Patent Foramen Ovale

Normal Fetal-Postnatal Transition

• During fetal life, the foramen ovale functions as a one-way valve allowing shunting of oxygenated umbilical venous blood from the right atrium to the left atrium, bypassing the fluid-filled lungs
• At birth, with the first breath and expansion of lungs, pulmonary vascular resistance decreases dramatically while systemic vascular resistance increases, reversing the pressure gradient across the atrial septum
• The septum primum acts as a valve, pressing against the ostium secundum and functionally closing the foramen ovale
• In approximately 25% of adults, anatomic fusion of the septum primum to the limbus of the foramen ovale fails to occur, leaving a potential pathway for shunting

Key Mechanism 1: Paradoxical Embolism and Right-to-Left Shunting

• Venous thrombi (typically from deep veins of lower extremities) normally lodge in the pulmonary circulation and are filtered by lung vasculature
• In PFO, when right atrial pressure exceeds left atrial pressure (Valsalva maneuver, coughing, straining), the septum primum is pushed leftward, creating an opening that permits paradoxical embolism
• Blood clots, air, or other thrombi bypass pulmonary filtration and enter systemic circulation, with predilection for cerebral vasculature
• This mechanism explains stroke in young patients without traditional atherosclerotic risk factors

Key Mechanism 2: Shunt Physiology and Hemodynamic Effects

• At rest, left atrial pressure typically exceeds right atrial pressure, so minimal left-to-right shunting occurs across most PFOs
• However, any condition increasing right atrial pressure relative to left atrial pressure (Valsalva, positive pressure ventilation, tricuspid regurgitation, pulmonary hypertension) can reverse the gradient
• The degree of shunting depends on PFO size (measured as tunnel length and maximal separation of septum primum from ostium secundum), with larger defects permitting greater shunt volume
• In most adults with PFO, hemodynamically significant shunting is absent at rest, explaining why most PFOs are clinically silent

Key Mechanism 3: Atrial Septal Aneurysm and Enhanced Thromboembolism Risk

• Atrial septal aneurysm (ASA) occurs in 2-7% of the general population and is present in up to 70% of PFO patients with cryptogenic stroke
• ASA represents excessive tissue bulging (≥10 mm) of the interatrial septum into either atrium, resulting from abnormal development of the septum primum and ostium secundum
• The combination of PFO with ASA creates a larger potential for shunting and a turbulent flow environment that promotes intracardiac thrombus formation and platelet aggregation
• The oscillating motion of the aneurysmal segment may contribute to endothelial injury and thrombogenesis

Anatomic/Developmental Causes

• Failure of normal foramen ovale closure: Incomplete fusion of septum primum to limbus of ostium secundum (embryologic basis for all PFOs)
• Associated congenital cardiac lesions: Ostium secundum atrial septal defect (ASD), sinus venosus ASD, atrial septal aneurysm, Marfan syndrome, Ehlers-Danlos syndrome, Eisenmenger syndrome

Thromboembolism Risk Factors

• Age: Cryptogenic stroke associated with PFO primarily in patients <55 years
• Hypercoagulable states: Antithrombin III deficiency, Factor V Leiden, prothrombin G20210A mutation, antiphospholipid antibody syndrome
• Immobilization: Prolonged bedrest, long airplane/car travel, recent surgery, orthopedic procedures
• Malignancy: Cancer-associated thromboembolism
• Atrial fibrillation: Increased risk of thrombus formation in left atrium
• Recent surgery: Particularly orthopedic procedures (hip/knee replacement, hip fracture repair)
• Dehydration and polycythemia: Increase blood viscosity and thrombotic risk
• Estrogen use: Oral contraceptives and hormone replacement therapy increase VTE risk

Associated Conditions Increasing Shunting

• Pulmonary hypertension: Increases right atrial pressure chronically
• Chronic lung disease: COPD, interstitial lung disease
• Tricuspid regurgitation: Augments right atrial pressure
• Right ventricular dysfunction: From any etiology

Asymptomatic PFO (Majority of Cases)

• Most patients with PFO detected incidentally on echocardiography remain completely asymptomatic
• Average discovery age often >50 years when echo performed for other indications
• No dyspnea, syncope, or exercise limitation attributable to the defect itself

Cryptogenic Stroke (Primary Clinical Manifestation)

