Value of electrocardiographic parameters and ajmaline test in the diagnosis of Brugada syndrome caused by SCN5A mutations.

Abstract

Background

The Brugada syndrome is an arrhythmogenic disease caused in part by mutations in the cardiac sodium channel gene, SCN5A. The electrocardiographic pattern characteristic of the syndrome is dynamic and is often absent in affected individuals. Sodium channel-blockers are effective in unmasking carriers of the disease. However, the value of the test remains controversial.

Methods and Results

We studied 147 members representing 4 large families with SCN5A mutations. Of these, 104 determined to be at possible risk for Brugada syndrome were selected for electrocardiographic and genetic evaluation. Twenty-four individuals displayed an ECG diagnostic of Brugada syndrome at baseline. Of the remaining, 71 received intravenous ajmaline. Of 35 positive genetic tests, 28/30 had Ajmaline positive, as opposed to 7/41 with negative Ajmaline test. The sensitivity, specificity, positive and negative predictive value of the drug challenge were 80% (28:35), 94.4%(34:36), 93.3%(28:30), and 82.9%(34:41) respectively. Penetrance of the disease phenotype increased from 32.7 to 78.6% with the use of sodium channel blockers. In the absence of ST segment elevation under baseline conditions, a prolonged PR interval, but not IRBBB or early repolarization patterns, indicate a high probability of an SCN5A mutation carrier.

Conclusions

In families with Brugada syndrome, the data suggest that ajmaline test is valuable in the diagnosis of SCN5A carriers. In the absence of ST segment elevation at baseline, family members with first degree AV block should be suspected of carrying the mutation. Ajmaline test is often the key to making the proper diagnosis in these patients.

Short Abstract

The identification of SCN5A as responsible for Brugada syndrome allows validation of the diagnostic tools and electrocardiographic parameters used to identify the syndrome in patients with no ST segment elevation. We analyzed 4 families carrying different SCN5A mutations. Penetrance of the disease phenotype increased from 32.7 to 78.6% with the use of sodium blockers, with a sensitivity of 80% and specificity of 94.4%. First degree AV block suggests that a family member may be a carrier of the disease gene. The presence of IRBBB was not different among genetic carriers versus non-carriers. Clinicians should not rely in this parameter to suspect a positive diagnosis.

Introduction

The Brugada syndrome is an arrhythmogenic disease characterized by the occurrence of sudden death in young individuals with a characteristic electrocardiographic pattern of ST segment elevation in leads V1 to V3¹. Sudden cardiac death is most commonly secondary to the development of polymorphic ventricular tachycardia and fibrillation. The disease displays an autosomal dominant pattern of transmission. To date, one gene has been identified. Mutations in SCN5A, the α subunit of the sodium channel, account for approximately 20% of familial cases of the disease¹. Causative mutations reduce the availability of sodium channel current. Spontaneous fluctuation of the electrocardiogram, to the point of normalization of the ST pattern, makes identification of individuals at risk for sudden death difficult. The electrocardiographic pattern of ST segment elevation in V1 to V3 can be unmasked in patients with a normal ECG with the use of potent sodium channel blockers². These tests, performed under close monitoring due to the possible inducibility of malignant arrhythmias, have become standard procedure in the differential diagnosis of malignant arrhythmias in patients with a structurally normal heart. This year, Rolf et al³ published a comprehensive assessment of the usefulness of ajmaline test in a large patient population, assessing the risks, diagnostic impact and protocol.

The lower sensitivity of some of the sodium channel blockers, the variable electrocardiogram and the lack of gold standard in the diagnosis of Brugada syndrome continue to burden our ability to properly risk stratify some of the individuals. It is not clear whether identification of the genetic defect is the best gold standard in this disease, but genetics is emerging as an accurate test, keeping in mind limitations like penetrance, expression, gender, autonomic factors, that will play a role in delineating the final phenotype of the individual and the risk of arrhythmia. In a family with Brugada syndrome, those who are not carriers of the familial mutation (except in those rare instances where there are two mutations in the family) can be considered spared of the familial disease. And likewise those who are carriers of an ion channel mutation in SCN5A are potentially at higher risk for the development of malignant arrhythmias. Genetic testing in Brugada syndrome permits accurate assessment of familial penetrance of the disease, and of the value of the ajmaline test and baseline electrocardiographic parameters in identifying genetic carriers in a family. Our principal objective in this study is to contrast the results of an ajmaline challenge and genetic screening in four large families with a known SCN5A mutation so as to assess the sensitivity and specificity of the pharmacological test. Our secondary aim is to assess the value of other electrocardiographic parameters in the identification of possible genetic carriers.

