In cases of autopsy-negative SUD, a cardiac and genetic evaluation of first and second-degree relatives and/or a molecular autopsy may elucidate the underlying mechanism of the sudden death. Although arrhythmias can be isolated events, some may represent manifestations of inherited arrhythmia syndromes with increased risk for syncope, cardiac arrest, or SCD in relatives when there is a family history of sudden death. Thus, first-degree relatives of the decedent should have a comprehensive cardiovascular evaluation including an extensive personal and family history, physical examination, 12-lead electrocardiogram, treadmill stress test, 24-hour Holter monitoring, and an echocardiogram.

In 2003, Behr et al. performed a detailed cardiovascular evaluation of 109 first-degree relatives of 32 cases of SUD and showed that 22% of these families had evidence of inherited cardiac disease with the majority having features suggestive of LQTS (10). Similarly, in 2005, Tan et al. found that 28% of families had an identifiable cardiac channelopathy including CPVT and LQTS following a clinical assessment of first-degree relatives of young SUD victims (25). In a 2008 follow-up study by Behr et al., a diagnosis of inheritable heart disease was made in 53% of first-degree relatives of SUD victims following a comprehensive clinical evaluation; 70% being diagnosed with either LQTS (53%) or BrS (17%). Strikingly, 30% of the families evaluated reported a family history of additional unexplained premature sudden deaths under the age of 45 years, and nearly 20% of the decedents had a prior history of syncope.

In 2010, van der Werf et al. identified a certain or probable diagnosis in 47 (33%) of 140 families of SUD victims (aged 1 to 50 years) following a clinical cardiac assessment. Ninety-six percent of these 47 families were diagnosed with an inherited cardiac disease (21% LQTS, 17% CPVT, 15% BrS, and 15% ARVC) (26). The diagnostic yield among families depended upon the age of the decedent ranging from a high of 70% when the decedent was between ages 1 and 10 years to a low of 21% when the decedent was between 41 and 49 years of age. Many of these sudden death victims had antecedent warning signs prior to their own death, including syncope in 15% and a family history of young sudden death in 29%. However, there was no prior clinical diagnosis of an inherited cardiac disease for either the decedent or any other family member. In 2014, van der Werf et al., reported a low rate of cardiac events among 405 (267 were <50 years of age) first-degree relatives from 83 of their diagnosis-negative families from their 2010 study (27). Eleven of the 405 (2.7%) relatives died of a cardiac or unknown cause during a median follow-up period of 6.6 years; 5 were over the age of 75 years. Only one individual was younger than 50 years (0.4% [1/267] of this age group), a 10-year-old female who died suddenly while watching television. Her cardiologic evaluation performed 3 months prior was unremarkable. This young girl was identified as belonging to a particularly malignant family where four of seven children have experienced either a SUD or successfully resuscitated cardiac arrest prior to their 17th birthday. In addition, van der Werf reported that sudden death occurred in four non–first-degree relatives in four families with all sudden deaths occurring during the fourth decade of life (27).

In 2012, Caldwell et al. diagnosed 45 (23%) patients from 38 (35%) families with an inheritable cause of sudden death following their cardiologic evaluation of 193 individuals from 108 families because of SUD or aborted cardiac arrest in a young relative (28). Among 146 relatives from 84 families where the index case was found to have a structurally normal heart, 31 (21%) individuals from 25 (30%) families were diagnosed. The most common diagnosis was LQTS (12/31, 39%) followed by HCM (6/31, 19%), CPVT (5/31, 16%), DCM (4/31, 13%), BrS (2/31, 6.5%), ARVC (1/31, 3%), and LV noncompaction (1/31, 3%). Evaluation of the relatives of SUD victims with postmortem evidence of structural heart disease revealed an inherited cause in 14/47 (30%) individuals in 10/24 (42%) families. The most common diagnosis among the 14 individuals was ARVC (6/14, 43%) followed by HCM (5/14, 36%) and DCM (3/14, 21%) (28).

Incomplete penetrance and variable expression are hallmarks of the various cardiac channelopathies which lead to “concealed” forms of these disorders (29). Therefore, clinical assessment of surviving family members of SUD victims may be insufficient to detect LQTS, CPVT, or BrS in unsuspected individuals. A molecular autopsy involving targeted cardiac channel genetic testing is a tool for the forensic pathologist/medical examiner/coroner in order to try to provide the answer to unexplained deaths in the young and subsequently benefit other family members.

