1 Introduction
When sudden death (SD) occurs in adults and elderly persons, coronary atherosclerosis
is the usual cause [1,2]. On the contrary, a large spectrum of cardiovascular diseases,
both congenital and acquired, may account for SD in the young [3–10]. These diseases
are frequently concealed and discovered with surprise only at postmortem by means
of a thorough macroscopic and microscopic investigation. This review will address
the spectrum of structural substrates of cardiac SD with particular emphasis given
to the possible role of molecular biology techniques in identifying subtle or even
merely functional disorders accounting for electrical instability.
2 Epidemiology, pathophysiology and substrates of cardiac SD
SD is defined as a natural, unexpected fatal event occurring within 1 h of the beginning
of symptoms, in an apparently healthy subject or one whose disease was not so severe
enough as to predict such an abrupt outcome [11]. In the USA, the annual incidence
of SD in people aged 35–74 years is 191/100 000 in men and 57/100 000 in women; almost
half of all SDs occur in people with known coronary artery disease [12]. In the Veneto
region, Northeast of Italy, we recently calculated an overall prevalence of SD of
0.8/100 000/year in the young, based only upon autopsy reports [13]. When focusing
the attention only on young athletes the prevalence was twice that in young non athletes,
i.e. 1.6/100 000/year; these figures are explained by the existence of cardiovascular
diseases which cause a risk of SD during effort, such as hypertrophic cardiomyopathy
(HCM), arrhythmogenic right ventricular cardiomyopathy (ARVC) and congenital coronary
anomalies.
As far as pathophysiology is concerned, cardiac arrest may be either mechanical, when
the heart and circulatory functions are suddenly impeded by mechanical factors (i.e.
cardiac tamponade, pulmonary thromboembolism, etc.) or arrhythmic (mostly ventricular
fibrillation) [9].
Based upon the Veneto Region study project on juvenile SD, cardiovascular diseases
accounted for more than 80% of cases and about one third of events were due to a congenital
heart defect present since birth [8,9,13]. Table 1 reports the main causes of cardiovascular
SD in the young in major series which have been published in English. In our experience,
the most common causes include premature coronary atherosclerosis (21%), ARVC (14%),
mitral valve prolapse (12%), non-atherosclerotic coronary artery disease (11%), myocarditis
(10%), conduction system disease (9%) and HCM (7%). It is noteworthy that 6% of all
SDs remained unexplained even after thorough macroscopic and histologic examination.
Whether these are truly idiopathic or unexplained due to an inability to identify
subtle pathologic substrates remains to be elucidated. It may be that structural abnormality
resides at molecular level, thus enhancing the need for molecular biology investigation
[14].
Table 1
Major causes of cardiac SD in young people
Authors, time interval
Population
Age (years)
Total no.
Causes
a
(%)
Drory et al. 1976–85
Israel
9–39
118
Atherosclerotic CAD (58)
Myocarditis (25)
HCM (13)
Conduction system (4)
Neuspiel and Kuller 1972–80
Allegheny County, US
1–21
51
Myocarditis (27)
DCM (24)
Conduction system (12)
Aortic dissection (6)
CAA (6)
Atherosclerotic CAD (4)
Topaz and Edwards 1960–83
St. Paul, Minnesota, US
7–35
50
MVP (24)
Myocarditis (25)
HCM (12)
CAA (4)
Aortic stenosis (4)
Phillips et al. 1965–85
American forces, US
17–28
20
Myocarditis (42)
CAA (15)
HCM (10)
MVP (5)
Atherosclerotic CAD (5)
Aortic stenosis (5)
Kramer et al. 1974–86
Soldiers, Israel
17–30
24
Myocarditis (29)
HCM (25)
MVP (13)
Atherosclerotic CAD (13)
Aortic dissection (8)
CAA (4)
DCM (4)
Conduction system (4)
Basso (present study) 1979–1999
Veneto region Italy
1–35
273
Atherosclerotic CAD (21)
ARVC (14)
Valve disease (12)
CAA (11)
Myocarditis (10)
Conduction system disease (9)
HCM (7)
a
Abbreviations: ARVC, arrhythmogenic right ventricular cardiomyopathy; CAA, coronary
artery anomaly; CAD, coronary artery disease; DCM, dilated cardiomyopathy; HCM, hypertrophic
cardiomyopathy; MVP, mitral valve prolapse.
