What is Hypertrophic Obstructive Cardiomyopathy (HOCM)?

What is HOCM (Hypertrophic Obstructive Cardiomyopathy)?

HOCM is an outdated term that should no longer be used; the preferred nomenclature is HCM (hypertrophic cardiomyopathy) with or without left ventricular outflow tract obstruction, because the term HOCM incorrectly implies that obstruction is an invariable feature of the disease when in fact one-third of patients never develop obstruction. 1

Historical Context and Nomenclature Problem

• The term "hypertrophic obstructive cardiomyopathy" (HOCM) was popular in the 1960s and 1970s, along with "idiopathic hypertrophic subaortic stenosis" (IHSS), but these terms are potentially confusing and misleading. 1
• At least 80 different names, terms, and acronyms have been used to describe this disease over the years, creating significant confusion in the medical and non-medical community. 1
• The term HCM was introduced in 1979 and has become the predominant formal designation because it encompasses both obstructive and nonobstructive hemodynamic forms. 1

What the Disease Actually Is

HCM is a genetic cardiovascular disease characterized by unexplained left ventricular hypertrophy (typically asymmetric) with a nondilated, hyperdynamic chamber in the absence of another cardiac or systemic disease capable of producing the magnitude of hypertrophy evident. 1

Diagnostic Criteria

• In adults, HCM is diagnosed by imaging showing maximal end-diastolic LV wall thickness ≥15 mm anywhere in the left ventricle. 1
• Wall thickness of 13-14 mm is considered borderline but can be diagnostic in family members of HCM patients or with a positive genetic test. 1
• In children, diagnosis requires LV wall thickness ≥2 standard deviations above the mean (z-score ≥2) adjusted for age, sex, and body size. 1

Prevalence and Epidemiology

• HCM affects approximately 1 in 500 people (0.2%) in the general population, equivalent to at least 600,000 people in the United States. 1
• Most affected individuals remain unidentified, likely without symptoms or shortened life expectancy. 1
• HCM is a global disease with similar prevalence across different parts of the world. 1

The Obstruction Component (What "HOCM" Tried to Describe)

Left ventricular outflow tract (LVOT) obstruction is present or develops over time in approximately two-thirds of HCM patients, but one-third remain nonobstructive throughout their lives. 1, 2

Classification of Obstruction

• Basal (resting) obstruction: LVOT gradient ≥30 mmHg at rest, present in approximately one-third of patients. 1, 2
• Labile (provocable) obstruction: LVOT gradient <30 mmHg at rest but ≥30 mmHg with physiologic provocation, present in another one-third of patients. 1, 2
• Nonobstructive HCM: LVOT gradient <30 mmHg both at rest and with provocation, present in the final one-third of patients. 1, 2

Mechanism of Obstruction

• Obstruction is caused primarily by systolic anterior motion (SAM) of the mitral valve leaflets with mid-systolic contact against the ventricular septum. 1, 2
• During ventricular systole, flow against the abnormally positioned mitral valve apparatus creates drag force that pushes the leaflets into the outflow tract. 1
• The obstruction is dynamic and varies with loading conditions, contractility, daily activities, food intake, and alcohol consumption. 1, 2

Clinical Significance of Gradients

• Gradients ≥30 mmHg define obstructive disease and are associated with increased risk of heart failure progression, severe symptoms, and stroke. 1, 2
• Gradients ≥50 mmHg represent the conventional threshold for considering septal reduction therapy (surgical myectomy or alcohol septal ablation) in symptomatic patients who fail medical management. 1, 2

Key Clinical Features

• The disease is characterized by myocyte hypertrophy and disarray, interstitial fibrosis, and left ventricular diastolic dysfunction. 3
• Hypertrophy is typically asymmetric, most commonly involving the basal interventricular septum, though virtually any pattern of LV wall thickening can occur. 1
• The LV chamber is nondilated and hyperdynamic, often with systolic cavity obliteration. 1
• When obstruction is present, it produces a loud apical systolic ejection murmur with unique clinical signs. 1

Genetic Basis

• HCM is caused by mutations in genes encoding sarcomere-associated proteins, most commonly MYH7 and MYBPC3, which together account for approximately 50% of cases. 3
• In approximately 40% of patients, the causal genes remain unidentified. 3
• Some individuals are "genotype positive/phenotype negative," carrying disease-causing mutations without overt hypertrophy. 1

Common Pitfall to Avoid

Do not use the terms HOCM or IHSS in clinical documentation or communication, as they perpetuate the misconception that obstruction is always present and can lead to inappropriate management decisions for the one-third of patients who are nonobstructive. 1 Instead, specify "HCM with LVOT obstruction" or "obstructive HCM" when obstruction is documented, and "nonobstructive HCM" when it is absent. 1