Prolonged ECG monitoring in patients with syncope

Holter monitoring in syncope.

Presenting symptoms over the course of a 24-hour Holter monitoring is rare to occur. The vast majority of patients have a syncope-free interval measured in weeks, months or years, but not days and, therefore, symptom-ECG correlation can rarely be achieved with Holter monitoring.

Presenting symptoms over the course of a 24-hour Holter monitoring is rare to occur. The vast majority of patients have a syncope-free interval measured in weeks, months or years, but not days and, therefore, symptom-ECG correlation can rarely be achieved with Holter monitoring.

In an overview (1) of the results of 8 studies of ambulatory monitoring in syncope, only 4% of patients (range between 1% and 20%) had a correlation of symptoms with arrhythmia. The true yield of conventional ECG monitoring in syncope may be as low as 1-2% in an unselected population.

An asymptomatic arrhythmia detected by Holter monitoring is often used to make a diagnosis by inference, but, without symptom-ECG correlation, there is a potential for ECG findings to be inappropriately maximised leading to unnecessary therapy, e.g. pacemaker implantation in a patient with vasomotor syncope.

Alternatively, there is a potential for symptoms to be inappropriately minimised by physicians if Holter monitoring fails to yield any evidence of an arrhythmia. Holter monitoring in syncope may be of more value if symptoms are very frequent (>1 per week).  Daily single or multiple episodes of loss-of-consciousness might increase the potential for symptom-ECG correlation. However, experience in these patients suggests that many have psychogenic blackouts.  Undoubtedly, in such patients, true negative findings of Holter monitoring may be useful in confirming the underlying cause.

External Loop Recorder in syncope.

Conventional event recorders are external devices equipped with fixed electrodes through which an ECG can be recorded by direct application to the chest wall.  Provided the patient can comply at the time of symptoms, a high-fidelity recording can be made.
Recordings can be prospective or retrospective (loop recorders) or both. Prospective external event recorders have a limited value in syncope because the patient must be able to apply the recorder to the chest during the period of unconsciousness and activate recording. Retrospective ECG allows activation of the device after consciousness has been restored.

External retrospective loop recorders, on the other hand show a relatively higher diagnostic yield in syncope. In one study (2), 25% of enrolled patients had syncope or pre-syncope recorded during the monitoring period up to 1 month. However, since patients usually do not comply with external retrospective loop recorders for more than a few weeks, symptom-ECG correlation cannot be achieved when the syncopal recurrence rate is less frequent. In a  recent study, (3) the external loop recorder was not useful for diagnosis of syncope in patients with 3?4 episoders (more than 2) of syncope during the previous 6 months, no overt heart disease and a negative tilt testing. External loop recorders in syncope may be valuable if symptoms are frequent (>1 per month).

Implantable Loop Recorder in syncope.

The Implantable Loop Recorder is placed subcutaneously under local anaesthesia, and has a battery life of 18-24 months. High fidelity ECG recordings can be made. Retrospective ECG allows activation of the device after consciousness has been restored. Automatic activation is also available in case of occurrence of predefined arrhythmias.

In the initial clinical experience, implantable Loop recorders were used for diagnosis in patients with unexplained syncope at the end of full conventional unsuccessfull work-ups. Symptom-ECG correlation was achieved in 88% of a small series of highly selected patients , within a mean of 5 months of implantation (4). In a larger series (5), correlation between symptoms (syncope or pre-syncope) and  ECG was achieved in 59% of 85 patients within a mean of 10 months of implantation. Syncope-ECG correlation was achieved in 27% of patients and presyncope-ECG correlation in 32%; presyncope was much less likely to be associated with an arrhythmia than syncope and did not prove to be an accurate surrogate for syncope in establishing a diagnosis.

Pooled data from 4 studies (4,5,6,7) for a total of 247 patients with unexplained syncope at the end of a complete conventional investigation show that a correlation between syncope and ECG was found in 84 patients (34%); of these, 52% had a bradycardia or asystole at the time of the recorded event, 11% had tachycardia and 37% had no rhythm variation.

One further study (8) randomised 60 patients with unexplained syncope to “conventional” testing with external loop recorders and tilt and electrophysiologicl testing or to prolonged monitoring with the Implantable Loop Recorder. The results were that a strategy of implantation of the Loop recorder in an initial phase of the work-up is more likely to provide a diagnosis than conventional testing (52% vs 20%). However, patients at high risk of life-threatening arrhythmias, as were those with an ejection fraction <35%, were excluded.

From the initial experience in patients with unexplained syncope, it appears that the Implantable Loop Recorder might become the reference standard to be adopted when an arrhythmic cause of syncope is suspected but not sufficiently proven to allow an aetiological treatment. There are several areas of interest that merit further clarification:

• patients in whom epilepsy was suspected but in whom treatment has proven ineffective (9);
• patients with recurrent and unexplained syncope without structural heart disease when the understanding of  the exact mechanism of spontaneous syncope may alter the therapeutic approach (6);
• patients who have a diagnosis of neurally-mediated syncope when the understanding of the exact mechanism of spontaneous syncope may alter the therapeutic approach (6);
• patients with bundle branch block in whom a paroxysmal AV block is likely despite a complete negative electrophysiologic evaluation (10);
• patients with definite structural heart disease and/or non-sustained ventricular tachyarrhythmias in whom a ventricular tachyarrhythmia is likely despite a completed negative electrophysiologic study (11);
• patients with unexplained falls (12).

The implantable loop recorder carries a high up-front cost of approximately €1.500.  However, if symptom-ECG correlation can be achieved in a substantial number of patients, then analysis of the cost per symptom-ECG yield showed that the implanted device may be more cost-effective that a strategy using conventional investigation (8).

Diagnosis

Whatever the type of ECG monitoring used (Holter, external or implantable loop recorder) the diagnostic criteria are similar. ECG monitoring is diagnostic when a correlation between syncope and an electrocardiographic abnormality (brady- or tachyarrhythmia) is detected.

Conversely, ECG monitoring excludes an arrhythmic cause when there is a correlation between syncope and no rhythm variation. In the absence of such correlations additional testing is recommended with the possible exception of ventricular pauses longer than 3 sec when awake, or periods of Mobitz II or 3rd degree atrioventricular block when awake or rapid paroxysmal ventricular tachycardia in which the diagnosis is considered however likely (13,14). Presyncope may not be an accurate surrogate for syncope in establishing a diagnosis and, therefore, therapy should not be guided by presyncopal findings.

ECG monitoring in syncope – where in the workup?

The role of ECG monitoring in syncope cannot be defined in isolation.  Physicians may be guided by the results of clinical history, physical examination and objective testing, for example, by tilt testing. Knowledge of what transpires during a spontaneous syncopal episode is the gold standard for syncope evaluation. For this reason it is likely that implantable monitors will become increasingly important in syncope and that their use will be anticipated in the diagnostic flow instead of or before many other conventional investigations are carried out.

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.