3 New molecular assays: perspectives of application in the study of SD
The principle of all molecular hybridization assays is the complementary base pairing
between two nucleic acid strands. In situ hybridization provides the direct detection
of nucleic acid in cellular material in which simultaneous morphological analysis
can be performed. Polymerase chain reaction (PCR) allows millions of copies of any
specific DNA sequence to be generated in a few hours. The reaction consists of an
in vitro enzymatic amplification of defined DNA sequence by repeated rounds of heat
denaturation, primer annealing and DNA polymerase-mediated primer extension. The amplified
DNA can then be seen as a distinct band after standard agarose gel electrophoresis,
and the specificity of detection can be increased by subsequent hybridization or DNA
sequencing. According to the nested PCR technique, a second pair of primers ‘internal’
to the original primer pair used in a subsequent series of amplification cycles. Using
this strategy, the sensitivity is enhanced from 100 to 1000 times, to such an extent
that even a single copy target can be detected in a complex background of 300 000
cells or more.
One of the most important and frequent applications of these novel techniques is for
the identification of microbial pathogens. Given the extreme sensitivity of these
techniques, particularly of PCR, a single copy of a gene can be readily detected even
from an extremely small amount of tissue. The decision to develop and apply PCR for
routine diagnosis of myocarditis must be considered in relation to the low cost, speed,
sensitivity and reliability of more conventional culture and/or serological methods
[15]. Different molecular strategies have now been developed to recognize and distinguish
an infective from a latent state of virus. In addition to qualitative analysis and
genome sequencing different methods have been developed for the quantification of
PCR product. Quantification of different viral genome has now successfully introduced
into routine diagnostic virology as a prognostic marker of severe infective disease.
Genome quantification may also allow to accurately assess the risk of infection and
to monitor the therapeutic response [16].
Moreover, PCR may detect virtually all the common genetically inherited diseases in
which the defective gene has been identified such as Duchenne muscular dystrophy,
Marfan syndrome and HCM.
PCR can also trace the inheritance of diseases in which the defective locus has only
been defined in terms of linkage to other cellular genes. Using PCR, allelic forms
of many cellular genes can be identified by sizing selected introns between the coding
exons. This approach should provide much more detailed linkage studies than are presently
possible, using restriction fragment length polymorphism analysis.
A variety of types of samples may be used for PCR analysis. Extracted nucleic acid
may be easily amplified, even in a partially purified sample. Extracted DNA or RNA
from formalin-fixed, paraffin-embedded samples obtained either at autopsy or at surgery,
are successfully used as templates for the PCR. This could permit an ever more frequent
application not only for perspective but also for retrospective studies of either
genetically determined or infective cardiovascular diseases. The recent description
of a successful identification 22 years later of inherited long QT syndrome in a 12-year-old
girl who had died suddenly, through molecular testing of archived paraffin-embedded
myocardial samples, is emblematic [17]. First of all, this result has potentially
great importance for forensic science by providing a plausible mechanism for cardiac
arrest in the setting of an otherwise structurally normal heart. Moreover, thanks
to this molecular biology finding, the presence of the disease-causing mutation was
subsequently demonstrated in one of the decedent's parents who also had a diagnostic
QTc with inherent treatment and prevention strategies.
However, it is important to stress that random search of any gene in formalin-fixed
material is not affordable and the results are often disappointing. Thus, such investigation
should be performed only in selected cases in which clinical or morphologic phenotype
should suggest a peculiar genotype.
In the following sections we will analyze those cardiovascular diseases either acquired
from an infectious pathogenesis or genetically determined, where availability of molecular
biology techniques will open new avenues of investigation. In some instances, such
as in the setting of dilated and ARVC, molecular pathology techniques will be of help
for both genetic screening and the search for viral genomes.
