A

Cardiac Structure

LV volumes and the pattern of hypertrophy can be well defined by echocardiography ( Figure 1 , Video 1 [ view video clip online], Table 1 ). Ventricular volumes in HCM are usually normal or slightly reduced. Traditionally, the biplane Simpson’s method has been applied to the measurement of LV volumes and ejection fraction (EF). Recently, real-time three-dimensional (3D) echocardiography has been shown to provide more accurate means of quantification, though there is a paucity of data on its accuracy in HCM. All imaging windows should be used to accurately define the areas of increased wall thickness. Hypertrophied segments often have slightly increased brightness in comparison with segments having normal end-diastolic wall thickness.

( Left ) Parasternal short-axis view from a patient with severe asymmetric HCM involving the anterior septal and anterior lateral walls. ( Right ) Apical four-chamber view from a patient with apical HCM. The arrow points to the hypertrophy in the distal lateral wall.

LV hypertrophy, although usually asymmetric, can also be concentric. The distribution of hypertrophy can be in any pattern and at any location, including the right ventricle. Although septal predominance is more common, hypertrophy can be isolated to the LV free wall or apex ( Figure 1 ). The presence of hypertrophy localized to the anterolateral wall can be missed, and careful imaging and extra care during interpretation are needed. When the extent of hypertrophy is difficult to visualize, having a high index of suspicion and meticulous imaging of the LV apex and/or the use of LV cavity opacification by intravenous contrast aids in the accurate diagnosis ( Videos 2 and 3 [ view video clips online]). In particular, apical HCM and apical aneurysms can be missed without contrast. Transthoracic echocardiography (TTE) combined with the intravenous injection of an echocardiographic contrast agent should be performed in patients with HCM with suspected apical hypertrophy, to define the extent of hypertrophy and to diagnose apical aneurysms and clots. It is possible to express the severity of hypertrophy using semiquantitative scores, which are based on wall thickness measurements by two-dimensional (2D) imaging in parasternal short-axis views at end-diastole. In the presence of adequate-quality images and expertise, 3D echocardiography provides the most accurate echocardiographic approach for quantifying LV mass.

B

Assessment of LV Systolic Function

LV EF is usually normal or increased in patients with HCM and should be assessed in all imaging studies. Of note, patients with HCM with significant hypertrophy can have small LV end-diastolic volumes and therefore reduced stroke volumes despite having normal EFs. Overt LV systolic dysfunction, termed the “dilated or progressive phase of HCM,” “end-stage HCM,” or “burnt-out HCM,” is usually defined as an LV EF < 50% and occurs in a minority (2%–5%) of patients. Prognosis is markedly worse in the presence of LV systolic dysfunction. Likewise, the development of an apical aneurysm is an uncommon but important complication that can be readily recognized with contrast echocardiography.

In addition to 2D and 3D imaging, Doppler methods have been used to assess for the presence of subclinical LV systolic dysfunction. Doppler tissue imaging measures the velocity of myocardial motion in systole and in diastole. Reduced systolic (Sa) and reduced early diastolic (Ea or e′) velocities can occur before the onset of overt hypertrophy. Doppler tissue imaging can also be used to measure myocardial strain and strain rate, which unlike tissue Doppler velocities are not affected by translation and tethering. Strain rate imaging has been shown to be useful in differentiating nonobstructive HCM from hypertensive LV hypertrophy. However, tissue Doppler–derived strain imaging has technical limitations due to its angle dependence. Speckle-tracking echocardiography (STE) directly assesses myocardial motion from B-mode (2D) images and is independent of angulation between the ultrasound beam and the plane of motion. Several studies have shown reductions in strain ( Figures 2 and 3 ) in patients with HCM compared with controls. In terms of rotational motion, STE allows for quantification of the twisting (or wringing) motion of the heart. Observing LV torsion in normal subjects from an apical perspective, the base rotates clockwise while the apex rotates counterclockwise, creating a coordinated “wringing” motion of the left ventricle. Rotation velocities of twisting and untwisting are usually similar in patients with HCM as a group and in control subjects ( Figure 4 ), although individual variations exist. Although the extent of rotation is usually normal, there can be differences in the direction of rotation. For example, mid-LV rotation in patients with HCM occurs in a clockwise direction, opposite to the direction seen in normal subjects.

