Hypertrophic cardiomyopathy and sudden death:
what determines the risk

Article written and reviewed by Dr Luca Moretti

Hypertrophic cardiomyopathy can increase the risk of ventricular arrhythmias and sudden cardiac death, but the risk is not the same for everyone with this diagnosis. Many patients do not have the markers that identify a probability high enough to justify an implantable defibrillator, whereas a minority require more aggressive prevention.
Assessment is therefore individualized, integrating personal and family history, echocardiography, Holter ECG, cardiac magnetic resonance imaging and, in some cases, validated risk models. No single finding, considered in isolation, fully describes a person's risk.

Factors that may increase risk include:

• previous cardiac arrest or sustained ventricular tachycardia;
• syncope considered likely to be arrhythmic and not explained by common causes;
• sudden death related to hypertrophic cardiomyopathy in close relatives;
• very marked left ventricular hypertrophy;
• nonsustained ventricular tachycardia recorded on Holter monitoring;
• left ventricular apical aneurysm;
• reduced left ventricular ejection fraction;
• extensive myocardial fibrosis on cardiac magnetic resonance imaging.

Hypertrophic cardiomyopathy is a heterogeneous disease: people with similar wall thickness can have very different arrhythmic profiles. Risk stratification is intended precisely to avoid two opposite errors: underestimating someone who needs protection or implanting a defibrillator in someone whose probability of benefit is too low compared with the risks of the device.

Why hypertrophic cardiomyopathy can cause serious arrhythmias

In hypertrophic cardiomyopathy, the heart muscle may show myocyte disarray, microvascular abnormalities and areas of fibrosis. This substrate can promote ventricular arrhythmias, especially when associated with other risk factors.
The mechanism therefore does not depend simply on the heart being "thicker". Maximum wall thickness is important, but so are the distribution of hypertrophy, the presence of scars, ventricular function, any aneurysms and the documented electrical history of the heart.

One reason modern assessment is multiparametric is that sudden death can occur through ventricular tachycardia or ventricular fibrillation even in people without significant everyday symptoms. Conversely, shortness of breath or chest pain may be very limiting but do not automatically indicate high arrhythmic risk.

The risk factors that carry the most weight

A previous cardiac arrest or sustained ventricular tachycardia represents the clearest situation: in these cases, the risk of another serious arrhythmia is high enough for a defibrillator to be a secondary prevention strategy.
In primary prevention, meaning before a serious event has already occurred, the decision is more complex. Recent syncope suspicious for an arrhythmic cause, a family history of sudden death attributable to hypertrophic cardiomyopathy and very marked hypertrophy are traditionally relevant factors. In U.S. guidelines, a maximum left ventricular wall thickness of about 30 mm or more is considered an important risk marker.

Holter ECG may show episodes of nonsustained ventricular tachycardia, meaning sequences of rapid ventricular beats that terminate spontaneously. They do not all carry the same weight: the rate, duration and repetition of episodes are interpreted together with the other data.
A left ventricular apical aneurysm and an ejection fraction below 50% are also relevant markers. The latter is particularly important because in hypertrophic cardiomyopathy an ejection fraction that appears "only mildly reduced" already represents significant dysfunction compared with the typical behavior of the disease.

Cardiac MRI and fibrosis: what they add to the assessment

Cardiac magnetic resonance imaging makes it possible to measure hypertrophy accurately, identify apical aneurysms that may be poorly seen on echocardiography and quantify late gadolinium enhancement, a marker of myocardial fibrosis.
A large amount of fibrosis is associated with greater arrhythmic risk and can help decision-making when traditional markers do not provide a clear answer. However, it is not appropriate to turn a single fibrosis percentage into an automatic yes-or-no decision about a defibrillator.

Guidelines use cardiac MRI as part of an overall assessment. The distribution of scars and the clinical context are also important. Therefore, a report stating "LGE present" does not by itself mean that the patient is at high risk: the amount and its integration with other factors are crucial.

How risk is calculated and why the systems are not identical

European guidelines use validated risk models that combine several variables and estimate the probability of a sudden-death event over a given time interval. In adults, the HCM Risk-SCD model considers, among other factors, age, maximum wall thickness, left atrial diameter, outflow tract gradient, nonsustained ventricular tachycardia, syncope and family history.
U.S. guidelines instead use an approach based more on the presence of major clinical risk markers, supplemented by cardiac MRI and other modifying factors.

The two approaches should not be interpreted as simple calculators to use independently. The numerical value supports clinical decision-making but does not replace specialist assessment, especially in very young patients, rare forms, carriers of particular genetic variants or when findings are borderline.
Risk stratification must also be repeated over time, generally with periodic reassessments, because an initially favorable profile may change with the appearance of arrhythmias, new symptoms or structural changes.

When an implantable defibrillator is considered

The implantable cardioverter-defibrillator, or ICD, automatically recognizes and treats certain potentially fatal ventricular arrhythmias. It is the main strategy for preventing sudden death in selected patients with hypertrophic cardiomyopathy who are considered at high risk.
After cardiac arrest or clinically significant sustained ventricular tachycardia, the indication is very strong. In primary prevention, however, the expected benefit is weighed against possible device-related problems, including infections, lead complications, inappropriate therapies and the need for replacements over a lifetime.

For this reason, an ICD is not automatically implanted in everyone with hypertrophic cardiomyopathy. The decision should be shared after clarifying estimated risk, age, lifestyle, the type of device available and the long-term practical consequences.
Physical activity should also be discussed individually. Modern recommendations do not impose a blanket ban on exercise for all people with hypertrophic cardiomyopathy; the specific assessment is discussed in the guide on cardiomyopathy and exercise.

Frequently asked questions about sudden-death risk

Is everyone with hypertrophic cardiomyopathy at risk of sudden death?
No. Risk is highly variable and must be estimated individually. The diagnosis alone does not equal a high-risk profile.

Which factors matter most?
Previous cardiac arrest or sustained ventricular arrhythmia, suspicious syncope, family history, very marked hypertrophy, nonsustained ventricular tachycardia, apical aneurysm, systolic dysfunction and extensive fibrosis are among the main factors.

What is Holter monitoring used for?
To identify intermittent arrhythmias, particularly nonsustained ventricular tachycardia that contributes to risk stratification.

Is a defibrillator necessary for everyone?
No. It is used when the expected risk justifies the benefits and possible device-related problems.

Does risk need to be reassessed?
Yes. Assessment is dynamic and should be repeated periodically because the clinical picture can change.