Familial sinoatrial node dysfunction. Increased vagal tone a possible aetiology

Sick sinus syndrome and atrial fibrillation in older persons - A view from the sinoatrial nodal myocyte

Sick sinus syndrome remains a highly relevant clinical entity, being responsible for the implantation of the majority of electronic pacemakers worldwide. It is an infinitely more complex disease than it was believed when first described in the mid part of the 20th century. It not only involves the innate leading pacemaker region of the heart, the sinoatrial node, but also the atrial myocardium, predisposing to atrial tachydysrhythmias. It remains controversial as to whether the dysfunction of the sinoatrial node directly causes the dysfunction of the atrial myocardium, or vice versa, or indeed whether these two aspects of the condition arise through some related underlying pathological mechanism, such as extracellular matrix remodeling, i.e., fibrosis. This review aims to shed new light on the myriad possible contributing factors in the development of sick sinus syndrome, with a particular focus on the sinoatrial nodal myocyte. This article is part of a Special Issue entitled CV Aging.

Pacemaker activity of the human sinoatrial node: effects of HCN4 mutations on the hyperpolarization-activated current

The hyperpolarization-activated 'funny' current, If, plays an important modulating role in the pacemaker activity of the human sinoatrial node (SAN). If is carried by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are tetramers built of four HCN subunits. In human SAN, HCN4 is the most abundant of the four isoforms of the HCN family. Since 2003, several loss-of-function mutations in the HCN4 gene, which encodes the HCN4 protein, or in the KCNE2 gene, which encodes the MiRP1 accessory β-subunit, have been associated with sinus node dysfunction. Voltage-clamp experiments on HCN4 channels expressed in COS-7 cells, Xenopus oocytes, or HEK-293 cells have revealed changes in the expression and kinetics of mutant channels, but the extent to which these changes would affect If flowing during a human SAN action potential is unresolved. Here, we review the changes in expression and kinetics of HCN4 mutant channels and provide an overview of their effects on If during the time course of a human SAN action potential, both under resting conditions and upon adrenergic stimulation. These effects are assessed in simulated action potential clamp experiments, with action potentials recorded from isolated human SAN pacemaker cells as command potential and kinetics of If based on voltage-clamp data from these cells. Results from in vitro and in silico experiments show several inconsistencies with clinical observations, pointing to challenges for future research.

The Contribution of HCN4 to normal sinus node function in humans and animal models

Although sinus node bradycardia is a very common clinical condition, the cellular mechanisms contributing to abnormal sinus node function are not clearly delineated. In recent publications, mutations in the hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channels have been associated with sinus bradycardia. These channels are thought to be crucial in generating the spontaneous sinus node action potential, in accelerating the heart rate during sympathetic drive, and decelerating heart rate during vagal stimulation. Humans carrying HCN4 mutations indeed display significant bradycardia. Recent studies generating HCN4 knock out mice suggested that although HCN4 is crucial in early development, other mechanisms may also play a role in the accelerated heat rate achieved during sympathetic drive. In this review, we focus on genotype-phenotype correlation of these mutations and discuss the relative contribution of various ion channels to sinus node function. We also discuss the importance of HCN in treating clinical conditions such as brady- and tachycardia.

Point mutation in the HCN4 cardiac ion channel pore affecting synthesis, trafficking, and functional expression is associated with familial asymptomatic sinus bradycardia

BACKGROUND

The hyperpolarization-activated nucleotide-gated channel--HCN4 plays a major role in the diastolic depolarization of sinus atrial node cells. Mutant HCN4 channels have been found to be associated with inherited sinus bradycardia.