LV global longitudinal strain by STE in a control subject ( left ) and a patient with HCM and hyperdynamic left ventricle ( right ). LV global strain is markedly reduced at 7% in the patient with HCM. AVC , Aortic valve closure.

( Left ) Radial strain in the LV short-axis view from six myocardial segments by STE in a control subject. ( Right ) Strain from a patient with HCM and hyperdynamic left ventricle. Radial strain is markedly reduced in all six segments in the patient with HCM. AVC , Aortic valve closure.

Twist by STE in a control subject ( left ) and a patient with HCM ( right ). Both exhibit an initial clockwise rotation followed by a counterclockwise rotation of 17°.

Although STE is a promising method to evaluate myocardial function, there are significant differences between strain values across the 17 LV segments in normal individuals. Therefore, the variation of regional strain across the left ventricle necessitates the use of site-specific normal ranges, and the routine use of STE is not recommended at the present time.

C

Assessment of LV Diastolic Function

LV and left atrial (LA) filling abnormalities have been reported in patients with HCM irrespective of the presence and extent of LV hypertrophy. The assessment of LV diastolic function in HCM can be limited by the relatively weak correlations between the mitral inflow and pulmonary venous flow velocities and invasive parameters of LV diastolic function. However, the atrial reversal velocity and its duration ( Figure 5 ) recorded from the pulmonary veins have a significant correlation with LV end-diastolic pressure.

Assessment of LV diastolic function in a patient with HCM with elevated LV end-diastolic pressure but normal LA pressure. Mitral inflow shows a short mitral A duration at the level of the mitral annulus, whereas the Ar velocity in pulmonary venous flow is increased in amplitude and duration. Lateral annular e′ velocity is normal, and the ratio of peak E velocity (at the level of mitral tips) to e′ velocity is <8, consistent with normal LA pressure. ( Right ) Tissue Doppler (TD) velocities. A , Peak mitral late diastolic velocity; a′ , late diastolic TD velocity; Ar , atrial reversal signal in pulmonary veins; E , peak mitral early diastolic velocity; e′ , early diastolic TD velocity; D , diastolic velocity in pulmonary veins; S , systolic velocity in pulmonary veins.

Previous studies have noted reasonable correlations between E/e′ ratio and LV filling pressures. This was found across a wide range of annular velocities, including in patients in whom lateral annular e′ velocity was >8 cm/sec ( Figures 5 and 6 ). A recent study noted modest correlations in patients with HCM with severely impaired LV relaxation and markedly reduced annular velocities. The E/e′ ratio has also been correlated with exercise tolerance in adults and children with HCM. In addition, septal e′ velocity appears to be an independent predictor of death and ventricular dysrhythmia in children with HCM.

Assessment of LV diastolic function in a patient with HCM with elevated LA pressure. Mitral inflow shows a restrictive inflow pattern (E velocity, 140 cm/sec). The arrow points to an L velocity in middiastole, which is observed in the presence of impaired relaxation and increased filling pressures. Lateral annular and septal annular tissue Doppler (TD) velocities (both e′ and a′) are markedly reduced consistent with severely impaired LV relaxation. The markedly increased E/e′ ratio is consistent with increased LA pressure > 20 mm Hg. The reduced mitral A velocity with its short deceleration time and the severely reduced a′ velocity are consistent with increased LV end-diastolic pressure. A , Peak mitral late diastolic velocity; a′ , late diastolic TD velocity; E , peak mitral early diastolic velocity; e′ , early diastolic TD velocity.