In 2004, Chugh et al. identified 12 cases of SUD following a comprehensive postmortem analysis of a consecutive series of 270 adult (age ≥20 years) cases of SCD occurring over a 13-year period (21). Postmortem genetic analysis of the LQTS-susceptibility genes revealed an identical KCNH2 mutation in 2 of these 12 (17%) cases of autopsy-negative SUD. Similarly, Di Paolo et al. performed LQTS molecular autopsies on 10 cases of juvenile (ages 13 to 29 years) SUD and identified KCNQ1 mutations in two individuals (30).

In 2007, Tester and Ackerman completed the largest molecular autopsy series of SUD to date (31,32). Comprehensive mutational analysis of all 60 translated exons in the LQTS-associated genes (KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2) along with targeted analysis of the CPVT1-susceptibility gene (RYR2) was conducted on a series of 49 medical examiner referred cases of SUD. Over one-third of these SUD cases had a presumably pathogenic cardiac channel mutation with mutations in RYR2, alone, accounting for nearly 15% of the cases (31,33). In this series, sudden death was the sentinel event in all but four mutation-positive SUD cases. However, many had a positive family history of cardiac events yet no family members had been diagnosed with either LQTS or CPVT. Overall, approximately half of the 17 decedents, with a cardiac channel mutation detected by postmortem genetic testing, exhibited potential warning signs, either personally or in the family. The repeated observations of unheeded warning signs of syncope and/or family history of sudden death in nearly 15% to 30% of young SUD victims emphasize the importance of heeding potential warning signs.

Population-based studies involving the evaluation of relatives and molecular autopsy investigations of the decedent show that identifiable and potentially treatable cardiac channelopathies (including LQTS, CPVT, and BrS) account for approximately one-third of autopsy-negative SUD in the young. In addition, approximately 10% to 15% of SIDS may be due to these cardiac channelopathies (19,34,35,36,37,38,39,40).

Since an accurate diagnosis from a postmortem molecular analysis of an SUD victim may benefit surviving family members who may also be susceptible to life-threatening arrhythmia syndromes, recent guidelines suggest that postmortem genetic testing should become a standard of care in the evaluation of SUD cases
(24,41,42,43). With now over 100 sudden death-susceptibility genes discovered, next-generation WES, allowing the simultaneous genetic analysis of an individual’s entire library of ∼20,000 genes, is an attractive, cost-effective ($1,000 to $2,000 per sample), and time conducive (few weeks) technique for a comprehensive postmortem genomic study (44). In fact, we recently provided the first ever proof-of-principle case report of a WES-based comprehensive molecular study of a previously healthy 16-year-old sudden death victim, where we identified a pathogenic MYH7 mutation, previously associated with familial HCM, sudden death, and impaired MHC-beta actin-translocating and actin-activated ATPase activity (44). Subsequently, Bagnall et al. completed a WES-based postmortem genetic analysis in their cohort of 28 SUD cases and identified 2/28 (7%) cases with rare variants among the most common LQTS genes (KCNQ1, KCNH2, and SCN5A) and another 6 rare variants among an additional 25 common cardiac channelopathy/cardiomyopathy genes in 6 (21%) SUD cases (45). While the comprehensive nature of WES may be beneficial, it creates a daunting task of scrutinizing thousands of nonsynonymous variants per case. In fact, in our recent exome-based molecular analysis of 14 autopsy-negative SUD cases, each case hosted ∼12,000 nonsynonymous (amino acid altering) variants with an average of 80 such variants localizing to 117 sudden death-susceptibility genes (46). Furthermore, 50% of the SUD cases hosted at least one ultra-rare variant among the 117 sudden death-susceptibility genes with nearly 43% having mutations in cardiomyopathy-associated genes, despite having an autopsy examination void of any overt structural pathology. Therefore, it must be keenly understood that WES findings must be interpreted with extreme caution, particularly in the setting of an autopsy-negative SUD. Without proper interpretation, WES-based postmortem analysis may potentially lead to an inappropriate genetic diagnosis in the SUD case and subsequently in surviving family members who may undergo confirmatory genetic testing, despite being phenotypically normal.

