What are the causes and management of vasovagal syncope?

Causes of Vasovagal Syncope

Vasovagal syncope is primarily caused by a reflex response that triggers vasodilation and bradycardia, resulting in systemic hypotension and cerebral hypoperfusion. 1

Primary Triggers and Mechanisms

Vasovagal syncope (also called reflex syncope or neurally-mediated syncope) occurs when specific triggers activate a reflex that causes:

• Vasodilation (vasodepressor response)
• Bradycardia (cardioinhibitory response)
• Or both (mixed response)

Common Triggers

• Emotional distress 1:

  • Fear
  • Severe pain
  • Emotional stress
  • Medical instrumentation (e.g., blood draws)
  • Blood phobia

• Orthostatic stress 1:

  • Prolonged standing
  • Standing in crowded, warm places
  • Sudden postural changes

• Situational triggers 1:

  • Coughing or sneezing
  • Gastrointestinal stimulation (swallowing, defecation, visceral pain)
  • Micturition (post-micturition)
  • Post-exercise
  • Post-prandial (after eating)
  • Other activities (laughing, brass instrument playing, weightlifting)

• Environmental factors 2:

  • Hot weather
  • Lack of food
  • Dehydration

Clinical Presentation

Prodromal Symptoms

Most patients experience prodromal symptoms before losing consciousness 1, 2:

• Diaphoresis (sweating)
• Warmth
• Nausea
• Pallor
• Dizziness
• Visual disturbances
• Hearing disturbances

However, approximately one-third of patients may experience sudden episodes with no prodromal features 2.

During the Event

• Loss of consciousness
• Pallor (reported in about half of cases)
• Sweating (reported in about 13% of cases)
• Myoclonus (reported in about 5% of cases)
• Unwitnessed episodes (occur in about 25% of cases)

After the Event

• Fatigue (most common after-effect)
• Potential for injury (over half of patients sustain injuries)
• Fractures (occur in about 13% of patients)

Diagnostic Approach

Initial Evaluation

The diagnosis of vasovagal syncope can often be made during initial evaluation when 1:

1. Precipitating events (fear, pain, emotional distress, instrumentation, prolonged standing)
2. Are associated with typical prodromal symptoms

Diagnostic Criteria

Classical vasovagal syncope is diagnosed when 1:

• Syncope occurs with identifiable triggers
• Typical prodromal symptoms are present
• No evidence of cardiac disease exists
• Other causes of syncope have been excluded

Differential Diagnosis

Important conditions to distinguish from vasovagal syncope include 1:

1. Cardiac syncope:

   • Arrhythmias (bradycardia, tachycardia)
   • Structural heart disease
   • Cardiac ischemia

2. Orthostatic hypotension:

   • Primary autonomic failure
   • Secondary autonomic failure (diabetes, amyloidosis)
   • Drug-induced
   • Volume depletion

3. Non-syncopal conditions:

   • Seizures
   • Metabolic disorders (hypoglycemia, hypoxia)
   • Psychogenic pseudosyncope
   • Transient ischemic attacks

Management Approach

Non-pharmacological Interventions

1. Education and reassurance 3, 4

   • Explain the benign nature of the condition
   • Identify and avoid triggers when possible

2. Lifestyle modifications 4, 5:

   • Adequate hydration
   • Increased salt intake (when not contraindicated)
   • Avoiding rapid position changes
   • Avoiding high room temperatures

3. Physical counterpressure maneuvers 4:

   • Isometric leg and arm exercises
   • May be less effective in older patients or those with short/no prodrome

Pharmacological Treatment

For patients with recurrent episodes despite conservative measures 4:

1. First-line medications:

   • Fludrocortisone (mineralocorticoid)
   • Midodrine (alpha-agonist)

2. Other medications with varying evidence:

   • Serotonin reuptake inhibitors (fluoxetine)
   • Anticholinergic agents
   • Beta-blockers (evidence is mixed; metoprolol was not helpful in pivotal trials)

Specialized Interventions

For refractory cases 6, 4:

1. Cardiac pacing:
   • Indicated for cardioinhibitory or mixed carotid sinus syndrome
   • Consider for cardioinhibitory vasovagal syncope with >5 attacks per year or severe physical injury and age >40 years
   • Closed-loop stimulation pacemakers show promise

Common Pitfalls and Caveats

1. Misdiagnosis: Vasovagal syncope can present atypically, especially in older patients where prodromal symptoms may be absent 1, 2.

2. Incomplete evaluation: Failing to exclude cardiac causes of syncope before diagnosing vasovagal syncope.

3. Overlooking situational triggers: Some patients may not recognize specific triggers like post-prandial state or mild dehydration.

4. Medication effects: Not considering the contribution of medications that can exacerbate orthostatic tendencies (diuretics, vasodilators, antidepressants).

5. Injury risk: Underestimating the potential for injury during syncopal episodes, which occurs in over half of patients 2.