CARDIAC CHANNELOPATHIES

Dr Binjo J VazhappillySR Cardiology

Calicut Medical College

• Cardiac channelopathies refers to genetic disorders characterized

by altered cardiac excitability, in the absence of structural cardiac

involvement.

• Also known as inherited arrhythmogenic diseases (IADs).

• Disorders due to genetic mutation affecting the genes that control

the excitability of myocardial cells.

Major cardiac channelopathies include

Long QT Syndrome

Brugada syndrome

CPVT

Short QT syndrome

Webster and Berul circulation 2013;127:126-140

Long QT Syndrome

• LQTS is an IAD characterized by abnormally prolonged QT interval.

• 13 genes are found to be linked to LQTS till now.

• Mutations in 3 genes KCNQ1 (LQT1), KCNH2 (LQT2) and SCN5A

(LQT3) accounts for approximately 75% of cases with a strong

clinical phenotype.

Prevalence Assumed to be 1/2500 live births . In an Italian study ,where ECG was performed in 44,596 infants ,

0.07% had a QTc >470 ms and 0.47% had QTc between 451 and 470 ms

Molecular screening showed disease-causing mutation in 43% of neonates with QTc >470 ms & 29% of those with QTc b/w 461 and 470 ms

Schwartz etal Circulation. 2009;120:1761-1767

CHANNELOPATHY

GENE

PROTEIN

LQT 1 KCNQ1 α-subunit of Iks

LQT 2 KCNH2 α-subunit of Ikr

LQT 3 SCN5A Sodium channel, α-subunit

LQT 4 ANK2 Cellular structural protein

LQT 5 KCNE1 β-subunit of Iks

LQT 6 KCNE2 β-subunit of Ikr

LQT 7 KCNJ2 α-subunit of Ik1

CHANNELOPATHY

GENE

PROTEIN

LQT 8 CACNA1C l-type Ca + channel, α-subunit

LQT 9 CAV3 Plasma membrane structural protein

LQT 10 SCN4B Sodium channel, β-subunit

LQT 11 AKAP9 Kinase anchoring protein (Yotaio)

LQT 12 SNTA1 Syntrophin α1 (affects sodium current)

LQT 13 KCNJ5 Inwardly rectifying potassium channel, α-subunit

Potassium Channel LQTS

• Potassium channel derangements account for majority of LQTS cases : Long QT 1, 2, 5, 6, 7 and 13.

• IKs : slowly activating delayed rectifier cardiac potassium channel subunit is encoded by KCNQ1 β subunit is encoded by KCNE1 Loss of function mutation result in LQT 1 and LQT 5

• Ikr : rapid delayed rectifier potassium channel subunit encoded by KCNH2 β subunit encoded by KCNE2 Loss of function mutation result in LQT 2 and LQT 6.

• IKI : another inwardly rectifying K+ channel

encoded by gene KCNJ2 Loss of function mutation result in LQT 7 (Tawil-Anderson syn)

• KCNJ5 mutation which is a loss-of-function mutation in an inwardly

rectifying potassium channel result in LQT 13

Sodium channel LQTS

• subunit of sodium channel is encoded by SCN5A and β subunit encoded by SCN4B.

• Gain of function mutations in SCN5A produce LQT3.

• Mutations in SCN4B produce LQT10.

Molecular basis for long QT syndrome

Topol EJ, Califf RM et al

Pathophysiology of LQTS• Prolonged repolarization results fm net reduction in outward

current, due to ↑ in inward Na + or Ca + current, ↓ in outward K+ current or both resulting in long QT interval.

Fast heart rate preceding TdP in LQT1 postulate delayed afterdepolarizations (DADs) as its arrthymogenic mechanism.

• Increased Ca+ loading in parallel with QT prolongation, facilitate DADs and DAD-dependent TdP.

Pause-dependent TdP is triggered by early afterdepolarizations (EADs).

• Pause leads to enhanced Ca+ release from intracellular stores and activate Ca+ dependent transmembrane currents.

Initiation of arrhythmia in LQTS

Pause dependent torsade de pointes

Mainly in LQT2

Non-pause dependent torsade de pointes

Mainly in LQT1

Clinical features

• May be asymptomatic• Symptomatic pts present with palpitations, presyncope, syncope or

cardiac arrest.• Neuronal deafness is associated with Jervell and Lange-Nielsen

syndrome

• In a study on 287 pts , 61% were symptomatic : 9% presented with a cardiac arrest, 26% with syncope ,10% with seizures , 6% had presyncore or palpitation

• 67% had symptoms related to exercise, 18% had symptoms during exercise and with emotion, 7% with emotion alone, 3% with loud noise and exercise and 2% with anesthesia.

