SAM refers to the abnormal systolic anterior motion of the anterior (and occasionally posterior) mitral valve leaflet during mid-systole. The role of SAM in producing the outflow gradient is controversial, although the degree of SAM and size of the outflow gradient are related. Three explanations have been offered for SAM:
A Venturi effect that draws the mitral valve toward the septum because of the early high-velocity jet of ejected blood and the decreased pressure at the outflow tract;
Pulling of the mitral valve against the septum by the contraction of the papillary muscles, which are abnormally located and orientated;
Pushing of the mitral valve against the septum, perhaps by the posterior wall because of its abnormal position in the LV outflow tract (see figures on next page).
Which patients with HCM are at greatest risk for sudden death?
Sudden death in patients with HCM is presumed (but not proven) to be due to a ventricular arrhythmia. It may occur in patients who are asymptomatic or whose clinical course has been otherwise stable. Markers of increased risk for sudden death include:
‚ Age 30 years at diagnosis
‚ Family history of HCM and sudden death
‚ History of syncope (in children)
‚ Nonsustained ventricular tachycardia on ambulatory ECG monitoring (in adults) Symptoms, degree of functional limitation, and severity of the outflow tract gradient do not correlate with risk of death. In young competitive athletes who die suddenly, unsuspected HCM is the most common diagnosis at autopsy.