4 Cardiovascular diseases at risk of SD: potential targets of molecular pathology
investigation
4.1 Infective cardiac disease
4.1.1 Myocarditis
Myocarditis accounted for up to 44% of fatal events in major series on cardiac SD
in the young [18]. The strongest evidence that subclinical myocarditis can be a cause
of ventricular fibrillation comes from an autopsy series on USA army recruits in which
42% of those who died suddenly had histological evidence of myocarditis [19]. Whether
the high incidence of myocarditis in this series is due to a selection bias towards
more tropical diseases as well as to an overestimation of inflammatory infiltrates
due to the absence of standardized diagnostic criteria remains intriguing. Although
myocarditis usually presents with signs of pump failure and ventricular dilatation,
ventricular arrhythmias have been described in patients with myocarditis and apparently
normal hearts. A recent flu-like illness is common, although the symptoms may be mild
and clinical signs of heart failure subtle or absent. Cardiac involvement is unpredictable
and may affect the conduction system, causing heart block, or the ordinary myocardium,
causing ventricular arrhythmias. SD may occur both in the active or healed phases
as a consequences of life-threatening ventricular arrhythmias that develop mostly
in the setting of an unstable myocardial substrate, namely inflammatory infiltrate,
interstitial edema, myocardial necrosis and fibrosis. The gross appearance of the
heart is not distinctive and its weight may be within normal values. A major limitation
in the autopsy diagnosis of myocarditis in several series has been the lack of standardized
histological criteria. A ‘starry-like sky’ feature (>14 leucocytes/mm2) or a patchy
inflammatory infiltrate, either polymorphous or lymphocytic, sometimes no more than
3 foci at a magnification of 6×, and not necessarily associated with myocyte necrosis,
are typical features. This subtle substrate, together with the possible inflammatory
involvement of the conduction system, seems highly arrhythmogenic and may account
for unexpected arrhythmic cardiac arrest. To increase diagnostic sensitivity of histology,
use of immunohistochemistry by means of a large panel of monoclonal and polyclonal
antibodies is mandatory to identify and characterize the inflammatory infiltrate.
Evidence of myocardial infection, whether bacterial or viral, has been rarely found.
Chlamydia pneumoniae myocarditis was implicated in the SDs of several young Swedish
elite orienteerers following RNA detection of this organism in the heart of one of
the victims [20]. A subsequent paper implicated this agent in one third of cardiac
SDs among 15 Swedish orienteerers who died unexpectedly between 1979 and 1992 [21].
Nonetheless, viral infections are the most plausible cause. Although enterovirus is
the most important causative agent in the pathogenesis of myocarditis, several studies
have shown that various other viruses, such as adenovirus, herpesvirus (cytomegalovirus,
herpes simplex virus, Epstein Barr virus) parvovirus, influenza virus A and B, and
hepatitis C virus can be involved in myocardial infective disease, particularly in
the pediatric population [15]. Viruses rarely cause evident cytopathic damage, so
that classical morphology often fails to identify the etiological agents. The diagnosis
of viral myocarditis has, for a long time, been based on viral culture and serology.
However, these investigations are time consuming and generally fall short in specificity
and sensitivity. More recently, molecular biology techniques as PCR and nested-PCR
have been shown to rapidly detect the presence of infective agents, in specific and
very sensitive way. Their application is now successfully carried out also on tissues
in which nucleic acid could be partially degraded such as autoptic or paraffin embedded
samples (Fig. 1) [22,23].
Fig. 1
A 17-year-old, previously asymptomatic, boy who died suddenly at rest. View of the
heart specimen removed at autopsy which is grossly normal. At histology, the left
ventricular myocardium shows a polymorphous inflammatory infiltrate associated with
myocyte necrosis (hematoxylin—eosin ×240). Immunohistochemistry reveals a massive
T lymphocytes infiltrate (CD43 ×240). Agarose gel electrophoresis showing enteroviral
RT-PCR results: Lanes: 1, DNA molecular-weight-marker (pUCBM21 DNA cleaved with Hpa
II and Dra I plus Hind III) 2, RT-PCR amplified products of patient's specimen (coxsackievirus
B3 KB infected cells) 3, negative control (uninfected cells) 4, RT-PCR amplified products
of positive control (coxsackievirus B3 KB infected cells).
4.1.2 Unstable coronary plaque
The coronary artery pathology in SD adult victims consists of single, double or triple
vessel atherosclerotic disease and usually includes a thrombotic occlusion of a coronary
segment, which accounts for sharp interruption of the regional myocardial blood flow
[1,2]. On the contrary, coronary SD in the young is usually due to a single subobstructive
plaque, located at the first segment of the anterior descending coronary artery, mostly
fibrocellular, devoid of atheroma, fissuring or thrombosis [7] (Fig. 2). In the setting
of acute thrombosis, superficial erosion seems to be a peculiar mechanism precipitating
plaque instability, unlike in adults where it is mainly due to rupture of the thin
fibrous cap of an atheromatous plaque [24]. Endothelial erosion may be the consequence
of either plaque inflammation or intimal smooth muscle cell proliferation [25]. The
inflammatory nature of atherosclerotic plaque components prompted the postulation
that either infection and/or autoimmune phenomena are involved in the onset and progression
of the disease [26]. Several reports, using molecular hybridization assays, have shown
a correlation between the occurrence of atherosclerosis and the presence of infective
microorganisms, like herpesviruses and Chlamydia pneumoniae
[27]. Although a cause—effect relationship is far from being demonstrated and these
microorganisms could act simply as ‘innocent bystanders’, some microbial infections
have been associated with acute coronary syndromes; in particular, an increase in
enterovirus-specific antibodies at the time of myocardial infarction diagnosis suggests
a role for this virus in triggering acute coronary syndromes [28,29], thus opening
new avenues of research in unstable coronary plaques.
