Noncompacted myocardium (NCM) was first reported by Grant in 1926 as a heterogeneous
myocardial disorder characterized by prominent ventricular trabeculation,
intratrabecular recesses, and bilayered myocardium composed by a compacted and a
noncompacted layer.
1,2
It can occur isolated or associated with other
cardiomyopathies, complex syndromes, metabolic disorders and congenital heart diseases,
such as Ebstein's anomaly, left ventricular (LV) or right ventricular outflow tract
obstruction, bicuspid aortic valve, cyanotic congenital heart diseases, and coronary
artery anomalies. Although NCM usually affects the left ventricle, it can also affect
both ventricles or the right ventricle alone.
3
The etiology of LV noncompaction is uncertain, and several etiological bases have
been
implicated. It is believed to be due to pathogenic mechanisms resulting in a failure
in
the final phase of myocardial morphogenesis, or myocardial compaction. Increasing
evidence has supported a genetic base by identifying mutation in the genes that encode
sarcomeric, cytoskeletal and nuclear membrane proteins.
4-6
Although considered rare by some authors, NCM incidence and prevalence are uncertain.
Ritter et al.
7
have reported a 0.05%
prevalence in all echocardiographic exams of a large institution. Patients with heart
failure (HF) have been reported to have a 4% prevalence of NCM.
8
Currently, it is controversial whether NCM is a distinct cardiomyopathy or a
morphological characteristic shared by different heart diseases. Thus, while the World
Health Organization/International Society and Federation of Cardiology considers NCM
an
unclassified cardiomyopathy, the American Heart Association considers it a primary
genetic cardiomyopathy.
9,10
The most recent classification of
cardiomyopathies proposed by the European Society of Cardiology Working Group on
Myocardial and Pericardial Diseases considers NCM an unclassified familial
cardiomyopathy.
11
The clinical presentation can occur at any age, being highly variable. Patients can
be
asymptomatic or have symptoms of severe HF, associated or not with lethal arrhythmias,
sudden cardiac death and thromboembolic events.
12
Many asymptomatic patients are identified incidentally by an echocardiography performed
for assessment of cardiac murmur or for familial screening after identifying an index
case.
12
Symptoms of HF occur in more than half of the patients with NCM, LV dysfunction being
reported in up to 84% of them. In addition, arrhythmias are common: atrial fibrillation
can affect 25% of adult patients, and ventricular tachyarrhythmias, up to 47% of
patients. The occurrence of thromboembolic events, such as stroke, transient ischemic
attack, pulmonary embolism and mesenteric ischemia, ranges from 0 to 38%, according
to
studies published.
3,13,14
Electrocardiographic abnormalities can be present in up to 90% of patients, being,
however, unspecific. The most common findings include intraventricular conduction
delay,
LV hypertrophy, ventricular repolarization changes, and Wolff-Parkinson-White
syndrome.
3,13,14
The increasing advancement in imaging techniques, in addition to the increasing
application of genetic tests for the diagnosis of NCM, significantly impacts on the
understanding of the mechanisms involved in the NCM genesis and its clinical
treatment.
Of the cardiac imaging techniques, echocardiography and cardiac magnetic resonance
imaging (CMR) are the major diagnostic tools. Because of its large availability and
easy
access, in addition to no need for contrast agents, no radiation exposure, and mainly
its low cost as compared to CMR, echocardiography is the first choice and most commonly
used method for the diagnosis of NCM.
8,12-14
Usually the diagnosis of NCM should be considered in the presence of a bilayered
myocardium composed by one thinner epicardial layer and one thick endocardial layer
with
prominent trabeculations and deep intraventricular recesses. The trabeculations are
mainly identified on two-dimensional (2D) mode, but can be evidenced on one-dimensional
or M mode. Color Doppler imaging shows blood flow in those recesses in continuity
with
the left ventricle.
8,12-15
Different echocardiographic criteria have been used to diagnose NCM, and the main
ones
used in clinical practice are as follows (Table
1):
Table 1
Criteria proposed for the diagnosis of noncompacted myocardium.