Methods

1. Clinical Analysis

We characterized four large European families comprised of 147 members (figure 1). The ECG was considered positive if the ST segment was elevated by at least 0.2 mV in at least one of the precordial leads before or after ajmaline (figure 2)⁴. First degree AV block was defined as a PR longer than 200 msec. TpTe was defined as the time in msec from the peak of the T wave to the end of the T wave. Right Bundle Branch Block (RBBB) was defined as the presence of a prolonged QRS complex (≥120 msec), rsr′, rsR′ or rSR′ pattern in V1 and/or V2 and wide and deep S waves in the left precordial leads, and a normal R peak time in leads V5 and V6, but ≥50 ms in lead V1⁵. Incomplete RBBB was defined as the presence of rSR′ pattern in V1 and/or V2 and QRS complex less than 120 msec. One hundred and sixteen members were relatives at possible risk of Brugada syndrome, of whom 8 were deceased with no information available (table 1). Each of them underwent a complete physical examination and a 12-lead ECG. Twenty-four had a positive ECG at baseline. Of the remaining 84 subjects with a negative ECG at baseline, 71 (29 males and 42 females) received intravenous class I blocker (ajmaline 1mg/kg over 5 minutes).

Figure 1.

Pedigrees of the four families with SCN5A mutations. Squares represent males and circles females.

Figure 2.

Example of positive and negative ajmaline test in two different patients with SCN5A mutation

Table 1.

Genetic data and response to ajmaline in the 4 families.

	Family 24-011	Family 24-228	Family 24-064	Family 24-104	Total
Family members	38	40	50	19	147
Spouses	8	8	12	3	31
Members at risk	30	32	38	16	116
No clinical or genetic information	4	2	2	0	8
Genotype N/A, ECG+	1	1	1	1	4
Genotype +	13	18	22	8	61
Genotype +, ECG +	3	9	6	2	20
Genotype +, AJM +	8	6	10	4	28
Genotype +, AJM −	1	2	4	0	7
Genotype +, AJM N/A	1	1	2	2	6
Genotype −	12	11	13	7	43
Genotype −, AJM +	0	2	0	0	2
Genotype −, AJM −	10	8	12	4	34
Genotype −, AJM N/A	2	1	1	3	7
Mutation	IV27S+7insGGG	R367H	R769C	T1620M

AJM: ajmaline test.

ECG +: individual with a positive electrocardiographic pattern at baseline

2. Genetic Analysis

The study was approved by the Regional Institutional Review Board and written consent was obtained from study participants. Blood samples (10 ml) were obtained from participating family members and spouses. Genomic DNA was isolated from peripheral blood leukocytes using a commercial kit (Gentra System, Puregene).

The exons of SCN5A were amplified and analyzed by direct sequencing. PCR products were purified with Exosap (USB) and were directly sequenced from both directions with the ABI PRISM BigDye Terminator Cycle Sequencing Reaction and the ABI PRISM 3100-Avant Automatic DNA Sequencer.

Statistical Analysis

Electrocardiographic data were analyzed by unpaired t test and a value of P<0.05 was accepted as significant. The significance of the incidence of 1^st degree AV block, complete RBBB, and incomplete RBBB between carriers without ST elevation and non-carriers was calculated by the Pearson chi-square test. The significance of the difference in PR, QTc, ST elevation, and Tp-Te between the same groups of patients was calculated by ANOVA with Scheffe’s test for post hoc analysis. Data are presented as mean±SD.

Results

Genetic data

All 4 families had mutations in SCN5A. Genotype was positive for the SCN5A mutations in 61 of 104 individuals (27 males, 34 females) (Table 1). All 20 patients with positive basal ECG and genetic testing had the mutation.

Electrocardiographic parameters

Electrocardiographic data are shown in table 2. There were 5 patients with RBBB (3 with ST segment elevation and two without) who were carriers of the genetic mutation. None of the non-carriers had a complete RBBB. Individuals with a sodium channel mutation had a longer PR interval than individuals without the mutation. First degree AV block defined as a PR interval > 200 ms was more frequent in patients with the mutation (8/41 vs 2/43) (P ≤ 0.05). QT, Tp-Te and the presence of incomplete RBBB, were no different between carriers and non-carriers.

Table 2.

Electrocardiographic parameters in patients and family members according to Genotype and presence of ST segment elevation in the precordial leads.