4.2 Genetically determined cardiac disease
4.2.1 Cardiomyopathy
HCM is heart muscle disease phenotypically characterized by either symmetric or asymmetric
left ventricular hypertrophy with a chaotic spatial arrangement of the myocytes (‘disarray’),
often marked by SD along the natural history. Heart dysfunction appears more in the
form of electrical instability than of impaired contractility, which, on the contrary,
may be enhanced. Risk factors are considered to be youth, previous syncopal episodes,
a malignant family history, myocardial ischemia, sustained ventricular tachycardia
on electrophysiological test and ventricular tachycardia on holter monitoring, attenuated
blood pressure changes on exercise and enhanced QRS fractionation of the extrasystole
[30]. Recently, molecular genetic studies demonstrated that HCM is a heterogeneous
disease, with several missense mutations in genes encoding for proteins of the cardiac
sarcomere, among which β-myosin heavy chain, cardiac troponin T, α-tropomyosin, myosin
binding C and actinin [31]. Despite data on genotype—phenotype correlations are still
preliminary, it seems that the phenotype varies not only with the type of mutation
but also within individuals carrying the same mutation. For instance the arginine-to-glutamine
mutation in 403 codon of β-myosin is associated with a poor prognosis, whereas the
arginine-to-tryptophan mutation appears more benign [32]. Moreover, the knowledge
that myosin-binding protein C mutations appear to be associated with age-related penetrance
in adulthood and that troponin T mutation is associated with up to 50% of non penetrance,
no or mild hypertrophy (HCM ‘without hypertrophy’) and a high risk of SD, even in
the absence of severe left ventricular hypertrophy, would have consequences for genetic
counselling. The recent pointing out of a high frequency of hepatitis C virus in the
hearts of patients with the apical form of HCM is intriguing and opens new avenues
of investigation [33].
Fig. 2
A 19-year-old athlete who died suddenly at rest. A transverse section at the level
of first tract of left anterior descending coronary artery reveals a concentric, fibrocellular
atherosclerotic plaque which is completely devoid of lipids (Heidenhain trichrome
×12).
At postmortem, the heart shows asymmetric left ventricular hypertrophy, usually in
the basal portion of the ventricular septum but also in the anterior free wall and
apex [34] (Fig. 3). The septal bulging together with a septal endocardial plaque and
anterior mitral valve leaflet thickening, may account for the left ventricular outflow
gradient. The combination of myocardial disarray and interstitial fibrosis represent
an ideal substrate of inhomogeneous intraventricular conduction with potential reentry
phenomena. More recently, a detailed pathology study on young subjects dying suddenly
demonstrated that the superimposition of ischemic damage, in the form of myocyte necrosis
and large fibrous scars mimicking healed infarction [34]. The ischemic damage occurs
in the absence of significant epicardial coronary artery disease, although small vessel
disease as well as intramural course of the left anterior descending coronary artery
have been noted. Elevated intramyocardial diastolic pressure may restrict intramural
arteries during diastole thus impairing coronary filling and myocardial perfusion.
The combination of myocardial disarray and replacement fibrosis has to be considered
the malignant arrhythmogenic substrate in HCM.
ARVC, which is also known as right ventricular dysplasia, is one of the leading causes
of SD in the young in our geographic area [6,35,36]. In these subjects the presence
of ECG abnormalities, like an inverted T wave in the right precordial leads (V1–V3),
increased QRS duration of >110 ms, late potentials detected by high resolution ECG
and ventricular arrhythmias, even in the shape of single premature ventricular beats
with left bundle branch block morphology, should raise suspicion of the disease and
lead to further non-invasive and invasive investigations. Patients with a history
of syncope, familial SD and precordial T wave inversion beyond V3 seem to have a worse
prognosis.
