How is hypertrophic cardiomyopathy diagnosed? What are the signs on ultrasound?

Author: Wang Fang, Chief Physician, Beijing Hospital

Reviewer: Zhu Dan, Chief Physician, Peking University Third Hospital

With people paying more attention to physical examinations and the widespread application of ultrasound technology, hypertrophic cardiomyopathy has become more and more common.

Under normal circumstances, the thickness of the left ventricle is generally 9-11 mm, and the thickness of the right ventricle is generally no more than 5 mm. When the thickness of the myocardium exceeds 15 mm, or even reaches 20 mm or 30 mm, this abnormal thickening is called pathological hypertrophy.

Ultrasound is a very good means of checking whether the heart structure is normal, and it can be said to be the gold standard for diagnosing hypertrophic cardiomyopathy. Generally speaking, in diagnosing primary hypertrophic cardiomyopathy, it is necessary to exclude myocardial hypertrophy caused by secondary factors such as hypertension and aortic valve stenosis, which have different manifestations under ultrasound.

Figure 1 Original copyright image, no permission to reprint

What is the difference between hypertrophic cardiomyopathy and myocardial hypertrophy caused by hypertension?

Long-term hypertension will increase the burden on the heart, leading to thickening of myocardial fibers and cardiac hypertrophy. This is an adaptive change of the heart to hypertension, but if it continues for a long time it will cause damage to heart function.

Under ultrasound, myocardial hypertrophy caused by hypertension is generally diffuse hypertrophy, while hypertrophic cardiomyopathy is asymmetric hypertrophy or localized hypertrophy. This is a very important distinguishing point between the two.

Second, hypertensive myocardial hypertrophy is a compensatory hypertrophy that occurs in order to enhance the contractile function of the heart. In this case, the contractile force of the myocardium is still strong, which is a normal compensatory hypertrophy, aimed at helping the heart to contract more effectively. In contrast, hypertrophic cardiomyopathy is a pathological hypertrophy of the entire myocardium caused by genetic abnormalities. Although the myocardium appears to be hypertrophic and muscular, its contractile force is actually weak, and its diastolic capacity is significantly reduced. Tissue Doppler ultrasound examination can be used to evaluate the diastolic function of the myocardium, providing a basis for differential diagnosis.

Myocardial hypertrophy caused by hypertension generally does not exceed 15 mm. For this type of mild myocardial hypertrophy, when ultrasound is difficult to judge, magnetic resonance imaging may be needed to help determine whether it is pathological hypertrophy or compensatory hypertrophy. Magnetic resonance imaging can help further determine, but it is not a necessary means.

What is the difference between hypertrophic cardiomyopathy and myocardial hypertrophy caused by aortic stenosis?

Aortic valve stenosis. Since the opening of the aorta is very small, the myocardium has to contract very hard to pump the blood out, so the myocardium will become compensatory and hypertrophic.

Under ultrasound, myocardial hypertrophy caused by aortic stenosis is uniform, which is a compensatory hypertrophy. The entire left ventricular myocardium is enlarged to counter the outlet obstruction caused by valvular stenosis. Hypertrophic cardiomyopathy is uneven hypertrophy.

Aortic valve stenosis can be caused by congenital bicuspid valve malformation, degeneration, rheumatism, etc. Ultrasound can clearly determine whether the aortic valve is stenotic. In patients with hypertrophic cardiomyopathy, there is generally no serious stenosis of the aortic valve.

It is relatively easy to distinguish from these two aspects through ultrasound.

Primary hypertrophic cardiomyopathy is generally divided into three categories: obstructive, non-obstructive, and latent obstructive. So, how does cardiac ultrasound distinguish between obstructive and non-obstructive hypertrophic cardiomyopathy?

Obstructive hypertrophic cardiomyopathy first manifests itself on ultrasound as significant hypertrophy of the basal segment of the ventricular septum.

Second, when the heart contracts, the mitral valve should be closed under normal circumstances. If the mitral valve does not close at this time, the anterior leaflet of the mitral valve moves forward to block the left ventricular outflow tract, aggravating the stenosis of the left ventricular outflow tract. This is called the SAM sign, or systolic anterior movement phenomenon, which is an obvious sign of obstructive hypertrophic cardiomyopathy. Because of left ventricular myocardial hypertrophy, especially the hypertrophy of the base of the ventricular septum, the left ventricular outflow tract is narrowed. Due to the accelerated blood flow, the outflow tract is relatively negative, which produces a siphon effect on the anterior leaflet of the mitral valve, resulting in forward movement during systole to block the left ventricular outflow tract and aggravate the stenosis of the left ventricular outflow tract.

Third, spectrum Doppler measurement shows that the non-obstructive left ventricular outflow tract has a negative systolic blood flow spectrum, which is wedge-shaped; while the obstructive left ventricular outflow tract has a jet signal during systole, with a higher flow rate. Its characteristic change is a negative filling jet during systole, faster blood flow, a pressure difference of ≥30mmHg, and a peak shifted backward, presenting an inverted "dagger-like" single peak shape. If the pressure difference exceeds 50mmHg, various clinical symptoms will appear, and there is even a risk of sudden death, which requires active intervention.

Therefore, under ultrasound, signs such as asymmetric myocardial hypertrophy, SAM sign, high-speed blood flow in the left ventricular outflow tract, and pressure difference ≥ 30 mmHg are seen, indicating obstructive hypertrophic cardiomyopathy.

Figure 2 Original copyright image, no permission to reprint

If no signs of obstruction are seen in a resting state, it may also be latent obstructive hypertrophic cardiomyopathy. In a resting state, it does not cause outflow obstruction, but once there is activity, more blood is needed, and when the pressure increases, potential obstruction will occur, and symptoms such as chest tightness, shortness of breath, and blacking out of the eyes will appear. At this time, a provocative test is needed to further clarify. Dobutamine can be used under ultrasound monitoring, which is equivalent to using medicine to make the heart move, simulating the condition of the heart in a state of exercise, so as to discover possible latent obstruction. Of course, no drugs are used. Let the patient do breath-holding exercises or squatting exercises, and then do an ultrasound examination immediately to see if there is any obstruction. These are all commonly used diagnostic methods. If a provocative test is performed and there is still no obstruction, then it can be determined to be non-obstructive hypertrophic cardiomyopathy.