An insertable cardiac monitor (ICM), or implantable loop recorder, is a device implanted in the chest subcutaneous tissues. It can be activated either automatically or by the patient/family member to store the electrocardiogram (ECG) recorded during an event [1]. The latest generation devices enable remote monitoring and facilitate early diagnosis by allowing daily automatic and patient-activated transmissions [2].

ICMs can play an important role in the detection of rare arrhythmias and in the diagnostic workup of syncope [3] and palpitations [4].

This study sought to evaluate the outcome of ICM implantation in a large, heterogeneous cohort of pediatric and young adult patients.

All consecutive patients who underwent ICM implantation between January 2010 and December 2019 were included in the study.

This study complies with the Declaration of Helsinki and was approved by the local ethics committee. Informed consent was obtained from all patients or their guardians.

All patients underwent ICM implantation after a comprehensive non-invasive evaluation [5, 6] including clinical visit, standard 12-lead ECG, Holter monitoring, exercise test, external loop recorder and imaging studies as appropriate. Additionally, an electrophysiologic study (EPS) was often performed in patients with suspected tachyarrhythmias or when the evaluation of the sinus node and atrioventricular node-His bundle conduction was required [7]. In patients 15 kg of weight, transesophageal EPS was performed by applying the same protocol when only atrial pacing was needed.

Since the new millennium, ICMs have been implanted in children and adolescents, with a diagnostic yield of 50–71% (9–13). ICMs have been found to be more diagnostic in symptomatic (94%) than asymptomatic patients (30%) [13]. However, a substantial number of symptoms remained unexplained [12]. Previous studies showed that complications (pocket erosion/infection) occurred in up to 7% of patients [10, 11, 12, 13]. The longest median follow-up durations were 18–19 months [10, 11].

Over the past decade, the new miniaturized ICMs have become available [8, 14]. Placidi et al. [8] first described the results of this new devices implanted in very young and small children. In that study, overall diagnostic yield was 47% but pocket erosion/infections occurred in 9% of patients [8]. In 2019, Bezzerides et al. [15] described 133 patients with ICMs and found a diagnostic yield of 58%, with positive diagnoses in 40% and negative diagnoses in 60% during a median follow-up of 12 months. Complications occurred in 4.5% of patients. Overall, 22% of patients received device treatment, 12% medication change and 6% EPS/ablation [15].

To the best of our knowledge, the current study reports the results of the largest cohort of pediatric and young adult ICM patients with the longest median follow-up (31 vs. 18 months [10]. The data obtained showed an overall diagnostic yield of 63%, which was higher in symptomatic patients (89%), in patients without structural heart disease (71%) and in those with CHD (61%). These findings are consistent with those reported for the pediatric population, with an overall diagnostic yield of 63% (vs. 58–67%) [9, 12, 15], a diagnostic yield in symptomatic patients of 89% (vs. 90%) [13], and in those with syncope of 70% (vs. 72%) [12]. The diagnostic yield was higher for patients with palpitations (76% vs. 25–43%) [12, 15]. This finding may be due to the longer follow-up.

Negative arrhythmic diagnoses were generally more frequent than positive diagnoses, that is, symptoms occurred without any arrhythmia registered. Interestingly, in patients who underwent ICM replacement for further monitoring, the diagnostic yield was lower and positive diagnoses were more frequent than negative arrhythmic diagnoses. The main symptoms were syncope and palpitations, which are common symptoms in children and adolescents. Cardiac syncope occurred in 5%, and undefined syncope in 3% of patients referred to a Pediatric Syncope Unit [16]. In the current study, symptomatic patients had a high diagnostic yield especially in the presence of palpitations. The diagnostic yield rose to 80% when patients had both syncope and palpitations. Conversely, presyncope showed a lower diagnostic yield. This result is new, not being reported in previous studies that included only few cases of near syncope [9, 12].

Our previous data revealed a 100% diagnostic yield of ICM in children with palpitations and negative EPS [8]. Data from this study in a larger population followed up for a longer period confirmed that high diagnostic yield. A negative arrhythmic diagnosis was established in the majority of patients. A positive diagnosis was made in few patients and was mainly due to atrio-ventricular node reentry tachycardia, which should be considered in case of suspicious symptoms and when other examinations, including EPS, proved inconclusive [17].

The presence of arrhythmias leading to ICM implantation was not significantly associated with a higher diagnostic yield. However, bradyarrhythmias (AVB and SND/sinus pauses) and SVT showed the best results.