• Cardinal presenting symptom: Acute focal neurologic deficit (weakness, speech difficulty, vision loss, sensory changes) in patient <55 years without traditional atherosclerotic risk factors
• Definition: Ischemic stroke without identifiable etiology despite standard evaluation (carotid imaging, cardiac evaluation, coagulation studies)
• PFO found in 40-50% of cryptogenic stroke patients versus 25% of control population
• Stroke typically occurs after a Valsalva maneuver (straining, coughing, Valsalva during defecation) or period of immobilization in preceding days to weeks

Transient Ischemic Attack (TIA)

• Transient focal neurologic symptoms resolving completely within 24 hours (typically within 1 hour)
• May presage recurrent stroke; approximately 10-15% of patients with PFO-related TIA experience recurrent events within 1 year if untreated

Migraine with Aura

• Controversial association; some studies suggest increased prevalence of migraine with visual aura in PFO patients
• Proposed mechanism: paradoxical embolism causing transient cortical hypoperfusion or right-to-left shunting of serotonergic substances
• Currently not standard indication for PFO closure; remains area of ongoing investigation

Dyspnea and Exercise Limitation (Rare)

• Occurs only with significant right-to-left shunting, typically requiring large PFO or concurrent conditions elevating right atrial pressure
• Dyspnea on exertion out of proportion to cardiac or pulmonary findings
• May represent platypnea-orthodeoxia: dyspnea and hypoxemia in upright position improving when supine (caused by positional changes in shunt magnitude)

Decompression Illness ("The Bends") in Divers

• Divers with PFO who ascend too rapidly develop nitrogen bubble formation in venous blood
• Normally, bubbles lodge in pulmonary vasculature; with PFO, bubbles paradoxically embolize to systemic circulation
• Presents with neurologic manifestations (spinal cord syndrome, cerebral arterial gas embolism) in addition to typical joint/skin manifestations of decompression sickness

Physical Examination

• Cardiovascular exam typically normal: No murmurs, thrills, or signs of heart failure in isolated PFO
• Pulse abnormalities absent: Normal arterial pulses and jugular venous pulsations
• Right ventricular heave absent: Unlike ASD, which may produce RV hyperkinesis
• Neurologic exam: Findings depend on location/extent of stroke (weakness, language disturbance, visual field defect, ataxia, gait disturbance)

Transthoracic Echocardiography (TTE)

• Initial screening modality but limited sensitivity (~60%) for PFO detection
• Visualizes interatrial septum, allows assessment of septum primum and ostium secundum
• Can identify atrial septal aneurysm (excessive tissue bulging ≥10 mm into either atrium)
• Doppler may show systolic flow reversal in pulmonary veins suggesting right-to-left shunting
• Advantage: non-invasive, provides complete cardiac assessment, identifies structural abnormalities
• Primary limitation: acoustic windows inadequate in approximately 10-15% of patients

Transesophageal Echocardiography (TEE) with Bubble Study

• Gold standard imaging modality for PFO diagnosis with sensitivity 88-99%
• Provides superior visualization of interatrial septum from posterior esophageal position
• Contrast-enhanced bubble study (agitated saline): Performs Valsalva maneuver while injecting agitated saline IV; appearance of microbubbles in left atrium within 3-4 cardiac cycles confirms right-to-left shunting
• Allows quantification of shunt: negative (no bubbles), trivial (1-10 bubbles per frame), small (11-25 bubbles), moderate (26-100 bubbles), large (>100 bubbles)
• Directly measures anatomic characteristics: tunnel length, size of opening, presence of aneurysm
• Superior assessment of concurrent cardiac pathology (left atrial thrombus, endocarditis, valvular disease)
• Limitations: semi-invasive, operator-dependent, sedation required, cannot be performed with absolute contraindications (esophageal pathology)

Transcranial Doppler (TCD) with Bubble Study

• Non-invasive alternative for quantifying right-to-left shunt
• Detects microembolic signals in middle cerebral artery during injection of agitated saline
• Useful for serial monitoring and grading shunt magnitude
• Less anatomic detail than TEE but no procedural risk

Cardiac Catheterization (Rarely Used)

• Not first-line diagnostic tool but occasionally performed if need to assess pulmonary pressures
• Can perform oxygen step-up test: increased oxygen saturation in right atrium relative to superior vena cava suggests significant right-to-left shunting
• Allows passage of catheter across PFO to confirm pathway

