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Myotonic dystrophy is a hereditary disorder with systemic involvement. Cardiac involvement occurs in 80% of MD1 patients and it often precedes the involvement of skeletal muscle. Cardiac involvement in patients with MD1 occurs as a degenerative process, with progressive fibrosis and fatty replacement of the myocardium, which involves not only the specialized conduction system but also areas, initially unaffected, of the atrial and ventricular myocardium. This anatomy-pathologic substrate may, on the one hand, facilitate the development of cardiac conduction diseases, ventricular tachycardia (VT), and sudden cardiac death (SCD) on the other hand, it may be responsible for ventricular dyssynchrony, leading to cardiomyopathy with systolic dysfunction.
- myotonic dystrophy
- cardiovascular disease
- neurological disease
- sudden cardiac death
- arrythmogenic risk
1. Cardiovascular Involvement in MD1
1.1. Conduction System Disease
An impairment of the conduction system is common cardiac abnormality in MD1 patients. First-degree atrioventricular block (AVB) (28.2–34.1%) and QRS complex >120 ms (18.4–19.9%) are the most frequent abnormalities found [18,19]. These abnormalities identifies a subgroup of MD1 patients in need of cardiac pacing, because they are considered independent predictors of a prolonged His-ventricle (HV) interval ≥70 ms in electrophysiological study (EPS) [20,21].
1.2. Atrial Fibrillation (AF)
AF has a prevalence of 11% in MD1 patients, about 70-fold higher than the general population [22]. If we consider cardiac-implanted electronic device-detected AF events, this value increases to 25% [23,24,25]. There are several risk factors which have been identified as predictors of AF development: male sex, low left ventricular ejection fraction (LVEF), electro-mechanical echocardiographic and electrocardiographic abnormalities [22,26]. MD1 patients with AF have higher overall mortality than those without AF [22]; however, the association with SCD is still controversial. Therefore, non-sustained supraventricular tachycardia is reported with a prevalence of 37%.
1.3. Ventricular Arrhythmias
The prevalence rates of non-sustained and sustained VT were 2.2% and 0.8%, respectively [27]. In MD1 patients who are in need of permanent cardiac pacing, a previous episode of non-sustained VT is considered the only independent predictor of sustained VT [19]; therefore, it is used as a criterion for the preference of an implanted cardioverter defibrillator (ICD) over a pacemaker (PM) [28,29]. For this reason, the early identification of a non-sustained VT in MD1 patients is an aim in the management and prevention of SCD [30]. Several studies have shown evidence of increased dispersion of ventricular repolarization (QTc dispersion, JTc dispersion, transmural dispersion of repolarization, QT variability index) and sympathovagal balance in patients with MD1 (heart rate variability) suggesting the potential interest of these measures to predict ventricular arrhythmias [31,32,33,34].
1.4. Sudden Cardiac Death
Pneumonia and cardiac arrhythmias are the most frequent primary causes of death. [35] SCD has an annual incidence of 0.53–1.16% [36,37], three-fold higher in MD1 patients than in age-matched healthy controls. Even if the mechanisms leading to SCD remain controversial, extreme bradycardia sch as the complete AVB, asystole, and VT may represent the most prevalent cause of SCD in MD1 patients [38].
Independent predictors of SCD are (i) clinical diagnosis of atrial tachyarrhythmia and electrocardiogram (ECG) with one of the following features: any rhythm other than sinus rhythm, PR interval ≥ 240 ms, QRS duration ≥ 120 ms, second- or third-degree atrioventricular block [37,39], age, family history of SCD, and left bundle branch block [40,41].
1.5. Cardiomyopathy and Heart Failure
Different from arrhythmias, little is still known about the epidemiology of left ventricular (LV) dysfunction and heart failure (HF) among MD1 patients [42]. The prevalence of LV systolic dysfunction (LVSD) (EF < 54%), assessed by trans-thoracic echocardiography (TTE), ranged from 0% to approximately 21% [42]. The causes of LVSD are not completely understood; however, they might include intra-ventricular (IV) and atrioventricular (AV) conduction time delay, atrial or ventricular arrhythmias, and ventricular myocardial fibrosis, until a dilatated cardiomyopathy. MD1 patients with prolonged PR or QRS intervals showed a four times higher risk of developing LVSD or HF [43,44]. Among MD1 patients with AF, the prevalence of LVSD accounted for up to 46% [42]. Contrast-enhanced cardiac magnetic resonance imaging (MRI) studies have detected myocardial fibrosis in 13–40% of MD1 patients [45,46,47].
