Editor's Note: This discussion is prompted by Herrera Siklody C. et al. Incidence of Asymptomatic Intracranial Embolic Events After Pulmonary Vein Isolation: Comparison of Different Atrial Fibrillation Ablation Technologies in a Multicenter Study. J Am Coll Cardiol 2011;58:681-688.

Catheter ablation is increasingly used for the treatment of symptomatic patients with atrial fibrillation refractory to antiarrhythmic drugs. Multiple randomized trials have shown a highly significantly superiority of catheter ablation over antiarrhythmic drugs with respect to restoration and maintenance of sinus rhythm restoration.⁽¹⁾ Moreover, catheter ablation significantly improves quality of life. The total complication rate of the procedure is approximately 2 and 5%, life threatening complications are relatively rare.^(2,3) Since the ablation procedure requires an extensive mapping and intense ablation within the left atrium there have always been worries about the thromboembolic potential of the procedure. The causes of thromboembolic events are multiple and include (I) char formation at the catheter tip, (II) mobilization of pre-existing left atrial thrombi, (III) thrombus formation in left atrial sheaths, (IV) the thrombogenic potential of left atrial endocardial lesions, and (V) electrical cardioversion during the procedure.

Although most clinical trials on catheter ablation of atrial fibrillation have reported embolic events, the incidence of this complication is fortunately relatively low. Clinically evident cerebral or peripheral embolic events were reported to occur in less than 1% of patients treated.^(2,3) However, recent studies have reported a high incidence of so-called silent strokes or asymptomatic intracranial embolic events after left atrial catheter ablation procedures.^(4-6) Silent strokes result from an embolic event that does not lead to any acute clinical symptoms. The highly sensitive method of diffusion weighted MRI was applied for ischemic insult detection in these studies. The main and consistent findings of the studies were that (I) compared to clinical apparent events, there was a significantly higher rate of asymptomatic events, and  (II) there were distinct differences in the frequency of such events for different ablation techniques and technologies.^(5,6) In the studies from Gaita et al. using irrigated radiofrequency ablation a 0.4% incidence of clinical apparent stroke but 11% incidence of silent stroke was reported from 234 consecutive patients.⁽⁴⁾

Interestingly, electrical or pharmacological cardioversion during the ablation session was predictive of increased risk for silent stroke. In a subsequent publication Gaita et al. reported on the comparative incidence of silent strokes after ablation with different ablation systems and energy sources.⁽⁵⁾ Silent stroke was observed with all ablation technologies investigated. However, open irrigated radiofrequency ablation and cryo- balloon ablation resulted in a significantly lower incidence of silent strokes compared to non-irrigated mutlielectrode catheter ablation (8.3% vs 5.6% vs 38.9%).⁽⁵⁾ These findings confirmed earlier reports from Herreda Sikoldy et al. showing an incidence of silent stroke of 7.4% after irrigated radiofrequency ablation, 4.3% after cryoballoon ablation, but 37.5% after non-irrigated multielectrode ablation.⁽⁶⁾ Both studies raise strong evidence that “ablation technology and energy source matters” since non-irrigated multielectrode ablation resulted is a significantly higher silent stroke rate as compared to other ablation techniques. Thus, regarding the mechanism(s) leading to silent stroke it is very likely that most thromboembolic events are due to energy application, i.e. charring at the ablation catheter, rather than due to mobilization of pre-existing left atrial thrombi or air embolization.

Commentary

What is the clinical significance of silent stroke? Should we be worried? Although there is no evidence as of now that silent stroke has an impact on cognitive function, the issue needs to be taken serious. A very recent study investigated the question of lesion persistence after ablation by additional MRI studies 2 weeks to one year after ablation. Fourteen patients had a total of 50 lesions (3.4 lesions per patient) detected early after non-irrigated multielectrode ablation of atrial fibrillation.⁽⁷⁾ None of the patients had neurological symptoms. Forty-seven of 50 lesions, all of them small of medium sized, were not detectable at follow-up evaluation. However, in three patients large postablation lesions (< 10 mm) persisted. Although these patients were fortunately also free from neurological symptoms we consider the finding of persistent lesions worrisome. It is currently unknown whether these patients although “asymptomatic” may have a decline in mental, intellectual, or even psychological function during long term follow up. Studies should be undertaken to further evaluate and understand the mechanism(s) leading to silent stroke and to assess strategies to reduce the incidence of silent stroke or even avoid such events. Such strategies may include modification of anticoagulation regimen. Interestingly, all studies that have reported silent stroke were performed after discontinuation of oral anticoagulation and it seems possible that performing ablation on effective oral anticoagulation may reduce silent stroke risk. In addition, technical modifications may be necessary for ablation technologies with a high risk of silent stroke to reduce the incidence.

References

1. Calkins H, Reynolds MR, Spector P, et al. Treatment of atrial fibrillation with antiarrhythmic drugs or radiofrequency ablation: Two systematic literature reviews and meta-analyses. Circ Arrhythm Electrophysiol 2009;2;349-361.
2. Cappato R, Calkins H, Chen SA, et al.: Updated Worldwide Survey on the Methods, Efficacy, and Safety of Catheter Ablation for Human Atrial Fibrillation. Circ Arrhythm Electrophysiol 2010;3;32-38.
3. Dagres N, Hindricks G, Kottkamp H, et al.: Sommer P, Gaspar T, Bode K, Arya A, Husser D, Rallidis LS, Kremastinos DT, Piorkowski C. Complications of atrial fibrillation ablation in a high-volume center in 1,000 procedures: still cause for concern? J Cardiovasc Electrophysiol 2009; 20:1014-9.
4. Gaita F, Caponi D, Pianelli M, et al. Radiofrequency catheter ablation of atrial fibrillation: a cause of silent thromboembolism? Magnetic resonance imaging assessment of cerebral thromboembolism in patients undergoing ablation of atrial fibrillation. Circulation 2010; 122: 1667-1673.
5. Gaita F, Leclercq JF, Schumacher B, et al. Incidence of silent cerebral thromboembolic lesions after atrial fibrillation ablation may change according to technology used: Comparison of irrigated radiofrequency, multipolar non-irrigated catheter and cryoballoon. J Cardiovasc Electrophysiol 2011; 22: 961-968.
6. Herrera Siklody C, Deneke T, Hocini M, et al. Incidence of asymptomatic intracranial embolic events after pulmonary vein isolation comparison of different atrial fibrillation ablation technologies in a multicenter study. J Am Coll Cardiol 2011; 58: 681-688.
7. Deneke T, Shin DI, Balta O, et al. Post-ablation asymptomatic cerebral lesions—longterm follow-up using magnetic resonance imaging. Heart Rhythm 2011; doi:10.1016/j.hrthm.2011.06.030.

Keywords: Atrial Fibrillation, Catheter Ablation, Magnetic Resonance Imaging, Stroke

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