Resolution of Refractory Multifocal Atrial Tachycardia in <scp>C</scp> ostello Syndrome Using Trametinib: A Case Supporting <scp>MEK</scp> Inhibitors as Targeted, Specific Antiarrhythmic

Arrhythmias affect approximately half of patients with Costello syndrome (CS, OMIM # 218040), with non-reentrant atrial tachycardia being the most common. This case describes an infant with Costello syndrome carrying the pathogenic HRAS c.34G>A (p.G12S) variant who developed early-onset, drug-refractory multifocal atrial tachycardia (MAT). Despite multiple antiarrhythmic therapies, rhythm control remained inadequate until trametinib was initiated, resulting in rapid resolution of MAT and allowing stepwise discontinuation of adjunct antiarrhythmics. Although MAT did not recur after the MEK inhibitor was stopped, the patient subsequently developed hypertrophic cardiomyopathy only after trametinib was discontinued, suggesting a potential disease-modifying cardioprotective effect during active treatment. This case supports MEK inhibition as a targeted antiarrhythmic strategy in Costello syndrome, with benefits that may extend beyond acute rhythm stabilization to include prevention of cardiac hypertrophy. While spontaneous resolution of MAT can occur in Costello syndrome, the rapid and sustained response in this patient strengthens the hypothesis that trametinib exerts a direct antiarrhythmic effect.