A Garson, M Dick et al Circulation. 1993;87:1866-1872

Triggers of arrhythmia

• Triggers include exercise, noise, emotion, sudden wakening from sleep by noise , swimming or diving.

• Swimming and exertion-induced cardiac events are strongly associated with LQT1.

• Auditory triggers and events during postpartum period occur in pts with LQT2.• Events occurring during periods of sleep or rest are most common in LQT3.

Triggers of LQTS

Schwartz et al

Jervell and Lange-Nielsen syndrome

• Autosomal recessive variant of long QT syndrome.• Due to homozygous or compound heterozygous mutations on

either the KCNQ1 or KCNE1 genes.

• Pts also suffer from congenital deafness.

• Most severe of major variants of LQTS.• 90% have cardiac events, 50% become symptomatic by age of 3 yrs and their average QTc is markedly prolonged (557± 65

ms)

Event-free survival comparing Jervell and Lange-Nielsen syndrome pts with other long-QTS

Timothy syndrome / LQT8

• Mutations in CACNA1C, encoding voltage-gated calcium channel results in Timothy syndrome or LQT8.

• Rare and extremely malignant variant.

• Pts had marked QT prolongation and associated syndactyly .• Presents with 2:1 atrioventricular block and macroscopic T-wave

alternans.• Among 17 children reported by Splawski et al, 10 (59%) died at a

mean age of 2.5 yrs.

ECG in LQTS

• QTc values exceeding 440 ms (in males) and 460 ms (in females) are

considered abnormal .

• LQT1 is associated with a broad-based T wave.• LQT2 with low-amplitude notched or biphasic T wave.• LQT3 with long isoelectric segment followed by a narrow-based T wave.

LQTS : Diagnostic Criteria

Score

≤1 point: low probability

1.5–3 points: intermediate probability

≥3.5 points: high probability.

Schwartz et al Circ Arrhythm Electrophysiol. 2012;

Keating Criteria Asymptomatic with QTc > 470 ms or Typical symptoms with QTc ≥ 450 ms

Sensitivity and specificity of QTc duration alone ,Schwartz score and Keating criteria

Nynke Hofman, Arthur A.M. Wilde et al EHJ

Risk stratification

Risk of a 1st cardiac event in pts younger than 40 years of age in the absence of any LQTS active treatment

Treatment

• All LQTS pts with h/o syncope and asymptomatic individuals with

definite QT prolongation should be treated withβ-blockers

• Drugs used are Propranolol ( 2-4 mg/kg/d),

Nadolol (1-2.5 mg/kg/d)

Metoprolol (2-4 mg/kg/d)

• Dose titration done with a target of 25% to 35% reduction of

maximal heart rate attained on therapy.

• Left Cardiac Sympathetic Denervation may be considered in

symptomatic patients even after betablocker therapy.

Treatment

• ICD implantation along with β -blockers is recommended for LQTS

patients with previous cardiac arrest or who are experiencing

syncope and/or VT while receiving beta blockers.

• Permanent pacing is indicated for sustained pause-dependent VT,

with or without QT prolongation.

• Avoid competitive sports, QT-prolonging drugs and lowered K+

levels .

β-blocker therapy in LQTS

• β-blocker therapy results in 42% to 78% reduction of aborted cardiac arrest or sudden cardiac death.

• Pts on β-blockers still have risk of sudden death. • In a study , no. of cardiac events before initiation of β-blocker

therapy was 0.97 events /pt/yr which decreased to 0.31 after initiation of therapy .

• β-blocker is most efficacious in LQT1 and less effective in LQT3. • In LQT3, combination of mexiletine with a noncardioselective β-

blocker (propranolol ) is used .

Brugada Syndrome

• Characterized by peculiar ECG pattern of ST-segment elevation in

leads V1 to V3 and incomplete or complete RBBB in the absence of

signs of acute MI.

• Autosomal dominant disorder with variable expression

• More common in men than in women.

• Usually diagnosed in adulthood ( Avg age at diagnosis is 41 yrs).

Genes Involved in Brugada CHANNELOPATHY GENE CHANNEL/PROTEIN Effect

BrS 1 SCN5A Cardiac sodium channel subunit

↓ Na+ current

BrS 2 GPD1L Glycerol-6-phosphate dehydrogenase

↓ Na+ current

BrS 3 CACNA1C L-type calcium channel subunit

↓ Ca2+ current

BrS 4 CACNB2 L-type calcium channel β subunit

↓ Ca2+ current

BrS 5 SCN1B Cardiac sodium channel β1 subunit

↓ Na+ current

BrS 6 KCNE3 Transient outward current β subunit

↑ K+ Ito current

BrS 7 SCN3B Cardiac sodium channel β3 subunit

↓ Na+ current

ECG patterns

Type-1 ≥ 2-mm J-point elevation, coved type ST-T segment elevation and

inverted T-wave in leads V1 and V2.Type-2 ≥ 2-mm J-point elevation, ≥ 1-mm St segment elevation, saddleback

ST-T segment and a positive or biphasic T-wave. Type-3 Same as type 2, except that the ST-segment elevation is <1 mm.

