Cardiac magnetic resonance imaging (CMR) can be a wonderful tool when myocarditis is in question. In this short video, you will learn what to look for on the CMR. If you’re not reading the CMR yourself, you will learn what questions to put on the referral and how to make sense of the CMR report.
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[00:00:00] In this lesson, we're going to learn how to assess myocarditis using CMR. Myocarditis refers to inflammation of the myocardium and it is a potentially serious condition, being both the cause of sudden cardiac death and also sometimes leading to a dilated cardiomyopathy. Myocarditis is most commonly due to a viral infection, either because of direct viral involvement to the myocardium or as a result of a
[00:00:30] post-viral immune-mediated reaction. However, myocarditis can also be seen in other forms of infection including bacterial and fungal infections and can also be triggered by exposure to toxications and certain drugs including chemotherapy agents. This is Lake Louise in Canada, and this is where a meeting was held to discuss the role of CMR in myocarditis. As a result of this meeting, a set of
[00:01:00] diagnostic CMR criteria were agreed for myocarditis and these are now known as the Lake Louise criteria. So, when would it be appropriate to perform a CMR study for suspected myocarditis? Well, the recommendations suggest that it's appropriate when someone presents with new onset or persisting symptoms suggestive of myocarditis. This might include breathlessness or orthopnea, palpitations,
[00:01:30] effort intolerance, malaise or chest pain. In addition, there needs to be evidence for recent or ongoing myocardial injury, in other words, we need evidence of ventricular dysfunction or new or persisting ECG abnormalities or an elevated troponin. In addition, we need a suspected viral etiology. Although, there are other non-viral causes of myocarditis, the CMR guidelines focus
[00:02:00] very much on the viral etiology. So, we need a history of a recent systemic viral disease or previous myocarditis or the absence of risk factors for coronary disease or a young patient aged below 35 years or symptoms which are not explained by coronary stenosis on angiography or a recent negative ischemic stress test. And if we have a patient who meets all three sets of criteria, then it's appropriate to
[00:02:30] undertake CMR scanning, to look for CMR evidence of myocarditis. So, what are the Lake Louise criteria for the CMR diagnosis of myocarditis? Well, if we have a patient who has clinically suspected myocarditis, based upon the criteria that we've just been discussing on the previous slides, then the CMR findings are consistent with myocardial inflammation if at least two of the following criteria are present.
[00:03:00] So, criteria number one is evidence of a regional or global myocardial signal intensity increase, in T2-weighted images. The second criterion that we would look for, is evidence of an increased global myocardial early gadolinium enhancement ratio, between myocardium and skeletal muscle, in gadolinium-enhanced T1-weighted images. And the third criterion is that there is at least one focal
[00:03:30] lesion, with non-ischemic regional distribution, of late gadolinium enhancement. So, we need at least two of these three criteria to be present in order to say that the CMR scan is consistent with myocardial inflammation. There are other supportive pieces of evidence that we may see such as left ventricular dysfunction or pericardial effusion. And if that's present, it doesn't prove myocarditis but it does provide additional
[00:04:00] support for diagnosis. So, let's take a look at an example. This is a 48-year-old female patient who presented with clinical features compatible with myocarditis. And this is her cine-CMR image from the four-chamber view. So, we have the left ventricle, left atrium, right ventricle, and right atrium. And what is notable in this image is the left ventricle is dilated and is severely impaired. Indeed,
[00:04:30] her left ventricular ejection fraction was just 20%. So, she has severely impaired left ventricular systolic function, which is one of the supportive pieces of evidence for the diagnosis of myocarditis. And then, this is a T2-weighted image and in this image we look for evidence of either regional or global increase in myocardial signal intensity. This is one of the Lake Louise criteria
[00:05:00] for the diagnosis of myocarditis. This is the left ventricle, just here, the right ventricle, is just here and the two atria are, here and the left ventricular myocardium appears quite bright. So, we have an increase in myocardial signal intensity in this T2-weighted image. And so, this patient has one of the Lake Louise criteria for making a CMR diagnosis of myocarditis.
[00:05:30] The patient also has a second criterion for a diagnosis of myocarditis and that's evident in these late gadolinium-enhancement images. So, this is a short access view of the left ventricle. We can see the left ventricle here, the right ventricle just adjacent to it. On this image again, we have the left ventricle and the right ventricle just adjacent to it. These are images at the same position in the same patient and it's just two different ways of obtaining late gadolinium-
[00:06:00] enhancement images. And we can see that most of myocardium is quite dark but here in the lateral wall, we can see some epicardial late gadolinium enhancement and that's visible on both the images. And so, what we're seeing is a focal lesion in a non-ischemic distribution. In other words, if this was due to myocardial infarction, we'd expect this late enhancement to be
[00:06:30] sub-endocardial rather than epicardial. So, the fact that its [unintelligible 00:06:35] presents is below the epicardium rather than the endocardium, indicates that it is not due to an ischemic injury. It's not due to an infarction, and that therefore, meets another of the Lake Louise criteria for the diagnosis of myocarditis. Here's a different patient. This is a late gadolinium-enhancement image obtained in the three-chamber view, so we have the left ventricle, left atrium,
[00:07:00] aortic valve, and aortic root. This is the left ventricular myocardium seen here. Again, most of it appears dark but there are patchy areas of late gadolinium enhancement present. And again, this is a non-ischemic distribution. In other words, it is not sub-endocardial, instead it is present in the mid-wall. So, this meets one of the criteria for the CMR diagnosis of myocarditis.
[00:07:30] So, in summary, how do we assess myocarditis using CMR? Well, it's all based around the Lake Louise criteria and we begin by assessing left ventricular volume and function. We quantify left ventricular volumes, stroke volume and ejection fraction, we can calculate cardiac index, and we measure left ventricular wall thickness, and calculate left ventricular mass. So, we perform a detailed assessment of left ventricular size and function. We look for evidence of myocardial inflammation
[00:08:00] or injury. So, we look for evidence of myocardial edema on the T2 imaging. We look for evidence of early myocardial contrast uptake and also evidence of late gadolinium enhancement in a non-ischemic distribution. And in some cases, we may see evidence of pericardial involvement as evidenced by a pericardial effusion. So, all of these features need to be incorporated into your CMR report
[00:08:30] in a myocarditis study. Myocarditis can be a challenging clinical diagnosis but hopefully, now, you'll feel more confident in using CMR to investigate this condition. Let's check what we've learned with some multiple choice questions.