virus is the most common viral cause of myocarditis.
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A 50-year-old man comes to the emergency department because his heart "is beating really quickly" and he has diffuse chest pain. He has never had symptoms like this before and has been generally healthy, except for a recent febrile illness about two weeks ago during which he had a fever, headache, and sore throat for two days. His sick contacts include his two children, who had similar symptoms before he developed a fever. He denies any recent joint pain or skin rashes. Cardiac examination shows non-reproducible chest pain with no murmurs, rubs, or gallops on auscultation. Which of the following is a reasonable treatment choice for his condition?
With myocarditis, myo- means muscle, card- means heart, and -itis means inflammation. So, myocarditis is inflammation of the myocardium, which is the muscular middle layer of the heart wall, which contracts and relaxes so the heart can pump blood all around the body.
Inflammation in the myocardium layer causes swelling, which damages the heart muscle cells’ ability to contract. That means that less blood gets pumped out of the heart with each heartbeat. If myocarditis is severe enough, it can lead to heart failure, which is when the heart can’t keep up with the demands of the body. Once the inflammation resolves, the heart contraction typically returns to normal, but occasionally, when the inflammation is really severe, it can cause fibrosis, or scar tissue, in the myocardium. Scar tissue doesn’t contract normally, so if that happens, it can cause long term problems with heart contraction.
In North America, viral infections, specifically Coxsackievirus B infections, are the main cause of myocarditis. Viral infections can trigger lymphocytic myocarditis, which is when lymphocytes - the B and T cells of the immune system - and water make their way into the interstitial space - the space in between heart muscle cells. There are plenty of other infectious causes as well, though. One of these is trypanosoma cruzi, a single-cell protozoan that causes Chagas disease throughout South America. In Chagas disease, under a microscope, it’s possible to see groups of amastigotes within the heart muscle cells, which are trypanosomes that are in the intracellular stage. As a result, the heart muscle cells necrose or die. There’s also Trichinella, a roundworm that moves from the intestines into various parts of the body, causing a variety of problems, including myocarditis. Myocarditis can also be seen in Lyme disease which is caused by the bacteria Borrelia burgdorferi, which is spread by deer ticks. Finally, in immunocompromised individuals, there’s Toxoplasma gondii, a single-cell parasite harbored by cats, that can cause myocarditis.
Those were the infectious causes, but there are also non-infectious causes of myocarditis as well. These include systemic lupus erythematosus, also known as lupus, and polymyositis, a generalized inflammation of the muscles, where the immune system starts to attack the myocardial layer of the heart. There’s also drug-associated myocarditis, which means there’s an adverse drug reaction that inflames the heart. Drug reactions cause a hypersensitivity myocarditis, which is when eosinophils get into the blood vessels in the myocardium. Finally, there’s giant cell myocarditis, which causes inflammation in the heart, from an unknown cause. The key finding is that macrophages - immune cells that engulf foreign substances - start to fuse together to form a single giant cell, hence the name.
Individuals with myocarditis can have chest pain that is sometimes positional, meaning that it can get better or worse depending on the body’s position. It can also cause arrhythmias, or irregular heartbeats, because the inflammation affects the pacemaker cells traveling through the myocardium. There can also be more general symptoms like fatigue, fever and shortness of breath. In severe cases, where myocarditis starts to develop into heart failure, there can be additional symptoms like fluid retention in the feet and ankles.