6,13
Criterion
Chin et al.
2
Jenni et al.
16
Stöllberger et al.
17
Number of patients
8
7
104
Phase of the cardiac cycle
End-diastole
End-systole
Trabeculations assessed at end-diastole; NCM and CM
assessed at end-systole
View
Short axis parasternal views and/or apical views
Short axis parasternal
Conventional and modified views
NCM/CM ratio
-
>2
-
Jenni et al.
16
consider for the
diagnosis of NCM the presence of two myocardial layers, a thin compacted one
(compacted myocardium - CM), and another thicker, noncompacted layer (NCM), with
deep endomyocardial recesses filled with blood flow on color Doppler, in the
absence of other cardiac abnormalities. A NCM/CM ratio > 2 is considered
diagnostic. The measures should be acquired at end-systole on short axis
parasternal view
16
(Figure 1).
Figure 1
Criteria proposed for the diagnosis of noncompacted myocardium. NCM:
noncompacted myocardium; CM: compacted myocardium; LV: left
ventricle; LA: left atrium.
Chin et al.
2
define NCM as the
presence of excessively prominent ventricular trabeculations and progressively
increased total thickness of the myocardial wall from the mitral valve and
towards the apical region, characterized by CM/(NCM + CM) ≤ 0.5, assessed
at end-diastole on short-axis parasternal views and/or apical views
17
(Figure 2).
Figure 2
Criteria proposed by Chin for the diagnosis of noncompacted
myocardium. NCM: noncompacted myocardium; CM: compacted myocardium;
LV: left ventricle; LA: left atrium.
Stöllberger et al.
17
define NCM as the presence of three or more trabeculations along the LV
endocardial borders, different from the papillary muscles, false tendons and
aberrant muscle bands, which move synchronized with the CM. In that study, the
trabeculations were better visualized at end-diastole, while the bilayered
myocardium was better assessed at end-systole
18
(Figure
1).
Recently, Paterick et al.
13
(Wisconsin) have proposed the diagnosis of NCM as a ratio NCM/CM > 2, with
measures taken at end-diastole on short-axis parasternal view. This criterion
requires clinical validation
13
(Figure 1).
Critical analysis
The diagnostic criteria described by Jenni et al.
16
and Chin et al.
2
are based on the measurement of NCM and CM thicknesses. However,
the criteria differ regarding the cardiac cycle point at which the measurements
should be taken. Chin et al.
2
propose the measurements of NCM and CM thickness be performed at end-diastole, while
Jenni et al. propose them at end-systole.
16
The criteria proposed by Jenni et al.
16
used an NCM/CM ratio > 2 at end-systole, generating higher
specificity and lower sensitivity as compared to the criteria by Chin et
al.,
2
who use the CM/(NCM +
CM) ratio ≤ 0.5. However, the increase in sensitivity is due to a decrease in
specificity, as compared to the criteria by Jenni et al.
16
A recent study has assessed the accuracy of the echocardiographic criteria described
by Chin et al.,
2
Jenni et
al.
16
and
Stöllberger et al.
17
for
the diagnosis of NCM in patients with HF as compared to a control group of normal
individuals. The size and the number of the trabeculations identified on apical view
at end-diastole were assessed, as were the measurements of the NCM layer thickness
on short-axis parasternal view at end-systole. Only concordant cases assessed by two
reviewers were considered positive.
18
In that study, the percentages of patients meeting the diagnostic criteria for NCM
were as follows: Chin et al.
2
criteria, 79%; Jenni et al.
16
criteria, 64%; and Stöllberger et al.
17
criteria, 53%. In that study, the Chin et al. criteria had
higher sensitivity, however with a higher percentage of false-positive diagnoses.
The correlation between the three echocardiographic criteria applied was weak, with
only 30% of patients meeting all three criteria. All individuals of the control
group had preserved ventricular dimensions and systolic function. Five control group
individuals (4 black and 1 white) met at least one criterion for the diagnosis of
NCM. This result emphasizes the limitation of the echocardiographic criteria to
diagnose NCM, particularly in black individuals, leading to an excessive diagnosis
of NCM.
