Influenza is one of the common causes of myocarditis. Myocarditis is the condition characterized by the infection of muscles of the heart. Flu causes myocarditis through various mechanisms.
How Does Flu Cause Myocarditis?
One of the major complications of influenza is acute myocarditis. The virus either attacks the cardiac muscles or it is the immune system and the inflammation in response to a viral infection that leads to cardiac muscle damage. The severity of myocarditis varies from patient to patient. In some patients, the condition goes asymptomatic while some patients develop severe complications to the tune that it may take away the lives though impairment in the cardiac function. It is essential that cardiac monitoring should be done in the cases of influenza infections to prevent the patient from developing complications. This would also include diagnosis and starting the treatment immediately. Viruses most commonly associated with developing myocarditis include Coronavirus, Epstein-Barr virus, Cytomegalovirus, Influenza (A, B), Coxsackie (A, B), and Adenovirus. The application of cellular and molecular biology, as well as the detailed knowledge of the viral structure and function, helps us in understanding the pathophysiology of the viral effects on the heart in greater detail. The understanding of cardiac damage, reducing the function of the heart and the progression of the disease helps physicians in providing high-quality treatment. (2) Following are the various mechanisms through which the viral infections affect the cardiac muscles leading to myocarditis:
Direct Viral Cause. The toxins secreted by the virus causes damage to the muscle cells of the heart. The duration of this damage is approximately within three days of the infection. The defense mechanism although slows the process of viral replication, but the replication of the virus in the heart muscles is sufficient enough to cause severe cardiac decompensation, and that too at a rapid rate. The factors that affect the rate of this damage are age, exercise, and nutrition. Although other factors such as pregnancy and status of sex hormone play their role in the development of myocarditis, the not important factors remain the immune system. If the immune system fails to control the initial phase of viral replication, then it may lead to cardiomyopathy. The research has concluded that the presence of viral genetic material is sufficient to cause damage of cardio muscles and is capable of disrupting the filamentous structure of the protein and this effect is independent of the myopathy caused by the immune system. (2)
Indirect Immune Cause. The prevention of viral replication by the immune system poses both the positive as well as negative consequences in context to myocardial health. As soon as the virus invades the body, the cells that response early are the Natural Killer (NK) cells. Studies indicate that low level of natural killer cells causes increased severity of myocarditis. However, along with the NK cells, the NK-like cells are also produced. These cells damage the cardiac muscles by releasing a substance known as perforin. After the production of NK cells, the cells produced by the immune system are T-cells. These cells kill the virus present in the myocytes, causing damage to the myocytes themselves. This is done due to molecular mimicry. Further, products from the damaged myocytes cause further damage to the other myocytes by T-cells. (2) (1)
Role Of Nitric Oxide. Nitric oxide is produced by the endothelial cells. Nitric oxide has a beneficial effect on the heart as well as the blood vessels as it causes cardiac muscle relaxation and manages diastolic function. However, during the severe inflammation, in the case of myocarditis, the expression of the inducible form of nitric oxide (iNOS) is increased and this result in the increased production of nitric oxide. Excessive production of nitric oxide increases oxidative stress resulting in necrosis and apoptosis of myocardial cells. (2)
Myocarditis caused due to viral infection through various mechanisms. This includes the direct attack of the flu virus on the muscle cells, damage to the muscle cells by the immune system and necrosis and apoptosis caused by the increased concentration of nitric oxide.