Morphology and diagnosis
During embryogenesis, the mesh of muscle fibres making up the heart normally becomes compacted while the intertrabecular sinusoids degenerate. This compaction process occurs in gestational weeks 5 – 8, and results in transformation of intertrabecular sinusoids to capillaries. It starts in the epicardium and progresses inward towards the endocardium, from the base of the heart to apex, and is more complete in the left ventricle (LV) than in the right ventricle (RV). Development of the coronary circulation occurs in the same period (1, 13) and is inhibited in the presence of noncompaction. The result is multiple persistent and prominent ventricular trabeculae, and deep intertrabecular recesses which communicate with the ventricular cavity and not with the coronary circulation (fig 1, fig 2). The ventricular cavity and the intertrabecular recesses are covered by a continuous endothelium (1, 2, 5). Predilection sites for noncompaction (normal findings in fish, amphibian and reptile hearts ) are the mid and distal segments of the lateral and inferior walls, as well as in the apical region (5, 9, 13). The extent of changes varies among patients (1, 4).
Literature presents the condition with many names; isolated LVNC, honeycombed myocardium, spongy myocardium, persisting myocardial sinusoids, myocardial disorganization and left- ventricular hypertrabeculation (4, 7). In principle they all describe the same condition.
LVNC was first described in 1932, in connection with autopsy of a newborn child who had aortic atresia and a coronary-ventricular fistula (14). Several publications and review articles have described the condition since then (1–6), (6, 15, 16). LVNC may present concomitantly with other congenital anomalies of the heart (such as ventricular septal defects or ventricular outflow obstruction), and isolated (without other congenital anomalies of the heart) with or without other rare extracardiac conditions (such as systemic myopathies or facial dysmorphism) (2–7), (9–11). The isolated type without extracardiac abnormalities will be discussed in the following, unless otherwise specified.
Increased knowledge about the pathogenesis, diagnostic characteristics and prognosis of LVNC has recently caused the condition to be classified as primary cardiomyopathy (17). Echocardiography or MRI of the heart (12) are considered to be diagnostic gold standards.
Several echocardiographic findings are considered to be typical (2, 5, 7, 9) – (11):
Absence of other cardiac anomalies (by definition).
Involvement of the apex and/or apical and/or midventricular segments of the inferior and/or lateral wall. More than 80 % of patients with LVNC display noncompaction in one or more of these predilection sites.
Three or more prominent myocardial trabeculae in the same image plane.
Deep intertrabecular recesses that communicate with the ventricular cavity.
The affected myocardial segments have two layers: a thin epicardial layer and a thickened trabecular endocardial layer.
The outer myocardial layer is compacted in a normal way, but the inner layer is noncompacted. When the wall segments are measured at end-systole, the relationship between the noncompacted layer and the compacted layer will typically be > 2.0.
There is no international consensus about strict diagnostic criteria for LVNC and one or more of the echocardiographic criteria listed above may also be found in other conditions or diseases that affect the LV. However, presence of all the above-mentioned criteria is reported to be highly specific for LVNC (11). When findings are equivocal, intravenous echocardiographic contrast agents may help to make the diagnosis (18). In magnetic resonance images of the heart, a noncompacted/compacted layer ratio ≥ 2.3 (measured in the ventricle’s end diastole) is considered typical for LVNC (12) (fig 3). Other cardiac imaging modalities such as computer tomography and ventriculography may also be useful (19).
Obvious trabeculation is a normal finding in the RV. However, in LVNC up to half of cases also have more pronounced trabeculation of the RV (1).
Histopathologic findings of LVNC are unspecific. Myocardial biopsies have been described as normal or with subendocardial fibrosis or fibroelastosis, myocardial fibrosis, myocardial hypertrophy and degeneration, myocardial scarring or signs of inflammation (4, 7, 8). Electron microscopy has shown normal architecture of the myofibrils, with or without mild enlarged mitochondria (7).
Unspecific ECG changes such as left-ventricular hypertrophy, left or right bundle branch block, hemiblock, or ST-T changes may be present (1, 3, 6, 8, 11).
Several differential diagnoses should be considered, such as prominent myocardial trabeculae (typically fewer than three) in a normal ventricle, hypertrophic cardiomyopathy (deep recesses that do not communicate with the ventricular cavity), dilated and restrictive cardiomyopathy, endocardial fibroelastosis, left-ventricular thrombi (particularly in the apex), intraventricular bands, intramyocardial haematomas or abscesses, aneurisms and cardiac metastases (1, 6) – (8, 13).