Obstructive vs. Nonobstructive Hypertrophic Cardiomyopathy: Understanding Your Diagnosis

You’ve just been diagnosed with hypertrophic cardiomyopathy (HCM). But you heard your doctor mention another word, too — either “obstructive” or “nonobstructive.” So, naturally, you’re curious what these terms mean and, more importantly, what they mean for you.

There are two main types of HCM: obstructive and nonobstructive. These types have a lot in common, but they also differ in important ways that can affect your treatment options and how you feel from day to day.

Cardiologist Milind Desai, MD, explains.

Defining obstructive vs. nonobstructive HCM

When you’re diagnosed with HCM — a type of heart disease that thickens your heart muscle — your provider will specify which type you have. “This is the first fork in the road,” Dr. Desai explains. “The type you have determines the direction of your care plan.”

You can have either of the following:

• Obstructive HCM: This means that your heart muscle is thick in the area leading up to your aortic valve, which controls blood flow out of your heart. This thickened muscle forms an “obstruction” that can reduce blood flow to your body.
• Nonobstructive HCM: In this case, there’s not an obstruction near your aortic valve. But your heart muscle may be thick or stiff in other areas, leading to blood flow problems.

Similarities

Obstructive and nonobstructive HCM have a lot in common, including:

• What causes them
• How they affect your heart’s structure (with one important difference!)
• Which symptoms you can develop

Causes

HCM, no matter the type, is often genetic. You’re born with certain gene variants that cause your heart muscle to thicken.

“This is an autosomal dominant condition,” Dr. Desai explains. “That means if you have it, there’s a 50% chance your child will be born with it, too.”

But exactly how those gene variants affect you can vary widely.

“Your mother might be genetically positive but never actually develop the disease — in other words, never show symptoms or have problems with blood flow,” he observes. “But you might inherit those genes and develop severe symptoms early in life.”

It’s also possible not to have a clearly identifiable genetic cause.

“In these cases, there may be other genes responsible that we haven’t found yet,” Dr. Desai clarifies. “There are also people who develop HCM later in life due to factors like longstanding high blood pressure.”

Structural changes

Both obstructive and nonobstructive forms of HCM change your heart’s structure and, in turn, its function. Both types cause:

• Thick, stiff walls
• A smaller cavity (the open space inside your heart’s main pumping chamber, the left ventricle)
• Damage to the small branches of your coronary arteries
• Arrhythmias, like atrial fibrillation (AFib) or ventricular tachycardia (VT)

But there’s one important difference. With obstructive HCM, the thick walls block (obstruct) blood flow out of your heart. That doesn’t happen with the nonobstructive form. More on that in a bit.

But no matter the type, you need close monitoring and treatment to help your heart work at its best.

Types of symptoms

Because both types of HCM affect your heart’s structure, they can lead to similar types of symptoms. These commonly include:

• Shortness of breath, especially when you’re pushing yourself
• Dizziness
• Fainting
• Fatigue
• Chest pain or discomfort
• The sensation that your heart is racing, pounding or flip-flopping (palpitations)

“While the types of symptoms are similar, the severity can change based on the form of HCM you have,” Dr. Desai says. “Generally, obstructive HCM causes more severe symptoms. But if you’re in the early stages of nonobstructive HCM, your symptoms may be milder or bother you less often.”

But that can change as the disease progresses.

“In advanced nonobstructive HCM, the heart muscle is very thick and stiff, and the cavity is very small,” he adds. “This scenario causes symptoms that are just as severe as the obstructive form.”

Differences

The main differences between obstructive and nonobstructive HCM are:

• How they affect blood flow through your heart
• How they’re treated

Impact on blood flow

With obstructive HCM, thick heart muscle blocks or reduces blood flow out of your heart — specifically, from your left ventricle, through your aortic valve and into your aorta. Nonobstructive HCM doesn’t do this. That’s the major, defining difference.

To envision how obstructive HCM affects blood flow, Dr. Desai offers an analogy: Picture a tunnel that gets lots of traffic, like the Lincoln Tunnel connecting New York and New Jersey.

“Imagine that there’s construction leading up to the tunnel,” he describes. “You see orange barrels lined up, narrowing the lanes. At 4 a.m., there aren’t many cars that need to get through, and traffic flows smoothly into the tunnel.”

But then rush hour hits.

“When you have lots of cars trying to get through, the narrowed lanes become a problem,” he continues. “And that’s what happens with obstructive HCM.”

Dr. Desai breaks down the exact steps:

1. Just like those orange barrels on the road, the thickened area of your heart muscle narrows the path for blood to flow as it approaches your aortic valve. Part of your nearby mitral valve might also get in the way.
2. If you’re taking it easy, blood can still reach your valve (and flow out to your body) without any issues. That’s like traffic at 4 in the morning.
3. At times when your body needs more blood (like when you’re in hot weather or exerting yourself), blood flow out of your heart must increase. That’s rush hour — when more “cars” are approaching the tunnel (your aortic valve).
4. When blood flow increases, it can’t easily pass through the “narrowed lanes.”
5. The obstruction can cause blood to back up and flow in the wrong direction, toward your lungs, which leads to shortness of breath. If your brain can’t get enough blood, you might feel dizzy or pass out. It’s your heart’s version of a traffic jam.

“Obstructive HCM is what I call dynamic,” Dr. Desai says. “That means it’s not a fixed obstruction. It causes symptoms at certain times, usually when your body’s demand for blood increases.”

So, what about nonobstructive HCM?

“There’s no obstruction causing your symptoms,” Dr. Desai says. “But your heart muscle is stiff, meaning it can’t easily expand to allow the chambers to fill with blood.”

To continue the traffic analogy, it’s like some of the entrance ramps are closed, limiting how many cars can fill up the lanes leading up to the tunnel. This means that even though there’s no obstruction, fewer cars pass through. And that can also lead to symptoms.

Treatment options

Your treatment plan will be guided by whether you have obstruction. Current treatment options for obstructive HCM include:

• Myosin inhibitors: These medications target the root cause of the disease. “They’re game changers,” Dr. Desai says. “They help reduce the obstruction, reduce symptoms and improve quality of life. You have a good chance of avoiding surgery.”
• Septal myectomy: This is a surgical option that’s appropriate for some people. “We essentially shave away some of the thick muscle, repair the mitral valve and relieve the obstruction,” Dr. Desai explains. “Most people have excellent outcomes.”

The situation is a bit different for nonobstructive HCM. There currently aren’t medications or surgeries that directly treat this condition.

“We do have lots of tools in our belt for managing nonobstructive HCM,” Dr. Desai says. “But we’re working on developing more precise and effective ones, too.”

For now, if you have the nonobstructive form, your doctor may recommend medications that help your heart work better. For example, certain drugs can help a stiff left ventricle fill with blood and pump that blood to your body.

And finally, treatments you may need for either type of HCM include:

• Medications to manage heart disease risk factors: Your doctor may prescribe meds to manage high blood pressure, high cholesterol or diabetes. They’ll also help you reach and maintain a healthy weight. All these things lower your risk of additional heart problems, like coronary artery blockages, on top of HCM.
• Medications or a device to manage arrhythmias: Your doctor may suggest medications or a device, like an implantable cardioverter defibrillator (ICD). “Treating arrhythmias reduces your risk of sudden cardiac death,” Dr. Desai notes.

Final thoughts

An HCM diagnosis is life-changing, no matter which form you have. But learning the nuances of obstructive HCM vs. nonobstructive HCM can help you understand exactly what’s happening inside your body. The more you know about your diagnosis, the more you can take an active role in your care.