Sick sinus syndrome

Sick sinus syndrome (SSS) comprises a variety of conditions involving sinus node dysfunction and commonly affects elderly persons. Patients may experience syncope, pre-syncope, palpitations, or dizziness. SSS has multiple manifestations on electrocardiogram, including sinus bradycardia, sinus arrest, sinoatrial block, and alternating patterns of bradycardia and tachycardia. Autosomal recessive type (SSS1) can be caused by compound heterozygous mutation in the SCN5A gene, and autosomal dominant type (SSS2) can be caused by heterozygous mutation in the HCN4. SCN5A encodes an alpha-subunit of the cardiac sodium channel carrying the current to form a rapid upstroke of action potential, whereas HCN4 codes for an alpha-subunit of hyperpolarization-activated cation channel.