The Cardiomyopathies
The evaluation of cardiomyopathy is complicated by the fact that few specific diagnostic criteria exist, and identification is often a process of exclusion. Further, many potential etiologies may be responsible for the myopathic process, and it may be possible to identify a specific etiology in only the minority of patients. Accordingly, a diagnostic strategy has evolved that initially seeks to place patients into one of three pathophysiologic categories: dilated, hypertrophic, or restrictive; then, the specific etiologies recognized as producing the individual pathophysiologic state are pursued. Thus, dilated cardiomyopathies are associated with myocyte loss and necrosis, a marked increase in LV volume, thinning of the myocardium, and profound systolic dysfunction.249 Hypertrophic cardiomyopathy (HCM) (Chap. 77) is recognized by increased myocardial thickness, particularly involving the interventricular septum, with preserved systolic function. Restrictive cardiomyopathies may be due to infiltration of the myocardium by abnormal substances or fibrotic tissue; these cause symmetrical degrees of wall thickening with modest or no diminution of systolic function and little change in cavity size.250 Echocardiography customarily serves as the cornerstone of such evaluations and provides data on cavity size, wall thickness, and systolic function. Thus, on echocardiogram, patients with dilated cardiomyopathy exhibit a marked increase in left LV and volume, little change in wall thickness, and severe contractile dysfunction.249 Patients with HCM exhibit a dramatic increase in LV wall thickness, with the septum characteristically disproportionate to the posterior wall, and often subaortic stenosis induced by systolic anterior motion of the anterior MV leaflet. Patients with restrictive cardiomyopathy are identified by a symmetric increase in wall thickness accompanied by modest changes in contractile function and LV cavity size.250 Hypertrophic Cardiomyopathy HCM is a primary abnormality of the myocardium that exhibits myocyte disarray and unprovoked hypertrophy, often affecting the septum disproportionately (Chap. 77). The disorder, which is often transmitted in an autosomal dominant pattern, has been linked to a number of abnormalities in genes that code for myocardial proteins.251 A number of classic echocardiographic findings occur in HCM (Fig. 15–106). The fundamental abnormality on echocardiogram in HCM is LVH, which is often severe. Although the hypertrophy may be confined to the septum, it may be concentric or involve any other portion of the LV.252 The customary classic finding is asymmetric septal hypertrophy (ASH), defined as a disproportionate thickness of the interventricular septum compared to the posterobasal wall with a ratio of greater than 1.3 to 1.253 In some cases the entire septum is hypertrophied, while in others the thickening may be localized to the proximal, mid-, or distal (apical) septum. Asymmetric hypertrophy of the proximal interventricular septum may lead to dynamic LV outflow tract obstruction—hypertrophic obstructive cardiomyopathy (HOCM) or idiopathic hypertrophic subaortic stenosis (IHSS). Although ASH is almost always present in cases of dynamic LV outflow tract obstruction, it is not a specific marker for HCM and may occur in some patients with RV hypertrophy (RVH), inferior MI, and a minority with hypertensive LVH.254 In general, the more extensive the hypertrophic process, the more severe the symptoms. Extent of hypertrophy, however, does not appear to correlate well with risk of sudden death, as patients with minimal hypertrophy may still be at significant risk.255
The second characteristic finding of HCM is systolic anterior motion of the MV, or SAM, which usually involves the anterior MV leaflet. Posterior-leaflet SAM also has been reported in HCM, as have a variety of MV deformities. Encroachment of the pathologically thickened septum upon the LV outflow tract creates a pressure drop by a Venturi effect, which draws the mitral leaflets toward the septum, creating dynamic (subaortic) LV outflow obstruction (Fig. 15–106). Recent work has also demonstrated the important effects of papillary muscle position and chordal tension on systolic mitral morphology and SAM.256 Because of distorted mitral coaptation during systole, SAM generally causes MR of variable severity. The severity and duration of SAM directly influence the degree of both outflow tract obstruction and MR. Like asymmetrical septal hypertrophy, SAM (especially systolic motion of the chordae) is not pathognomonic for HCM, having been reported in other conditions such as hypovolemia, anemia, and states where LV outflow tract narrowing and hyperdynamic contraction are present. The third manifestation of classic HCM is midsystolic closure of the aortic valve (Fig. 15–106E).257 This finding is best seen on M-mode recordings, occurs only in the presence of outflow tract obstruction, and is probably a manifestation of the sudden pressure drop during mid- and late systole caused by SAM. As with ASH and SAM, midsystolic aortic closure is not specific for HCM and can occur in MR, aortic root dilatation, ventricular septal defect, and discrete subaortic stenosis. When HCM is present, however, midsystolic aortic valve closure suggests significant outflow tract obstruction. The fourth important abnormality of HCM is observed on Doppler examination of the LV outflow tract (LVOT). Normally, Doppler interrogation of this area produces a spectral tracing that peaks early in systole and has a maximum velocity of less than 1.7 m/s. In many patients, HCM creates a high-pressure gradient coincident with SAM, which is detected by Doppler as a high-velocity systolic jet in the LVOT. As opposed to valvular aortic stenosis, however, the maximal velocity in obstructive HCM peaks late in systole, creating a characteristic "saber-tooth" pattern (Fig. 15–107). Although the subaortic gradient can be estimated using the modified Bernoulli equation, the assumptions used in this equation may not apply to HCM, as intraventricular gradient calculations can be spuriously high because of the phenomenon of pressure recovery.258 Similar Doppler patterns also may be seen occasionally within the LV in patients with HCM if systolic obliteration of the hypertrophied LV causes localized areas of high flow velocity in the more distal portions of the ventricular cavity.
