Discussions

Herein, we characterised the clinical course of the first child who endorsed spontaneous recovery from complete atrioventricular block. Advanced atrioventricular block is a potentially fatal interruption of the cardiac conduction system, with a global incidence of 1 in 20,000 live births. ^{Reference Barra, Providência, Paiva, Nascimento and Marques1} Mortality rate, albeit in the absence of associated congenital heart disease on most occasions, was estimated to be 8~16% in children with congenital complete atrioventricular block if left untreated. ^{Reference Jaeggi, Hamilton, Silverman, Zamora and Hornberger2} Most patients are symptom-free until later stage of disease progression. Therefore, the diagnosis is primarily based on ECG and Holter monitoring, in conjunction with echocardiography to assess cardiac structure and function.

Previous literature has interrogated a plethora of possible triggers for paediatric atrioventricular block, including but not limited to prior infection, infiltrative disease, non-ischemic cardiomyopathy, autoimmunity, Bezold–Jarisch reflex, iatrogenic effect after the administration of β-blockers, calcium channel inhibitors, and anti-arrhythmia medications. Although the identification of secondary cause and corresponding specified management are pivotal to restore conductive fluency, those who were eventually classified into idiopathic complete atrioventricular block principally required permanent pacemaker implantation to prevent consequent cardiovascular collapse, Torsade de pointes, or asystole. The risk factors of atrioventricular block ranged from maternal lupus and anti-SSA(Ro)/anti-SSB(La) antibody to elevated systolic blood pressure and hyperglycaemia. ^{Reference Kerola, Eranti and Aro3} Genetically, NKX2.5 mutation was related to conduction defects, ^{Reference Benson, Silberbach and Kavanaugh-McHugh4} while SCN5A variant to atrial standstill and conductive suspension. ^{Reference Schott, Alshinawi and Kyndt5} Hence, the pathogenesis is intertwined with junctional or ventricular escape machinery secondary to the fibrosis of the conductive system. Still, the entire mechanistic understanding is in its infancy.

The diagnostic algorithm to evaluate young patients with advanced atrioventricular block has been well constructed and was practiced when assessing the present case. Notably, no overt cause was delineated after the exhaustion of every inquiry toward the common etiologies, hampering the establishment of further intervention aside from close observation. Moreover, American College of Cardiology/American Heart Association guidelines does not recommend permanent pacemaker implantation on patient sustaining 3rd degree atrioventricular block (class III) unless in the presence of symptomatic bradycardia, comorbid with atrial fibrillation, >3 seconds asystole, escape rhythm <40 bpm, or status post recent cardiac surgery. ^{Reference Kusumoto, Schoenfeld and Barrett6} The management in acute stage as well as the frequency or duration of future follow-ups remained ambiguous. Besides, pattern of exacerbation or subsiding of atrioventricular block has yet been elucidated, making the alteration of the natural clinical course arduous.

To our knowledge, no spontaneous improvement of advanced atrioventricular block has been published. Even though multiple reports documented the recovery of baseline atrioventricular block or post-operative atrioventricular block after congenital heart surgery, no single clinical index was eligible to predict possible regression or of prognostic implication. ^{Reference Socie, Nicot and Baudinaud7} Unfavourable outcome was associated with atrioventricular block onset unless the reversible cause was eliminated. Other anecdotal case reports described the abatement of complete atrioventricular block due to primary giant cell myocarditis ^{Reference Hanawa, Izumi and Saito8} or sarcoidosis ^{Reference Park, Park and Cho9} after corticosteroid therapy. Besides, one patient recovered from anterior wall myocardial infarction complicating complete atrioventricular block after coronary reperfusion by thrombolysis. ^{Reference Sasikumar and Kuladhipati10} Whilst in the present case, no clinical indicator was available at presentation to dictate subsequent therapeutic arrangement. In addition, long-term monitoring is warranted to analyse whether regressed atrioventricular block would recur and the development of future complications, if any. This in turn may retrospectively facilitate the identification of original aetiology.