Sudden death is the complication that shapes how hypertrophic cardiomyopathy (HCM) is followed, and it can be the first manifestation of the disease. Risk is not read from the thickness of the wall alone, and the central management decision is whether a patient needs an implantable cardioverter defibrillator (ICD), a device that detects a dangerous ventricular rhythm and shocks the heart out of it.
The arrhythmic substrate
On tissue imaging, fibrosis appears at the right ventricular insertion points and as patchy areas inside thickened segments, as explained in the cardiac MRI note. Fibrosis is the key imaging finding for risk stratification: more fibrosis means more ventricular arrhythmia and sudden death, because scar borders sustain re-entrant tachycardia — and patients without obstruction remain at risk when fibrosis is extensive.
Wall thickness also tracks risk, with 30 mm or more a recognised risk factor carrying roughly 18 sudden-death events per 1000 patient-years (about 1.8% per year) against near-zero in healthy people, graded upward from 15 mm. But thickness and fibrosis are not tightly coupled: modest hypertrophy with fibrosis can still be arrhythmic.
Clinical risk factors
Established risk factors are:
- prior arrest or sustained ventricular tachycardia, which mandates secondary-prevention implantation
- family history of sudden death
- maximal thickness of 30 mm or more
- unexplained syncope, especially on exertion
- non-sustained ventricular tachycardia on Holter monitoring — even short runs of a handful of beats, with risk rising with burden
- an abnormal blood pressure response to exercise
Notably, a patient with only a 15 mm septum but daily Holter runs can still qualify: substrate outweighs centimetres.
Estimating risk
The HCM Risk-SCD calculator formalises 5-year risk from age, maximal thickness, left atrial size, resting outflow gradient, family history, non-sustained tachycardia, and unexplained syncope. A risk of 6% or more (some sources 5%) at 5 years favours implantation; 4–6% calls for shared decision-making; below 4% generally argues against.
Two caveats matter. The calculator gives population-derived estimates, not individual prediction, so low never means zero. It also omits fibrosis despite extensive enhancement being among the strongest individual predictors, so many clinicians let fibrosis tip borderline cases.
For patients who otherwise fall in the low-risk group, European guidance gives extensive late enhancement — 15% or more of left ventricular mass — and a left ventricular ejection fraction below 50% a role in that shared decision. Marked left ventricular mass increase by CMR also worsens prognosis, though normal total mass never excludes segmental disease.
Exercise testing adds blood pressure behaviour and the VE/VCO2 slope — the rise in ventilation for each unit of carbon dioxide produced — whose elevation independently predicts sudden death even in modest, asymptomatic hypertrophy.
Exercise and sport
Intensive competitive sport is generally discouraged in definite HCM, but current guidance supports individualised participation decisions after expert evaluation with shared decision-making. A defibrillator prevents death but not incapacitation during sport, which is part of why intensive competition needs careful counselling. Low-intensity recreation such as golf, bowling, or hiking is generally safe and worth encouraging. For the elite athlete earning a living through sport, the doctor’s role is clear counselling, clearly documented, so that any decision to compete is shared and informed.