Hypertrophic cardiomyopathy is a genetic disorder characterized by increased cardiac muscle mass. This disorder has broad phenotypic expression, including, among others, asymmetric septal hypertrophy, midcavity hypertrophy, and apical hypertrophy. In recent years, it has been recognized that hypertrophic cardiomyopathy is not characterized solely by ventricular hypertrophy but that a number of abnormalities of the mitral apparatus (papillary muscles, leaflets, chords, and annulus) may also occur. These figure prominently in the echocardiographic evaluation and surgical planning of patients with hypertrophic cardiomyopathy and serve as the focus of this review.
Hypertrophic cardiomyopathy (HCM) is a genetic disorder characterized by increased myocardial mass. Its estimated prevalence among adults is approximately 1 in 500. To date, >1,400 causal mutations have been identified involving several genes that encode sarcomeric proteins. HCM has broad phenotypic expression, including, among others, asymmetric septal hypertrophy, midventricular hypertrophy, and apical hypertrophy. In addition, a number of abnormalities of the mitral apparatus (i.e., papillary muscles [PMs], leaflets, chords, and annulus) have also been identified and are listed in Table 1 . These abnormalities figure prominently in the pathophysiology, echocardiographic evaluation, and surgical planning of HCM and serve as the focus of this review article.
| PMs |
| PM hypertrophy |
| Anterior PM fusion |
| Medial PM displacement |
| Accessory PMs |
| Apical PM displacement |
| Doubly bifurcated PMs |
| PM fibrosis (remodeling) |
| Mitral valve leaflets |
| Increased mitral leaflet length and area |
| AML percussion injury |
| Mitral annulus |
| Mitral annular calcification |
| Reduced systolic annular excursion |
| Chordae tendineae |
| Chordal rupture |
| Excess chordal laxity |
Normal Anatomy and Mechanics of the Mitral Apparatus
The mitral valve leaflets share a convexoconcave line of closure that spans between their commissures. The anterior mitral leaflet (AML) is significantly longer than the posterior mitral leaflet (PML), but it has a smaller perimeter that encircles just one third of the annulus. The maximal length of the AML, from the base to the free margin of A2, does not normally exceed 3.0 cm, and that of the PML, similarly measured at P2, does not normally exceed 1.5 cm.
The normal left ventricle usually contains two PMs that attach to the middle third of its posterior wall beneath the mitral valve commissures, with tips overhanging more basal left ventricular (LV) wall segments. It is interesting to note that the PMs arise from the trabeculae carnae rather than from the subjacent compacted myocardium, as is often depicted in illustrations of the heart. The anterolateral PM usually has a single head and is generally best imaged in the apical four-chamber view, and the posteromedial PM often has two heads and is most readily appreciated in the apical two-chamber view. Last, accessory PMs can be found in about 15% of otherwise normal hearts.
End-diastolic PM width, measured in the short-axis view using transthoracic echocardiographic imaging, is normally ≤1.1 cm. The fibers contained within the PMs are oriented parallel to its long axis. Their contraction, therefore, renders the PMs shorter and wider, such that their cross-sectional area increases by about 25% during systole. Normal end-diastolic PM length is approximately 3.0 cm, with a fractional shortening of about 20%. The combined mass of the PMs is normally ≤7 g/m 2 when measured using cardiac magnetic resonance (CMR) imaging. With echocardiographic strain imaging, normal PMs demonstrate synchronous development of negative longitudinal strain during systole. PM synchrony facilitates coordinated apposition of the mitral valve leaflets in anticipation of the rise in LV pressure, which then presses them together, forming a competent overlapping zone of coaptation that measures about 1.0 cm in height. This affords some measure of coaptation reserve should the leaflets become effaced by systolic anterior motion (SAM) of the AML, PM-chordal tethering, or annular enlargement.
The PMs serve a number of functions. During systole, they facilitate ejection by bulging into the submitral space such that the LV ejection flowstream is repositioned anteriorly into the outflow tract (OT) ( Figure 1 ). PM contraction also makes a contribution, albeit minor, to LV stroke volume. Finally, systolic contraction of the PMs takes up any chordal slack that might otherwise permit the mitral leaflets to prolapse into the left atrium during the apical descent of the annulus.
LV OT Obstruction
LV OT (LVOT) obstruction caused by SAM of the mitral valve occurs in about 70% of patients with HCM coming to clinical attention. Early studies suggested that OT obstruction is related to the Venturi effect. Accordingly, a narrow OT, the consequence of septal hypertrophy, was believed to sufficiently accelerate blood flow such that the attendant drop in pressure could produce enough lift to draw the mitral valve anteriorly toward the septum. However, it has been demonstrated that the onset of SAM actually precedes the systolic acceleration of blood flow in the OT. Moreover, the mean velocity in the LVOT at the start of SAM is approximately 90 cm/sec, which is insufficient to create lift. In light of this, as well as other observations, it is now generally accepted that SAM and LVOT obstruction are unrelated to the Venturi effect.
More recently, attention has shifted to the possibility that SAM and OT obstruction are caused by leaflet drag. Accordingly, it has been suggested that posterior displacement of the LV ejection flowstream, the result of the mass effect produced by septal hypertrophy, so positions its trajectory such that its angle of attack aligns with the free margins of the mitral valve leaflets ( Figure 2 ), which can then be swept (dragged) anteriorly toward the septum, resulting in OT obstruction.