Fig. 3
A 16-year-old boy with previous history of syncope who died suddenly on effort. Long
axis cut of the heart specimen: note the asymmetric septal hypertrophy with subaortic
bulging and septal endocardial fibrous plaque; histology of the interventricular septum
reveals typical myocardial disarray with interstitial fibrosis (Heidenhain trichrome
×47).
The disease is characterized pathologically by a peculiar myocardial atrophy with
fibro-fatty substitution of the RV free wall in an otherwise apparently normal heart
(Fig. 4). Histology discloses the disappearance of the RV myocardium with the fibro-fatty
or fatty replacement, extending from the epicardium towards the endocardium. The intraventricular
conduction delay, consequent to fibro-fatty replacement, is a source of electrical
instability, due to reentrant phenomena, in the shape of ventricular arrhythmias with
left bundle branch block morphology, indicating a right ventricular origin. Familiarity
with an autosomal dominant inheritance has been demonstrated in nearly 50% of cases
[37]. Six loci have been identified by linkage analysis so far, two mapping to chromosome
14 and the remaining to chromosomes 1, 2, 3 and 10, one each. Recently, we identified
a mutation of the cardiac ryanodine receptor gene in families who map to chromosome
1q42–q43 [38]. A recessive form associated with palmoplantar keratosis has also been
also reported on chromosome 17 and a defective gene encoding for a cell-to-cell adhesion
molecule, i.e. plakoglobin, has been found [39]. Apoptosis has been demonstrated to
account for myocyte death both in autopsy and biopsy material [40]. RV aneurysms,
left ventricular involvement, focal myocarditis as well as bouts of apoptosis, most
probably worsen ventricular electrical vulnerability and lower the ventricular fibrillation
threshold. Evidence of inflammatory infiltrates associate with myocyte death is a
frequent finding in several pathological series [41]: whether inflammation is primary
or secondary to cell death remains to be established. To test the infective etiopathogenetic
theory, we recently did a nested PCR investigation which failed to detect any enteroviral
genome in biopsies of ARVC patients with both recent and chronic clinical onset of
the disease [42]. However, it may be that other types of virus are involved and need
to be searched for.
Dilated cardiomyopathy is a genetically and clinically heterogeneous disease. The
natural history demonstrates that death occurs not only due to progressive congestive
heart failure or as a complication of thromboembolism, but also abruptly due to arrhythmic
cardiac arrest. Although death is obviously expected along the natural history, in
a few cases arrhythmic SD may be the first manifestation of the disease and the diagnosis
is achieved only at postmortem by observing a heavy heart with dilated ventricles
and no inflammatory myocardial or coronary artery disease. As far as molecular basis
of dilated cardiomyopathy, at least 30% of cases are inherited, with a significant
percentage of the remaining cases being acquired (i.e. myocarditis, autoimmune, etc.).
Inherited forms may have autosomal dominant, autosomal recessive, X-linked, or mitochondrial
transmission, with evident genetic heterogeneity [43]. The persistence of a viral
infection is thought to have a pathogenetic role in different chronic myocardial diseases
with unknown etiology [44,45]. Different molecular techniques have produced controversial
results with respect to the rate of enteroviral positivity in myocardial samples of
patients with dilated cardiomyopathy. Several studies showed no, or a very low percentage
of, enteroviral PCR positivity in patients suffering from the end stage of dilated
cardiomyopathy [22]. Viral clearance or involvement of other cardiotropic viruses,
such as adenovirus, human cytomegalovirus, coronavirus and hepatitis C virus, could
explain the apparently negative findings.
Fig. 4
A 35-year-old man with a previous history of palpitations and negative T waves in
V1–V2 who died suddenly at rest. Gross view of the RV outflow tract showing massive
fatty replacement of the free wall myocardium; At histology, the RV infundibulum shows
transmural fatty replacement of the atrophic myocardium (Heidenhain trichrome ×3).
4.2.2 Marfan syndrome
The disease is familial in the majority of patients, whereas 30% of cases are sporadic.
