Cryptogenic strokes and TIAs are those in which no obvious cause is found by patient history, carotid Doppler studies, or cardiac conditions such as atrial fibrillation, myocardial infarction, or valve diseases.  Patent foramen ovale (PFO), shown on the right, has been associated with cryptogenic stroke allowing paradoxical embolism from the veins to the brain through a right-to-left shunt (RLS).  Currently, PFO is considered when stroke occurs in young people.  However, PFO is found in all ages; 34% of adults in the first three decades of life declining to 20% in the 9th and 10th decades¹ and ranging from 1 to 19mm in diameter.  Cryptogenic stroke patients, assessed with traditional single gate transcranial Doppler (sgTCD), have 12 to 1 odds of having a large PFO compared to a non-stroke group².  Also, migraine patients with aura have a 3 to 1 odds of having a PFO compared to a non-migraine group³.  Conditions for venous thrombosis and pulmonary embolism also exist widely and deep vein thrombosis is a common finding in the vascular laboratory.  Therefore, the conditions for paradoxical embolism are widely prevalent at all ages.

Atrial septal defect (ASD) is a permanent opening through the interatrial septum that often persists into adulthood.  Blood flows back and forth through the defect depending on the back and forth pressure gradient between the atria.  This defect usually places a load on the right ventricle that, however, may be tolerated for many years.  If the mean right atrial pressure is chronically elevated these patients have a significant desaturation of the arterial blood.

The urgency to diagnose PFO and ASD is driven by the advent of safe transcatheter closure devices and the popularity of TCD over invasive transesophageal echocardiography (TEE) has enhanced the search for PFO and ASD.  sgTCD has demonstrated high accuracy in ruling in, and ruling out, PFO when compared to the gold standard, TEE ^{4, 5, 6, 7, 8} .  Particularly, sgTCD was able to detect large shunts, which are more clinically relevant ^{9, 10, 11}.  Using intravenous injections of agitated saline, the suspended bubbles pass through the PFO from the right to the left atrium and are easily detected by TCD as audible chirps and microembolic spectra in the cerebral arteries.  A Valsalva strain (forced expiratory effort against a closed glottis) facilitates passage of the microbubbles through the PFO by raising the pressure in the right atrium over that of the left atrium.  Agitated saline contrast agent has been used safely for many years in echocardiography and TCD.

Newly developed power m-mode TCD ¹² (pmTCD) detects 66% more bubble microemboli than traditional single gate TCD.  This increased delectability allows an expanded 6-level grading scale.  The accuracy of pmTCD against TEE is 96%.  If pmTCD testing is positive, TEE may be indicated to confirm the type and location of the shunt and to detect other cardiac abnormalities.  If pmTCD is negative or Grade I, there is no need to search further for a right to left shunt as a cause of stroke.  Power m-mode TCD is complimentary to TEE by providing its expanded grading system and works well in combination with transthoracic echocardiography.

Clinical Indications for PFO Testing

Patients with a TIA, stroke, or with brain infarctions in whom:

1. No significant carotid artery disease is found i.e. > 50% ipsilateral stenosis,

2. No atrial fibrillation or other heart disease is found which is prone to embolization, and

3. PFO closure is anticipated if it is found.

Additional indications include selected migraine patients, transient global amnesia, brain abscesses, divers with decompression sickness, peripheral embolism, and patients with arterial O₂ desaturation.