CHAPTER 57: CLASSIFICATION OF CARDIOMYOPATHIES

INTRODUCTION

Clinical taxonomy aids understanding and facilitates appropriate categorization of diseases through the use of logical groups and hierarchies on the basis of predefined characteristics that are useful for the diagnosis and management of human ailments. The classification results in standardization of disease nomenclature. In the premolecular era, this process was predominantly driven by sorting clinical phenotypes (eg, heart failure, cardiomyopathies, arrhythmias) and pathology (eg, cancer). For clinically and etiologically complex diseases such as cardiomyopathies, the principles of classification have been historically based on morphofunctional phenotypes shared by similar disorders with potentially overlapping treatments. Traditionally, the cardiomyopathies have been considered idiopathic diseases. In the past two decades, clinical and molecular genetics and advanced imaging have helped elucidate the etiopathogenetic basis of cardiomyopathy, and it is more than likely that the term idiopathic will gradually become obsolete. Most cardiomyopathies have a genetic basis, and for many of those that are not heritable, the mechanisms are now known. More than 100 disease genes have now been confirmed as candidate genes, and the number is continually increasing. Advances in biotechnology have allowed for rapid identification of underlying causes.

CLASSIFICATION OF CARDIOMYOPATHIES: A HISTORICAL PERSPECTIVE

Heart muscle disorders are relatively young diseases. In a famous editorial published in Heart in 1982, John F. Goodwin reviewed the 30-year history of cardiomyopathies,¹ starting from the early 1950, when Brigden published the St. Cyres lecture on “Uncommon Myocardial Diseases: The Non-coronary Cardiomyopathies.”² The evolution from description to definition and clustering of similar cardiomyopathies led to the first classification of cardiomyopathies in 1972, when Goodwin and Oakley defined cardiomyopathies as the heart muscle diseases of unknown cause and described them as dilated (DCM), hypertrophic (HCM), and restrictive (or obliterative) (RCM) cardiomyopathy types.³ In 1980, the World Health Organization (WHO) adopted the classification of cardiomyopathies as “heart muscle diseases of unknown cause” (Fig. 57–1) to distinguish cardiomyopathy from cardiac dysfunction as a result of known cardiovascular entities such as hypertension, ischemic heart disease, or valvular disease.⁴ In 1995, the WHO/International Society and Federation of Cardiology (ISFC) Task Force on the Definition and Classification of the Cardiomyopathies expanded the classification to include all diseases affecting heart muscle and to take into consideration etiology as well as the dominant pathophysiology.⁵ The WHO/ISFC task force added two more classes: arrhythmogenic cardiomyopathy (ACM; ie, arrhythmogenic right ventricular cardiomyopathy [ARVC]) and unclassified cardiomyopathy.⁵ In 1998, the ISFC evolved to become the World Heart Federation (WHF), and until recently, did not indulge in developing scientific statements, and the recommendations for both diagnosis and management of cardiomyopathies were not updated. The morphofunctional phenotype has continued to be the unique basis for classification of cardiomyopathy.