Primary diagnosis of atrioventricular pseudo-block in a neonate with definitive diagnosis of long QT syndrome: diagnostic considerations and therapeutic approaches

INTRODUCTION: Long QT syndrome (LQTS) is a potentially lethal medical condition that might never be diagnosed and cause sudden cardiac death. It is mainly caused by mutation in electrolyte transporter genes. Due to the significant difference in the treatment approach of heart block and other rhythm disorders that mimic this condition, it is necessary to discriminate these conditions. The occurrence of pseudo-block in electrocardiography features but without disturbance in the function of the conduction system can mask the definite diagnosis of the real underlying disorder, and this issue leads to the selection of an unfavorable treatment protocol and sometimes the sudden death of the patient. CASE PRESENTATION: We described an infant who showed evidence of atrioventricular (AV) block in initial electrocardiography (ECG) on his first day, but in further evaluations, the final diagnosis of LQTS was raised. The patient recovered after performing the treatment protocol, which included Mexiletine and beta-blockers. After the genetic test of the parents and the patient, it was determined that a defective allele of the gene had caused the condition. CONCLUSION: Our report shows the importance of timely differentiation between heart block and LQTS in neonates and choosing the correct treatment approach to faster patient recovery and prevent sudden death. CLINICAL KEY MESSAGE: Primary diagnosis of LQTS in neonates might not be a straightforward process due to resembling AV pseudo-block and can cause misleading diagnosis and treatment. Long QT syndrome has several nonspecific presentations. They might be asymptomatic until adulthood and be diagnosed after sudden cardiac death. Preventive measures such as timely initiation of medications, ICD or PPM implantation, and continuous observation by caregivers are the mainstay of survival and quality of life improvement.