Restrictive Cardiomyopathy (RCM) is a rare type of cardiomyopathy that is characterized by stiffening of the heart muscle, which impairs its ability to fill with blood. In RCM, the muscle cells in the heart become replaced with abnormal tissue, such as scar tissue. This causes the muscle walls in the lower chambers of the heart (the ventricles) to become stiff, thereby restricting the heart's movement.

While the heart's pumping action is not typically affected, its ability to relax and fill with blood is compromised. The impaired filling function means the ventricles receive less blood than normal, reducing blood flow throughout the body and the heart. Consequently, blood backs up as it cannot enter the heart normally, leading to increased pressure and enlargement of the upper chambers of the heart (the atria). 

RCM can be caused by variants in genes that encode for proteins that are involved in the structure and function of the heart muscle. RCM can be inherited in an autosomal dominant or autosomal recessive manner, depending on the specific gene involved. In some cases, RCM can also be caused by new (de novo) gene changes, which occur spontaneously and are not inherited from either parent.

Mutations in the TNNI3 gene are the most common cause of familial RCM. TNNI3 encodes for troponin I, which is involved in regulating the contraction of heart muscle cells. Other genes that have been linked to RCM include MYH7, MYBPC3, and TNNC1. 

Genetic testing is not always necessary for diagnosing RCM. The condition can also be diagnosed based on clinical symptoms, such as an enlarged liver or reduced heart function.

While RCM is a genetic disorder, it is important to note that not all cases of RCM are caused by genetic changes. Some cases may be due to other factors, such as amyloidosis or sarcoidosis.