Echocardiography
has been used extensively both as an aid to diagnosis and for research
into the pathophysiology of hypertrophic cardiomyopathy (HOCM) otherwise
known as idiopathic hypertrophic subaortic stenosis (IHSS). It is
characterized by an abnormal arrangement of the myocardial cells,
which instead of lying in parallel rows, form whorl-like patterns.
It most commonly affects the interventricular septum, but may also
involve the entire myocardium or occur in isolated areas undetectable
except by detailed histopathologic examination.
 |
| Fig. 20 |
In severe cases, the gross pathologic changes are striking (Fig.
20). The septum is massively hypertrophied, almost obliterating
the left ventricular cavity and sometimes invading the right ventricle
as well. The hypertrophied region often involves the left ventricular
outflow tract, and causes obstruction to blood being ejected into
the aorta.
 |
| Fig. 21 |
In a classic case, the septum is very thick, frequently over
2.0 cm, and usually moves poorly. The left ventricular cavity
is small. The posterior left ventricular wall has normal thickness,
and moves vigorously (Fig.
21). The left atrium is mildly enlarged as a result of elevated
left ventricular filling pressure. The aortic valve cusps show
an abnormal motion pattern, typically opening normally at the
onset of ejection then fluttering to a semi-closed position (Fig.
22).
 |
| Fig. 22 |
This pattern is similar to that of a fixed sub-aortic obstruction
(as described in an earlier unit of this series) but usually in
hypertrophic obstructive cardiomyopathy the cusps tend to remain
fully open for longer and the fluttering as they partially close
is coarser and more irregular. The motion of the mitral valve
is also abnormal (Fig.
23). Because the ventricle is small, the anterior leaflet
usually contacts the septum when it opens in early diastole.
 |
| Fig. 23 |
In addition, echoes that appear to arise from the mitral apparatus
are seen to move up towards the septum during systole. The origin
of these echoes, and the reason for the so-called systolic anterior
motion (SAM) are controversial. It was initially thought that
they arose from the mitral leaflets, but in many cases it appears
from two-dimensional studies that they represent mitral chordae
tendineae. One theory for the abnormal motion is that the high
ejection velocity outflow tract causes sufficient pressures drop
to suck upward the mitral valve through the Venturi effect.
 |
| Fig. 24 |
In support of this argument is the observation that certain patients
with hypertrophic obstructive cardiomyopathy do indeed have some
mitral regurgitation. Another possibility is that the septal hypertrophy
distorts the shape of the ventricle. (Fig.
24) shows a parasternal long axis from a patient with marked
asymmetric septal hypertrophy and demonstrates that the left ventricular
septal surface is concave in relationship to the cavity rather
than convex as is normal individuals.
 |
| Fig. 25 |
(Fig.
25) shows the phenomenon of SAM where the chordae and leaflet
tips approximate the septum in systole and result in left ventricular
outflow tract obstruction. The thickened myocardium is often highly
reflective of ultrasound in these patients and appears very bright
on the two-dimensional echocardiogram. None of the above findings
is totally specific to hypertrophic obstructive cardiomyopathy.
Thickening of the septum can be found in any condition that causes
right ventricular hypertrophy, and left ventricular hypertrophy
can be unevenly distributed in the myocardium to give disproportionate
thickening of the septum. Infiltrative diseases can cause localized
thickening of the septum. It is also possible for the pattern
of premature aortic valve closure to be indistinguishable from
that of a fixed sub-aortic obstruction or even mitral regurgitation.
The SAM pattern of mitral valve motion can be found in patients
with mitral valve prolapse, where two-dimensional echocardiographic
studies show the mitral chordae to have a whip-like action after
the mitral valve closes at the onset of systole. Severe left ventricular
hypertrophy can cause the mitral chordae to be displaced anteriorly
during systole, though in this case they tend to follow the contour
of the posterior endocardium.
In addition, there has been an excessive, and unwarranted, reliance
on the fact that the ratio of the thickness of the septum to the
posterior free left ventricular wall must exceed 1.3:1, as was published
in the early echocardiographic literature. More recent studies have
shown many different manifestations of this disorder, including
severe concentric ventricular hypertrophy. Two-dimensional echocardiography
has also identified another subset of patients where the hypertrophy
is limited to the apical portions of the septum and free left ventricular
walls. Careful examination of the apex is, therefore, requisite
in all patients suspected of having obstructive cardiomyopathy.
In these latter cases, SAM is frequently absent.
The diagnostic problems found in hypertrophic obstructive cardiomyopathy
are a good example of one of the pitfalls inherent in any technique,
namely of inferring that an echocardiographic abnormality reflects
a particular pathological process. If all the features described
above are present, it is most probable that the patient has hypertrophic
cardiomyopathy; if only one or two are present, other possible causes
should be investigated fully before this conclusion is drawn.
Even with these limitations, a careful echocardiographic examination
is now the diagnostic procedure of choice for identifying patients
with this entity. The constellation of findings of small left ventricular
cavity size, asymmetric (localized or occasionally symmetric) left
ventricular hypertrophy, highly reflective myocardium and SAM almost
always means that this disorder is present. When SAM is absent,
physiologic maneuvers or pharmacologic challenges may provoke this
phenomenon and increase the diagnostic certainty. These include
the administration of amyl nitrite or isoprenaline, and assuming
an upright posture after a period of squatting. All are designed
to reveal the presence of a latent dynamic outflow obstruction.
The extent of systolic mitral valve motion abnormality provides
some indication of the severity of the obstruction. Two-dimensional
studies often show that in mild cases the abnormal motion is confined
to the mitral chordae, but with severe obstruction the extent of
the abnormal motion increases and involves the mitral leaflets as
well. In an attempt to quantify this observation, an "obstruction
index", based upon the proportion of systole during which the mitral
apparatus appears to be in contact with the septum, has been developed.
However, although there is a moderately good correlation between
this and the outflow gradient, the relationship is not close enough
to be of much predictive value.
Echocardiography has virtually displaced all other diagnostic modalities
for identification of hypertrophic obstructive cardiomyopathy. One
of the additional contributions of echocardiography to our understanding
of hypertrophic cardiomyopathy has been its ability to readily evaluate
large numbers of patients and their relatives. Such studies show
that the disease is genetically transmitted, by demonstrating its
presence in asymptomatic relatives of patients known to have the
disorder (the mode being autosomal dominant, with variable penetration).
It has been suggested that the obstructive and non-obstructive forms
of hypertrophic cardiomyopathy represent different diseases, but
from an echocardiographic point of view the latter simply represents
a milder form. The key finding is usually that of a thickened, relatively
immobile septum with no other apparent cause.
2004