The role of implantable loop recorders (ILRs) in the diagnosis and risk stratification of cardiac diseases has gained increasing relevance; this has been reflected in international recommendations for their implantation.1,2 In patients with unexplained syncope, the diagnostic yield of a conventional strategy consisting of an external loop recorder, tilt-table testing and an electrophysiological study (EPS) has been shown to be inferior to prolonged monitoring with an ILR.1,3,4

The role of an ILR has also been shown in patients with syncope and bundle branch block in whom paroxysmal atrioventricular block is suspected; patients with cryptogenic stroke, for identifying atrial fibrillation (AF), guiding anticoagulation decisions, and monitoring arrhythmia burden following ablation; patients with presumed epilepsy in whom antiepileptic treatment has proven ineffective; patients with unexplained falls; and patients with red flag symptoms and primary electrical diseases or cardiomyopathies, such as hypertrophic cardiomyopathy (HCM), arrhythmogenic right ventricular cardiomyopathy (ARVC) or dilated and non-dilated left ventricle cardiomyopathy (DCM/NDLVC).1,2,5

In this issue of the Portuguese Journal of Cardiology, Figueiredo et al.6 present their experience regarding the incremental value of ILRs in arrhythmia detection in cardiomyopathies, reinforcing the importance of these devices in risk stratification of patients with structural heart disease. A total of 45 patients were included: HCM was present in 31 patients (69%), DCM/NDLVC in 12 patients (26%) and transthyretin amyloid cardiomyopathy (ATTR-CM) in 2 patients (4%).

Not infrequently, patients with cardiomyopathies fall into a “gray zone” of arrhythmic risk: they do not meet criteria for implantable cardioverter-defibrillator (ICD) or pacemaker implantation, yet they pose significant challenges in clinical decision-making. ILRs allow reclassification of arrhythmic risk based on electrocardiographic findings, whether symptomatic or asymptomatic.

The 2018 ESC guidelines on syncope support its implantation in patients with HCM with recurrent episodes of unexplained syncope who are at low risk of sudden cardiac death (SCD), according to the HCM Risk-SCD score. Similarly, ILR implantation is indicated in patients with ARVC and recurrent unexplained syncope who are at low SCD risk based on a multiparametric assessment. ILRs are also recommended in patients with inheritable arrhythmogenic disorders, such as Brugada Syndrome or congenital Long QT Syndrome, who present with unexplained syncope and are not candidates for ICD implantation.1

The 2022 European Society of Cardiology's Guidelines for the management of patients with ventricular arrhythmias and the prevention of SCD have refined the indications for ICD implantation, incorporating factors beyond left ventricular ejection fraction, such as late gadolinium enhancement, syncope, inducible ventricular tachycardia on EPS and genetic testing results.2 Nevertheless, some patients who do not meet criteria for ICD implantation still fall into a “gray zone”. Figueiredo et al. reinforce the importance of ILR in identifying additional markers that may either prompt a different management strategy or modify the previously calculated risk score. Notably, the 2023 ESC guidelines for the management of cardiomyopathies do not have any recommendations on the role of ILR, highlighting an area where further evidence and clearer guidance may be warranted.7

Likewise, consensus documents on amyloid cardiomyopathy are largely silent regarding the role of ILR in this condition.8 Clinical decisions generally follow the approach used for patients with syncope. However, it is well established that arrhythmic risk in these patients is high, and ILR offers an opportunity for the early diagnosis of arrhythmia with therapeutic implications.9,10

Recent studies using ILRs have demonstrated a high prevalence of arrhythmia and conduction disturbances in patients with Fabry Disease despite normal initial 24-hour Holter monitoring.11

In this context, it is imperative to define structured risk stratification criteria for selecting patients who may derive the greatest benefit from ILR implantation, thereby enabling continuous electrocardiographic monitoring and timely detection of clinically significant arrhythmias.

Figueiredo et al. also showed that the main ILR-guided diagnosis was AF. Although the duration of episodes that warrant anticoagulation remains a matter of debate, in this study, an AF episode was defined as having a minimum duration of 10 minutes in Reveal LINQ™ and LINQ II™ devices (Medtronic), and as ‘only longest episode’ in Confirm Rx™ devices (Abbott), an acceptable and not excessively permissive “time window”, considering that this is a population with structural heart disease and consequently a high thromboembolic risk.

Over a mean follow-up of 19±13 months, 44% of patients had, at least, one ILR-guided diagnosis: most commonly AF, diagnosed in 24% of the cohort. Notably 20% of patients (n=9) received an ICD, with one patient later receiving appropriate ICD therapies. Importantly, as demonstrated in this study, ILR findings have the potential to influence clinical management and decision-making. These include, more specifically, initiation of anticoagulation, modification of antiarrhythmic therapy, device implantation (ICD or pacemaker) or referral to EPS or an ablation procedure.

The main limitation of this study relates to the small number of patients, which may reflect the overly restrictive indications for the implantation of these devices in this very specific subpopulation. As evidence continues to grow regarding their role in arrhythmia diagnosis and arrhythmic risk stratification, the indications are likely to expand.