A comprehensive approach is recommended when predicting LV filling pressures in patients with HCM, taking into consideration the above velocities and ratios, as well as pulmonary artery pressures and LA volume, particularly in the absence of significant mitral regurgitation and atrial fibrillation, as the latter two conditions lead to LA enlargement in the presence of a normal LA pressure.

LA size provides important prognostic information in HCM. LA enlargement in HCM is multifactorial in origin, with important contributions from the severity of mitral regurgitation, the presence of diastolic dysfunction, and possibly atrial myopathy. because LA volume has been shown to be the more accurate index of LA size, LA volume indexed to body surface area should be assessed in accordance with ASE guidelines.

There are three main mechanical functions of the left atrium: (1) reservoir function (during ventricular systole and isovolumic relaxation), (2) conduit function (during early diastole), and (3) contractile (booster pump) function (during atrial systole). The assessment of LA function via Doppler echocardiographic techniques has been performed by indirect methods using pulmonary venous inflow signals and LA volumes by 2D and 3D echocardiography during the different atrial phases. Other indirect measurements of LA function have included the calculation of LA ejection force and kinetic energy, which are increased in patients with obstructive HCM and are reduced (though not normalized) after relief of obstruction.

Strain imaging of the left atrium allows for more direct assessment of LA function. Longitudinal strain of the LA by tissue Doppler and 2D strain during all three atrial phases was assessed in HCM. LA strain values were reduced in all three atrial phases and were significantly lower in patients with HCM compared with those with secondary LV hypertrophy. In general, 2D atrial strain is more reproducible and less time-consuming than tissue Doppler strain, but it is not recommended at the present time for routine clinical application.

D

Dynamic Obstruction and Mitral Valve Abnormalities

Primary structural abnormalities of the mitral valve apparatus in HCM include hypertrophy of the papillary muscles, resulting in anterior displacement of the papillary muscles, and intrinsic increase in mitral leaflet area and elongation. In addition, abnormalities of the mitral valve apparatus predispose the leaflets to be swept into the LVOT by drag forces created by a hyperdynamic EF. This results in systolic anterior motion (SAM) of the mitral valve or chordate, which is the mitral valve abnormality that is characteristic of obstructive HCM. Of note, significant obstruction is caused by valvular rather than chordal SAM. SAM is defined as systolic motion of the mitral leaflets into the LVOT ( Figure 7 ) resulting in turbulent flow, appreciated as a mosaic pattern by color flow Doppler. SAM also results in distortion of mitral leaflet coaptation, resulting in mitral regurgitation ( Figure 7 ). The maximal instantaneous gradient, reflecting the severity of LVOT obstruction, is determined by measuring the peak LVOT velocity. This is measured by continuous-wave Doppler. Care should be taken to avoid contamination of the LVOT signal with the mitral regurgitation jet ( Figure 8 ).

( Left ) M-mode recording of SAM and mitral leaflet septal contact ( arrows ). ( Right ) SAM on 2D echocardiography ( arrow ). In the same panel, color Doppler shows the high velocities across the LVOT in mosaic color and the eccentric mitral regurgitation jet that is directed posterolaterally.

Continuous-wave (CW) Doppler recordings of peak velocity across the LVOT ( cross : 4.5 m/sec) ( left ) and peak velocity of mitral regurgitation signal ( arrow : 6.3 m/sec) ( right ). The concave-to-the-left contour of the Doppler CW jet causes a decrease in the LVOT orifice size as systole progresses and as the mitral valve is pushed further into the septum. Identification of this contour can be useful to differentiate high CW jets of dynamic LVOT obstruction from mitral regurgitation and from valvular aortic stenosis.

