Stroke is a major cause of disability and death. Many strokes have a likely explanation, such as carotid disease, poorly controlled hypertension, diabetes, hyperlipidemia, smoking, inherited blood-clotting conditions and atrial fibrillation (AF).
However, a quarter of patients have none of the defined risk factors and yet face the consequences of stroke or stroke with resolution of symptoms within 24 hours (transient ischemic attack, or TIA). These strokes with no specific risk factor or cause found (crypotegic strokes) create anxiety, as it is unclear what can be done to prevent additional strokes. Cryptogenic strokes have been shown to have a higher rate of recurrence than other strokes.
AF causes approximately one-sixth of all strokes, and anticoagulation substantially lowers the risk of stroke in patients with AF. AF can be intermittent, short-lived and asymptomatic, making it challenging to identify the arrhythmia with conventional electrocardiography, ambulatory monitoring and a 30-day event recorder. However, if a patient is detected to have AF and has a CHA2DS2-VASc score higher than 2, then there is compelling evidence that anticoagulation, with either warfarin or novel anticoagulants, reduces the stroke risk by approximately 65 percent.
A study by Jeff S. Healey, M.D., and others conducted in patients with implanted pacemakers documented that stroke risk was significantly increased in those individuals with AF lasting more than six minutes. The study was published in The New England Journal of Medicine in 2012.
Asymptomatic AF as a risk factor for stroke is frequently considered but difficult to prove. "An implanted device is vastly superior to surface monitoring in detecting AF owing to continuous analysis over years, generally better signal-to-noise ratio and lack of compliance issues frequently encountered with the use of external recorders," says Komandoor Srivathsan, M.D., director of Heart Rhythm Services at Mayo Clinic's campus in Arizona. "Even among implantable devices, intracardiac electrogram acquisition through a pacemaker or an implantable cardiac defibrillator has more sophisticated rhythm recognition capabilities because of the spatial clarity of the obtained electrogram when compared with the subcutaneous recording of an implanted loop recorder."
Many cryptogenic strokes are thought to be related to unrecognized AF. The use of implantable cardiac monitoring (ICM) to identify asymptomatic AF and associated stroke risk was explored in a study by Tommaso Sanna, M.D., and others, published in The New England Journal of Medicine in 2014. This study indicated that an additional 10 percent of patients with AF can be detected at 12 months of ICM compared with other monitoring techniques, and more widespread use of ICM in individuals with cryptogenic stroke might allow for earlier identification of AF and anticoagulation treatment.
This hypothesis is tempered by the following issues:
- Ten patients have to have ICM to detect one additional patient with AF. Assuming therapeutic anticoagulation is of equal benefit to symptomatic AF patients at the time of stroke, approximately 100 patients will have to be implanted with a device to prevent one recurrent stroke (currently cost-prohibitive).
- Infection, extraction and reimplantation add to morbidity and cost.
- Device detection issues and the occurrence of asymptomatic 30-second episodes of AF remain a concern.
- In the same 2014 issue of The New England Journal of Medicine, David J. Gladstone, M.D., Ph.D., and others reported that a 30-day event recorder was able to detect AF in a significant number of patients and minimized the need for ICM.
- Competing risk factors are frequent in the age group of patients who experience stroke, and the etiology of a specific stroke ascribable to a cause is nebulous in many instances.
- The majority of cryptogenic strokes remain cryptogenic even after these investigations.
Nevertheless, this study adds important information that some cryptogenic stroke patients likely have AF, and AF should be high on the list of differential diagnoses in these individuals. The prevention of recurrent strokes in these patients is feasible through enhanced detection provided by ICM, but the subset of patients who would benefit from intensive monitoring needs to be better defined.
Additional studies will need to confirm that anticoagulation is of equal benefit to those with cryptogenic stroke due to asymptomatic AF prior to its widespread adoption. The ICM size has recently been greatly reduced to the size of a large capsule, and implantation techniques have become simpler. Implantation of these smaller devices may eventually become an in-office, outpatient procedure, which would change the cost-benefit equation.
- Implantable cardiac monitoring with a subcutaneous loop recorder can detect asymptomatic, subclinical atrial fibrillation in 10 percent of patients with cryptogenic stroke at one year.
- The rate of detection of asymptomatic atrial fibrillation in cryptogenic stroke through implantable cardiac monitoring is low.
- The number of patients who need to receive implants to detect atrial fibrillation in cryptogenic stroke and prevent recurrence is currently too high to be cost-effective.
For more information
Healey JS, et al. Subclinical atrial fibrillation and the risk of stroke. The New England Journal of Medicine. 2012;366:120.
Sanna T, et al. Cryptogenic stroke and underlying atrial fibrillation. The New England Journal of Medicine. 2014;370:2478.
Gladstone DJ, et al. Atrial fibrillation in patients with cryptogenic stroke. The New England Journal of Medicine. 2014;370:2467.