Inclusion of the right ventricular muscle bundle during interventricular septal measurement improves diagnostic accuracy for hypertrophic cardiomyopathy

IntroductionMeasurement of the interventricular septum (IVS) is a key diagnostic and prognostic parameter in the evaluation of hypertrophic cardiomyopathy (HCM). Right ventricular muscle bundles (RVMB) that parallel the IVS complicate septal measurement on both echocardiography and magnetic resonance imaging. Current guideline statements reference left ventricular wall thickness measurements more than 15 mm as part of the diagnostic criteria for HCM. The medical literature lacks published data on the impact of including RVMB as part of the IVS measurement and its influence on diagnostic accuracy for HCM. MethodsWe measured the IVS and RVMB separately on echocardiography in 97 consecutive subjects referred for both echocardiography and magnetic resonance imaging (MRI) as part of the initial evaluation for HCM. Subjects were categorized as having or not having HCM based on current practice guidelines. Patients with HCM were sub-categorized as having septal involvement (HCM-Sep) or primarily apical hypertrophy (HCM-Ap). This was done because subjects with obvious HCM-Ap could be diagnosed with HCM irrespective of IVS thickness. ResultsCompared to subjects who did not have HCM, those with HCM-Sep had both increased IVS (15.4 {+/-} 2.7 vs 9.8 {+/-} 1.9 mm, p<0.001) and RVMB thickness (5.2 {+/-} 3.1 vs 1.9 {+/-} 1.9 mm, p<0.001). Within the group of subjects that either had HCM-Sep or did not have HCM, inclusion of the RVMB in the septal measurement increased the sensitivity for HCM from 63% to 100%, whereas specificity decreased from 100% to 87%. RVMB thickness more than 5 mm was seen in 46% of subjects with HCM-Sep but was absent in all subjects with HCM-Ap and those without HCM. The RVMB was visible on long axis imaging in 55% of subjects without HCM, 75% of subjects with HCM-Ap and 85% of subjects with HCM-Sep. ConclusionsInclusion of the RVMB in the measurement of IVS thickness on echocardiography may improve overall diagnostic accuracy for HCM. In addition, RVMB thickness is increased and is more often visible on parasternal long axis imaging in subjects with HCM, consistent with being part of the HCM pathology. This is particularly true in those with HCM-Sep. These data have implications for standardization of echocardiographic and MRI reporting in HCM.