What causes hcm

What is Hypertrophic Cardiomyopathy (HCM)?

Hypertrophic cardiomyopathy (HCM) is a disease in which the heart muscle (myocardium) becomes abnormally thick (hypertrophied). This thickening can make it harder for the heart to pump blood to the rest of the body. In many cases, HCM is an inherited condition, meaning it is passed down through families. The thickened heart muscle can also interfere with the heart's electrical system, leading to arrhythmias (irregular heartbeats).

The term 'hypertrophic' refers to the thickening of the heart muscle. In HCM, the left ventricle, the heart's main pumping chamber, is most often affected. The walls of the ventricle become stiff and may bulge into the chamber, reducing its capacity to fill with blood. In some individuals, the thickening of the heart muscle can obstruct the outflow of blood from the left ventricle, a condition known as obstructive HCM. In others, there is no significant obstruction, termed non-obstructive HCM.

What Causes Hypertrophic Cardiomyopathy?

Genetic Basis of HCM

The overwhelming majority of HCM cases, estimated to be around 60-70%, are caused by inherited genetic mutations. These are not typically acquired during a person's lifetime but are present from birth. The specific genes involved are those that provide instructions for making proteins that make up the sarcomeres, the basic contractile units of heart muscle cells. When these proteins are abnormal due to a genetic mutation, the heart muscle cells grow abnormally, leading to thickening of the heart walls.

More than 1,500 different genetic mutations have been identified that can cause HCM. These mutations are often found in genes encoding for:

• Beta-myosin heavy chain (MYH7)
• Myosin-binding protein C (MYBPC3)
• Cardiac troponin T (TNNT2)
• Alpha-tropomyosin (TPM1)
• Cardiac troponin I (TNNI3)

The inheritance pattern for HCM is typically autosomal dominant. This means that a person only needs to inherit one copy of the altered gene from either parent to develop the condition. If one parent has HCM, each child has a 50% chance of inheriting the gene mutation and developing the condition. However, not everyone who inherits a mutation will develop significant symptoms or the full spectrum of the disease; this phenomenon is known as variable penetrance.

Non-Genetic Factors and HCM

While genetics are the primary cause, other factors can play a role in the development or progression of HCM, particularly in cases where a clear genetic mutation isn't identified or in the context of secondary forms of the disease. These are often referred to as secondary hypertrophic cardiomyopathy.

• Hypertension (High Blood Pressure): Long-standing, uncontrolled high blood pressure can cause the heart muscle to thicken over time as it works harder to pump blood against increased resistance. This type of thickening is usually more uniform than in genetic HCM.
• Aortic Stenosis: Narrowing of the aortic valve requires the left ventricle to generate higher pressures to pump blood into the aorta, which can lead to hypertrophy.
• Athletic Training: Intense, prolonged endurance training can lead to physiological cardiac hypertrophy, sometimes called athlete's heart. This is a normal adaptation and is usually reversible when training stops, but it can sometimes be difficult to distinguish from HCM, especially in athletes with a genetic predisposition.
• Other Medical Conditions: Certain rare genetic syndromes and metabolic diseases, such as Fabry disease or Noonan syndrome, can also be associated with heart muscle thickening.

Understanding the Mechanism of Thickening

The genetic mutations responsible for HCM disrupt the intricate structure and function of the sarcomeres. These proteins are responsible for the coordinated contraction and relaxation of the heart muscle. When these proteins are faulty, the heart muscle cells try to compensate, often by increasing their size and rearranging their internal structure. This uncontrolled growth leads to the characteristic thickening of the heart walls, particularly the interventricular septum (the wall between the two ventricles).

The consequences of this thickening are multifaceted:

• Reduced Chamber Size: The thickened walls can make the heart chambers smaller, reducing the amount of blood they can hold and pump with each beat.
• Stiffness: The hypertrophied muscle becomes stiff, impairing the heart's ability to relax and fill properly with blood during diastole (the relaxation phase of the heartbeat).
• Outflow Obstruction: In obstructive HCM, the thickened septum can bulge into the left ventricular outflow tract, the path blood takes to exit the heart, creating a blockage. This obstruction can worsen during exercise.
• Arrhythmias: The abnormal muscle structure and electrical properties of the heart can lead to abnormal heart rhythms, such as atrial fibrillation or ventricular tachycardia, which can increase the risk of sudden cardiac death.
• Mitral Valve Dysfunction: The thickened septum can also affect the mitral valve, sometimes causing it to leak (regurgitation) and allow blood to flow backward into the left atrium.

Diagnosis and Screening

Given the genetic nature of HCM, screening is crucial for family members of individuals diagnosed with the condition. Echocardiography (ultrasound of the heart) is the primary diagnostic tool used to identify the characteristic thickening of the heart muscle. Genetic testing can also confirm the presence of specific mutations known to cause HCM and is particularly useful for identifying at-risk relatives who may not yet show signs of the disease.

It's important to distinguish between genetic HCM and secondary forms of hypertrophy. While the heart muscle thickens in both, the underlying cause and management strategies differ significantly. Understanding the cause is fundamental to providing appropriate care and managing the risks associated with HCM.