Thick where it should not be
Hypertrophic cardiomyopathy (HCM) is an abnormal thickening of the heart muscle that is not explained by the heart having to work harder. That last clause is the whole point. A weightlifter's heart or a hypertensive heart can develop left ventricular hypertrophy for a good reason — it is pushing against a load. In HCM the muscle thickens because of a genetic fault in the contractile proteins, with no load to justify it.
Often the thickening is lopsided, hitting the septum hardest — this is asymmetric septal hypertrophy. The muscle is also disorganised at the microscopic level ("myocyte disarray") and threaded with fibrosis, which sets the stage for both poor filling and electrical instability.
Obstruction and stiffness
Two problems flow from the thick wall. First, the cramped, stiff chamber fills poorly — a problem of diastolic function, the heart's ability to relax. The squeeze may look strong, but a heart that cannot fill cannot eject much either. Second, in many patients the bulging septum narrows the path blood must take to leave the heart, an effect called left ventricular outflow tract obstruction. The harder the heart squeezes, the more it obstructs itself — a cruel paradox.
Symptoms include exertional breathlessness, chest tightness even with clean arteries, palpitations, and fainting — the last is an ominous sign because it may mean a dangerous rhythm or sudden, severe obstruction. Many people with HCM, though, have no symptoms at all and are found only on a family screen or a routine ECG.
The sudden-death question
HCM is the most common cause of sudden cardiac death in young athletes. The disorganised, scarred muscle can short-circuit into ventricular tachycardia or ventricular fibrillation without warning. Importantly, the risk does not track neatly with symptoms — some of the highest-risk people feel completely well. So a major job of the cardiologist is to estimate that risk for each patient and decide who needs an implantable defibrillator.
- Risk markers a cardiologist weighs: extreme wall thickness, unexplained fainting, a family history of sudden death, runs of VT on monitoring, and scar on cardiac MRI.
- Treat symptoms by slowing and calming the heart with a beta-blocker or a non-dihydropyridine calcium channel blocker; never use drugs that empty the heart and worsen obstruction.
- For severe obstruction unresponsive to drugs, shave or shrink the thick septum — surgical myectomy or alcohol septal ablation through a catheter.