Data regarding LV diastolic dysfunction (LVDD) in MD1 patients are quite lacking. To date, mild diastolic dysfunction has been observed in 5–50% of MD1 patients [48]. The diastolic dysfunction in MD1 might be related to AF, fibrotic degenerative changes in the myocardium (likely affecting LV relaxation), and impaired calcium metabolism in cardiomyocytes. No association between LVDD and AV or IV conduction defects has been observed [49]. Whether AV/IV conduction defect may cause LV mechanical impairment or whether both electrical and mechanical impairments may be the common result of fibrosis of the myocardium and conduction system still needs to be clarified.
The prevalence of symptomatic HF in MD1 subjects ranges from 0% to 7.1% [37]; however, HF symptoms should be underestimated due to the limited level of activity of MD1 patients. The early diagnosis of HF disease is of pivotal importance since it increases the risk of all-cause death by four times, and the risk of cardiac death by six times [44].
Therefore, because there are no trials which demonstrate any benefits, it is reasonable that treatment for HF should be started early. In particular, angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor antagonists (ARB) could be of particular benefit in MD1 for anti-fibrotic properties [50] with LVEF < 50%. Beta-blockers should be reserved for patients without AV conduction abnormalities or recipients of PM and/or ICD; the up-titrate drug dosage should be applied based on individual response and toleration. Cardiac resynchronization therapy is recommended for patients with persistent symptomatic HF (New York Heart Association functional class III) due to LVSD (LVEF < 35%) with large QRS (>150 ms) with left bundle branch block pattern and a normal sinus rhythm while receiving optimal guideline-directed medical therapy [51,52].
1.6. Hypotension
It is generally recognized by clinicians that MD1 subjects have low blood pressure (BP) values. However, only a few non-systematic studies have shown that consecutive MD1 patients have significantly lower BP values than healthy control subjects [53]. It is still not clear whether low BP may be related to the pathophysiology of the disease or the autonomic cardiac dysfunction, predominantly parasympathetic, that is common in MD1 subjects, or if it may be a specific complication of the disease related to the genetic mutation [54]. However, low BP values have recently been demonstrated to be a marker of disease severity and to contribute, when added to other clinical, electrocardiographic, and respiratory parameters, to stratifying MD1 patients at risk of death [55].
1.7. Stroke and Systemic Embolism
The prevalence of both symptomatic and asymptomatic ischemic strokes in MD1 patients was about 6.5% [56]. The AF/flutter was found in 55% of MD1 patients with ischemic stroke. All patients with stroke had CHADS2 [Congestive heart failure, Hypertension, Age ≥ 75 years, Diabetes mellitus, prior Stroke or TIA or thromboembolism (doubled)] and CHA2DS2-VASc [Congestive heart failure, Hypertension, Age ≥ 75 years (doubled), Diabetes mellitus, prior Stroke or TIA or thromboembolism (doubled), Vascular disease, Age 65 to 74 years, Sex category; HF: heart failure; TIA: transient ischemic attack; TE: thromboembolism; MI: myocardial infarction; PAD: peripheral artery disease] scores higher than two [57]. An expert consensus opinion of the AHA for the management of MD1 patients suggests the use of the CHA2DS2-VASc score to stratify thromboembolic risk; however, it also outlines the need to carefully consider their increased fall risk due to underlying neuromuscular disease and muscle weakness. Since studies comparing vitamin K antagonists (VKAs) and direct oral anticoagulants (DOACs) in this clinical setting are lacking, a careful evaluation of renal function is warranted, eventually based on the dosage of cystatin C, because serum creatinine may be low to non-detectable in a setting of low muscle mass (which is not uncommon in MD1 subjects) [51,58].
2. Non-Invasive Cardiac Evaluation
To optimize the clinical management of MD1 patients, neurologists should identify referent cardiologists/electrophysiologists with expertise in neuromuscular disorders early [51].
Cardiologic evaluation is comprehensive of ECG, TTE, and 24-h Holter ECG monitoring and is highly recommended at the time of disease diagnosis. A cardiologic clinical history investigation should focus on eventual warning symptoms, including healing, dizziness, pre-syncope, syncope, or breathlessness. Moreover, even for completely asymptomatic subjects, annual cardiologic visits with ECG are recommended, since MD1 is a progressive disease [52]. Given the increased prevalence of AF in MD1 patients and its association with higher overall mortality, we suggest performing careful electrocardiographic monitoring by 24-h Holter ECG at least annually in the overall MD1 population and daily remote monitoring (RM) of those with cardiac implantable electronic devices (CIEDs). For MD1 patients at increased risk of AF, according to electrocardiographic and echocardiographic risk parameters, an external loop recorder should be considered.