AVB was the main arrhythmia leading to implantation. Patients with high-degree AVB showed mainly positive diagnoses, most often leading to pacemaker implantation (Table 2). Patients with low-degree AVB showed mainly negative arrhythmic diagnoses, requiring no treatment. These findings show that a definite diagnosis and appropriate therapy could most often be established in patients with high-degree AVB, even if asymptomatic. In patients with low-degree AVB, in whom ICM was more often implanted than in high-degree AVB because of symptoms, negative diagnoses were frequent, thus excluding the need for therapy and allowing patient’s reassurance. This is the case of symptomatic patients (syncope/presyncope, etc.) in whom ECG recording showed 1st degree or Mobitz 1 2nd degree AVB, but also unexpected asymptomatic (for example, nocturnal) 2:1, 3:1 or paroxysmal complete AVB. In these cases, ICM monitoring often showed that symptoms were not correlated with arrhythmias and these higher-degree AVB episodes did not require pacemaker implantation [18]. Conversely, episodes of symptomatic intermittent high-degree AVB led to pacemaker implantation.

The diagnostic performance was lower for PVCs and VT. In case of PVCs, this might occur because symptomatic patients with PVC had often symptoms not related to PVC, and monitoring did not show more severe arrhythmias. Furthermore, the Medtronic Reveal LINQ${}^{\text{TM}}$ cannot automatically detect or count the PVC, but PVC episodes can be detected by patient-activated recording. On the contrary, in case of VT, ICM programming can justify the low diagnostic yield: if VT rate is lower than the tachycardia limit, asymptomatic episodes might be undetected. Conversely, if the tachycardia limit is too low, sinus tachycardia can saturate the ICM memory and the device might not record true tachycardias.

Sensing defects can occur as well. During asystolic pauses, ventricular oversensing (muscular movements, noise artifacts) may cause underestimation of pause durations (Fig. 3). A pause episode is detected by the device when the interval between the last ventricular complex before and the first ventricular complex after the pause exceeds the planned interval. The device terminates a pause episode when it records 12 ventricular markers following its detection. However, if there are further pauses in an episode without 12 ventricular markers in between, the device will consider only the duration of the first pause. The further pauses can be detectable on ECG but are not included in the duration counter. For this reason, patients/parents were instructed not only to register the symptom but also to make a patient-activated transmission as soon as possible after an episode.

Fig. 3.

ICM findings: oversensing. (A) Sinus arrest. Oversensing of noise, registered as ventricular activity (ventricular sensing, VS and fibrillation sensing, FS, that concluded the automatic detection of the pause, underestimating its duration). (B) 3rd degree AVB with long RR pauses. Rare oversensing of P waves detected as ventricular sensing (VS).

Among structural heart disease, CHD patients showed the highest diagnostic yield (61%), whereas those with cardiomyopathies the lowest. This finding seems quite consistent with that previously reported in pediatric and adult CHD patients (71%) [10] and higher than that found in adult CHD patients (41%) [19].

Conversely, channelopathies did not show a high diagnostic yield, as reported in adults [20]. Nowadays ICMs can be implanted in asymptomatic children with severe arrhythmias, including LQTS, CPVT, Brugada syndrome, or variants of unknown significance. This approach is new but justified by the small device dimensions and by the possibility of obtaining an early recognition of dangerous arrhythmias and rhythm monitoring during unclear symptoms. Therefore, this approach can guide management, and continuous cardiac monitoring may provide psychological protection to parents. However, it was shown that symptoms were not reliable surrogates for arrhythmia in children with inherited arrhythmia syndromes [21].

This is a retrospective, single-centre study. Population selection might be biased by the fact that our institution is a third level centre and a Pediatric Syncope Unit, and acts as a referral centre for syncope/presyncope. This might result in higher proportions of patients with this symptom compared with a real-world scenario. Although the number of patients is quite large for a pediatric cohort, subgroup numbers may be too small for robust comparisons. Albeit some analyses were performed on the results of EPS and ICM monitoring, the study was not intended to compare or demonstrate a superiority of any of these procedures.

In a large pediatric population after long-term follow-up, ICMs showed a high diagnostic yield in the overall cohort, which was significantly higher in (1) symptomatic patients, mostly with palpitations, (2) patients without structural heart disease, and (3) patients with syncope rather than those with presyncope.

Patients with positive/negative diagnoses were significantly older than those without an arrhythmia diagnosis. However, there were no significant differences according to age subgroups.

Although most patients had negative EPS before implantation, ICMs were diagnostic in a high proportion of patients. The diagnostic yield was higher in CHD patients than in patients with cardiomyopathies and channelopathies. Prolonged monitoring after ICM replacement did not increase the diagnostic yield.

MSS and FD—methodology; MSS, FD, LP, CDM, DR, IC, MCI, CP, IT, FAS, MC—validation; MSS and FD—visualization; MSS, FD, LP, CDM, DR, IC, MCI, CP, LR, IT, FAS, MC—formal analysis, investigation and data curation; MSS, FD, LP, CDM, DR, IC, MCI, CP, LR—writing - original draft preparation; MSS, FD, LP, CDM, DR, IC, MCI, CP, LR—writing - review and editing.

The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review as part of the Ricerca Corrente RRC-2020-2366948.

The authors thank Elisa Del Vecchio for her valuable collaboration in the editorial revision.

This research received no external funding.

The authors declare no conflict of interest.