Congenital LQTS is the prototypic cardiac channelopathy with an estimated prevalence of 1 in 2,000 to 2,500 persons. Clinically, LQTS is characterized by abnormal cardiac repolarization resulting in QT interval prolongation (Fig. 20.2A) which predisposes patients to torsade de pointes (TdP, Fig. 20.2B). Palpitations seldom
represent the sole indicator of an episode of TdP. The most common form of LQTS is autosomal dominant LQTS, previously known as Romano–Ward syndrome (51,52).

Hundreds of mutations in 16 distinct LQTS-susceptibility genes have been identified and generally involve either loss-of-function potassium channel mutations or gain-of-function sodium channel mutations. Most recently, gain-of-function mutations in the CACNA1C-encoded L-type calcium channel alpha-subunit have been implicated in pure, “non-Timothy syndrome” LQTS and mutations in three distinct genes (CALM1, CALM2, and CALM3) encoding for the identical calmodulin protein are LQTS-causative (53,54,55). Except for five rare subtypes that stem from perturbations in key cardiac channel interacting proteins (ChIPs) or structural membrane scaffolding proteins (ankyrin B-, calmodulin-, caveolin 3-, yotiao-, and syntrophin alpha-LQTS) (54,55,56,57,58,59), LQTS is a pure “channelopathy” resulting from mutations in cardiac channel alpha and beta subunits. The majority of LQTS is due to mutations in either the KCNQ1-encoded I_Ks potassium channel (LQT1, 30% to 35%), the KCNH2-encoded I_Kr potassium channel (LQT2, 25% to 30%), or the SCN5A-encoded I_Na sodium channel (LQT3, 5% to 10%) (60,61).

The past two decades of LQTS research have provided numerous genotype–phenotype relationships (Fig. 20.3). Genotype–ECG relationships include broad-based T waves in LQT1, low-amplitude or notched T waves in LQT2, and long ST isoelectric segment with normal T-wave morphology in LQT3 (62,63). Gene-specific arrhythmia triggers have been observed including: exertion in LQT1 (particularly swimming), auditory triggers and the postpartum period in LQT2, and events during sleep in LQT3 (64,65,66,67,68). Importantly, the response to beta-blockers is influenced strongly by the underlying genotype where patients with LQT1 realize superior protection than patients with either LQT2 or LQT3 (69).

In the presence of a clinical diagnosis of LQTS, the yield of LQTS genetic testing is about 75% (71). Generally accepted indications for LQTS genetic testing are summarized in Table 20.1. However, genetic tests for LQTS and the other cardiac channelopathies must be scrutinized and interpreted with great caution and must not be viewed as binary tests (42,70,72). These genetic tests are probabilistic tests. Some mutations will be definite disease-causative mutations while other genetic variants may not be pathogenic (70). We estimate that at least 10% of the mutations published as pathogenic may lack sufficient evidence to warrant that designation.

Individuals with LQTS may or may not display the hallmark repolarization abnormality of QT interval prolongation (Fig. 20.2A). In fact, approximately 40% to 50% of patients with genotype-positive LQTS have a normal resting QTc (29). This observation reinforces the critical role of genetic testing. In general, a heart rate corrected QT interval (QTc) >480 ms in postpubertal females or >470 ms in postpubertal males should prompt a thorough investigation for LQTS. These values represent the 99th percentile in the distribution of QTc values. Previously, a QTc >440 ms (males) or
>450 ms (females) has been considered “borderline” QT prolongation. In fact, the 2009 AHA/ACC/HRS guidelines denote that a QTc ≥450 ms in adult males and ≥460 ms in adult females be considered “prolonged QTc.” Although 50% of patients with genetically proven LQTS have a QTc <460 ms, these cutoff values would result in an unacceptably high rate of false-positives if used as part of a screening program. Using these cutoff values, an estimated 15% to 20% of the entire population would have “borderline QT prolongation” and 5% to 10% of adults would satisfy the guidelines definition of “prolonged QTc” (Fig. 20.4). In addition, these QTc values are based upon an accurate measurement of the QTc (73). It is absolutely critical to measure the QTc manually. Relying on the computer-derived QTc is unacceptable. Calculation of an average QTc from either lead II or V5 is recommended. Simply using the beat yielding the maximum QTc yields too many false-positives. Furthermore, these QTc distributions do not apply to a maximum QTc value derived from a 24-hour ambulatory ECG recording.