CHAPTER 59: HYPERTROPHIC CARDIOMYOPATHIES

DEFINITION AND EPIDEMIOLOGY

Hypertrophic cardiomyopathy (HCM) is defined as left ventricular (LV) hypertrophy in the absence of abnormal loading conditions, such as severe hypertension or valve disease, sufficient to provoke the observed phenotype.¹ It occurs in approximately 1 in every 500 adults,^2,3,4,5,6,7 with a slightly higher prevalence in males, but with no significant differences across ethnicities or geographical locations. Ventricular hypertrophy frequently develops during periods of rapid somatic growth, but can appear de novo at any time from infancy to old age.

The term “hypertrophic cardiomyopathy” embraces a range of disorders that can be grouped into familial/genetic and nonfamilial/nongenetic subtypes^1,8 (Fig. 59–1). The prevalence of individual disorders (phenocopies) varies with age. In 50% to 60% of adolescents and adults, HCM is inherited as an autosomal dominant trait caused by mutations in cardiac sarcomere protein genes. A further 5% of patients have metabolic or storage disorders, neuromuscular disease, chromosome abnormalities, and genetic syndromes such as cardio-facial-cutaneous disorders or phakomatoses. When all these disorders are excluded, 25% to 30% of cases remain unexplained.

HISTORICAL PERSPECTIVE

Patients with unexplained ventricular hypertrophy have been described for more than 100 years,⁹ but it was not until the publication of Sir Russell Brock’s paper on “functional obstruction of the left ventricle” in 1957¹⁰ and Donald Teare’s¹¹ description of asymmetrical myocardial hypertrophy in 1958 that HCM was established as a distinct disease. From the 1960s onward, diagnostic criteria for the disease evolved with the prevailing technologies used to interrogate the heart. An initial focus on clinical signs and symptoms and cardiac catheterization meant that dynamic LV outflow tract obstruction (LVOTO) was central to the diagnosis of the disease.^{12,13,14,15,16,17,18,19,20} In the 1970s, the asymmetrical distribution of hypertrophy was emphasized by the use of M-mode echocardiography,^{21,22,23,24,25} but with the advent of two-dimensional echocardiographic imaging and, later, cardiac magnetic resonance imaging, the complex spectrum of ventricular involvement is now better appreciated.^26,27

Early clinical reports suggested that HCM is inherited in 30% to 50% of patients.²⁸ This was confirmed in prospective clinical genetic studies using two-dimensional echocardiography that suggested 50% to 60% of cases are inherited as an autosomal dominant trait.^29,30 In 1989, the first genetic mutation for HCM ...