The current issue of Annals of Internal Medicine contains a paper highlighting the importance of familial dilated cardiomyopathy. In a series of “idiopathic” dilated cardiomyopathy (DCM) the investigators found evidence of DCM or subclinical abnormalities in left ventricular function in relatives of about one third of patients. Previous estimates have cited similar or even higher prevalence of familial etiology of DCM. This points out that familial DCM is a very important etiology of heart failure and may be the most common cause of idiopathic DCM.
In my experience, when we encounter a new case of DCM we do the standard work up to screen for valve disease, coronary disease, thyroid disease, hemochromatosis as well as toxins (particularly ethanol) and if that screen is negative we say “it must have been a viral myocarditis.” I always wonder if that diagnosis is correct. I have no doubt it exists but I think it tends to be overdiagnosed.
That raises the question of just how common viral myocarditis really is as a cause of DCM. Controversy surrounds the issue of how many patients actually have myocarditis, and how many cases of myocarditis are actually viral. This small series found a viral etiology in around 20% to 30% of DCM, all Coxsackie B.
So, what can we make of this? Here are some key points.
1) Familial cardiomyopathy is a common cause of DCM.
2) Familial DCM is important to recognize because family members may benefit from screening via electrocardiography and echocardiography.
3) Gene testing in DCM should be limited to specific (and infrequent) situations. This review of the genetics of cardiomyopathy discusses genetic testing and makes some useful practical points.
4) Viral myocarditis is a known cause of cardiomyopathy but its relative importance is controversial. The American College of Cardiology Guidelines state that myocardial biopsy is indicated only in unusual specific circumstances, and the guidelines do not recommend viral serologic testing.