What are the neurogenic causes of syncope?

Neurogenic Causes of Syncope

The primary neurogenic causes of syncope include neurally-mediated (reflex) syncope, autonomic failure syndromes, and cerebrovascular disorders that lead to cerebral hypoperfusion and transient loss of consciousness. 1

Neurally-Mediated (Reflex) Syncope

Neurally-mediated syncope occurs when a triggered reflex causes inappropriate vasodilation and bradycardia, resulting in systemic hypotension and cerebral hypoperfusion:

• Vasovagal syncope (common faint) - The most common form of reflex syncope, triggered by:

  • Emotional stress, fear, pain, or blood phobia 1, 2
  • Prolonged standing or orthostatic stress 2
  • Characterized by prodromal symptoms including lightheadedness, dizziness, nausea, diaphoresis, and pallor 3, 2

• Carotid sinus syncope - Occurs when mechanical manipulation of the carotid sinuses triggers the vasovagal reflex 1, 2

  • More common in older adults 1
  • Can be reproduced by carotid sinus massage 1

• Situational syncope - Associated with specific scenarios:

  • Cough or sneeze-induced syncope 1, 2
  • Gastrointestinal stimulation (swallowing, defecation, visceral pain) 1, 2
  • Micturition syncope (particularly post-micturition) 1, 2
  • Post-exercise syncope 1, 2
  • Post-prandial syncope 1, 2
  • Other triggers (brass instrument playing, weightlifting) 1

• Glossopharyngeal neuralgia - Can trigger the vasovagal reflex 1

• Nicotine-related vasovagal syncope - Nicotine can trigger the vasovagal reflex through several pathways 3

Autonomic Failure Syndromes

Autonomic failure syndromes cause neurogenic orthostatic hypotension (OH), defined as a sustained reduction of systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of assuming upright posture 1:

• Primary autonomic failure syndromes:

  • Multiple system atrophy 1
  • Pure autonomic failure 1, 4
  • Parkinson's disease with autonomic failure 1
  • Lewy Body dementia 1

• Secondary autonomic failure syndromes:

  • Diabetic neuropathy 1
  • Amyloid neuropathy 1
  • Other peripheral neuropathies 1, 4

• Postural Orthostatic Tachycardia Syndrome (POTS) - A milder form of autonomic insufficiency characterized by orthostatic intolerance with excessive tachycardia upon standing 1, 4

Clinical Presentation of Neurogenic Orthostatic Hypotension

Symptoms of neurogenic OH differ from those due to dehydration, drugs, and cardiac/reflex syncope:

• Persistent and often progressive generalized weakness 1
• Fatigue 1
• Visual blurring 1
• Cognitive slowing 1
• Leg buckling 1
• "Coat hanger" headache (triangular headache at the base of the neck due to trapezius ischemia) 1

These symptoms may be provoked or exacerbated by:

• Exertion 1
• Prolonged standing 1
• Meals 1
• Increased ambient temperature 1

Diagnostic Approach for Neurogenic Causes of Syncope

• Autonomic evaluation is useful to improve diagnostic and prognostic accuracy in patients with:

  • Parkinsonism or other central nervous system features 1
  • Peripheral neuropathies 1
  • Progressive autonomic dysfunction without central or peripheral nervous system features 1
  • Postprandial hypotension 1
  • Known or suspected neuropathic POTS 1

• Neurological evaluation with simultaneous EEG and hemodynamic monitoring during tilt-table testing can help distinguish among syncope, pseudosyncope, and epilepsy 1

Management Considerations

For neurogenic orthostatic hypotension:

• Non-pharmacological measures:

  • Acute water ingestion (≥240 mL) for temporary relief 1
  • Physical counter-pressure maneuvers (leg crossing, lower body muscle tensing) 1
  • Compression garments (at least thigh-high, preferably including abdomen) 1

• Pharmacological therapy:

  • Midodrine can be beneficial in patients with syncope due to neurogenic OH 1, 5
  • Fludrocortisone is commonly used but needs more controlled studies 5
  • Other options include octreotide, indomethacin, or ergotamine 5

Important Caveats

• The care of patients with neurogenic OH is complex, especially in individuals with neurodegenerative disease 1
• Many treatments for neurodegenerative disease can worsen orthostatic hypotension, requiring careful balance of benefits and risks 1
• Medication review is essential as many pharmacologic agents may cause or worsen orthostatic hypotension 4
• Falls during syncope can cause significant injury, highlighting the importance of proper management 3
• Neurogenic syncope tends to recur, affecting quality of life and potentially restricting employment or activities 6