Distinguishing a dynamic LVOT gradient from fixed LVOT obstruction by a subvalvular membrane is important. In addition, concomitant aortic valve stenosis should be excluded by examination of the aortic valve anatomy, including transesophageal echocardiography (TEE) if necessary, and the use of pulsed-wave Doppler at the aortic annular level, paying particular attention to early systole, as the aortic valve may demonstrate premature leaflet closure or fluttering due to the LVOT obstruction. Examination of the LVOT for diseases causing fixed obstruction, such as a membrane, is another important reason to consider TEE. These patients should be identified, as they are surgical candidates. Helpful clues for the presence of fixed subvalvular stenosis on TTE include an early peaking LVOT signal by continuous-wave Doppler similar to that of aortic stenosis, as well as aortic regurgitation, which is uncommon in patients with HCM who have not had surgical myectomy.

Midcavitary obstruction can occur with and without LVOT obstruction in ventricles with hyperdynamic function and/or concentric hypertrophy. This is frequently observed in elderly patients with a sigmoid septum. The site of obstruction is determined by pulsed-wave and color Doppler showing high velocities at the site of obstruction (velocity aliasing by pulsed-wave Doppler). LVOT obstruction contributes to dynamic systolic dysfunction in obstructive HCM, as manifested by the midsystolic drop in LV ejection velocities at the entrance of the LVOT and the reduced longitudinal strain, both of which improve with treatment of obstruction.

A number of abnormalities contribute to SAM. These include the anterior displacement of the papillary muscles and the reduced posterior leaflet restraint. These mechanisms were highlighted in both in vitro and in vivo studies of mitral valve models that mimicked the anteriorly displaced papillary muscles in obstructive HCM. Anterior displacement of the papillary muscles shifts the mitral leaflets anteriorly toward the LVOT and leads to chordal and leaflet laxity. As drag forces generated by the left ventricle pull the anteriorly displaced and elongated leaflets into the outflow tract in early systole, the distal one half to one third of the leaflets form an angle anteriorly into the LVOT, creating a “funnel” composed of both leaflets ( Figure 7 ). The coaptation point between the anterior and posterior leaflets is typically eccentric because of the greater anterior leaflet motion relative to the posterior leaflet.

The drag forces that create SAM play an important role in the generation of an LVOT gradient. The extent of septal hypertrophy and resultant narrowing of the LVOT also contribute to the LVOT gradient. In addition to the role of drag forces on the mitral valve leaflets created by LV contraction, Venturi forces created as flow enters the narrowed LVOT may contribute to obstruction. But SAM often begins before the aortic valve opens, at a time when LVOT velocities are low. Moreover, the velocity of LVOT Doppler flow at SAM onset does not differ from velocities observed in the outflow tract of normal subjects. This indicates that though Venturi forces are present in the outflow tract, they are not a major contributor to SAM. Recognition by echocardiography of the importance of drag forces as the dominant cause of SAM led to a modification of myectomy, which is now extended past the tips of the mitral valve and in some cases to the base of the papillary muscles.

Anomalous insertion of the papillary muscles in which one or both heads of the papillary muscles insert directly (with absent chordae tendineae) into the ventricular aspect of the mitral leaflets can occur in up to 13% of patients with HCM and can contribute to LVOT obstruction ( Figure 9 , Video 4 [ view video clip online]). The recognition of these abnormalities can be facilitated using off-axis views and consideration of TEE if valvular pathology cannot be discerned. The echocardiographic report should contain a clear statement about the papillary muscle size (if hypertrophy is present) and if there is direct insertion into the mitral leaflets contributing to LVOT obstruction.

Anomalous insertion of the papillary muscle, which inserts directly into the anterior mitral leaflet ( arrow ).

E

Mitral Regurgitation in HCM

Because the anterior leaflet motion is greater than that of the posterior leaflet during SAM, an interleaflet gap occurs, resulting in a posteriorly directed jet of mitral regurgitation, which can be significant (moderate or greater depending on the extent of the gap). The gap is created between the leaflets because of the failure of the posterior leaflet to move toward the outflow tract as much as the anterior leaflet. This is because the anterior leaflet has the greater surface area and hence greater redundancy and mobility. The degree of mitral regurgitation relates to the extent of mismatch of anterior to posterior leaflet length and the decreased mobility of the posterior leaflet to move anteriorly. The mismatch can be quantified by measuring the coaptation length between the two leaflets, which is shorter with the above-described posterior leaflet abnormalities. Dynamic obstruction also affects the severity of mitral regurgitation, such that mitral regurgitation is dynamic in HCM and is affected by the same factors that influence the severity of obstruction.

