Can high-grade right Posterior Cerebral Artery (PCA) steno-occlusive disease with normal flow in the remaining posterior and anterior circulation cause presyncope or syncope?

High-Grade Right PCA Stenosis and Presyncope/Syncope

High-grade right posterior cerebral artery (PCA) steno-occlusive disease alone is unlikely to cause presyncope or syncope when the remaining posterior and anterior circulation shows normal flow. Syncope and presyncope are primarily caused by global cerebral hypoperfusion rather than focal posterior circulation compromise.

Pathophysiology of Syncope and Presyncope

Syncope and presyncope result from temporary global cerebral hypoperfusion that reduces cerebral oxygen delivery. According to guidelines:

• Syncope requires a decrease in blood return to the heart, leading to decreased cardiac output, decreased global cerebral perfusion, and subsequent loss of consciousness 1
• A drop in cerebral oxygen delivery of as little as 20% can cause light-headedness 2
• Presyncope presents with symptoms including pallor, sweating, lightheadedness, visual changes, and weakness 1

Why Isolated PCA Stenosis Rarely Causes Syncope

1. Localized vs. Global Hypoperfusion:

   • Syncope requires global cerebral hypoperfusion affecting the reticular activating system
   • Isolated PCA stenosis typically causes focal posterior circulation symptoms rather than global hypoperfusion 3
   • With normal flow in remaining vessels, collateral circulation typically maintains adequate perfusion

2. Compensatory Mechanisms:

   • Studies show that in patients with moderate PCA stenosis, cerebral autoregulation mechanisms often remain intact 4
   • Dynamic cerebral autoregulation is typically preserved in posterior circulation until stenosis becomes severe 5

3. Typical Symptoms of PCA Territory Ischemia:

   • Visual field defects (hemianopsia)
   • Visual hallucinations
   • Memory deficits
   • Sensory symptoms
   • NOT typically syncope or presyncope 3

Conditions That DO Commonly Cause Syncope/Presyncope

According to guidelines, the common causes of syncope include:

1. Cardiac Causes:

   • Arrhythmias (bradyarrhythmias, tachyarrhythmias) 1
   • Structural heart disease (aortic stenosis, hypertrophic cardiomyopathy) 1
   • Myocardial ischemia/infarction 1

2. Reflex (Neurally Mediated) Syncope:

   • Vasovagal syncope
   • Situational syncope
   • Carotid sinus hypersensitivity 1

3. Orthostatic Hypotension:

   • Decreased central venous pressure
   • Volume depletion
   • Autonomic dysfunction 2

4. Vascular Steal Syndromes:

   • Subclavian steal syndrome can cause syncope through reversal of vertebral artery flow 1
   • However, this affects the entire posterior circulation, not just one PCA

When PCA Disease Might Contribute to Syncope

In rare circumstances, PCA disease might contribute to syncope if:

1. Multiple Vessel Involvement:

   • When PCA stenosis is part of more extensive vertebrobasilar disease
   • When combined with significant stenosis in other cerebral vessels 6

2. Hemodynamic Failure:

   • Severe PCA stenosis with ipsilateral systolic flow velocity >85 cm/second may indicate hemodynamic failure 7
   • This could potentially contribute to syncope if combined with other hemodynamic stressors

3. Poor Collateral Circulation:

   • In patients with inadequate collateral flow, focal symptoms may be more pronounced 3

Clinical Approach

If a patient presents with presyncope/syncope and has isolated high-grade right PCA stenosis:

1. Look for more common causes of syncope first:

   • Cardiac evaluation (ECG, Holter monitoring, echocardiogram)
   • Orthostatic vital signs
   • Detailed history for vasovagal triggers

2. Consider PCA stenosis as contributing factor only if:

   • Symptoms occur with positional changes affecting posterior circulation
   • Visual symptoms precede presyncope
   • Other causes have been ruled out
   • Multiple vascular territories are affected

Conclusion

While high-grade PCA stenosis is a significant finding that requires appropriate management to prevent stroke, it is rarely the primary cause of presyncope or syncope when isolated and when the remaining cerebral circulation is normal. Clinicians should focus on more common causes of global cerebral hypoperfusion when evaluating syncope.