Placement of precordialleads in higher intercostal spaces can unmask theBrugada ECG pattern

Pathophysiology

• Debate is still going on whether pathophysiology is due to repolarization or depolarization disorder.

Repolarization hypothesis by Yan and Antzelevitch Transmembrane voltage gradient b/w RV epicardium and

endocardium due to heterogenous loss of AP dome in epicardium and not in endocardium.

Heterogeneity of transmembrane voltage potentials result in phase 2 reentry and triggered VF.

Pathophysiology

Cardiovascular Research 67 (2005) 367 – 378

Yan and Antzelevitch- Faulty repolarization

Depolarization hypothesis

• In BS pts RVOT endocardium shows activation slowing and is the last to depolarize.

• Delay in AP of RVOT causes an electrical gradient from more positive RV to RVOT, leading to ST-elevation in right precordial leads.

• When RVOT depolarizes later (during repolarization of RV), this gradient is reversed and net current flows towards RV, resulting in a negative T-wave in right precordial leads.

Depolarization Hypothesis

Cardiovascular Research 67 (2005) 367 – 378

conduction delay in RVOT

Clinical Presentation

• Clinical spectrum ranges from asymptomatic to SCD.

• Patients may present with late onset of VF, despite having

abnormal ECG pattern for decades.

• Syncope or seizures may occur due to self-terminating VF episodes.

• Agonal respiration and difficulty in arousal at night also may be

due to self-terminating VF episodes.

• Majority of BS pts are young, b/w 20 and 40 yrs of age at

presentation.

Brugada : Diagnostic criteria

Appearance of type 1 ST segment elevation (coved type) in > 1 rt precordial lead (V1 - V3) in the presence or absence of a sodium channel blocker, plus at least one of the following:

Documented ventricular fibrillation. Polymorphic ventricular tachycardia (VT). Family h/o sudden cardiac death at less than 45 years of age. Family h/o of type 1 Brugada pattern ECG changes. Inducible VT during electrophysiology study. Unexplained syncope. Nocturnal agonal respiration .

Type 2 and type 3 ECG are not diagnostic of Brugada syndrome

Second Consensus Conference : Europeon Heart Rhythm Society

• Brugada pattern : Pts with typical ECG features who are

asymptomatic and not having other clinical criteria.

• Brugada syndrome : Pts with typical ECG features and clinical

criteria (who have experienced sudden cardiac death or a sustained

ventricular tachyarrhythmia or who have one or more of the other

associated clinical criteria )

Drug challenge

• Done in pts with resting ECG type 2 or 3 Brugada pattern and having

family h/o sudden cardiac death at < 45 yrs and/or a family h/o

type 1 Brugada pattern ECG

• Drugs used

Flecainide : 2 mg/kg over 10 min iv or 400 mg PO

Procainamide : 10 mg/kg over 10 min iv

Ajmaline : 1 mg/kg over five minutes iv

Pilsicainide : 1 mg/kg over 10 minutes iv

Drug challenge

Indications for termination of the drug challenge include:

Development of a diagnostic type 1 Brugada pattern

≥2 mm increase in ST segment elevation in pts with type 2 Brugada

ECG pattern

Development of ventricular premature beats or other arrhythmias

Widening of the QRS ≥30 percent above baseline

Risk stratification

Treatment

• ICD is the only effective treatment to reduce mortality in BrS• ICD indication Class 1 BrS pts with previous cardiac arrest. Class IIa BrS pts with spontaneous pattern with h/o syncope. BrS pts with documented VT that has not resulted in cardiac arrest.

Drugs : Quinidine or amiodarone may be used for pts not willing for ICD / reduced life expectancy.

Electrical storm: Isoprotenol or Quinidine may be used.

Short QT Syndrome

• Rare condition with short-QT interval (<320 ms).

• Presents symptomatically with recurrent syncope, sudden cardiac

death and atrial fibrillation.

• Mutations in 6 different genes (3 gain of function and 3 loss of

function) are identified .

Genes in SQTSCHANNELOPATHY GENE

CHANNEL /PROTEIN

SQT 1 KCNH2 IKr

SQT 2 KCNQ1 IKs

SQT3 KCNJ2 IK1

SQT4 CACNA1C l-type ca+ channel, α-subunit

SQT5 CACNB2 l-type ca+ channel, β-subunit

SQT 6 CACNA2D1 l-type calcium channel subunit

Basis of Arrhythmogenesis

• Abbreviation of action potential in SQTS is heterogeneous with

preferential abbreviation of either epicardial or endocardial cells as

compared with sub-endocardial M cells, resulting in dispersion of

repolarization .