Not all mitral regurgitation associated with HCM is related to SAM. Patients with HCM can have intrinsic valvular abnormalities, such as mitral valve prolapse, leaflet thickening secondary to injury from repetitive septal contact or turbulent regurgitation jet, chordal rupture, chordal elongation or thickening, and infectious etiologies. Importantly, the presence of a central or an anteriorly directed jet should prompt careful evaluation of the mitral valve apparatus by TEE to identify intrinsic valvular abnormalities.

There are specialized situations, such as in the operating room or intensive care unit, in which the pathophysiologic settings can mimic obstructive HCM. An example of this is the postoperative repair of a myxomatous mitral valve in a patient with basal septal hypertrophy or sigmoid septum, in which the left ventricle is underfilled coming off bypass. In this situation, a number of factors converge and produce SAM along with LVOT obstruction. These include elongated mitral leaflets, a narrow LVOT, a small LV cavity, and hyperdynamic EF. In general, these can be reversed with volume loading, afterload increase, and stopping inotropic agents. Similarly, SAM with dynamic obstruction can be seen in patients on inotropic drugs, who are volume depleted, and in the elderly with basal septal hypertrophy or as part of the clinical presentation of stress-induced cardiomyopathy.

F

Myocardial Ischemia, Fibrosis, and Metabolism

In general, there is a limited role for echocardiography in diagnosing myocardial ischemia in HCM. Large areas of regional fibrosis can lead to segmental dysfunction manifested by reduced strain. However, a reduction in strain also occurs in segments without replacement fibrosis and has a reduced specificity for this diagnosis.

Measurement of coronary flow reserve in the left anterior descending coronary artery is feasible with transthoracic imaging. Abnormal flow reserve can be due to macrovascular and microvascular coronary artery disease (CAD). The technique requires experience, and an abnormal flow reserve has low positive predictive value in identifying patients with epicardial CAD. It is not yet feasible to use echocardiography for studying myocardial metabolism.

G

Echocardiography for Guidance of Septal Reduction Procedures

i

Surgical Myectomy

Direct cardiac visualization during myectomy is hampered by both the transaortic approach and the empty heart, potentially leading to imprecision in the extent of the myectomy. These limitations may result in either an inadequate resection, resulting in persistent LVOT obstruction, or too large a resection, which may inflict ventricular septal defect, complete heart block, or both. Therefore, intraoperative TEE has become an essential accompaniment to surgical myectomy, as it contributes to surgical planning, aids in determining the adequacy of repair, and detects complications.

Both the safety and efficacy of septal myectomy are improved with intraoperative TEE, which provides a road map of septal anatomy and geometry to the surgeon. Important information obtained from TEE includes the maximum thickness of the septum ( Figure 10 ), the distance of maximum thickness from the aortic annulus, the location of the endocardial fibrous plaque (friction or impact lesion), and the apical extent of the septal bulge. Moreover, functional and intrinsic mitral valve abnormalities are well characterized by TEE. Importantly, TEE can identify mitral valve abnormalities and guide the necessary repairs or replacement. In particular, TEE can more clearly identify the direct insertion of papillary muscles into the middle or base of the anterior mitral leaflet. Surgical techniques have been developed to address this pathology and avoid postoperative residual obstruction, including the release and selective resection of anomalous papillary muscle connections. Also, selected patients coming to surgery have very long redundant mitral valve leaflets. In these selected patients, anterior mitral leaflet plication has been successfully used to limit SAM. Horizontal anterior leaflet plication has emerged as a safe and useful technique when used in selected patients who are identified preoperatively by echocardiography and in the operating room by direct inspection. It decreases leaflet length and slack and stiffens the leaflet against deformation. Immediately after cardiopulmonary bypass, TEE is repeated to assess evidence of residual obstruction, or more than mild mitral regurgitation, so that further resection or repair can be performed.

