A 19-year-old man with congenital long QT syndrome has cardiac events exclusively during sleep and at rest, never during exercise or auditory arousal. Genetic testing is pending. Which subtype and mechanism fits this pattern?

• A LQT1, loss of function of IKs
• B LQT3, gain of function mutation in SCN5A causing persistent late sodium current ✓
• C LQT2, loss of function of IKr triggered by sudden noise
• D LQT5, loss of function of IKs accessory subunit

Explanation

The three major LQT phenotypes have distinct trigger profiles: LQT1 events occur with exercise, especially swimming; LQT2 events with emotional stress or sudden auditory stimuli; LQT3 events characteristically occur during sleep or rest at low heart rates. LQT3 results from a gain-of-function SCN5A mutation producing incomplete inactivation of the sodium channel and a sustained late inward current, which prolongs the action plateau. Beta blockers are less protective in LQT3, whereas mexiletine may help.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.