Hypertrophy variants and echocardiography
The myocardium in hypertrophic cardiomyopathy thickens unevenly. Several hypertrophy variants are distinguished: asymmetric (septal), symmetric, apical, midventricular (midventricular), mass-like (mass-like), and noncontiguous (noncontiguous).
On baseline B-mode echocardiography in the long-axis view, classic basal interventricular septum hypertrophy is frequently visualized. Doppler ultrasound can reveal diastolic dysfunction (the E wave decreases and becomes comparable in amplitude to the A wave), as well as measure the systolic gradient (e.g., 30 mmHg at rest).
In approximately 25% of septal and asymmetric forms, left ventricular outflow tract (LVOT) systolic obstruction develops. Its mechanism is explained by the Venturi effect: high-speed blood flow draws the anterior mitral leaflet toward the hypertrophied septum. Obstruction may be constant or provoked by physical exertion.
However, echocardiography has limitations:
- Apical forms are difficult to visualize because the cardiac apex lies too close to the low-frequency ultrasound transducer.
- Mass-like forms create challenges in differential diagnosis with true cardiac tumors.
Role of MRI and detection of fibrosis
When acoustic windows on echocardiography are poor, cardiac magnetic resonance (CMR) imaging becomes an extremely useful tool. Cine-MRI (Cine-MRI) provides precise measurement of wall thickness, left ventricular mass quantification, detects right ventricular involvement, mitral regurgitation, and radial or circumferential strain abnormalities (strain).
A unique capability of MRI is the detection of interstitial fibrosis using late gadolinium enhancement (Late gadolinium enhancement, LGE). Fibrotic tissue appears as "bright" myocardium against a normal "dark" background. This occurs due to an expanded extravascular collagen space and impaired contrast washout.
The pathogenesis of fibrosis is likely related to microvascular ischemia. Stress perfusion CMR with vasodilators in patients with acute chest pain and focal non-ischemic edema often reveals reduced myocardial blood flow reserve precisely in those areas where LGE fibrosis will later develop.
Typical LGE patterns in HCM:
- Localization at the anterior and inferior right ventricular insertion points (septal insertion).
- Diffuse or selective enhancement within the interventricular septum.
- Patchy enhancement (patchy) with large intramural foci.
- Relative sparing of the subendocardial layer (a crucial feature distinguishing it from post-infarction scar).
Clinically, fibrosis strongly correlates with prognosis. It provides a substrate for malignant arrhythmias (via re-entry), increasing the risk of sudden cardiac death in young patients (<40 years) and heart failure progression in older patients (>40 years).
T1 mapping and differential diagnosis
Histologically, fibrosis in HCM is often global and diffuse. Consequently, it may go undetected on standard LGE images: simply because no "normal" reference myocardium remains for comparison. Furthermore, the spatial resolution of LGE limits the assessment of microscopic fibrosis.
T1 mapping overcomes this limitation:
- Native T1 values in HCM are elevated, directly correlating with wall thickness and serving as a marker of severity.
- Extracellular volume (ECV) is increased due to extracellular matrix expansion and myocardial fiber disorganization (myocardial disarray).
ECV is a reliable tool for monitoring and risk stratification, helping distinguish HCM from physiological hypertrophy in athletes (where ECV is decreased due to cellular hypertrophy).
MRI is also indispensable for the differential diagnosis of the hypertrophic phenotype in other conditions:
- Anderson-Fabry disease: typically subepicardial contrast enhancement in the inferolateral wall.
- Amyloidosis: diffuse infiltration impedes myocardial signal suppression, creating a diffuse bright signal or a patchy intramural pattern.
- Sarcoidosis: small foci of patchy (patchy) enhancement reflecting the interstitial distribution of granulomas.
Computed tomography (CT)
Computed tomography also provides a fast and easy assessment of cardiac wall thickness. CT clearly demonstrates diffuse left ventricular myocardial hypertrophy, which in this pathology is often accompanied by predominant involvement of the anterior wall. An important advantage of modern CT scanners is that these data can be acquired with a very low radiation dose of only 1–3 mSv.