It maps to chromosome 15q15–q21.3 [46,47] and the defective gene encodes fibrillin-1,
which is the major constituent of microfibrils of the extracellular matrix. Marfan
patients usually die suddenly because of aortic dissection with cardiac tamponade
(type I–II with rupture within the pericardial cavity) and exhibit typical cardiovascular
features consisting of mitral valve prolapse, annuloaortic ectasia, with or without
fusiform aneurysm of the ascending aorta, and aortic incompetence. Aortic dissection
in Marfan syndrome may be also observed without dilatation of the aorta, so that its
occurrence may be unpredictable on the clinical grounds. The basic defect of spontaneous
laceration of the ascending aorta consists in elastic disruption and cystic medial
necrosis in the tunica media leading to aortic wall fragility. A severe pattern of
cystic medial necrosis, quite similar to that observed in the aortic tunica media
of patients with Marfan's syndrome, has been reported in patients who died suddenly
with bicuspid aortic valve, either isolated or associated with isthmal coarctation,
suggesting a possible genetic background with a developmental defect involving both
the aortic valve and the ascending aorta (Fig. 5). Although familial forms have been
described the genetic defect has not been found so far.
4.2.3 Supravalvular aortic stenosis
This is a rare genetic disease with autosomal dominant inheritance which has been
linked to chromosome 7 and results from a defect in the elastin gene which accounts
for an hour-glass obstruction of the ascending aorta and left ventricular hypertrophy
[48,49]. The phenotype is marked by elastosis of the aortic tunica media, intimal
thickening and dysplastic cusps [50]. Left ventricular hypertrophy, isolation of the
coronary ostia because of fusion of semilunar cusps with the aortic wall, as well
as stenotic intraaortic course of the proximal coronary arteries, are all factors
that account for the coronary ischemia in these patients [51]. Williams syndrome is
an autosomal dominant disorder that is characterized by supravalvular aortic stenosis,
peripheral pulmonary stenosis, obstructive coronary lesions, abnormal facies and mental
retardation. This syndrome was found associated with a deletion of a region of the
same chromosome 7 (7q11.23) that includes the elastin gene and is thought to be a
contiguous gene disorder caused by a deletion of multiple adjacent genes.
Fig. 5
A 24-year-old boy who died suddenly at rest due to massive cardiac tamponade. Gross
view of the ascending aorta: note the intimal tear just above a functionally normal
bicuspid aortic valve; Histology of the ascending aorta shows severe atrophy and fragmentation
of elastic lamellae (Weigert van Gieson ×60).
4.2.4 Ion channel disease
Arrhythmic SD may occur in the absence of gross structural cardiac pathology (idiopathic
ventricular fibrillation or ‘mors sine materia’) due to ion channel disorders. In
this setting, besides the availability of ECG tracings and full family and personal
history, molecular investigation will allow the definite cause of SD to be found.
The long QT syndrome is a familial disease with high cardiac electrical instability,
presenting with syncope due to ventricular tachyarrhythmias or with cardiac arrest
on exercise or emotional stress, often under the age of 15 [52]. The cause of death
at necropsy cannot be ascertained unless there are prior ECG data. Genetic analysis
reveals multiple abnormalities in genes related to both cardiac potassium and cardiac
sodium channels. At present, important genotype—phenotype correlations and the identification
of gene-specific arrhythmogenic triggers are slowly emerging. Alterations of ion pumps
and currents account for the lengthening of the action potential and prolonged QT
interval on ECG, and the propensity to ventricular fibrillation. The mortality in
untreated symptomatic cases exceeds 60% within 15 years. Clearly ECG screening of
surviving relatives is the only clinical way to establish the diagnosis in asymptomatic
carriers. On the basis of pattern of transmission, two major clinical syndromes have
been described: the more common autosomal dominant form with a pure cardiac phenotype
(Romano—Ward) [53] and the rarer autosomal recessive form characterized by the association
with congenital deafness (Jervell and Lang-Nilesen) [54].
The Romano—Ward LQTS results from a mutation in genes encoding cardiac ion channels
or auxiliary ion-channel subunits: KvLQT1 (at the LQT1 locus), HERG (at LQT2), SCN5A
(at LQT3), HKCNE1 (encoding minimal potassium-channel β subunit [MinK] at LQT5), and
HKCNE1 (encoding MinK-related peptide 1 [MiRP1] at LQT6) [55]. Moreover, families
linked to none of these genes have been described thus suggesting the existence of
other disease genes. The autosomal recessive variant of LQTS arises in patients who
inherit abnormal KvLQT1 or minK alleles from both parents and expresses itself with
remarkably long QT intervals.
