HCM is the most common form. In HCM, the muscles of the wall of the left ventricle have increased in thickness (hypertrophy). This causes increasing stiffness in the left ventricle, making it unable to fill efficiently. Also, the space in the left ventricle is getting smaller and smaller, resulting in less blood being pumped around and the area in the left atrium being enlarged. This increases the risk of thrombosis. An increase in pressure in the left atrium increases the pressure in the pulmonary vessels, which leads to fluid accumulation in the lungs and chest. HCM can also cause a thickening of the muscles that attach the heart valves and an abnormal movement of the heart valves, also called SAM (Systolic Anterior Motion). HCM can be caused by external factors. A thyroid that works too fast can lead to the symptoms of HCM. This is easily diagnosed by a blood test. HCM does not necessarily have to be hereditary, but in the case of HCM at a young age, a genetic defect should be considered.