Brugada Syndrome

Brugada Syndrome (BrS) is an inherited arrhythmogenic disorder caused predominantly by loss-of-function mutations in the SCN5A gene, which encodes the alpha subunit of the cardiac sodium channel Na_v1.5. This channel is responsible for the rapid inward sodium current (I_Na) that generates phase 0 of the cardiac action potential. Reduced I_Na leads to diminished depolarization reserve, particularly in the right ventricular outflow tract (RVOT) epicardium, creating a substrate for lethal ventricular arrhythmias.

SCN5A mutations are identified in approximately 20–25% of clinically diagnosed BrS patients, making it the most commonly implicated gene. Over 300 SCN5A variants have been described, most resulting in truncated or non-functional channels through premature stop codons, frameshift mutations, or missense variants that impair channel trafficking or gating. Additional genes have been implicated in smaller proportions of cases, including CACNA1C and CACNB2 (encoding L-type calcium channel subunits), SCN1B and SCN3B (sodium channel beta subunits), GPD1L, KCNE3, KCNJ8, HCN4, TRPM4, and SCN10A. In the majority of patients (~75%), no causative mutation is identified, suggesting polygenic inheritance, epigenetic factors, or undiscovered genetic mechanisms.

Two principal hypotheses explain the arrhythmogenic mechanism in BrS:

The repolarization hypothesis, championed by Antzelevitch, proposes that loss of I_Na in the RV epicardium leads to unopposed transient outward potassium current (I_to) during phase 1 of the action potential. Because I_to density is highest in the epicardial cells of the RVOT, this creates a pronounced action potential notch and even loss of the action potential dome in the epicardium, while the endocardium maintains a normal dome. The resulting transmural voltage gradient produces the characteristic ST-segment elevation on ECG. Heterogeneous loss of the dome across the epicardium — where some cells lose the dome and adjacent cells retain it — creates the conditions for phase 2 reentry, in which the dome from a cell that retained it propagates to a cell that lost it, triggering a closely coupled premature ventricular complex that can initiate ventricular fibrillation (VF).

The depolarization hypothesis suggests that conduction delay in the RVOT due to reduced I_Na is the primary mechanism. Slowed depolarization creates voltage gradients between the RVOT and the remaining RV myocardium, producing the Brugada ECG pattern. This hypothesis is supported by the finding of fractionated and delayed electrograms in the RVOT epicardium during invasive mapping studies. In reality, both mechanisms likely coexist and contribute to the phenotype.

The RVOT predominance of the Brugada phenotype is attributed to the higher density of I_to in the RV epicardium compared to the LV, the thinner wall of the RVOT (which accentuates transmural gradients), and the unique embryologic origin of the outflow tract from neural crest cells. Fibrosis and structural abnormalities have been demonstrated in the RVOT epicardium of BrS patients on detailed imaging and histopathologic studies, blurring the boundary between BrS and arrhythmogenic right ventricular cardiomyopathy (ARVC).

Fever is a well-recognized trigger for unmasking the Brugada ECG pattern and precipitating VF. Temperature-dependent sodium channel dysfunction — further reduction of I_Na at febrile temperatures — explains this phenomenon. Patients with BrS must be counseled to treat fever aggressively with antipyretics. Other triggers include large meals (vagotonic), alcohol excess, and certain medications (sodium channel blockers, psychotropic drugs, and anesthetics).

Autonomic modulation plays a central role: vagal enhancement (rest, sleep, post-prandial state) augments the Brugada pattern and increases arrhythmic risk, which explains the predominance of events during nighttime and at rest. Conversely, adrenergic stimulation (exercise, isoproterenol infusion) tends to normalize the ECG pattern by enhancing I_Ca,L and restoring the action potential dome. This autonomic dependence is leveraged therapeutically — isoproterenol is used to suppress VF storms in BrS.

The diagnosis of Brugada Syndrome hinges on recognition of the characteristic ST-segment elevation pattern in the right precordial leads (V1–V3). Two distinct morphologies have been defined, and only one is considered diagnostic:

The Type 1 (coved) pattern is the only ECG pattern that is diagnostic of BrS. It is characterized by a prominent, concave or straight ST-segment elevation ≥2 mm (0.2 mV) in one or more of leads V1–V3, followed by an inverted T wave with little or no isoelectric separation. The descending limb of the ST segment is smooth and gradually merges into the negative T wave, creating the classic "coved" morphology. The J-point elevation is ≥2 mm, and the ST segment descends with a convexity upward. This pattern may be present spontaneously or may only be unmasked by pharmacologic provocation or fever.

The Type 2 (saddleback) pattern features a high-takeoff J-point elevation ≥2 mm with an initial concavity (giving a "saddleback" appearance), followed by a positive or biphasic T wave. Importantly, a Type 2 pattern alone is not diagnostic of Brugada Syndrome — it requires conversion to a Type 1 pattern with pharmacologic challenge to confirm the diagnosis. The Type 2 pattern is relatively common in the general population and must be distinguished from normal variants, early repolarization, and athletic heart patterns.

