Noncompaction Cardiomyopathy

Noncompaction cardiomyopathy is thought to be caused by arrest of normal embryogenesis of the endocardium and myocardium. The myocardium starts out as a loose network of interwoven fibers separated by deep recesses. These recesses communicate with the ventricular cavity. They eventually compact down, proceeding from the epicardium to endocardium and from the base of the heart to the apex. This happens around the same time as the coronary circulation develops, and these recesses turn into capillaries.

Arrest of compaction can lead to either:

• Noncompaction of the ventricular myocardium: Persistence of recesses, which stay in communication with both the ventricular cavity and the coronary circulation, or
• Isolated noncompaction of the ventricular myocardium (also known as left ventricular hypertrabeculation): Persistence of recesses, which communicate only with the ventricular cavity, not the coronary circulation.

The left ventricle is always affected, but because the right ventricle normally has a trabeculated appearance, differentiating a normal but highly trabeculated right ventricle from a pathological noncompacted ventricle may be difficult. This has led some to dispute the existence of right ventricular noncompaction.

Non-compaction can be found in normal subjects, where it is more commonly found in the apical segments (91% of subjects) compared to mid-cavity levels (78% of subjects) and basal segments (21% of normal subjects). For this reason, a ratio of >2.0 between the thickness of the non-compacted and compacted myocardial layers in systole is considered diagnostic on echocardiography. A ratio of >2.3 between the thickness of the non-compacted and compacted myocardial layers in diastole is considered diagnostic on cardiac MRI, with, with sensitivity, specificity and positive and negative predictive values of 86%, 99%, 75%, and 99%, respectively.

The ratio is calculated using long-axis (horizontal and vertical long-axis and left ventricular outflow tract) views during diastole. In each view, the segment with the most pronounced trabeculation (excluding the apex) is chosen to calculate the ratio. The highest ratio wins. A segment is considered non-compacted if the visual appearance clearly suggests the presence of two myocardial layers with different degrees of tissue compaction.

Complications include systemic embolization and regional wall motion abnormalities.

References

• Petersen SE, Selvanayagam JB, Wiesmann F, Robson MD, Francis JM, Anderson RH, Watkins H, Neubauer S. Left ventricular non-compaction: insights from cardiovascular magnetic resonance imaging. J Am Coll Cardiol. 2005 Jul 5;46(1):101-5.
• Weiford BC, Subbarao VD, Mulhern KM. Circulation. 2004 Jun 22;109(24):2965-71. Noncompaction of the ventricular myocardium. Circulation. 2004 Jun 22;109(24):2965-71.