Hypertrophic cardiomyopathy (HCM) is a condition where areas of heart muscle become thickened and stiff.
The thickening makes it harder for the heart to contract and pump blood. Depending on where the thickening is, it can affect how blood flows out of the heart (referred to as 'HCM with obstruction' or 'HOCM').
The following diagram (courtesy of Wikipedia) illustrates the difference between a normal heart and one with hypertrophic cardiomyopathy.