Pharmacologic provocation testing is performed when the baseline ECG shows a Type 2 pattern, a borderline Type 1 pattern, or a normal ECG in a patient with clinical suspicion (unexplained syncope, family history of sudden death, VF survivor). Sodium channel blockers are administered under continuous ECG monitoring in a controlled setting. Ajmaline (1 mg/kg IV over 5 minutes) is the most sensitive agent and is preferred in Europe. Procainamide (10 mg/kg IV over 20 minutes) is commonly used in North America. Flecainide (2 mg/kg IV over 10 minutes, or 400 mg orally) is an alternative. The test is positive if a Type 1 coved pattern appears in ≥1 right precordial lead. The test should be stopped immediately if a Type 1 pattern appears, significant QRS widening (>130% baseline) occurs, ventricular arrhythmias develop, or the patient becomes symptomatic.

High right precordial lead placement significantly increases diagnostic sensitivity. Recording leads V1 and V2 in the 2nd and 3rd intercostal spaces (one or two interspaces higher than standard 4th intercostal space placement) can unmask a Type 1 pattern that is not visible with standard lead placement. This technique should be performed routinely when BrS is suspected, during provocation testing, and during serial ECG monitoring of known BrS patients. The 2013 HRS/EHRA/APHRS consensus statement recommends high lead placement as part of the standard diagnostic workup.

The differential diagnosis of Brugada phenocopies includes conditions that can produce a Brugada-like ECG pattern without the underlying genetic channelopathy. These include right bundle branch block, acute right ventricular ischemia or infarction (especially RV MI), pulmonary embolism, pericarditis, metabolic derangements (hyperkalemia, hypercalcemia, acidosis), hypothermia, mediastinal tumors compressing the RVOT, arrhythmogenic right ventricular cardiomyopathy, pectus excavatum, and various medications (lithium, tricyclic antidepressants, antihistamines, cocaine). A Brugada phenocopy is defined by the absence of a true channelopathy and resolution of the ECG pattern when the underlying condition is corrected.

The role of the electrophysiology study (EPS) in Brugada Syndrome remains one of the most debated topics in clinical electrophysiology. Unlike SVT ablation where the EPS is both diagnostic and therapeutic, the EPS in BrS has been primarily investigated as a risk stratification tool to identify patients at higher risk of spontaneous VF who may benefit from ICD implantation.

Programmed Ventricular Stimulation

Programmed ventricular stimulation (PVS) is performed from the RV apex and/or RVOT using standard protocols with up to three extrastimuli at two drive cycle lengths (typically 600 ms and 400 ms). The endpoint is induction of sustained ventricular fibrillation or polymorphic ventricular tachycardia lasting >30 seconds or requiring cardioversion. The prognostic value of VF inducibility is highly controversial.

The landmark PRELUDE study found that VF inducibility during PVS did not predict arrhythmic events in asymptomatic patients with a spontaneous Type 1 pattern. Similarly, a meta-analysis by Prossano et al. showed limited predictive value, with a sensitivity of ~60% and specificity of ~65% for future events. However, proponents of EPS argue that inducibility identifies a subgroup with a more arrhythmogenic substrate, and some data suggest that inducibility with ≤2 extrastimuli (a more conservative protocol) has better specificity. The current 2022 ESC Guidelines assign EPS a Class IIb recommendation for risk stratification in patients with a spontaneous Type 1 pattern who are otherwise asymptomatic — it may be considered but is not mandatory.

Conduction Abnormalities

EPS in BrS patients frequently reveals conduction system disease consistent with the underlying sodium channel dysfunction. A prolonged HV interval (>55 ms) is found in 20–30% of patients and reflects infra-Hisian conduction delay. Some studies have suggested that an HV interval >65 ms independently predicts arrhythmic events, though this finding has not been consistently replicated. An incomplete or complete RBBB pattern is common and reflects preferential conduction delay in the right ventricle. Prolonged QRS duration during the baseline EPS correlates with more extensive substrate.

Additional EPS findings may include prolonged sinus node recovery time (SNRT), reflecting SCN5A-related sinus node dysfunction, and prolonged atrial effective refractory periods with increased vulnerability to atrial fibrillation. Indeed, AF occurs in up to 10–20% of BrS patients and may be an underrecognized comorbidity.

Epicardial RVOT Substrate Mapping

Detailed substrate mapping of the RVOT epicardium — performed during epicardial ablation procedures — has revealed the electrophysiologic hallmarks of the Brugada substrate. These include fractionated electrograms (multiple deflections within a single electrogram, reflecting disorganized local activation), delayed potentials extending beyond the QRS offset into the ST segment, low-voltage zones (<1.0 mV bipolar) concentrated on the anterior epicardial surface of the RVOT, and late potentials detected on signal-averaged ECG. These abnormal electrograms cluster in the area corresponding to the region of maximal ST elevation and are most prominent after administration of sodium channel blockers (ajmaline or flecainide), which "unmask" the substrate by further reducing I_Na in already-vulnerable tissue.