Neuroimaging in Cryptogenic Stroke

• Brain MRI with diffusion-weighted imaging (DWI): Identifies acute ischemic stroke; multiple infarcts in different vascular territories suggest paradoxical embolism source (contrast to single vessel disease pattern)
• MRA of brain and neck: Excludes atherosclerotic disease of carotid/vertebral arteries and intracranial vasculature
• Transesophageal ultrasound or CT: May identify atrial thrombus in left atrium/left atrial appendage

Hypercoagulability Workup (Selected Patients)

• Thrombophilia panel: Factor V Leiden, prothrombin G20210A mutation, antithrombin III level, protein C/S activity
• Antiphospholipid antibodies: Lupus anticoagulant, anticardiolipin antibodies, anti-beta2 glycoprotein-I antibodies
• Complete blood count: Polycythemia as thrombotic risk factor
• Homocysteine level: Elevated homocysteine increases thrombotic risk
• Indicated in: patients <40 years with cryptogenic stroke, recurrent venous thromboembolism, family history of thrombosis

Diagnostic Criteria for PFO

• Anatomic criteria: Probe-patent opening between septum primum and ostium secundum, permitting passage of probe (intraoperative finding) or demonstration of atrial septal discontinuity on TEE
• Functional criteria: Right-to-left shunt demonstrated by contrast-enhanced bubble study with microbubbles appearing in left heart chambers
• Clinical criteria for PFO-related stroke: Cryptogenic stroke in patient <55 years with PFO and no other identifiable stroke etiology, ideally with Valsalva-associated symptom onset

Asymptomatic PFO (Incidental Finding)

• No intervention indicated based on current guidelines (Class IIb recommendation)
• Routine antiplatelet or anticoagulation therapy not recommended for asymptomatic PFO
• Conservative management: Patient education regarding stroke risk factors, cardiovascular risk factor modification (smoking cessation, hypertension control, diabetes management)
• Anticoagulation/antiplatelet therapy NOT indicated even in presence of PFO alone
• Serial follow-up: Periodic clinical assessment; no mandated imaging surveillance required
• Diving restrictions: Should counsel that recreational diving carries increased risk due to decompression-related arterial gas embolism

Cryptogenic Stroke with PFO

• Antiplatelet monotherapy: Aspirin 81-325 mg daily preferred initial agent
• Mechanism: Inhibits platelet cyclooxygenase, reducing thromboxane A2 production and platelet aggregation
• Reduces recurrent stroke risk by approximately 20% compared to placebo
• Well-tolerated, widely available, inexpensive
• Alternative antiplatelet agents: Clopidogrel (75 mg daily) or ticagrelor for aspirin-intolerant patients
• Anticoagulation with warfarin or DOAC: Reserved for specific scenarios (concurrent atrial fibrillation, hypercoagulable state, recurrent events despite antiplatelet therapy)
• Mechanism: Vitamin K antagonist (warfarin) or direct thrombin/Xa inhibitor (DOAC) reduces thrombin generation and prevents thrombus propagation
• Typical INR target 2-3 if warfarin chosen
• Apixaban, dabigatran, rivaroxaban acceptable DOACs for stroke prevention

• Indicated for: Recurrent cryptogenic stroke despite medical therapy, first stroke with high-risk PFO features or concurrent thrombophilia, young patient with strong desire to prevent recurrence
• Procedure: Catheter-based closure using various devices (Amplatzer PFO Occluder, Gore Cardioform, Helix, StarFlex)
• Performed via femoral vein access under TEE guidance
• Device positioned across PFO, expanding to seal opening permanently
• Success rate: >95% anatomic closure rate, but 10-25% residual shunting observed
• Efficacy: RESPECT trial (2017) showed superior stroke prevention with closure versus medical therapy in cryptogenic stroke patients; subsequent trials (REDUCE, Gore REDUCE) confirmed benefit
• Number needed to treat (NNT): approximately 50 patients over 2 years to prevent one stroke
• Timing: Generally performed within 6 months of index stroke event
• Periprocedural anticoagulation: Heparin during procedure, followed by dual antiplatelet therapy (aspirin + clopidogrel) for 6 months, then aspirin monotherapy indefinitely
• Limitations: Approximately 10-25% residual shunting remains, device-related complications (device embolization, atrial perforation in <1%), left atrial thrombus formation (rare but serious)

• Reserved for: Failed percutaneous closure, concurrent cardiac surgery for other indication, anatomically unsuitable PFO
• Approach: Median sternotomy or thoracoscopic approach; primary suture closure or tissue patch closure
• Success rate >99% for complete closure
• Higher perioperative morbidity than percutaneous approach

