What are the causes of near syncope (presyncope)?

Causes of Near Syncope (Presyncope)

Near syncope results from the same pathophysiological mechanisms as syncope—transient cerebral hypoperfusion—but without progressing to complete loss of consciousness, and should be evaluated identically to syncope as both conditions carry similar short-term serious outcomes and mortality rates. 1, 2

Pathophysiological Framework

Near syncope occurs when cerebral perfusion decreases below normal (50-60 ml/100g tissue/min) but remains sufficient to prevent complete loss of consciousness. 3, 1 A drop in systolic blood pressure to approximately 60 mmHg typically triggers syncope, but lesser decreases cause presyncope. 3, 1

The mechanism involves either:

• Decreased cardiac output from reduced venous filling, arrhythmias, or structural heart disease 3, 1
• Decreased peripheral vascular resistance from excessive vasodilation or impaired vasoconstriction 3, 1

Primary Etiological Categories

1. Neurally-Mediated (Reflex) Syncope

This is the most common cause, accounting for 21.2% of cases. 1

• Vasovagal syncope: Triggered by emotional stress, pain, prolonged standing, or fear 3, 4
• Situational syncope: Associated with specific triggers including:
  • Micturition (post-micturition) 3
  • Defecation 3
  • Cough or sneeze 3
  • Post-prandial 3
  • Post-exercise 3, 5
  • Swallowing or gastrointestinal stimulation 3
• Carotid sinus hypersensitivity: Triggered by mechanical manipulation of carotid sinuses 3

2. Orthostatic Hypotension (9.4% of cases) 1

Primary autonomic failure syndromes: 3

• Pure autonomic failure 3
• Multiple system atrophy 3
• Parkinson's disease with autonomic failure 3

Secondary autonomic failure: 3

• Diabetic neuropathy 3
• Amyloid neuropathy 3

Drug-induced orthostatic hypotension (particularly common in elderly): 3

• Diuretics 3
• Beta-blockers 3
• Calcium antagonists 3
• ACE inhibitors 3
• Nitrates 3
• Antipsychotic agents 3
• Tricyclic antidepressants 3
• Dopamine agonists/antagonists 3

Volume depletion: 3

• Hemorrhage 3
• Diarrhea 3
• Addison's disease 3

3. Cardiac Arrhythmias

These carry the highest morbidity and mortality risk. 1, 6

Bradyarrhythmias: 3

• Sinus node dysfunction (including bradycardia-tachycardia syndrome) 3
• Atrioventricular conduction system disease 3
• Mobitz type II block 3
• High-grade or complete AV block 3

Tachyarrhythmias: 3

• Paroxysmal supraventricular tachycardias 3
• Ventricular tachycardia 3
• Inherited syndromes (long QT syndrome, Brugada syndrome) 3
• Drug-induced proarrhythmias (torsades de pointes) 3
• Implanted device malfunction 3

4. Structural Cardiac/Cardiopulmonary Disease

These conditions obstruct cardiac output mechanically: 3

• Obstructive cardiac valvular disease (especially aortic stenosis) 3, 5
• Hypertrophic obstructive cardiomyopathy 3, 5
• Acute myocardial infarction/ischemia 3
• Atrial myxoma 3
• Acute aortic dissection 3
• Pericardial disease/tamponade 3
• Pulmonary embolus/pulmonary hypertension 3

5. Cerebrovascular Causes (Rare)

Syncope is an unusual manifestation of neurological processes and should only be pursued if suggested by history or physical examination. 3

• Severe bilateral carotid or basilar artery disease (rarely occurs without focal neurological signs) 3
• Vascular steal syndromes 3
• Increased intracranial pressure (subarachnoid hemorrhage, brain tumors—usually obvious with headache and focal findings) 3

High-Risk Features Requiring Urgent Evaluation

The following features suggest cardiac causes and warrant immediate assessment: 1

• Age >60 years 1
• Male gender 1
• Known underlying cardiac disease 1
• Palpitations prior to episode 1
• Occurrence during exertion 1
• Occurrence in supine position 1
• Family history of sudden cardiac death 1

Special Considerations in the Elderly

Age-related physiological changes predispose elderly patients to near syncope: 3

• Reduced baroreceptor response 3
• Diminished heart rate response to orthostatic stress 3
• Decreased ability to preserve sodium and water 3
• Autonomic dysfunction 3
• Multiple medications with synergistic effects 3
• Presentation may be atypical (falls, gait disorders, amnesia) 3

Clinical Pitfalls to Avoid

Do not dismiss presyncope as less serious than syncope—both conditions share similar prognoses and require identical evaluation. 1

Avoid ordering routine head CT for uncomplicated presentations without head trauma or focal neurological deficits (yield <1%). 1

Do not overlook cardiac causes in patients with neuromuscular disorders (Duchenne dystrophy)—these patients may develop cardiomyopathy, complete AV block, or ventricular arrhythmias requiring pacemaker or defibrillator therapy. 3

Recognize that seizure-like activity can occur with cardiac syncope—upward gaze deviation, asynchronous myoclonic jerks, and brief automatisms result from global cerebral hypoperfusion and do not indicate a neurological cause. 3