Diastolic dysfunction has been long recognized in HCM. Doppler interrogation of LV inflow often reveals a relaxation abnormality, with a reduced early diastolic (E) velocity, a prolonged deceleration slope of the E wave, and an increased velocity of the atrial systolic (A) component.259 Color Doppler imaging can be used to demonstrate intraventricular flow characteristics. Dilated Cardiomyopathy In cases of dilated cardiomyopathy (DCM), the heart is typically greatly enlarged and systolic function is markedly depressed (Chap. 66).260 Four-chamber dilatation is a common but not uniform finding, as some patients may have relatively preserved RV size (this may confer an improved prognosis).261 Marked LV enlargement and generalized dysfunction can also be caused by severe ischemic heart disease, chronic alcohol abuse, various infectious myocarditides, anthracyclines and other cardiotoxic agents, nutritional deficiencies, and hereditary myopathies.262 Severe ischemic disease is often segmental and has been reported to spare the posterior wall frequently, while the LV dysfunction of DCM is usually global. The typical constellation of echocardiographic findings in DCM include an increased LV end-diastolic diameter and volume with decreased fractional shortening, thinning of the LV walls (Fig. 15–108), increased E point-septal separation, LA enlargement, and limited mitral and aortic valve opening (due to low stroke volume). Intracardiac thrombi are frequently observed and are most often found in the LV apex. M-mode imaging of the mitral leaflets may demonstrate a "B bump," or notch just before systolic valve closure, indicating elevated LV diastolic pressure (Fig. 15–8). The cardiac valves are usually normal, but mitral annular dilatation and secondary MR are common.
Doppler echocardiography often reveals an abnormally low-velocity time integral in the LV outflow or inflow tracts. Diastolic MR due to elevated LV diastolic pressure also may be present. Diastolic dysfunction is common, and pulsed-wave Doppler interrogation of mitral inflow may show an abnormal relaxation, restrictive, or pseudonormal pattern depending on LV diastolic pressures and loading conditions.263 A restrictive pattern of mitral inflow Doppler confers a poor prognosis in patients with DCM. Restrictive Cardiomyopathy Restrictive cardiomyopathy may be idiopathic or secondary to infiltrative diseases such as amyloidosis, hemochromatosis, hypereosinophilic syndrome and Loeffler endocarditis, sarcoidosis, radiation toxicity, glycogen storage diseases, and Gaucher disease (Chap. 83). Typical 2D echocardiographic features of these diseases include (1) a diffuse increase of ventricular thickness in the absence of marked ventricular chamber dilation and (2) marked biatrial enlargement250 (Fig. 15–109). Systolic function is often modestly decreased. As with the other cardiomyopathies, these echocardiographic findings are nonspecific. Doppler examination may show a mitral inflow relaxation abnormality early in the course of restrictive cardiomyopathy, but restrictive pattern (E much greater than A, with shortened E deceleration time) is a more classic finding, which often evolves with time and indicates both a high LA pressure and poor prognosis.265,266
Amyloidosis is generally the most commonly encountered restrictive cardiac disease. In addition to biventricular hypertrophy, amyloidosis is also associated with diffuse thickening of the interatrial septum and cardiac valves. In advanced disease, depressed systolic function is also common. An abnormal "speckled" pattern or "ground glass" appearance of the myocardium has been described on 2D echocardiography, but this sign is absent in many cases and therefore has minimal clinical usefulness. The finding of a restrictive mitral inflow pattern (and an abnormally high diastolic component of pulmonary vein inflow) on Doppler echocardiography has been identified as a marker of advanced disease and poor prognosis.266,267 In addition to increased myocardial thickness, endocardial thickening and fibrosis and restricted atrioventricular leaflet motion are common features of Loeffler endocarditis and endomyocardial fibroelastosis.264 Intraventricular thrombi are also common in these processes. |