Shanghai Score System

The Shanghai Score provides a structured diagnostic framework for BrS, integrating ECG findings, clinical history, family history, and genetic data into a points-based system. A score ≥3.5 points is considered probable/definite BrS. Key scoring elements include: spontaneous Type 1 ECG pattern (3.5 points), drug-induced Type 1 pattern (2 points), documented VF or polymorphic VT (3 points), unexplained syncope with suspected arrhythmic mechanism (2 points), nocturnal agonal respiration (2 points), family history of BrS with Type 1 ECG (2 points), family history of SCD <45 years with negative autopsy (0.5 points), and SCN5A pathogenic variant (0.5 points). This system helps standardize the diagnosis, particularly in borderline or ambiguous cases.

Catheter ablation for Brugada Syndrome has emerged as a transformative therapy since the landmark work of Nademanee et al. in 2011, demonstrating that epicardial substrate ablation of the RVOT can normalize the Brugada ECG pattern and eliminate VF recurrence. Unlike ablation for focal arrhythmias, BrS ablation targets the arrhythmogenic substrate rather than a specific circuit or focus.

Epicardial RVOT Ablation

The procedure is performed via subxiphoid percutaneous pericardial access to reach the epicardial surface of the RVOT. The substrate-based approach targets abnormal electrograms on the anterior RV epicardium, specifically in the area corresponding to leads V1–V3. Using a combination of voltage mapping and electrogram analysis, the operator identifies zones of fractionated electrograms, late potentials (extending beyond QRS offset), and low-voltage areas. These abnormal electrograms are concentrated on the anterior and lateral epicardial surface of the RVOT and proximal RV free wall.

Drug-enhanced mapping is a critical step: intravenous ajmaline or flecainide is administered during the procedure to pharmacologically augment the substrate. After sodium channel blocker infusion, the area of abnormal electrograms typically expands significantly (often doubling in size), revealing additional regions of vulnerable tissue that may not be apparent at baseline. All areas demonstrating abnormal electrograms after drug challenge are targeted for ablation.

Mapping Fractionated and Late Potentials

The hallmark electrograms at ablation target sites include: (1) fractionated potentials with ≥3 deflections and duration >70 ms, reflecting slow and disorganized conduction through fibrotic or dysfunctional myocardium; (2) late potentials that extend beyond the surface QRS offset by >20 ms, representing delayed activation in the substrate region; (3) double potentials indicating areas of functional conduction block; and (4) low-voltage zones with bipolar voltage <1.0 mV surrounded by normal myocardium. Radiofrequency energy is delivered at each site with abnormal electrograms until local signals are eliminated or converted to normal morphology.

Combined Endo-Epicardial Approach

While the predominant substrate is epicardial, up to 15–20% of patients have endocardial abnormalities in the RVOT septum that contribute to the arrhythmogenic substrate. A combined endocardial and epicardial approach is increasingly advocated to achieve complete substrate elimination. Endocardial mapping is performed first via standard femoral venous access, followed by epicardial access if abnormal signals are identified on the epicardium. Some centers perform routine combined mapping for all BrS ablation cases to ensure no residual substrate is left untreated.

Normalization of the Brugada Pattern

The primary endpoint of ablation is normalization of the Brugada ECG pattern. After successful substrate ablation, the coved ST elevation in V1–V3 resolves, and repeat sodium channel blocker provocation no longer induces a Type 1 pattern. This normalization has been shown to be durable in long-term follow-up studies, with Nademanee reporting sustained normalization and freedom from VF in >90% of patients at 5-year follow-up. Elimination of abnormal electrograms on repeat mapping and non-inducibility of VF during PVS are additional procedural endpoints.

ICD Implantation

The implantable cardioverter-defibrillator (ICD) remains the cornerstone of therapy for high-risk BrS patients. Class I indications for ICD implantation include: (1) survivors of cardiac arrest due to VF or hemodynamically significant polymorphic VT (secondary prevention), and (2) patients with a spontaneous Type 1 ECG pattern and documented sustained ventricular arrhythmias. Class IIa indications include patients with a spontaneous Type 1 pattern and a history of syncope judged to be of arrhythmic origin. Asymptomatic patients with a spontaneous Type 1 pattern represent a management challenge — the annual event rate is ~0.5%, and ICD complications (inappropriate shocks, lead failures, infections) accumulate over decades in this typically young population. Risk stratification using clinical features, the Shanghai Score, and potentially EPS guides individualized decision-making.

Quinidine as Pharmacologic Therapy

Quinidine is the primary pharmacologic therapy for BrS. As an I_to blocker, quinidine directly counteracts the ionic mechanism underlying the Brugada substrate by reducing the transient outward potassium current that drives the transmural voltage gradient. Quinidine has been shown to normalize the Brugada ECG pattern, suppress VF inducibility during EPS, and reduce spontaneous arrhythmic events. It is indicated for: (1) patients with recurrent ICD shocks or VF storms as adjunctive therapy, (2) patients who are not candidates for or refuse ICD implantation, (3) pediatric patients in whom ICD implantation is challenging, and (4) as bridge therapy while awaiting ICD implantation. The typical dose is 300–600 mg twice daily, with monitoring of the QTc interval (quinidine also blocks I_Kr and can prolong QT). The QUIDAM trial confirmed its efficacy in reducing VF inducibility.