TEE of septal measurements before myectomy ( left ) (thickness, 2.9 cm) and after myectomy ( right ) (thickness, 1.5 cm). LA , Left atrium; RV , right ventricle.

Uncommon complications, including iatrogenic ventricular septal defects, may occur, and immediate recognition by TEE can lead to successful repair. Although the exact mechanism is unknown, aortic regurgitation (usually of mild severity) can occur, perhaps due to direct injury to the leaflets or destabilization of the annulus by beginning the myectomy too close to the right coronary cusp.

ii

Alcohol Septal Ablation

Alcohol septal ablation is an alternative to surgery when medical therapy has failed or is not tolerated. This technique involves the injection of alcohol into a proximal septal perforator branch of the left anterior descending coronary artery to produce a localized myocardial infarction of the thickened proximal ventricular septum involved in causing dynamic obstruction ( Figure 11 ). The use of myocardial contrast echocardiography (MCE) with the injection of echocardiographic contrast agent into the proposed target septal arteries to delineate the vascular distribution of the individual perforator branches is one of the important modifications to septal ablation and is key to the success of the procedure, as defined by at least a 50% reduction in LVOT gradient ( Figure 12 , Table 2 ).

Transesophageal echocardiographic images from a patient who underwent alcohol septal ablation. Before ablation, 2D image shows narrowed LVOT with SAM ( top left ). ( Top right ) Two-dimensional images after ablation. Color Doppler before ablation shows high-velocity signals in mosaic color with eccentric mitral regurgitation directed posterolaterally ( bottom left ). After ablation, velocities are much lower across the LVOT, and mitral regurgitation appears trivial ( bottom right ). The arrow points to the catheter across the LVOT, which is used to measure LV pressure during the procedure. LA , Left atrium; LV , left ventricle.

Myocardial contrast echocardiographic (MCE) images from two patients with HCM undergoing alcohol septal ablation. ( Left ) Opacification of the LV side of the basal septum ( arrow ), which is involved in the contact with the anterior mitral leaflet and the desired location to induce infarction. ( Right ) Opacification of the RV side of the septum ( arrow ), which is not the location that affects dynamic obstruction.

Table 2

Advantages of MCE during alcohol septal ablation

1. Shorter intervention time

2. Shorter fluoroscopy time

3. Fewer occluded vessels

4. Smaller amount of ethanol used

5. Smaller infarct size

6. Lower likelihood of heart block

7. Higher likelihood of success

Because there is considerable individual variation in the number, size, and vascular territory of the septal perforators, it is important to determine the vessel or vessels that should receive the alcohol injection. The initial method to identify the target septal perforator was to evaluate the gradient decrease during probatory balloon inflation. This has now been replaced at most centers by intraprocedural MCE under transthoracic or transesophageal echocardiographic guidance.

After the target septal perforator is identified and cannulated, a balloon catheter is advanced into the vessel and inflated to prevent backflow. Subsequently, 1 to 2 cm ³ of a diluted echocardiographic contrast agent (e.g., Definity, Lantheus Medical Imaging, North Billerica, MA; Optison, GE Healthcare, Milwaukee, WI; Levovist, Berlex Laboratories, Montville, NJ) is injected through the balloon catheter followed by a 1-mL to 2-mL saline flush during continuous imaging. The contrast agent should be diluted with normal saline to optimize myocardial opacification and minimize attenuation. Details of the dilution vary with the contrast agent used. Agitated radiographic contrast can be used instead of an ultrasound contrast agent. The optimal target territory of the basal septum should also include the color Doppler region of maximal flow acceleration in the area of mitral leaflet and septal contact. Typically, MCE produces a demarcated area with increased echo density in the basal septum and an acoustic shadowing effect. In addition, it is important to document the absence of perfusion of myocardial segments remote from the targeted areas for ablation, including the LV anterior wall, right ventricular (RV) free wall, and papillary muscles.