The Brugada syndrome is a clinical and ECG syndrome, characterized by apparent right
bundle branch block with right precordial ST segment elevation and an apparently normal
heart, which has been described in cases of SD by Brugada and Brugada, unfortunately
without postmortem reports [56]. These ECG characteristics may depend on exaggerated
transmural differences in action potential configuration, especially in the right
ventricular outflow tract. By studying a family with a SD case, confirmation of an
organic substrate was given by Corrado et al. [57], who reported not only fibro-fatty
dystrophy in the RV free wall but also involvement of the conduction system with sclerotic
interruption of the right bundle branch. The coexistence of both ‘septal’ and ‘parietal’
right conduction defects might account for the ECG pattern of right bundle branch
block and persistent ST segment elevation with ventricular electrical instability.
Conversely, in the absence of structural heart disease, the ECG abnormalities could
arise from ion currents dysfunction, such as I
to L-type Ca2+ channel [I
Ca (L)] and I
Na. At least one variant of the Brugada syndrome is caused by mutations in cardiac
sodium channels gene SCN5A, the same gene which is implicated in the LQT3 syndrome
[58]. The mutation of SCN5A causes loss of function in the Brugada syndrome and gain
of function in the LQT3. In particular, it results in the Brugada syndrome in reduced
sodium current density, thus accounting for the loss of the epicardial action potential
dome with transmural dispersion of repolarization, which in turn may predispose to
the development of ventricular fibrillation. However, other families with Brugada
syndrome have been tested showing no defects on the cardiac sodium channel, thus suggesting
genetic heterogeneity in analogy with other inherited cardiac diseases.
5 Final consideration on the pathologist's role in the diagnosis of causes and prevention
of SD
An accurate diagnosis of the underlying morbid entity at risk of SD either at postmortem
or in living patients with aborted SD and life-threatening arrhythmias, is the prerequisite
to adopt therapeutic and preventive strategies, by establishing whether the disease
is acquired due to infectious agents or hereditary.
In the latter situation, the final diagnosis may be the starting point for a widespread
investigation of the family, to detect asymptomatic carriers and to reassure non-carriers.
ECG and echocardiographic studies are essential preliminary investigations carried
out in all first degree family members. Unfortunately, they can detect only a small
proportion of gene carriers, particularly in children, where phenotypic expression
may still be absent. Genetic screening is theoretically the most effective tool in
the early recognition of asymptomatic genetic carriers. The potential benefit for
patients and family members is self-evident, since in the majority of cases SD may
be the first and sole manifestation of a familial disease. Moreover, the risk of cardiac
arrest is demonstrably higher with some specific mutation. However, there are some
concerns about genetic screening. First, at present it is an expensive, laborious
procedure, and can be done only at a few tertiary referral centers, with a specific
research interest in molecular genetics. There are pressures to study families because
of competition in science. Some people question whether genetic knowledge is beneficial,
neutral or harmful, and whether parents have the right to make decisions for their
children, especially when a disease is not manifest until middle age and little can
be offered in terms of treatment.
As a consequence, pathologists who see most of the index cases have a great responsibility.
If nothing is done and another SD occurs, the family rightly feels aggrieved. If the
pathological diagnosis is wrong, many expensive and time-consuming investigations
are carried out without benefit. This implies that all young SDs should undergo autopsy
by expert pathologists, whether or not working for a coroner or medical examiner.
Moreover, by applying molecular biology techniques to the study of blood and fresh
tissue obtained through endomyocardial biopsy, the pathologist will also have an essential
role in providing a correct in vivo diagnosis of genetically determined or infective
heart diseases potentially at risk of SD, thus allowing the more rational management
of patients.
Having achieved a precise diagnosis and recognized a potentially hereditary disease
as the cause of life-threatening arrhythmias and/or SD, it is then advisable to inform
through the general practitioner and to start a sequence of investigations from a
detailed family history to referral of parents, siblings and offspring to a cardiologist
for screening. A full genetic study should be carried out only when the family asks
for it. HCM, long QT syndrome and other ion channels diseases, Marfan's syndrome and
ARVC are, at present, cardiac familial disease at risk of SD; all are theoretically
amenable to genetic screening and all have the potential to devastate families by
expected SD and the families have the right to the best available advice. A precise
pathological diagnosis of the underlying heart disease, which also takes advantage
of advanced molecular biology techniques, will be the source of vital information
for the community, relatives and future generations.