"Its all about the fall", JACC case report revisits the classical dilemma of syncope vs. seizure.

The relevant laboratory tests, ECG, electroencephalogram, echocardiogram and brain magnetic resonance imaging were all unremarkable. Neurological consultation suggested psychogenic nonepileptic seizures (PNES) as the possible diagnosis.

But during the same admission, during telemetry, the patient experienced a witnessed recurrent event. An abrupt atrioventricular (AV) block (AVB) developed, followed by TLOC, and then convulsions. Telemetry reflected the physiology, including motion artifact corresponding to convulsions that occurred after the onset of AVB. AV conduction spontaneously resumed, and the patient awoke in a confused state. (Figure)

Transcutaneous pacing pads were placed, and an emergent transvenous pacing wire was inserted. On the basis of telemetry findings, through exclusion of other causes, a diagnosis of extrinsic idiopathic AVB (EI-AVB) was made. The patient later underwent dual-chamber pacemaker implantation programmed in the Managed Ventricular Pacing (MVP) mode.

At 3 months of follow-up, the patient had no syncope. However, she had 7 ventricular pacing events, with intrinsically conducted ventricular rates of 77 to 98 beats/min and AV delays of 130 to 160 ms immediately before ventricular pacing, consistent with episodes of EI-AVB treated with ventricular pacing. The MVP feature in this patient elucidates the necessity of pacemaker therapy in a patient with a very low ventricular pacing burden.

Despite markedly different pathophysiological characteristics, the clinical presentations of syncope and seizure can be difficult to differentiate from each other, particularly when patient-provided history is limited by loss of consciousness and episodes may be unwitnessed. In fact, misdiagnosis in TLOC may be as prevalent as 30% and is associated with increased morbidity.

Paroxysmal AVB can be divided into 3 subtypes:

1. Extrinsic vagal AVB (EV-AVB), also known as vasovagal syncope or neurocardiogenic syncope),

2. Intrinsic AVB (I-AVB, also known as Stokes-Adams syndrome) and,

3. EI(Extrinsic intrinsic) -AVB.

The patient's telemetry tracings excluded the more common EV-AVB and I-AVB. Although there was subtle PR interval prolongation on the last conducted beat, there was no PP interval prolongation before the block, as would be found in EV-AVB.

Contrary to what is often observed in I-AVB, there was no evidence of the following: native conduction disease (e.g., bundle branch block); an initiating premature atrial or ventricular complex, causing conduction tissue to become unexcitable secondary to slow partial depolarization and sodium channel inactivation (i.e., phase 4 block); or a subsequent ventricular complex resetting the membrane potential, thus prompting resolution of AVB.

Finally, the pattern observed was not consistent with phase 3 block, where a premature beat or faster atrial rate leads to AVB. Thus, the diagnosis of EI-AVB was made.

EI-AVB is an underrecognized cause of TLOC that occurs in patients with low baseline circulating plasma adenosine levels (APLs). The low APL renders patients hypersensitive to surges in endogenous adenosine, which induces AVB. A higher level of suspicion of uncommon causes, long-term ambulatory monitoring, and improved predictive tools to help guide work-up for TLOC may help reduce misdiagnosis and underdiagnosis.

Source: JACC Case Reports: Epublished DOI: 10.1016/j.jaccas.2021.04.011