This was thought to be a disease affecting both the heart muscle and the endocardium and it was given various names such as: idiopathic hypertrophy of the heart, endocardial sclerosis, cardiac enlargement of unknown cause, etc. Some of these hearts also had overt congenital anomalies, especially aortic stenosis and coarctation of the aorta.The term "endocardial fibroelastosis" was introduced by Weinberg and Himmelfarb in 1943. In their pathology laboratory they noted that usually the endocardium was pearly white or opaque instead of normally thin and transparent and microscopically showed a systematic layering of collagenous and elastic fibers. They felt their new term was more adequately descriptive, and, indeed it was quickly and widely adopted. Clinicians began applying it to any infant with a dilated, failing heart, in spite of the fact that the only way to definitively establish the presence of EFE was to see it at autopsy. EFE had quickly become the name of a disease, and it continues to be used by many physicians in this way, though many patients with identical symptoms do not have the endocardial reaction of EFE.In the latter decades of the twentieth century, new discoveries and new thinking about heart muscle disease gave rise to the term "cardiomyopathy". Many of the cases of infantile cardiac failure were accordingly called "primary cardiomyopathy" as well as "primary EFE", while those with identifiable congenital anomalies stressing the heart were called "secondary EFE".
The proponents of mumps as a cause pointed to this as the effect of the recent implementation of widespread mumps immunization.Evidence that viral infection may play a role as a cause or trigger of EFE was greatly reinforced by the study directed by Towbin in the virus laboratory of Texas Childrens Hospital. They applied the methods of todays genetics to old preserved specimens from autopsies of patients with EFE done well before mumps immunization began and found mumps genome in the tissues of over 80% of these patients. It seems undeniable that transplacental mumps infection had been in the past the major cause of EFE, and that immunization was indeed the cause of EFE having become rare.Non-infectious causes of EFE have also been studied, spurred by the opening of new avenues of genetics research. Now there are specific named genes associated with certain cardiomyopathies, some of which show the characteristic reaction of EFE. A typical example is Barth syndrome and the responsible gene, tafazzin.Developments in echocardiography, both the technology of the machines and the skill of the operators, have made it no longer necessary to see the endocardium at autopsy. EFE can now be found non-invasively by the recording of increased endocardial echos.