METHODS AND RESULTS

Sixteen members of a family with sinus bradycardia were evaluated. Evaluation included a clinical questionnaire, 12-lead ECGs, Holter monitoring, echocardiography, and treadmill exercise testing. Eight family members (5 males) were classified as affected. All affected family members were asymptomatic with normal exercise capacity during long-term follow-up. Electrophysiological testing performed on 2 affected family members confirmed significant isolated sinus node dysfunction. Segregation analysis suggested autosomal-dominant inheritance. Direct sequencing of the exons encoding HCN4 revealed a missense mutation, G480R, in the ion channel pore domain in all affected family members. Function analysis, including expression of HCN4 wild-type and G480R in Xenopus oocytes and human embryonic kidney 293 cells, revealed that mutant channels were activated at more negative voltages compared with wild-type channels. Synthesis and expression of the wild-type and mutant HCN4 channel on the plasma membrane tested in human embryonic kidney 293 cells using biotinylation and Western blot analysis demonstrated a reduction in synthesis and a trafficking defect in mutant compared with wild-type channels.

CONCLUSIONS

We describe an inherited, autosomal-dominant form of sinus node dysfunction caused by a missense mutation in the HCN4 ion channel pore. Despite its critical location, this mutation carries a favorable prognosis without the need for pacemaker implantation during long-term follow-up.

[Familial sick sinus syndrome associated to strabismus and persistent left superior vena cava]

We report a family in which the mother and her three sons suffered sick sinus syndrome and strabismus. Two members had a persistent left superior vena cava with drainage into coronary sinus. The illness in all members of this family was oligosymptomatic, and well tolerated with mild symptoms like dizziness, fatigue and exercise dyspnea associated with nodal rhythm. Three of them, had episodes of paroxysmal atrial fibrillation. All patients remain asymptomatic after pacemaker implantation.

Familial idiopathic atrial fibrillation with bradyarrhythmia

A 10-year-old boy is reported who presented with idiopathic atrial fibrillation and bradyarrhythmia. After history of intrauterine and postnatal bradycardia, atrial fibrillation was first documented electrocardiographically at 16 months of age. An underlying structural heart disease was not evident. At the age of 10 years, implantation of a permanent ventricular demand pacemaker was indicated after syncope due to severe bradyarrhythmia. The family history revealed five persons of four generations with bradyarrhythmias. Idiopathic atrial fibrillation known since childhood was documented in three close relatives. A high grade AV block resulting in bradyarrhythmias and the occurrence of ST-T-changes in precordial leads could be demonstrated in all affected family members suggesting a diffuse general conduction abnormality in these patients. In this family, idiopathic atrial fibrillation seems to be transmitted as an autosomal dominant trait.

CONCLUSION

Atrial fibrillation without underlying heart disease is extremely rare in children. Careful electrocardiographic follow-up of these patients and the evaluation of ECG recordings of all family members is recommended.

Familial symptomatic sinus bradycardia: autosomal dominant inheritance

Symptomatic sinus bradycardia, due to either sick sinus syndrome or vagotonia, can be familial, affecting several members of a family. We report an 18-year-old male patient with palpitations and limited exercise capacity who was noted to have severe sinus bradycardia. His resting heart rate was 40/min, with normal PR and corrected QT intervals, and sinus pauses up to 6 seconds during sleep. Exercise treadmill test and pharmacologic autonomic blockade during electrophysiologic studies abolished the bradycardia, suggestive of vagotonia rather than intrinsic sinus node dysfunction. This patient's father and a female cousin had a similar clinical history but associated with syncope and severe sinus bradycardia. The mode of transmission appeared to be autosomal dominant. All three have permanent demand pacemakers implanted and are asymptomatic.

Autosomal dominant inheritance of sinus node disease

A family with sinus node disease is presented. The mother was severely affected by sinus bradycardia and required a permanent atrial pacing system. The father is asymptomatic and has no evidence of conduction disturbances. All their offspring (one son and two daughters) are affected with variable degrees of severity. The occurrence of the disease in this family is suggestive of autosomal dominant inheritance with variable penetrance.

Familial sinus node disease and degenerative myopia--a new hereditary syndrome?

