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The natural history of patients with HCM is
characterized by slow progression of symptoms (ie angina and/or dyspnoea) and left
ventricular hypertrophy. Severe functional limitation is unusual and encountered in only
about 20% of patients. Approximately 50% of deaths are sudden1.
Although complete heart block may complicate myectomy, the development of symptomatic
conduction disease in an unoperated patient is rare. Preexcitation syndromes may be
present in a small proportion of patients (2%) but this appears to be seldom responsible
for sudden death in HCM. A short PR interval with a broad QRS complex might mimic the
presence of a WPW syndrome in some patients with HCM. However, this finding may be
explained by an abnormal septal activation of the hypertrophied muscle. Recently, we have
studied a cohort of 65 patients (unpublished observations) with HCM and either documented
or suspected ventricular tachyarrhythmias undergoing invasive electrophysiologic study.
Surprisingly, in about 20% of patients enhanced AV nodal conduction was observed, allowing
1:1 conduction with ventricular rates up to 280 bpm in the absence of an accessory
pathway. This finding may be of clinical importance as a possible trigger mechanism of
sudden arrhythmic death in HCM as these high ventricular rates during paroxysmal atrial
fibrillation may be detrimental in patients with HCM and impaired diastolic filling.
Outflow tract obstruction and coronary spasm have been suggested as a causal factor.
However, these mechanisms have not yet been shown to be of importance. Further hypotheses,
currently also not proven, have implicated acute changes in diastolic filling, acute
myocardial ischemia or impaired autonomic regulation as causes of sudden death. Currently,
not much information is available concerning the mechanism of sudden death in HCM making
risk stratification difficult.
Presently, no convincing data are available for proper risk stratification of patients
with HCM2,3. This inability to correctly identify
patients at risk of sudden cardiac death in HCM reflects our poor understanding of the
mechanisms of sudden death in this entity. Although some initiating mechanisms have been
identified in single cases such as atrial fi-brillation with rapid ventricular conduction,
altered baroreflex control of peripheral blood flow and others, the extent of myocardial
disarray and a potential for ischemic events in patients with HCM have to be better
characterized before appropriate pharmacological or surgical interventions are undertaken.
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