Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiomyopathy, affecting approximately 1 in 500 people. As the yield of genetic testing is only about 35-60%, the diagnosis of HCM is still clinical and based on the demonstration of unexplained and usually asymmetric left ventricular (LV) hypertrophy by imaging modalities.
HCM has clinical and phenotypic heterogeneity, including a broad hemodynamic spectrum comprising LV obstruction under resting conditions, dynamic (labile) gradients without rest obstruction, and gradients absent both at rest and with provocation (ie, nonobstructive HCM). Most attention has historically focused on patients with outflow obstruction, given that subaortic gradients are the most common mechanism responsible for heart failure symptoms in this disease. HCM patients with nonobstructive disease appear to experience a relatively benign clinical course, with a low risk for advanced heart failure symptoms, other disease complications, and HCM-related mortality and largely without the requirement for major treatment interventions. A small minority of nonobstructive HCM patients progress to heart transplantation.
(See the image below depicting hypertrophic cardiomyopathy.)
Cardiomyopathy, hypertrophic. Axial electrocardiographically (ECG) gated spin-echo MRI in a patient shows marked septal (S) and less-prominent posterior wall thickening.
In the past, echocardiography was the sole imaging modality used for the diagnosis and management of HCM. However, other imaging modalities such as cardiac magnetic resonance (CMR) have played an increasing role in the diagnosis, management, and risk stratification of HCM, particularly when the location of left ventricular hypertrophy is atypical (apex, lateral wall) and when the echocardiographic imaging is suboptimal.
Current American College of Cardiology and American Heart Association (ACC/AHA) diagnostic criteria for hypertrophic cardiomyopathy include the identification of a hypertrophied left ventricle (LV) with wall thickness typically 15 mm or greater in adults without another underlying cause.
Comprehensive transthoracic echocardiographic (TTE) and Doppler studies should be performed in the initial evaluation of all patients with suspected HCM, as well as during follow-up, particularly when there is a change in cardiovascular symptoms or an event. Echocardiography can be used to confirm the size of the heart, the pattern of ventricular hypertrophy, the contractile function of the heart, and the severity of the outflow gradient. It has the advantages of high resolution and no known risk. However, there remain patients in whom the diagnosis of HCM is suspected but the echocardiogram is inconclusive, mostly because of suboptimal imaging from poor acoustic windows or when hypertrophy is localized to regions of the LV myocardium not well visualized by echocardiography.
The advantages of CMR are enhanced spatial resolution, image quality, and inherent 3-dimensional nature, which have allowed identification of morphologic variants of HCM, some of which can be missed by echocardiography. In addition to this greater diagnostic accuracy, CMR provides excellent demarcation between the myocardium and blood pool, allowing the most accurate calculation of LV mass and volumes.
The high contrast resolution of ECG-gated MRI provides excellent information about cardiac anatomy.
Spin-echo MRI or cine magnetic resonance angiography (MRA) can be used to demonstrate ventricular anatomy and wall thickness. Cine MRA is used to evaluate ventricular function, ventricular end-diastolic and end-systolic volumes, valvular dysfunction, and outflow tract obstruction. In some cases, the signal intensity through the thickened myocardium varies.
In addition, contrast-enhanced CMR with late gadolinium enhancement (LGE) has the capability to identify areas of myocardial fibrosis/scarring with novel data, demonstrating that the extent of LGE by CMR may play an important role in risk stratification of patients with HCM.
The 12-lead ECG is useful largely for raising the suspicion of HCM in family members without LV hypertrophy and in identifying patterns such as Wolff-Parkinson-White syndrome, which may suggest certain phenocopies of HCM. Patterns mimicking myocardial infarction may provide evidence of the diagnosis and may be present in young individuals before there is manifest evidence of wall thickening on echocardiography. The 12-lead ECG is abnormal in 75-95% of patients with HCM, but these abnormalities do not correlate with severity or pattern of hypertrophy as determined by echocardiography.
Cardiac catheterization and angiography can be performed to evaluate hemodynamic and morphologic abnormalities associated with HCM, along with associated coronary artery anomalies. However, these are invasive procedures and should be used only if other tests cannot provide adequate information or if alcohol ablation of septal branches is planned.