IHSS (Idiopathic Hypertrophic Subaortic Stenosis) is an acronym often encountered when researching heart conditions related to muscle disorders. It refers to a specific condition where the heart muscle, primarily the left ventricle, has grown abnormally thick. This thickening profoundly alters the heart’s mechanics, leading to various symptoms and complications.
Defining the Condition and Nomenclature
The term IHSS (Idiopathic Hypertrophic Subaortic Stenosis) is largely outdated and considered obsolete in modern cardiology. The preferred medical term is Hypertrophic Cardiomyopathy (HCM). The original name was problematic because the “subaortic stenosis” part, implying a blockage of blood flow, is not present in all patients.
HCM describes a disorder where the myocardium, or heart muscle, becomes abnormally thickened without a clear cause like high blood pressure. This thickening often affects the interventricular septum, the wall dividing the heart’s left and right sides.
The “idiopathic” component of the old name, meaning the cause is unknown, is also less accurate today. Current understanding points to genetic mutations, primarily in the genes that code for the heart muscle’s contractile machinery, as the underlying cause in about 60% of cases. The disease is typically inherited in an autosomal dominant pattern.
Mechanical Effects on Heart Function
The physical consequence of the thickened heart muscle affects both the heart’s pumping and filling phases. The first effect is the potential for Left Ventricular Outflow Tract (LVOT) obstruction, which occurs in the most common form of the disease. The thickened septum can protrude into the LVOT, the pathway blood takes to exit the left ventricle and enter the aorta.
During contraction (systole), this protrusion impedes blood flow, creating a high-pressure gradient. The obstruction is often exacerbated by Systolic Anterior Motion (SAM) of the mitral valve, where the front leaflet is pulled into the narrowed outflow tract. This obstruction forces the heart to pump harder to eject blood.
The second effect is Diastolic Dysfunction, a universal feature of HCM. The thickened muscle is stiff and noncompliant, meaning it cannot properly relax during diastole (the heart’s filling phase). This stiffness prevents the left ventricle from adequately filling with blood.
Since the chamber cannot relax, pressure inside the ventricle rises, backing up into the lungs and often causing shortness of breath. This impaired filling reduces the amount of blood the heart can deliver to the body, impacting cardiac output.
Symptoms and Diagnostic Tools
Symptoms of Hypertrophic Cardiomyopathy vary widely, with some individuals remaining asymptomatic for years. Common complaints include shortness of breath, particularly with physical activity, and chest pain (angina). Angina can occur even without coronary artery blockages because the thickened muscle has increased oxygen demand.
Other signs include palpitations (a rapid or fluttering heartbeat), often related to abnormal heart rhythms like atrial fibrillation. Fainting (syncope) or near-fainting spells can also occur, sometimes triggered by exertion or a sudden drop in blood pressure. These symptoms result from mechanical issues, such as obstruction or arrhythmias, preventing adequate blood flow to the brain.
The primary diagnostic tool for HCM is the echocardiogram, which uses sound waves to create a moving image of the heart. This test allows a cardiologist to visualize the thickness of the heart walls and measure any outflow tract obstruction. For adults, a left ventricular wall thickness of 15 millimeters or greater, without another cause, is diagnostic.
An electrocardiogram (ECG) is also standard, often showing abnormal electrical patterns due to structural changes. Cardiac Magnetic Resonance Imaging (CMR) provides a detailed view of the heart structure and helps identify scar tissue, which is important for risk assessment. Genetic testing is frequently used, especially for screening family members.
Treatment and Management Strategies
The goals of treatment for Hypertrophic Cardiomyopathy are to reduce symptoms, improve the heart’s ability to fill, and lower the risk of sudden cardiac death.
Pharmacologic Management
Initial management involves pharmacologic intervention, with beta-blockers being a first-line treatment. These medications slow the heart rate, allowing more time for the heart to fill and reducing the muscle’s oxygen demand. Calcium channel blockers, such as verapamil, are also used to help the stiff muscle relax more fully, improving diastolic function. A newer class of medication, cardiac myosin inhibitors, specifically targets the underlying mechanism by reducing the over-activity of the heart muscle and helping to relieve obstruction in symptomatic patients.
Lifestyle and Interventions
Lifestyle modifications are an important part of management, particularly the avoidance of intense competitive sports and strenuous exertion. These activities can increase outflow tract obstruction and the risk of life-threatening arrhythmias. Patients are also advised to stay well-hydrated, as low fluid volume can worsen obstruction.
For patients who remain symptomatic despite medication or who have significant obstruction, interventional procedures may be necessary. Septal reduction therapy includes surgical septal myectomy, which involves removing a small piece of the thickened septum to widen the outflow tract. Alcohol septal ablation is a less invasive procedure that achieves a similar result by injecting alcohol into the artery supplying the thickened area, causing controlled tissue death.
Preventing Sudden Cardiac Death
An Implantable Cardioverter-Defibrillator (ICD) is used for patients considered at high risk for sudden cardiac death. The ICD constantly monitors the heart rhythm and can deliver an electrical shock to restore a normal rhythm if a dangerous arrhythmia, like ventricular fibrillation, occurs. This device has significantly improved survival rates for high-risk individuals with HCM.