Briefly: answers to topical questions
What are the main mechanisms and causes of myocardial fibrosis development in hypertrophic cardiomyopathy?
The probable mechanism of fibrous tissue development in hypertrophic cardiomyopathy is impaired intramural myocardial blood supply, i.e., microvascular ischemia.
- In acute chest pain and focal non-ischemic intramural edema, vasodilator stress perfusion CMR often reveals reduced myocardial blood flow reserve; these regions correspond to late-stage fibrosis zones on LGE.
- Interstitial fibrosis is detected on MRI due to an expanded extravascular collagen space and impaired contrast washout: late gadolinium enhancement highlights fibrotic tissue as "bright" myocardium against normal "dark" myocardium.
- Histologically, fibrosis in HCM is often global and diffuse, which is why it may not be detected on LGE due to the lack of a "normal" reference myocardium for comparison.
What MRI features distinguish fibrosis in hypertrophic cardiomyopathy from a post-infarction scar?
The primary MRI feature distinguishing fibrosis in hypertrophic cardiomyopathy from a post-infarction scar is the relative sparing of the subendocardial layer.
| Sign | Hypertrophic cardiomyopathy | Myocardial infarction |
|---|---|---|
| LGE pattern | Relative sparing of the subendocardial layer; patchy intramural contrast enhancement | Subendocardial enhancement with variable transmural extension |
| LGE localization | Diffuse or selectively localized in the interventricular septum; at the anterior and inferior insertion points; patchy intramural enhancement | Literature indicates subendocardial enhancement with variable transmural extension |
What MRI features are characteristic of the differential diagnosis of Anderson-Fabry disease?
Anderson-Fabry disease is characterized by a specific contrast distribution pattern on late gadolinium enhancement CMR.
- Anderson-Fabry disease (Anderson-Fabry disease) — typically subepicardial enhancement in the inferolateral wall (inferolateral wall).
This pattern is used in the differential diagnosis of the hypertrophic phenotype from storage diseases.
What are the imaging features of cardiac involvement in amyloidosis on magnetic resonance imaging?
Imaging features of cardiac involvement in amyloidosis on MRI include marked thickening of both ventricular walls, heterogeneous signal intensity on T1-weighted slices, and the presence of pericardial effusion.
- Late gadolinium enhancement (Late gadolinium enhancement) — failure of myocardial signal suppression (myocardial suppression / null myocardial signal) due to diffuse infiltration. Results in a diffuse high-intensity signal or a patchy intramural pattern.
- AL amyloidosis — global subendocardial contrast distribution and significantly elevated native T1 values.
- ATTR amyloidosis — failure in myocardial nulling (failure in myocardial nulling) and higher extracellular volume values (ECV).
MRI differentiation of myocardial infarction scar from fibrosis in HCM: infarction affects the myocardium starting from the endocardium, whereas in HCM the subendocardial layer remains intact.
Class and exam questions
Why does echocardiography sometimes fail to detect apical hypertrophy?
This is due to ultrasound physics. The cardiac apex lies too close to the low-frequency transducer, creating a blind spot and significantly hindering visualization.
What is the Venturi effect in HCM?
This is a hydrodynamic effect where high-velocity blood flow in the outflow tract pulls the anterior mitral leaflet toward the hypertrophied septum, causing systolic obstruction.
How to distinguish an "athlete's heart" from hypertrophic cardiomyopathy?
The primary MRI criterion is the extracellular volume (ECV) assessment. In athletes, it is decreased due to muscle cell growth, whereas in HCM, it is increased due to fibrous tissue expansion.
What else to study on this topic
- Mechanisms of formation and clinical significance of the systolic gradient in the LVOT
- Vasodilator stress-perfusion MRI in microvascular ischemia
- Dilated cardiomyopathy (DCM): exclusion criteria for ischemic and valvular etiologies
- Assessment of transmitral blood flow (E and A waves) in diastolic dysfunction
- Features of myocardial signal nulling in cardiac amyloidosis
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