A family is reported in which the father and both offspring suffered from sick sinus syndrome (SSS) accompanied by degenerative myopia in the two elder of the three family members. In all three individuals, the basic manifestations of the SSS were total sinus arrest with secondary AV junctional rhythm at a normal rate. Only the 7-year-old boy also exhibited paroxysmal atrial flutter and ventricular asystole lasting up to almost 7 s. Since he had, furthermore, recently sustained a left hemiparesis, a ventricular demand pacemaker was implanted. Chronic atrial fibrillation was eventually established in the father in whom observations were available for 16 years. Myopia of -11 D and -4.5 D, respectively, existed in the father and his 14-year-old daughter. The familial association of this defect with SSS suggests a hitherto undescribed genetic syndrome, presumably with underlying autosomal dominant transmission of the two traits.

Familial automaticity-conduction disorder with associated cardiomyopathy

An unusually large family of European descent was afflicted over four generations by an automaticity and conduction disorder with an associated dilated cardiomyopathy of variable expression. Ten living members affected with the disorder and three presumed affected but dead members were identified. Typically, the disorder presented as a sinoatrial bradyarrhythmia/tachyarrhythmia syndrome, followed by atrial enlargement and, variably, ventricular enlargement and dysfunction. Three family members required pacemaker implantation. Longevity did not seem to be greatly affected, but the demonstrated potential for embolic cerebrovascular events stresses an associated morbidity. The familial incidence was best explained by autosomal dominant inheritance with incomplete penetrance (greater in males and usually occurring first in adolescence) and variable expressivity. The large size of the family, frequency and profile of disease manifestations and disease tracking through at least four generations are unusual features of the familial disease described.

Cardiac rhythm abnormalities in patients presenting with transient non-focal neurological symptoms: a diagnostic grey area?

Eighty-nine patients attending neurology clinics with transient non-focal neurological symptoms were studied by routine electrocardiography and 24-hour monitoring of the electrocardiography and 24-hour monitoring of the electrocardiogram. In comparison with 109 control subjects there was no significant overall excess of arrhythmias (age-adjusted odds ratio 1.7, kappa2 = 2.67) except in the subgroup of patients under the age of 30 (odds ratio 11.6, p less than 0.05). Bradyarrhythmias, but not tachyarrhythmias, were significantly more common in the patients (odds ratio 7.4, p less than 0.001),. Since patients can rarely be studied while they are having symptoms a working diagnosis must be based on a balance of probabilities: arrhythmias in young patients, or bradyarrhythmias in any patient, are likely to be clinically relevant. Ambulatory electrocardiographic monitoring contributed to the diagnosis in at least 25 of the patients, Nevertheless, the extent to which further investigations are pursued, and the form of treatment ultimately adopted, must also be influenced by the frequency and severity of the patients' symptoms.

Diffuse conduction abnormalities in an adolescent with familial sinus node disease

We report a case of familial sinus node disease with associated conduction abnormalities in the atrioventricular node and distal conduction system Spontaneous atrial pacemaker activity was absent though the atrium could be depolarized. The pacemaker activity of the heart resided in the atrioventricular junction. The AV node showed impaired automaticity and abnormal conduction properties which partially improved after vagal blockage.

Sinuatrial disease in young people

A survey at three cardiac centres disclosed nine patients under the age of 25 years with sinus node dysfunction in the absence of other forms of heart disease. All were male and seven were above the 90th centile for height. Ambulatory monitoring was performed on all the first-degree relatives of six of the patients and three families showed conducting system disturbances in the form of sinuatrial disorders or delayed atrioventricular conduction. A genetic factor may be involved in the aetiology of sinuatrial disease in young people.

Familial sinus node dysfunction with autosomal dominant inheritance

A large family with sinus node dysfunction is described, spanning three generations and with an autosomal dominant trait. There was an obvious association between the grade of mental retardation and the severity of the sinoatrial disorder.

Pathological studies in sinoatrial disorder (sick sinus syndrome)

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