18
In that study, if the
control group was formed by individuals with HF, those results might have been even
more discrepant.
The criterion proposed by Paterick et al.
13
showed good correlation with the CMR findings, and,
according to those authors, that criterion provided more accurate measurements of
NCM and CM layer thickness. However, those criteria have not been validated,
requiring additional confirmation and comparison with other populations with cardiac
structural disease before they are adopted as a feasible diagnostic
option.
13
Despite the increasingly frequent diagnosis of NCM, the echocardiographic criteria
applied for that purpose are based on studies with limited numbers of patients and
different methodologies. The point of the cardiac cycle at which the measurements
of
NCM and CM thickness are taken influences directly the relationship between the two
layers assessed. Myocardial thickness is maximal at systole, and minimal at
diastole, which directly affects the ratio between NCM and CM. In addition, the
echocardiographic view on which those measurements are taken should be considered.
Most criteria suggest that the measurements be taken on short-axis parasternal view;
however, in daily clinical practice, measurements are more often taken on apical 4-
and 2-chamber views. Finally, there is no consensus on the ratio between NCM and CM
to be adopted as the diagnostic criterion, because of the lack of uniformity
accepted for diagnosis.
In addition, some studies have shown a considerable number of young athletes meeting
the NCM diagnostic criteria, emphasizing the lack of specificity of the current
diagnostic criteria when applied to highly-trained athletes.
19
Although infrequent, NCM has been reported in the right ventricle. However, in such
cases, the diagnostic criteria are even more restricted as compared to those applied
to the left ventricle on echocardiography, because of the limitation of the right
ventricle echocardiographic analysis due to its complex geometry, which cannot be
contemplated on only one echocardiographic view.
20,21
Thus, despite the increasing knowledge on NCM by echocardiography professionals, the
diagnostic bases of the criteria applied are frail. Therefore, studies with a larger
number of patients diagnosed with NCM are required, in addition to uniformization
of
the views used for the measurements, and the identification of the most suitable
point in the cardiac cycle for that purpose. Such studies should ideally compare
healthy individuals and patients with HF, because some "normal" patients can meet
the echocardiographic criteria for the diagnosis of NCM, with no apparent clinical
finding and benign prognosis, considering that the prevalence of NCM seems higher
in
patients with HF. In addition, the definition of the diagnostic criteria should take
into consideration the particularities of specific populations, such as
highly-trained athletes and black individuals, as well as the right ventricular
morphological characteristics.
Conclusions
Echocardiography is the first choice and most commonly used cardiac imaging method
for the diagnosis of NCM. Higher knowledge and understanding of NCM is the first
step to increase diagnostic accuracy in echocardiography laboratories. However, the
echocardiographic criteria used so far for that purpose are highly varied and have
been based on studies with a reduced number of patients.
Advanced techniques, such as three-dimensional echocardiography, use of contrast
agents to better define the endocardial borders, mainly in the apical region of
patients with limited acoustic window, as well the analysis of myocardial strain by
use of speckle tracking, are promising methods, with potential to increase the
diagnostic accuracy of echocardiography in patients with NCM. The use of such
techniques in clinical practice has increased in past years; however, the
improvement of imaging methods requires study and constant redefinition of the
echocardiographic criteria for the diagnosis of NCM.
22-25
The high prevalence of NCM in low-risk populations, such as athletes and normal black
individuals, suggests that the increase in LV trabeculations and recesses can
represent a pattern of response to the chronic increment of preload. Thus, because
of the current limitations for the diagnosis of NCM, integration of clinical and
electrocardiographic assessments, as well as a multimodality approach with
echocardiography and CMR, is suggested.
26-28
In addition to the multimodality imaging approach, future perspectives include
changes that suit different ethnicities and functional assessment based on
multicenter and international collaboration, incorporating genetic data for a more
accurate diagnosis of NCM.
26-28