In patients treated before the introduction of intraprocedural MCE, the main reason for unsatisfactory gradient reduction was suboptimal scar location. Intraprocedural guidance using MCE can lead to changes in the perforator vessel selected for ethanol injection and even cancellation of the procedure, and some of these patients may be referred for surgery. This may be the case when the target septal perforator also supplies papillary muscles or in settings when it is not possible to cannulate the target septal vessel.

At most centers, TTE is used for intraprocedural guidance. Multiple views, including apical four-chamber and three-chamber views and parasternal short-axis and long-axis views, are recommended to delineate opacification of both target and nontarget regions. Limitations of TTE include the difficulty of continuous monitoring during the procedure and suboptimal images in the supine position on the catheterization table. Some groups prefer TEE because it generally provides higher quality images. TEE usually requires general anesthesia, which can alter loading conditions and therefore LVOT gradients. If TEE is used, the apical four-chamber view (deep gastric at 0°) and longitudinal view (midesophageal, aortic valve level, 120°–130°) should be used. These views may be supplemented by the transgastric short-axis view to assess for possible perfusion of the papillary muscles or the right ventricle. The deep transgastric view is useful for measuring the intracavitary gradient with TEE, though it is usually more challenging than with TTE. There are preliminary data on intracardiac imaging during septal ablation. Intracardiac imaging provides high-quality near-field imaging and can be performed by interventional cardiologists. Because of the complex nature of the LVOT anatomy, 3D echocardiography can provide additional information. However, the added benefit of 3D TEE during alcohol ablation has not yet been defined.

Intraprocedural echocardiography is also useful for evaluating the results of the procedure. The region of the basal septum, which is infarcted by the alcohol infusion, is typically intensely echo dense. This region of the septum should also have reduced thickening and excursion. There is usually a reduction or elimination of mitral regurgitation when it is due to SAM. Most important, there should be elimination or reduction of dynamic obstruction.

H

Screening and Preclinical Diagnosis

At the present time, echocardiography is the most practical technique for HCM screening. Although it is felt that the most active phase of hypertrophy development occurs during adolescence, it is appreciated that late-onset hypertrophy (into the fifth or sixth decade of life) can also occur. Therefore, periodic screening is recommended at intervals of every 12 months during adolescence and every 5 years in adults, as well as at the onset of symptoms suggestive of HCM. All myocardial segments, not only the septum, should be carefully examined for evidence of hypertrophy on these screening examinations. Cardiovascular magnetic resonance (CMR) should be considered in patients with technically challenging echocardiograms, and in patients in whom electrocardiographic results is or have become abnormal, with still normal results on echocardiography.

Studies in transgenic animal models have noted the presence of abnormal myocardial function before the development of hypertrophy. These observations have led to the investigation of Doppler tissue imaging in the preclinical diagnosis of HCM in individuals carrying sarcomeric protein mutations encoding HCM. Some studies have shown annular e′ velocity to be promising, whereas one study noted that a′ velocity is abnormally reduced in preclinical HCM. Limitations to this approach include the lower specificity in older individuals or those with coexisting disease. Furthermore, it is difficult to interpret Doppler data and provide counsel to subjects who carry the mutation but who still have normal velocity values. Given the variable penetrance, these subjects may never develop HCM including abnormal myocardial function. Alternatively, it is possible that the abnormality in cardiac function is present but at a mild degree that is not amenable to diagnosis by myocardial imaging. Therefore, abnormal Doppler velocities do not establish the diagnosis of HCM but can help identify gene carriers who may benefit from closer follow-up.

