Abstract
A 58-year-old woman complained of general fatigue and was diagnosed with sick sinus syndrome (SSS) by ambulatory electrocardiogram, which demonstrated sinus arrest at midnight and paroxysmal atrial fibrillation (AF) at nighttime. Since her plasma cholinesterase (ChE) activity had been persistently zero, she was diagnosed with ChE deficiency. She refused permanent pacemaker implantation, and treatment with positive chronotropic drugs is ongoing. A novel association of ChE deficiency with SSS is theoretically possible rather than coincident, considering that ChE plays a key role in cholinergic influences on the sinus node leading to sinus bradyarrhythmia and on the atria, causing vagally mediated AF.