EXPLANATION


Neurally mediated syncope (NMS) or Neurally Mediated Reflex Syncope (NMRS) traditionally refers to syncope owing to an imbalance of sympathetic and parasympathetic tone. Enhanced parasympathetic tone due to vagal nerve stimulation can lead to dramatic slowing of the sinus node or atrioventricular nodes resulting in prolonged sinus bradycardia, junctional rhythm, pauses, sinus arrest, asystole or second or third-degree atrioventricular block. Triggering factors for NMS vary widely and include orthostatic stress, emotional stress, urination, coughing, swallowing, physical exercise, and stimulation of the carotid sinus in susceptible patients.


Typically, a patient with NMS will feel warmth, nausea, and lightheadedness and may appear pale before abruptly losing consciousness. Occasionally, patients do not experience any symptoms prior to syncope. Prior to a diagnosis of NMS, other causes of syncope must be ruled out by means of history, physical examination and appropriate tests. Frequently, the cause of syncope is identified during initial clinical evaluation and no further testing is needed. When the cause of syncope is not clear, a stepwise approach is required for diagnosis. The differential diagnosis for syncope is broad and includes structural heart disease, neurologic disease, cardiac disease, and arrhythmias such as life-threatening ventricular tachycardia, rapid supraventricular tachycardia and prolonged asystole. Prolonged asystole is the most common arrythmia (> 50%) leading to syncope in patients without significant structural heart disease and a normal ECG. If an arrhythmia is suspected, a 24-hour Holter or long-term event monitor can be used to establish the diagnosis. Invasive electrophysiology studies can be performed to distinguish whether syncope is caused by an arrhythmia such as ventricular tachycardia, sinus node dysfunction or intracardiac conduction delay. Often, syncope is due to a mixed pathology.


In the absence of structural heart disease and with a suspicion of NMS, a head-up-tilt-table (HUTT) test is performed to confirm the diagnosis. This is a provocative test with an orthostatic challenge to determine a patient’s susceptibility to syncope. The test is considered positive if symptoms are reproduced along with objective evidence of a sudden decrease in blood pressure or decrease in heart rate.


Medical therapy for NMS consists of beta blockers, alpha agonists, selective serotonin reuptake inhibitors, fludrocortisone, and anticholinergics. Pacemaker therapy can be helpful for patients refractory to medical therapy. According to the American College of Cardiology/American Heart Association, minimal carotid sinus pressure that induces asystole greater than three seconds in the absence of medications that depress sinoatrial or atrioventricular (AV) node conduction is considered a class I indication for pacemaker placement when the result is recurrent syncope. NMS with severe bradycardia reproduced during HUT testing is a class IIb indication for pacemaker insertion. The North American Vasovagal Pacemaker study published in 1993, which involved 54 patients with frequent syncopal spells and positive HUT tests, demonstrated that recurrence of syncope was significantly reduced in patients with pacemakers (22%) versus those without pacemakers (70%). Though pacemaker therapy is used in the treatment of NMS, the decision to implant a pacemaker is difficult and challenging in young patients with structurally normal hearts.


A novel therapy for NMS is cardioneuroablation (CNA) and was first described by Pachon in 2005 in 21 patients with a mean age of 48 years and diagnoses varying from NMS, functional high grade atrioventricular block and sinus node dysfunction. The CNA procedure is performed percutaneously and is based on radiofrequency ablation. The therapy targets vagal efferent postganglionic neurons innervating the sinoatrial (SA) or atrioventricular (AV) node. The postganglionic neurons are primarily located in discrete epicardial structures known as fat pads. Long-term vagal denervation of the atria, SA and AV nodes can be achieved by radiofrequency catheter ablation of these fat pads.


In 2017, Aksu and colleagues conducted a literature review to assess efficacy of CNA for treatment of NMS, which included five observational studies and five case reports. The review demonstrated reduced vagal tone lasting for at least 12 months after the procedure with improved tolerance of repeated head-up tilt testing. An additional study by Pachon et al published in 2011 demonstrated a significant decrease in atropine test positivity following CNA. However, in long-term follow up, patients demonstrated increased atropine test positivity suggesting either partial reinnervation and/or partial ablation.


Although initial reports of CNA have been positive, no formal guidelines exist regarding specific indications or contraindications for the procedure. The 2018 European Society of Cardiology guidelines do acknowledge the novel procedure as a possible treatment modality for NMS, but state that current evidence is insufficient to confirm the efficacy of vagal ganglia ablation. A 2022 prospective, randomized control trial by Piotrowski and colleagues included patients with documented symptomatic cardioinhibitory or mixed vasovagal syncope and positive atropine test. Patients who underwent CNA had less frequent syncope and better quality of life at 24-month follow-up.


Reports of complications from CNA have been sparse. A case report by Kumthekar et al published in 2020 describes a pediatric patient that developed paroxysmal atrial fibrillation after CNA. The arrhythmia was well controlled with medical therapy, and subsequently resolved. A study of long-term outcomes after CNA, published by Sun et al in 2016, noted one patient with inappropriate but transient sinus tachycardia.


Choice A is the correct answer as high-grade third-degree AV block in a structurally normal heart is more likely to be related to NMS and would be a strong indication for CNA. Choices B and C are not typical arrhythmias related to NMS but rather cardiac disease. Choice D, sinus arrhythmia, is not correct as it is a commonly encountered variation of normal sinus rhythm.


REFERENCES


1. Pachon JC, Pachon EI, Pachon JC, et al. "Cardioneuroablation"--new treatment for neurocardiogenic syncope, functional AV block and sinus dysfunction using catheter RF-ablation. Europace. 2005;7(1):1-13. doi: 10.1016/j.eupc.2004.10.003


2. Pachon JC, Pachon EI, Cunha Pachon MZ, et. al. Catheter ablation of severe neurally meditated reflex (neurocardiogenic or vasovagal) syncope: cardioneuroablation long-term results. Europace. 2011;13:1231–1242. doi:10.1093/europace/eur163


3. Zaqqa M, Massumi A. Neurally mediated syncope. Tex Heart Inst J. 2000;27(3):268-272.


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9. Gregoratos G, Cheitlin M, Conill A, et al. ACC/AHA Guidelines for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices: Executive Summary—a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Pacemaker Implantation). Circulation.1998;97:1325-1335. doi:10.1161/01.cir.97.13.1325


10. Connolly SJ, Sheldon R, Roberts RS, et al. The North American Vasovagal Pacemaker Study (VPS). A randomized trial of permanent cardiac pacing for the prevention of vasovagal syncope. J Am Coll Cardiol. 1999;33:16-20. doi:10.1016/s0735-1097(98)00549-x


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