2.1. Twelve Lead ECG
Twelve-lead ECG is an essential tool for risk stratification of life-threatening arrhythmic disorders in MD1 patients. It is indicated in all patients upon confirmation of MD1 diagnosis, and annually thereafter, due to the risk of disease progression [51,52]. A PR interval > 200 ms and/or QRS duration > 100 ms should be an indication of the need to perform an EPS for detecting a prolonged HV interval (>70 ms) in need of cardiac pacing [20,59,60]. However, it should be noted that up to 66.1% of MD1 patients with these electrocardiographic findings may have normal HV intervals [61]. Moreover, in MD1 patients with QRS > 120 ms and PR > 240 ms, a pacemaker may be considered to reduce the risk of SCD [29,37].
2.2. 24 h Holter ECG Monitoring
Ambulatory electrocardiographic monitoring is a useful tool for the identification of paroxysmal second or third-degree AV blocks, or intermittent bundle branch blocks, that do not appear at rest ECG. It is indicated at the time of MD1 diagnosis and in the case of the occurrence of either ECG abnormalities (AV or intra-ventricular blocks) or symptoms including heeling, dizziness, pre-syncope, and syncope [52]. Moreover, it may be useful to identify asymptomatic episodes of non-sustained VT or paroxysmal AF, which may impact the patients’ prognosis and need careful management [19,62].
2.3. Transthoracic Echocardiogram
TTE is the most widely used imaging tool to obtain structural and functional information about the heart. MD1 patients should undergo a cardiac imaging examination at baseline and every 1 to 5 years thereafter if the initial imaging study is normal [52]. Particular attention should be given to subjects with baseline electrocardiographic abnormalities or arrhythmias, since systolic dysfunction seems to be more common in these subgroups [49]. Moreover, new echocardiographic techniques, such as three-dimensional (3D) TTE or speckle tracking analysis, should be performed to empower bi-dimensional TTE diagnostic and prognostic ability [48].
2.4. Cardiac Magnetic Resonance
Contrast-enhanced cardiac MRI is a highly sensitive, non-invasive tool for the detection of functional and structural myocardial abnormalities. Besides parameters easily available with TTE examination, cardiac MRI may detect eventual myocardial damage suggestive of scarring through late gadolinium enhancement (LGE) [63]. Moreover, it may quantify interstitial fibrosis through the extracellular volume (ECV) fraction technique [64] and can detect even subtle myocardial deformation or contractility impairment (as per localized degenerated myocardial tissue) through the cardiac strain technique [65].
3. Invasive Cardiac Evaluation and Treatment
3.1. Electrophysiological Study
EPS should increase the accuracy of SCD risk stratification in MD1 patients with electrocardiographic abnormalities (PR interval > 200 ms or QRS > 100 ms) through the evaluation of HV interval prolongation (>70 ms), which identifies those in need of prophylactic cardiac pacing. To date, little is known about the timing and the role of programmed ventricular stimulation for arrhythmic risk stratification [30,74]. The ACADEMY 1, a prospective single-center study about the electrophysiological study-guided ICD strategy in the prevention of arrhythmic cardiac death in MD1 patients, suggests the inducibility of VT has a limited value in the arrhythmic risk stratification among MD1 patients [75,76].
3.2. Loop Recorder
An implanted loop recorder (ILR) should be considered as an option for detecting clinically asymptomatic conduction disorders or spontaneous VT and for helping in the decision about the best choice of device to prevent SCD. It should be useful in MD1 patients with first-degree AV, fascicular, or bundle branch block and HV interval < 70 ms [77].
3.3. Pacemaker
Permanent cardiac pacing is indicated in patients with any second- and third-degree AVB or His-ventricle (HV) interval > 70 ms, regardless of the symptoms, and it may be considered in those with QRS > 120 ms and PR > 240 ms. Atrial pacing in the Bachmann bundle region was associated with a reduction in atrial electromechanical delay and the risk of R-wave oversensing on the atrial lead [78], compared with right atrial stimulation; however, it showed no benefit for the prevention of AF onset [79]. The activation of right atrial preference pacing [80] and minimal ventricular pacing [81] algorithms seems to be an efficient strategy to reduce the risk of AF in MD1 patients implanted with a PM. An increase in the incidence of AF has been shown in patients with a higher rate of right ventricular pacing and a lower rate of atrial stimulation [82].
3.4. Implantable Cardioverter-Defibrillator (ICD)
ICD implantation may be considered for all MD1 patients with permanent pacing indication and spontaneous or EPS inducible VT, regardless of their symptoms or LVEF. Cardiac resynchronization therapy (CRT) may be an option in MD1 patients with bundle branch block (especially left bundle branch block) who need permanent pacemaker implantation; however, there are currently only a few case reports about CRT therapy in MD1 patients [83,84].
3.5. CIED Remote Monitoring
RM combined with interrogation of the cardiac implanted device (PM, ICD, or ILR) and at least annual evaluation should be adopted to improve the clinical management of asymptomatic arrhythmias and to reduce the family-provided healthcare costs for MD1 patients with motor disability [85].
This entry is adapted from the peer-reviewed paper 10.3390/jcdd11020063
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