	Baseline ECG +	Genotype + no ST elevation	Genotype −	P value
	Total (n=24)	Total (n=41)	Total (n=43)
PR (msec)	206±32	204±13	164±12	P≤0.001
1st deg AV block	5/24	8/41	2/43	P<0.05
QRS (msec)	105±10	106±4	91±2	P=0.08
RBBB	3/24	2/41	0/43	P=0.16
Rsr′	7/24	6/41	13/43	P=0.09
QTc (msec)	412±22	413±14	406±13	P=0.347
ST(V2) (mV)	0.21±0.06	0.05±0.007	0.05±0.004	P=0.08
TpTe (msec)	92±3	82±1	81±1	P=0.30
Ajmaline test	N/A	n=35	n=36
Positive		28/35	2/36	P<0.001
Negative		7/35	34/36	P<0.001
not done		6/41	7/43

RBBB=Right bundle branch block; Tp−Te=interval between peak and end of T wave. Statistical comparison (P value) was made between Genotype (+) without ST elevation and Genotype (−) groups of patients. Baseline ECG +: individual with a positive electrocardiographic pattern at baseline

Ajmaline test

Of the 71 individuals who received the ajmaline test, 30 developed the typical electrocardiogram pattern (positive ajmaline test) and 41 did not (negative ajmaline test) (table 2). Twenty eight patients with positive and 7 patients with a negative ajmaline test had the mutation. Therefore two patients with positive ajmaline test did not have the mutation, and 7 patients with a negative test had the mutation. Penetrance of the disease phenotype increased from 32.7 to 78.6% with the use of sodium channel blockers. The sensitivity, specificity, positive and negative predictive value of the ajmaline test were 80%, 94.4%, 93.3% and 82.9% respectively.

Discussion

This paper provides a comprehensive assessment of the use of ajmaline test and electrocardiographic parameters, other than the ST segment elevation, to identify family member carriers of a mutation in SCN5A.

We showed in 2000 that the appearance of ST segment elevation in the right precordial leads during infusion of intravenous sodium channel blocker correlated well with the presence of an SCN5A mutation⁶. The power of this test to uncover patients at risk for sudden death has been a matter of controversy. Recent publications have suggested that sodium channel blockers may not be as useful in making the diagnosis as previously claimed⁷ and has lead to the search for electrocardiographic parameters that may be of help in identifying concealed carriers. Consistent with the observation that some patients with SCN5A-mediated Brugada syndrome have conduction impairment, and that there is a link between sodium channel mutations and progressive conduction disease, Smits et al⁸ showed a prolonged PR interval in Brugada syndrome patients with SCN5A mutations when compared to those without an identified mutation. While this might suggest that family members with no ST segment elevation displaying conduction disease may be carriers of the genetic mutation, the study did not address this issue since it only assessed individuals affected with the disease, and not their family members. Despite the absence of scientific evidence, when faced with a family with Brugada syndrome, some physicians have taken minimal conduction alterations and incomplete RBBB, very common electrocardiographic patterns in the population, as possible indicators of a mutation carrier, with important implications in these usually young individuals.

Our findings support the contention that the electrocardiographic signature of the syndrome is dynamic and often concealed and that it can be unmasked by potent sodium channel blockers such as ajmaline. We indicate in this paper that the sensitivity of ajmaline test in carriers of SCN5A mutations is 80% and the specificity 94.4%. Two individuals are positive despite the lack of the familial mutation. Whether they are false positive or carriers of a second mutation in another gene is unknown. The use of ajmaline test increases the penetrance of the disease phenotype from 32 to 78%, confirming the usefulness of the test in identifying carriers.

In a family with Brugada syndrome, the electrocardiogram at baseline is of limited value in the absence of ST segment elevation. Clinicians should only make a positive diagnosis when the electrocardiogram presents a “coved-type” ST segment elevation. In our data 40% of the patients with SCN5A mutations present with this pattern at baseline. Clinicians should suspect family members of being carriers if they show first degree AV block, and/or saddle-back ST segment elevation without structural heart disease. A saddle-back pattern may be present in 19% of patients with SCN5A mutations who do not display a coved-type ECG. However, as shown in figure 3, these abnormalities may be present in the general population and may confound the diagnosis. For this reason a positive diagnosis for Brugada syndrome should never be based solely on the presence of a saddle-back pattern and further testing is recommended. Likewise, while RBBB was only present in carriers in our study, the data did not reach statistical significance. These patients also require further workup. Finally, physicians should not draw any conclusions as to whether an individual is a carrier based on the presence of IRBBB or minimal alterations in conduction.

Figure 3.

Electrocardiogram at baseline showing precordial leads of 4 different patients without SCN5A mutation. Examples of non-diagnostic patterns which could confound the diagnosis.

Limitations

Studies have been performed comparing the use of ajmaline and other class I sodium blockers. These studies indicate that ajmaline is the most potent agent to unmask the pattern. It is therefore possible that other sodium channel blockers may yield a lower sensitivity.

Our data assess the use of a sodium block test in a selected population of SCN5A carriers. Because the gold standard used is the genetic data, it is not possible at this point to know whether these data will hold in families with other genetic defects.