Introduction: The initial evaluation of a transient loss of consciousness consists of, first of all, the recognition of a syncopal versus non-syncopal origin of the episode and, in case of syncope, immediate risk stratification and identification of a potential etiology based on cli-nical presentation, history, physical examination and ECG. Depending on these findings further examinati-ons may be performed.

Methods: A 62 year old woman presented with four syncopes, the first episode having occurred one year before, and the others within the past month. All the episodes were orthostatic, three of them having typi-cal prodrome (light-headedness, sweating) and one episode without prodrome. In addition to this, the pa-tient had short episodes of dizziness without syncope. After the first syncope, she was diagnosed with arte-rial hypertension based on only one measurement for which she was being treated with Carvedilol and Zofenopril, hence the last three episodes occurred un-der this medication. These clinical features suggested a reflex mechanism and the orthostatic hypotension as the precipitating factor during the past month. The initial examination and ECG were normal. The first measure to be taken was the discontinuation of vaso-active drugs. A Tilt test was performed, during which the syncope was reproduced after nitrate provocation. The heart rate response and atrioventricular conduc-tion were atypical: during the initial sinus tachycardia at a rate of 103 bpm a 2:1 AV block with narrow QRS complexes was revealed, followed by 11 seconds of si-nus arrest with junctional escape rhythm. In order to make the differential diagnosis between the 2:1 AV block recorded during the Tilt test and a Mobitz I AV block with atypical Wenckebach periods, an ECG stress test was performed, the block reappeared at a sinus rate of 100 bpm and was symptomatic. A 48 h Holter was recorded which revealed the same AV block. These fin-dings supported the presumption of a paroxysmal atri-oventricular conduction disturbance not related to the reflex parasympathetic stimulation. An electrophysio-logical study was performed (EPS): the AV conduction was relatively normal with AH interval 60 ms and HV interval 58 ms and a Wenckebach point of 370 ms; in-fra-hisian block and transient complete AV block were recorded spontaneously. The treatment consisted of DDD pace-maker implantation (class I indication); at month re-evaluation the patient was symptom-free. Arterial hypertension was confirmed and amlodipine was the treatment of choice.

Results: The first issue to be discussed concerning the case is the initial risk stratification, as the clinical fea-tures were typical for low risk reflex syncope, with the exception of the particular episode without prodrome which, in absence of abnormal ECG, is also considered low risk. In this situation, guidelines recommend no further testing. The second issue is the difficult recogni-tion of infra-hisian conduction block in the presence of narrow QRS complexes. Most frequently, infra-hisian block is associated with broad QRS. The uniqueness of this case was the disclosure of a severe AV conduction disturbance at relatively low heart rate, not related to a parasympathetic drive. The 2:1 AV block during the stress test indicates an infra-hisian localization of the block, despite the narrow QRS. In this case, the infra-hisian block was spontaneous and was unveiled „acci-dentally” during EPS, as the programmed stimulation revealed normal AV conduction intervals.

Conclusions: Careful history taking of all the attacks can offer diagnostic clues. The paroxysmal infra-hisian AV block appears very rarely with narrow QRS com-plexes. An apparently benign transient conduction dis-turbance in older patients may indicate a more severe conduction disorder and not rarely do they have intri-cate mechanisms of syncope.