A

Cardiac Structure

Gated blood-pool radionuclide angiography can provide measurements of LV volumes and EF and RV volumes and EF. Thickened myocardium without a definable cause, usually in an asymmetric pattern with predominant septal involvement, can easily be identified by radionuclide angiography. Gated single photon-emission computed tomography (SPECT) can also provide similar data ( Table 3 ). However, echocardiography and CMR have higher spatial resolution and provide accurate measurements. Accordingly, the use of nuclear imaging for the sole purpose of assessment of cardiac structure is no longer recommended.

B

Radionuclide Angiography for LV Systolic Function

Gated blood-pool radionuclide angiography provides reliable and reproducible measurements of LV EF in patients with HCM. In most patients, radionuclide angiographic findings suggestive of HCM include normal or supranormal EF, disproportionate septal thickening, and systolic ventricular cavity obliteration. A small subset of patients develop LV systolic dysfunction late in the course of disease; in such patients, EF falls below normal and can be easily detected by radionuclide angiography. However, the routine application of radionuclide angiography for the sole purpose of EF assessment is often not needed given the availability of echocardiography and CMR.

C

Radionuclide Angiography for LV Diastolic Function

Quantitative parameters of LV filling are derived from the time-activity curve, which closely approximates the changes in LV volume during diastole. High–temporal resolution methods are preferred to avoid underestimation of LV filling. Peak filling rate is the most widely used radionuclide angiographic parameter of diastolic function and represents the maximum value of the first derivative of the time-activity curve. Improvement in LV filling and reduction in symptoms have been observed after therapy with calcium channel blockers, such as verapamil, though these drugs can lead to aggravation of diastolic dysfunction in some patients with increased LV early diastolic filling parameters due to increased LV filling pressures.

Echocardiography, which allows beat-to-beat measurement of diastolic filling patterns as well as the less load dependent indices of LV relaxation, is the technique of choice for assessing diastolic function in HCM.

D

Dynamic Obstruction and Mitral Valve Abnormalities

Nuclear techniques cannot show the presence of SAM or assess the severity and location of dynamic obstruction. However, the scan can show the presence of a hyperdynamic left ventricle with cavity obliteration.

E

Mitral Regurgitation in HCM

It is not possible to visualize the mechanisms behind mitral regurgitation using nuclear imaging. However, it is possible to quantify the severity of mitral regurgitation in patients with isolated lesions (i.e., no concomitant significant valvular regurgitation aside from mitral regurgitation) as the difference between LV and RV stroke volumes. Echocardiography is the recommended and preferred modality for that objective, given the limitations of the other techniques.

F

Myocardial Ischemia, Fibrosis, and Metabolism

i

SPECT

Ischemia in patients with HCM, in the absence of epicardial coronary artery stenosis, may be due to intramural small-vessel abnormalities, abnormal myocellular architecture, massive hypertrophy, and abnormalities of the intramural microcirculation leading to inadequate myocardial blood flow, particularly during increased myocardial oxygen demand with exertion. Myocardial oxygen demand is also increased by LV hypertrophy and outflow tract obstruction in many patients. Myocardial ischemia can be induced by exercise, vasodilators such as adenosine, and dobutamine. However, because of concerns of inducing and possibly aggravating the severity of dynamic obstruction with untoward hemodynamic effects, dobutamine is not preferred in conjunction with perfusion imaging in patients with HCM. The presence and severity of ischemia can be assessed by reversible abnormalities in regional thallium uptake ( Figure 13 ) and is a well-established pathophysiologic feature of HCM in adults. It has been associated with potentially lethal arrhythmias, adverse LV remodeling, and systolic dysfunction, even in the absence of epicardial disease. In addition to the above mechanisms, impaired LV relaxation and increased LV end-diastolic pressure can compress the coronary microcirculation and further restrict coronary artery blood flow. Recurrent myocardial ischemia can cause myocardial injury and scarring (characterized as fixed defects), which can potentially reduce the threshold for ventricular arrhythmias. In particular, fixed defects have been associated with syncope, larger LV cavity dimensions, and reduced exercise capacity.

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