The major abnormality of the heart in Hypertrophic Cardiomyopathy is an excessive thickening of the muscle. The distribution of muscle thickening or hypertrophy is variable. The left ventricle is almost always affected and in some patients the muscle of the right ventricle also thickens.
Asymmetric Septal Hypertrophy
Figure 5 shows a common presenation of HCM where the muscle thickening occurs predominantly in the "septum" or the dividing wall between the right and left sides of the heart. This form is called "asymmetric septal hypertrophy".
Figure 6 shows a common presentation creating "obstruction" where the muscle thickening occurs mainly in the upper part of the septum. Note that the mitral valves position creating "SAM" Systolic Anterior Motion.
It can be seen from figure 5 that the hypertrophy is greatest in the upper septum, in the area where blood flows out of the heart into the aorta or "outflow tract". The muscle thickening in this region may be sufficient to narrow the outflow tract, Figure 6. In such cases during the ejection of the blood flow from the heart, the mitral valve touches the septum (there should normally be a considerable gap between these structures, Figure 5). This narrowing of the outflow tract interferes with the normal ejection of blood. It causes turbulent blood flow and sometimes obstruction to flow. The turbulent flow produces a murmur which is audible with a stethoscope. In such patients, the abnormal position of the mitral valve may cause it to leak. This is called "mitral regurgitation" and may also cause a murmur, Figure 6.
In some cases of asymmetric septal hypertrophy obstruction to the outflow of blood from the heart may occur as shown here. Note that the mitral valve now touches the septum blocking the outflow tract ("systolic anterior motion of the mitral valve" or "SAM"). Some blood is leaking back through the mitral valve ("mitral valve regurgitation").
Some cases will present with "concentric" hypertrophy - which means equal amounts of thickness occure throughout the left ventricle. In this case the muscle thickening is of equal severity throughout the whole left ventricle.
Some cases may be called "apical" HCM, in this presentation the muscle thickening occurs predominantly at the tip (apex) of the left ventricle.
There may be many other presentations and patterns of hypertrophy in the disease.
Other Patterns of Muscle Thickening
In approximately 25% of patients the muscle thickening is evenly distributed throughout the ventricle. This is known as "symmetric" or "concentric" ventricular hypertrophy.
In a small proportion of patients (approximately 10%), myocardial thickening is predominantly at the tip or "apex" of the heart. This appeared to be more common pattern of hypertrophy in Japan than in the West. however as our understanding of HCM contiunues to advance this does not seem to hold a great deal of importance or value. The patter of hypertophy is not as important as the presents of hypertrophy itself. Patients with Concentric and Apical Hypertrophic Cardiomyopathy usually do not have a murmur at rest but may with excercise.
Function of the heart in Hypertrophic Cardiomyopathy
The thickened muscle usually contracts well and ejects a higher then normal amount of the blood from the heart (this is measured with something called an ejection fraction of EF. However the muscle in HCM is often stiff and relaxes poorly. This requires higher pressures than normal to expand with the inflow of blood. The amount of blood which the heart can hold is therefore reduced and this in turn will limit the amount of blood which can be ejected with the next contraction.
Occasionally patients present with minimal to no hypertrophy, but sever restrictions to the normal inflow of blood into the ventricles. The differentiation from Restrictive Cardiomyopathy may be different and accurate diagnosis relies on the presence of other features of the two conditions. Careful evaluation by an HCM Center can help to detirmine the root cause of they cardiomyopathy.
Muscle Cells Under the Microscope
Examination of the heart muscle in Hypertrophic Cardiomyopathy under a microscope shows that the normal parallel alignment of muscle cells has been lost. The cells appear disorganized. This abnormality is called "myocardial disarray". It is probable that myocardial disarray interferes with normal electrical transmission and predisposes to irregularities of the heart beat.
Normal parts of the Heart in Hypertrophic Cardiomyopathy
Finally, it is important to note that parts of the heart commonly affected in other conditions e.g. the heart valves and main coronary arteries (blood vessels that supply the heart) are generally normal in Hypertrophic Cardiomyopathy.