Introduction

Typical AVNRT is the most common clinically relevant supraventricular tachycardia. Patients with AVNRT usually present with palpitations, shortness of breath, or lightheadedness. Although several case studies reporting syncope in the context of AVNRT with other comorbid conditions such as long QT syndrome, hypertrophic cardiomyopathy, and an enlarged aorta compressing the right atrium resulting in underfilling of the right ventricle during the AVNRT have been published [1,2,3,4], the syncope secondary to AVNRT with a structurally normal heart is uncommon [5]. Previous reports suggest that compared to young, elderly patients had more syncope or presyncope episodes with AVNRT, likely due to frailty and the high prevalence of organic heart disease [6]. In this report, we present a rare case of an elderly patient who experienced a syncopal storm, due to typical AVNRT, which was documented to be associated with profound hypotension in the absence of structural heart disease.

Case details

A 78-year-old man presented with four episodes of syncope within 15 min on the same day, all of which were captured by closed-circuit television (Fig. 1). The first episode was while he sat in the compound of his building. He fell back, and fortunately, his head landed on a grassy patch. He decided to go up to his apartment. As he stood outside the elevator, he abruptly fell back, his head narrowly missing the concrete wall. Inside the elevator, he awkwardly crumpled down to the floor, fortunately without major injury. As he got out of the elevator, he again fell (Additional file 1: Videos 1–4).

Fig. 1
figure 1

Four falls. Left upper panel, sitting in the compound. Right upper panel, waiting for the elevator. Left lower panel, inside the elevator. Right lower panel, while leaving the elevator

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He had a history of hypertension and was on amlodipine. On enquiry, syncope was sometimes preceded by palpitations. The physical examination was unremarkable, without orthostatic hypotension or carotid hypersensitivity. The ECG was unremarkable. Echocardiography revealed a structurally normal heart. A 24-h Holter showed few monomorphic premature ventricular complexes, with one ventricular triplet; there was no pause or bradycardia. The exercise stress test was negative for inducible ischemia. Magnetic resonance imaging with angiography of the brain was normal.

What is the next step?

There was a diagnostic dilemma in this case about whether to proceed with an implantable loop recorder (ILR) or an electrophysiological study. In view of the nasty episodes, it was risky to wait for yet another fall. Because the syncope event had high-risk features and sudden brief palpitations preceded syncope, it was determined that an EP study rather than an ILR would be appropriate in this case. The baseline AH and HV intervals were normal. Surprisingly, typical AVNRT at 190 bpm was easily and repeatedly induced with atrial extra stimuli. During the AVNRT, the systolic BP fell from 160 to 40 mm Hg with the development of presyncope (Fig. 2). The tachycardia was terminable with atrial extrastimuli and with ventricular overdrive pacing. Despite vigorously programmed ventricular stimulation procedures from two right ventricular locations, introducing up to three extrastimuli, and using short–long–short sequences, no ventricular tachyarrhythmia was induced. The slow pathway was successfully modified with radiofrequency ablation. Post-procedure, no more tachycardia was inducible, and at a 6-month follow-up, the patient was asymptomatic.

Fig. 2
figure 2

During the tachycardia, the systolic BP dropped from 160 mm Hg (arrow) to less than 40 mm Hg (star) with the development of presyncope

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Discussion

AVNRT is the most common supraventricular tachycardia. Patients with AVNRT most commonly present with palpitations, sweating, dyspnea, and lightheadedness. A sensation of ‘neck pounding’ is one of the characteristic symptoms of typical AVNRT, due to atrial contraction against the closed tricuspid valve, causing rhythmic abrupt rises in venous pressure, also known as the ‘frog sign.’ Some patients with AVNRT experience diuresis during or after the tachycardia; the mechanism is probably related to an elevated mean right atrial pressure and plasma level of the atrial natriuretic peptide during the arrhythmia. Syncope is an uncommon but ominous presentation of AVNRT. In a study of 167 patients with supraventricular tachycardia who were referred for radiofrequency ablation, 64 had AVNRT; the most common symptom was palpitation (98%), while syncope was the least common (16%) [5].

It was initially thought that a heart rate ≥ 170 beats per minute was the only independent risk factor for syncope. However, this has been contested due to the lack of correlation between reported syncope and tachycardia cycle length [7, 8]. Syncope in AVNRT is more likely to occur at the onset of the arrhythmia due to sudden atrial distention and vigorous contraction against closed AV valves during AVNRT, leading to a strong reflexogenic stimulus resulting in impaired vasomotor function with delayed vasoconstrictive compensation resulting in syncope [9].

In some cases, the cause may be due to neurally mediated (neurocardiogenic) responses rather than a direct result of the tachycardia. In others, concomitant drug therapy (e.g., vasodilators) may undermine the compensatory vascular response [10]. In our case, syncope was likely due to a high heart rate, loss of atrioventricular synchrony, and an impaired autonomic vasomotor response, leading to cerebral hypoperfusion.

In our patient, the syncopal storm with four episodes in quick succession was exceptional. Either he developed recurrent episodes of AVNRT, or it was one prolonged episode with recurrent severe hypotension.

Conclusions

The syncopal storm is a very uncommon presentation of a common arrhythmia (typical AVNRT) in clinical practice. Multiple episodes of syncope in rapid succession are very rare. In most instances, patients will either be in a serious state or will be resting after an episode or two. Our patient, with a structurally normal heart, kept recovering well after every episode to be able to walk till the next episode. Finally, the culprit was AVNRT, which was documented to be associated with critical hypotension. After successful ablation, the patient was asymptomatic at intermediate-term follow-up.