• Dispersion of repolarization serves as substrate for initiation and

maintenance of reentry.

• In a case series1 of 29 pts 62 % (18 out of 29) were symptomatic Cardiac arrest – 34 %(initial symptom in 28%) Palpitations – 31 % Syncope – 24 % Atrial fibrillation – 17 %• Electrocardiographic findings Abnormally short QT interval Absence of ST segment Tall and peaked T waves Prolonged Tpeak-Tend interval and Tpeak-Tend/QT ratio.

1.Carla Giustetto, Fernando Di Monte etal EHJ(2006) 27, 2440–2447

Proposed Diagnostic Criteria: SQTS

Michael H. Gollob, MD, Calum J. Redpath et al JACC Vol. 57, No. 7, 2011

Management

• ICD implantation recommended for both primary and secondary

prevention of SCD in pts with SQTS.

• Pharmacological therapy with QT prolonging drugs.

• Quinidine is recommended in SQT1 syndrome.

• Disopyramide and amiodarone are also shown to prolong QT in

SQTS pts.

Catecholaminergic polymorphic ventricular tachycardia (CPVT)

• CPVT is a disorder of intracellular calcium handling causing adrenergic-dependent arrhythmias and sudden death.

• Pts have normal resting ECG and develop ventricular ectopy progressing to bidirectional or polymorphic VT during exercise or catecholamine infusion .

• Pts present with life-threatening VT or VF occurring during emotional or physical stress.

• Affected patients may have a family h/o juvenile sudden death or stress-induced syncope .

• CPVT can also present sporadically following a de novo mutation.

Genetic basis

• Mutations in 2 genes are identified : ryanodine receptor gene (RyR2) and calsequestrin 2 gene (CASQ2).

• RyR2 mediates release of Ca+ from SR which is is required for myocardial contraction.

• RyR2 mutation result in Autosomal Dominant form of CPVT

• Calsequestrin 2 protein is a protein in sarcoplasmic reticulum which binds large amounts of calcium.

• CASQ2 mutation result in Autosomal Recessive form of CPVT.

Mechanism for Arrhythmogenesis

• Delayed after depolarization (DAD) dependent triggered activity.

• Mutant ryanodine receptor is leaky and it releases excess of

calcium during diastole.

• This activates sodium-calcium exchanger that extrudes calcium

ions out from the cell.

• This generates a net inward current results in DAD.

• When large enough, DADs trigger extrasystolic action potential.

Mechanism for Arrhythmogenesis

Treatment

• Avoiding competitive sports • Beta blockers for all pts with spontaneous or documented stress-

induced ventricular arrhythmias.• For survivors of cardiac arrest or pts with syncope or sustained VT

or VF despite therapy with beta blockers, ICD is recommend.• Flecainide or verapamil may be given for pts who are symptomatic

with ICD and beta blockers .• Left sympathetic denervation for pts who remain symptomatic after

maximal medical therapy.

Other Channelopathies

Arrhythmogenic Right Ventricular Cardiomyopathy ( ARVC )

Subset of ARVC is caused by defects in cardiac ryanodine receptor (RyR2)

May represent a variant of CPVT rather than a subset of CPVT . Majority are caused by defects in desmosomal proteins PKP2 encoding plakophilin 2 : most common DSP encodes desmoplakin DSG2 encodes desmoglein 2 Hallmark of ARVC is fibrofatty replacement of the myocardium Arrhythmias may precede histological evidence of disease

Familial AF Familial clustering may occur in AF Account for a minority of pts with lone AF

KCNH2 IKr ↑ outward K+ current

KCNQ1 IKs ↑ outward K+ current

KCNJ2 IK1 ↑ outward K+ current

GJA5 Gap-junction protein connexin 40 Impared conduction

Sinus Node Dysfunction and Conduction Defects

Mutations of HCN4 gene which codes for cardiac pacemaker

current (If) results in sinus node dysfunction.

Other mutation which are associated with conduction defects are

SCN5A , NKX2.5 and GATA 5

Summary

• Cardiac channelopathies represent a group of disorders with inherited arrhythmogenic potential and structurally normal heart.

• Majority are due to mutations in genes encoding Na+ , K+ , Ca + channels of heart.

• In LQTS arrhythmia is triggered by exercise , emotion or noice.• Brugada syndrome is diagnosed by type 1 Ecg and documented

event.• Beta blockers are useful in LQT1 and CPVT.• ICD is indicated in survived cardiac arrest pts and in high risk

patients.

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