HCM: Midcavity Obstruction

History: A 43-year-old man had an abnormal ECG as part of a routine life insurance physical. An echocardiogram suggested the diagnosis of cardiomyopathy. The echo windows were reportedly poor and an MRI was requested for further evaluation.

Technique: Short and long axis ECG-gated FIESTA images were acquired to evaluate ventricular structure and function

Findings: There is severe left ventricular hypertrophy (LVH) between the midventricle and apex (note the apex itself is spared). The maximum LV end-diastolic wall thickness is 2.5 cm. Ventricular function is globally hyperdynamic. The ejection fraction is 79%. There is mild mitral regurgitation which is best appreciated on the four chamber view.  (The regurgitant volume was 30 ml and the regurgitant fraction was 20%). These findings are characteristic of hypertrophic cardiomyopathy – midcavity obstruction type. There is no evidence of left ventricular outflow tract obstruction (LVOT) obstruction.

Discussion: The term cardiomyoapthy refers to a spectrum of myocardial disorders of varying etiology and pathopysiology that have in common diminished systolic and/or diastolic ventricular function. There are a number of types of cardiomyopathy including ischemic, congestive, infiltrative, restrictive, and hypertrophic.

Hypertrophic cardiomyopathy (HCM) is a disease characterized by myocardial hypertrophy that is out of proportion to the hemodynamic load. In other words, ventricular hypertophy that cannot be explained by other causes such as systemic hypertension or aortic valve stenosis.

The overall prevalence of HCM is low – on the order of 0.1% of the population. The cause of HCM is under active investigation. In ~50% of patients there is autosomal dominant transmission, so it is important to screen asymptomatic relatives. The cause in the remainder of patients is unknown. HCM can manifest at any age, from the very young to the elderly.

In ~25% of patients with HCM the septal hypertrophy is sufficient to cause left ventricular outflow tract (LVOT) obstruction. In these patients a dark jet is seen in the LVOT during systole on bright blood images (e.g. FIESTA or FastCine). Measurements of the LV end-diastolic wall thickness are typically made anteroseptally and inferolaterally in all patients in part to screen for HCM. In patients with HCM, the ratio of the anteroseptal wall thickness to the posterlateral wall thickness is typically > 1.3-1.5:1.

There are a number of other morphologic patterns of HCM. These are shown in the figure below.

Patients with HCM characteristically have abnormal diastolic dysfunction. The thick left ventricle is abnormally stiff, which results in impaired left ventricular filling. As a result the left ventricular end-diastolic pressure is increased, causing pulmonary congestion and dyspnea. These symptoms occur despite the fact that systolic left ventricular function in these patients is usually well-preserved. In fact, global systolic function is often hyperdynamic with typical ejection fractions of greater than 70%. Patients with HCM are at increased risk for sudden death due to arrhythmia. There are a variety of possible treatments for HCM. These include medical therapy (e.g. beta-blockers), devices (e.g. pacemakers and internal defibrillators), and interventions (e.g. alcohol ablation of the septum or surgical myectomy).

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