Hypertrophic Obstructive Cardiomyopathy (HOCM)

What happens in HOCM

In hypertrophic cardiomyopathy, the heart muscle becomes abnormally thick (15 mm or more in adults, compared with about 6 to 11 mm normally), most often in the septum, the wall dividing the left and right ventricles. The muscle cells are disorganized and the heart becomes stiff.

In the obstructive form, the bulging septum narrows the left ventricular outflow tract (LVOT), the channel blood passes through to reach the aorta. As blood speeds through this narrow gap, it pulls the front leaflet of the mitral valve toward the septum (called systolic anterior motion), making the blockage worse and causing a leak back through the mitral valve.

Doctors measure the obstruction as a pressure difference (gradient) across the outflow tract. A gradient of 30 mmHg or more defines obstruction, and 50 mmHg or more is the level at which symptoms usually justify treatment.

The obstruction is dynamic: it changes from minute to minute. It worsens when the heart is less full or squeezes harder, for example during exercise, after standing up quickly, with dehydration, after a large meal or alcohol, and with medicines that dilate blood vessels.

Symptoms

Many people have few or no symptoms for years. When they occur:

• Shortness of breath on exertion, the most common symptom
• Chest pain with exercise, because the thick muscle needs more oxygen than it gets
• Dizziness or fainting, especially during or just after exertion
• Palpitations from extra beats or atrial fibrillation
• Fatigue and reduced exercise capacity

The first sign can be sudden cardiac arrest, most often in young people during intense exertion, which is why family screening matters.

The HOCM murmur

Doctors often hear a harsh murmur along the left side of the breastbone. It characteristically gets louder when you strain (the Valsalva maneuver) or stand up from squatting, and softer when you squat or raise your legs. This is the opposite of the murmur of aortic stenosis, which it can resemble.

Causes and inheritance

Most HOCM is caused by a variant in a gene for a sarcomere protein, the contractile machinery of heart muscle. MYH7 and MYBPC3 are the most common. The pattern is autosomal dominant: each child of an affected parent has a 50% chance of inheriting the variant, though not everyone who inherits it develops thickening (variable penetrance).

Conditions that can mimic HCM include long-standing high blood pressure, aortic stenosis, athlete's heart, and storage or infiltrative diseases such as Fabry disease and amyloidosis. Doctors look for these before confirming the diagnosis.

Diagnosis

1. Echocardiogram: measures wall thickness, the outflow gradient at rest and during Valsalva, mitral valve motion and leaks. If the resting gradient is low, an exercise echo looks for provoked obstruction.
2. ECG: usually abnormal, with signs of thickening and deep T waves.
3. Cardiac MRI: gives precise thickness and shows scarring (late gadolinium enhancement), which helps predict arrhythmia risk.
4. Ambulatory monitor (24 to 48 hours or longer): looks for atrial fibrillation and runs of ventricular tachycardia.
5. Exercise test: checks capacity, blood pressure response and symptoms.
6. Genetic testing: confirms the cause in many families and allows targeted testing of relatives.

Family screening

First-degree relatives (parents, siblings, children) should have an ECG and echocardiogram. If a gene variant is found in the family, relatives can have a genetic test; those without the variant can usually stop screening. Children who carry it are typically checked every 1 to 3 years, and adults every 3 to 5 years.

Treatment

Medicines

• Beta-blockers (first choice) slow the heart and reduce how hard it squeezes, lowering the gradient.
• Nondihydropyridine calcium channel blockers (verapamil, diltiazem) if beta-blockers are not tolerated.
• Disopyramide added for persistent symptoms.
• Cardiac myosin inhibitors such as mavacamten directly reduce excess muscle contraction, lower the gradient and improve symptoms. They need regular echocardiograms to make sure the heart's pumping function does not fall too low.

Medicines to avoid or use with caution include nitrates, many blood-pressure vasodilators, high-dose diuretics and digoxin, which can worsen obstruction.

Septal reduction procedures

For people with significant symptoms and a gradient of 50 mmHg or more despite medicines:

• Septal myectomy: open-heart surgery that removes part of the thick septum. At experienced centers it relieves obstruction in most patients with low risk, and the mitral valve can be repaired at the same time.
• Alcohol septal ablation: a catheter procedure that injects alcohol into a small artery feeding the septum, creating a controlled scar that thins it. Recovery is quicker, but the risk of needing a pacemaker is higher.

Preventing sudden death

An implantable cardioverter-defibrillator (ICD) is recommended for people who have survived cardiac arrest or sustained ventricular tachycardia, and considered for those with risk factors: unexplained fainting, a family history of sudden HCM death, very thick walls (30 mm or more), extensive scarring on MRI, or a reduced pumping function. Learn more about sudden cardiac death.

Atrial fibrillation

AFib is common in HCM and carries a high stroke risk, so blood thinners are recommended for anyone with HCM who develops it, regardless of usual risk scores.

Living with HOCM

• Stay hydrated, especially in heat, during illness or with alcohol.
• Avoid sudden exertion from rest and stand up slowly.
• Exercise: current guidelines support regular moderate exercise for most people with HCM. Decisions about competitive sport are made individually with a specialist.
• Pregnancy is usually well tolerated with specialist care; discuss it in advance, since some medicines must be changed.
• Check with your cardiologist before taking new medicines, including erectile dysfunction drugs and decongestants.

With modern treatment, most people with HOCM have a near-normal life expectancy.

Complications

• Heart failure from stiffness or, rarely, late thinning of the walls
• Atrial fibrillation and stroke
• Ventricular arrhythmias and sudden cardiac arrest
• Mitral valve leak
• Infective endocarditis on the mitral valve (uncommon)

Frequently asked questions

What is the difference between HCM and HOCM?

HCM (hypertrophic cardiomyopathy) is the disease of thickened heart muscle. HOCM is HCM in which the thickening blocks blood flow out of the left ventricle, either at rest or during exertion. About two-thirds of people with HCM have obstruction. The obstructive form is more likely to cause symptoms but has specific treatments.

What are the first symptoms of HOCM?

The first symptoms are usually shortness of breath, chest pain or dizziness during exercise, palpitations, or fainting with exertion. Some people have no symptoms and are diagnosed after a murmur, an abnormal ECG or family screening. Fainting during exercise always needs urgent evaluation.

Can HOCM be cured?

The genetic condition cannot be cured, but the obstruction can be removed or greatly reduced with septal myectomy or alcohol septal ablation, and myosin inhibitors can control it with medicine. With treatment and an ICD when needed, most people live a long, active life.

Is it safe to exercise with HOCM?

Most people can and should do regular moderate exercise such as brisk walking, cycling or swimming. Very intense or competitive sport needs a personal risk assessment with an HCM specialist. Avoid exercising when dehydrated, warm up and cool down gradually, and stop if you feel faint or have chest pain.

Is HOCM hereditary?

Usually yes. Most cases are caused by a gene variant passed down in an autosomal dominant pattern, giving each child a 50% chance of inheriting it. Parents, siblings and children of someone with HOCM should be screened with an ECG and echocardiogram, and genetic testing if a family variant is known.