient continues to receive hypoxic levels of oxygen (partial obstruction) or becomes anoxic (total obstruction), with a decreasing likelihood of successful resuscitation. Hence, resuscitation of unconscious patient should primarily focus on relief of this obstruction.
- Complete airway obstruction in which the victim becomes anoxic, leads to irreversible neurological damage within 4-6 minutes and to cardiac arrest within 5-10 minutes.
References
- Medical Emergencies in the Dental Practice (7th Edition), Stanley F. Malamed, Daniel L Orr II, Mosby Elsevier.
- Medical Emergencies in Dental Practice, Orrett E. Ogle, Harry Dym, Robert J. Weinstock, Quintessence Publishing Co, Inc.
- Medical Emergencies Essentials for the Dental Professional (2nd Edition), Ellen B. Grimes, Pearson Education Inc.
- Image credit: Adobe Stock (https://stock.adobe.com/images/unconscious-young-man-lying-on-floor/526244956).
*This article is an excerpt from the above mentioned books and Medical Sutras does not make any ownership or affiliation claims.