What is IHSS (Idiopathic Hypertrophic Subaortic Stenosis)?

IHSS (Idiopathic Hypertrophic Subaortic Stenosis)

IHSS is an outdated term from the 1960s-1970s that has been replaced by the preferred nomenclature "Hypertrophic Cardiomyopathy (HCM)" because the old term incorrectly implied that left ventricular outflow tract obstruction is always present, when in fact one-third of patients have no obstruction. 1

Historical Context and Nomenclature Evolution

• The term IHSS (idiopathic hypertrophic subaortic stenosis) is now considered obsolete and potentially confusing because it suggests that left ventricular outflow tract (LVOT) obstruction is an invariable component of the disease 1

• At least 80 different names, terms, and acronyms have been used historically to describe this condition, reflecting confusion in the medical community 1

• HCM became the predominant formal term in 1979 and is now universally accepted because it encompasses both obstructive and non-obstructive hemodynamic forms of the disease 1

• Other outdated terms include HOCM (hypertrophic obstructive cardiomyopathy), which carries the same misleading implication about obligatory obstruction 1

Why the Name Change Matters Clinically

Approximately one-third of HCM patients have no obstruction either at rest or with physiologic provocation, making terms like IHSS and HOCM inaccurate 1

The distribution of obstruction patterns in HCM is:

• One-third have basal (resting) obstruction with gradients ≥30 mm Hg 1
• One-third have labile, physiologically provoked gradients (≥30 mm Hg with provocation but <30 mm Hg at rest) 1
• One-third remain non-obstructive under all conditions 1

Current Preferred Terminology

The American College of Cardiology/American Heart Association and European Society of Cardiology all recommend using "Hypertrophic Cardiomyopathy (HCM)" with specification of whether obstruction is present 1

• The term should be qualified as "HCM with LVOT obstruction" or "non-obstructive HCM" when describing specific patients 1

• Although IHSS and HOCM persist occasionally in informal usage, they now rarely appear in the medical literature 1

What the Condition Actually Is

HCM is characterized by unexplained left ventricular hypertrophy (typically ≥15 mm maximal wall thickness in adults) associated with non-dilated ventricular chambers, in the absence of another cardiac or systemic disease capable of producing that magnitude of hypertrophy 1

• It is a common genetic cardiovascular disease with prevalence of approximately 1:500 in the general population 1

• The disease is inherited in an autosomal dominant pattern with equal distribution by sex 1