Primary electrical channelopathies are genetic ion channel disorders that predispose children with structurally normal hearts to malignant ventricular tachyarrhythmias, syncope, and sudden cardiac death.
Figure 10.1: Characteristic T-wave morphologies across Long QT Syndrome subtypes: LQT1 (broad-based), LQT2 (notched/bifid), and LQT3 (long isoelectric ST with late peaked T). Rendered with solid white background.
1. Clinical Presentation & Environmental Triggers
- LQT1 ($KCNQ1$): Syncope triggered by physical exertion, specifically swimming / diving or emotional stress.
- LQT2 ($KCNH2$): Syncope triggered by auditory stimuli (alarm clocks, doorbells, ringing telephones).
- LQT3 ($SCN5A$): Events occurring during rest or sleep / bradycardia.
- CPVT ($RyR2$): Emotion or exercise-induced polymorphic/bidirectional VT in childhood with completely normal resting 12-lead ECG.
2. Diagnostic Electrocardiographic Criteria
Schwartz Diagnostic Score for Pediatric LQTS:
| Diagnostic Parameter | Clinical Finding | Score Points |
|---|---|---|
| $QTc$ (Bazett) | $\ge 480\text{ ms}$ | 3 |
| $460 - 470\text{ ms}$ | 2 | |
| $450\text{ ms}$ (in males) | 1 | |
| T-Wave & Rhythm Signs | Torsades de Pointes | 2 |
| T-wave alternans | 1 | |
| Notched T wave in $\ge 3$ leads | 1 | |
| Low resting heart rate for age ($<2\text{nd}$ %ile) | 0.5 | |
| Clinical History | Syncope with physical/emotional stress | 2 |
| Syncope without stress | 1 | |
| Congenital sensorineural deafness (Jervell & Lange-Nielsen) | 0.5 | |
| Family History | Family member with definite LQTS | 1 |
| Unexplained sudden cardiac death in 1st-degree relative $<30$ yr | 0.5 |
Interpretation: Score $\ge 3.5$ = High probability; $1.5 - 3.0$ = Intermediate; $\le 1.0$ = Low probability.
Figure 10.2: Torsades de Pointes showing classic polymorphic sinusoidal twisting around the isoelectric axis.
Figure 10.3: Type 1 Brugada Pattern: Coved ST-segment elevation >= 2 mm followed by an inverted T wave in right precordial leads.
3. Multimodality Investigations
- Exercise Treadmill Testing (ETT): Evaluates $QTc$ behavior during recovery (failure of $QTc$ to shorten or paradoxical prolongation at 4th minute of recovery $\ge 480\text{ ms}$ indicates LQTS).
- Next-Generation Genetic Sequencing: Direct sequencing of $KCNQ1, KCNH2, SCN5A, RyR2, CASQ2, CACNA1C$.
4. Evidence-Based Clinical Management Protocols
- First-Line Pharmacotherapy: Non-selective Beta-Blockers (Nadolol $1-2\text{ mg/kg/day}$ or Propranolol $2-4\text{ mg/kg/day}$). Metoprolol is inferior and not recommended in LQTS.
- Mexiletine / Ranolazine: Added in LQT3 as a late sodium channel current ($I_{Na,L}$) blocker.
- Flecainide: Highly effective in CPVT by suppressing calcium sparks from mutated $RyR2$ channels.
- Left Cardiac Sympathetic Denervation (LCSD): Video-assisted thoracoscopic excision of lower third of left stellate ganglion for refractory syncope.
- Implantable Cardioverter-Defibrillator (ICD): Indicated for survivors of cardiac arrest or breakthrough syncope on maximal medical therapy.