Why PFO Increases Stroke Risk
A patent foramen ovale (PFO) increases stroke risk primarily through paradoxical embolism—venous thrombi bypass the pulmonary circulation and travel directly from the right atrium to the left atrium through the PFO, then into the systemic arterial circulation and brain 1.
Mechanism of Stroke in PFO
The pathophysiology involves several interconnected mechanisms:
Primary Mechanism: Paradoxical Embolism
- Right-to-left shunting allows venous clots to bypass normal pulmonary filtration
- When right atrial pressure transiently exceeds left atrial pressure (during Valsalva, coughing, or normal respiratory variation), blood flow reverses through the PFO 1
- Venous thrombi—often from deep vein thrombosis—can then embolize directly to cerebral arteries 2, 3
Additional Mechanisms
Beyond simple paradoxical embolism, PFO-related strokes may occur through 4, 5:
- In situ thrombus formation: Clot develops on the rim of the PFO defect itself or on an associated atrial septal aneurysm
- Atrial arrhythmias: Paroxysmal atrial fibrillation (which may be unrelated to the PFO but coexist)
- Passage of vasoactive substances: Humoral factors that normally undergo pulmonary degradation bypass the lungs
- Hemodynamic changes: Atrial remodeling associated with chronic shunting
Risk Stratification: Not All PFOs Are Equal
Critical caveat: PFO is present in 15-25% of the general adult population 6, 7, yet most people with PFO never have a stroke. The association is strongest in younger patients (<55-60 years) with cryptogenic stroke 2, 8.
High-Risk Anatomical Features
The stroke risk is substantially elevated when PFO occurs with 6:
- Atrial septal aneurysm (>10mm excursion): Odds ratio 15.59 for stroke in patients <55 years 2
- Large shunt size: Substantial right-to-left shunting (>25-30 microbubbles on contrast echo) 6
- Spontaneous shunting: Flow without Valsalva maneuver indicates higher baseline right-to-left pressure gradient
Prothrombotic States Amplify Risk
Hypercoagulable conditions synergistically increase PFO-related stroke risk 9, 10:
- Prothrombin G20210A mutation: Most strongly associated with increased cerebral ischemia in PFO patients
- Elevated D-dimer (>1.0 mg/L): Increases stroke recurrence risk 4-fold in PFO patients (adjusted HR 4.04) but shows no effect in non-PFO patients 10
- Antiphospholipid antibodies and Factor V Leiden: Lesser but measurable associations
Why Some PFOs Don't Cause Strokes
Population studies show no increased stroke risk in asymptomatic individuals with PFO 3, 4. The Northern Manhattan Study (NOMAS) and Olmsted County SPARC study found PFO was not an independent predictor of stroke in unselected populations 3, 4.
This apparent paradox resolves when recognizing that:
- Most PFOs have minimal shunting under normal physiologic conditions
- Stroke requires the confluence of PFO + venous thrombosis + sufficient right-to-left pressure gradient
- In many cryptogenic strokes with PFO, the PFO is incidental rather than causal 6
Clinical Implications for Diagnosis
The diagnosis of PFO-attributable stroke requires excluding all other stroke etiologies 1:
- Prolonged cardiac monitoring (≥30 days) to exclude atrial fibrillation
- Transesophageal echocardiography to exclude aortic atherothrombosis, left atrial thrombus, and cardiac tumors
- Carotid imaging to exclude cerebrovascular disease
- Stroke imaging showing nonlacunar embolic pattern (not small vessel disease) 6
The PASCAL classification system helps determine causality: patients classified as "probable" or "possible" PFO-related stroke show net benefit from closure, while those classified as "unlikely" show net harm (more atrial fibrillation without stroke reduction) 11.
Evidence Quality Note
The mechanistic understanding comes from observational data and case reports showing thrombus-in-transit through PFO 3, 4. However, deep vein thrombosis is identified in only a small minority of PFO-stroke patients 3, 4, suggesting either transient/resolved venous thrombosis or alternative mechanisms in many cases. The strongest evidence for causality comes from randomized trials showing PFO closure reduces recurrent stroke in carefully selected patients 6, supporting (but not definitively proving) the paradoxical embolism hypothesis.