Migraine with Aura (Controversial)

• No proven benefit of PFO closure for migraine management based on randomized trials
• MIST trial and subsequent studies failed to demonstrate migraine improvement with closure
• Not an indication for PFO closure per current guidelines
• Antiplatelet therapy may provide modest migraine benefit independent of PFO closure

Secondary Prevention After First Event

• Most guidelines recommend initial trial of antiplatelet therapy with close clinical follow-up

Complications of the shunt itself

• Recurrent ischemic stroke/TIA: repeat paradoxical embolism through the same channel, favored by atrial septal aneurysm, large shunt at rest, and a long tunnel. Signaled by new focal deficit; acute stroke is a time-critical emergency — activate stroke pathway and assess thrombolysis/thrombectomy eligibility per AHA/ASA acute ischemic stroke guidance before any PFO-directed decision.
• Systemic paradoxical embolism outside the brain: same mechanism, different bed — acute limb ischemia (pulseless, cold, painful extremity), mesenteric ischemia (pain out of proportion to exam), renal or splenic infarct (flank pain, hematuria). All are surgical/interventional emergencies.
• Cerebral arterial gas embolism in divers: nitrogen bubbles bypass the pulmonary filter. Neurologic deficit within minutes of surfacing is an emergency requiring high-flow oxygen and urgent recompression (hyperbaric oxygen).
• Platypnea–orthodeoxia: upright posture redirects caval flow across the septum; the clue is desaturation that worsens sitting up and improves supine, refractory to supplemental oxygen.
• Refractory hypoxemia with right-heart pressure elevation: pulmonary embolism, pulmonary hypertension, RV infarct, or positive-pressure ventilation reverses the interatrial gradient; suspect when hypoxemia is disproportionate to lung findings.

Complications of treatment

• Post-procedure atrial arrhythmia: device irritation of atrial tissue; new-onset atrial fibrillation/flutter is the most consistently reported closure complication in the randomized closure trials, usually early and often self-limited — but it mandates rhythm evaluation and reconsideration of anticoagulation.
• Device thrombus or infective endocarditis on the device: presents as recurrent embolism or fever/bacteremia; diagnosed by TEE.
• Device embolization, cardiac erosion, or perforation with tamponade: rare but emergencies — hypotension, pulsus paradoxus, distended neck veins; needs immediate echo and pericardiocentesis/surgery.
• Residual shunt after closure, and bleeding from antiplatelet/anticoagulant therapy, including intracranial hemorrhage.

• The stem archetype: young patient (<55), stroke after straining, coughing, or heavy lifting, recent long flight or immobilization, no atherosclerotic risk factors, normal carotids. Think paradoxical embolism through a PFO.
• Single best next step in cryptogenic stroke: complete the embolic source workup before blaming the PFO — TEE with agitated-saline bubble study plus lower-extremity venous duplex to find the DVT, and prolonged ambulatory rhythm monitoring to exclude occult atrial fibrillation (AHA/ASA 2021 secondary prevention guideline). A PFO is only causal after everything else is negative.
• The bubble-timing rule: microbubbles in the left atrium within 3–5 cardiac cycles = intracardiac shunt (PFO/ASD). Delayed appearance after several beats = intrapulmonary shunt — think pulmonary AVM and hereditary hemorrhagic telangiectasia. This is the most commonly tested discriminator.
• PFO is not ASD: PFO has a normal exam. A fixed, widely split S2 with a pulmonary outflow murmur and RV heave means an ostium secundum ASD with left-to-right shunt, not a PFO. Do not put a murmur on a PFO.
• The association examiners want: atrial septal aneurysm plus large shunt marks the high-risk PFO most likely to be causal and most likely to benefit from closure.
• Closure candidacy: percutaneous closure is reasonable in patients roughly 18–60 years old with a nonlacunar cryptogenic stroke and high-risk PFO anatomy, after shared decision-making including the risk of new atrial fibrillation (AAN 2020 practice advisory).
• Common distractors to avoid: closing a PFO for migraine with aura (trials were negative — not an indication); treating an incidental asymptomatic PFO with aspirin or closure (neither is indicated); and choosing anticoagulation over antiplatelet therapy in PFO stroke without atrial fibrillation or a thrombophilia — anticoagulation has not shown superiority and adds bleeding risk.