Number 26, 2005
Cardiovascular effects of exercise

Imaging: hypertrophic cardiomyopathy and athlete's heart

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Ingo Paetsch, Eike Nagel
Department of Cardiology, German Heart Institute, Berlin, Germany
Correspondence: Eike Nagel, Department of Cardiology, German Heart Institute, Augustenburger Platz 1, 13 353 Berlin, Germany. Email: eike.nagel@DHZB.de

Introduction
Imaging can play a crucial role in the differential diagnosis between hypertrophic cardiomyopathy and the athlete's heart. Even though the final diagnosis may be made only after reduction of training in some cases, imaging may provide essential information in most patients. Echocardiography is the first method to be used. Magnetic resonance techniques provide information, such as a more accurate determination of wall thickness and muscle mass, the detection of myocardial fibrosis, and the ability to assess intramural myocardial motion components, including cardiac twisting and untwisting. These data may aid better discrimination between those with hypertrophic cardiomyopathy, and athletes (see flow chart for the different diagnostic steps in Figure 1). In addition the criteria for differential diagnosis between dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy is summarized in Table I.

Table I. Differential diagnosis of dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy in athletes.

Magnetic resonance techniques
As most physicians are well informed concerning echocardiographic techniques, but may have a less specific knowledge on magnetic resonance imaging, the major magnetic resonance methods that are applied in the patients under discussion will be reviewed briefly here.

Assessment of left ventricular dimensions, wall thickness and muscle mass
Magnetic resonance imaging is superior to all other imaging methods for the assessment of left ventricular dimensions, wall thickness and muscle mass. This is because of its three-dimensional visualization independent of any imaging windows, the high contrast between blood and myocardium, and its high temporal and spatial resolution. Today, steady-state free precession techniques are used for this purpose. In these, in contrast to earlier techniques (turbo-gradient echo), the contrast between blood and the myocardium is independent of the inflow of blood (with fresh magnetization) into the imaging slice. Contrast is generated by the inherent differences of magnetization of blood and myocardium. The application of this technique has improved blood–myocardial contrast significantly in long-axis views and in patients with reduced ejection fraction. In addition, steady-state free precession techniques yield an unprecedented accuracy for the depiction of the trabeculae, as the presence of blood between these myocardial structures results in bright signal, independent of its flow, resulting in an even more accurate determination of wall thickness and muscle mass. Magnetic resonance is regarded as the reference standard for the assessment of left ventricular muscle mass [1,2].

Detection of fibrosis
A new technique applied in most patients with suspected cardiomyopathy is the so-called ‘late enhancement’ (or ‘delayed hyperenhancement’) technique [3]. The standard contrast agents used for magnetic resonance imaging (gadolinium chelates) diffuse from the vascular bed into the interstitium, but not into the cell. Any increase in distribution volume or a reduction in wash-out of the contrast agent will lead to a local enrichment of the drug, which can be visualized with magnetic resonance imaging. Although this technique was originally proposed for the detection of necrosis in myocardial infarction, increasing data confirm its applicability to the detection of any form of fibrosis within the myocardium.

Myocardial tagging
Myocardial tagging is a magnetic resonance technique that allows the placement of ‘tags’ on the myocardium at end-diastole by regionally modulating the magnetization (Figure 2). The tags remain fixed to the myocardium during systolic contraction and diastolic relaxation, and allow for the analysis of intramyocardial motion components (eg, endocardial or epicardial motion or rotation). A major drawback of this technique is the tedious analysis, which has hindered its application in clinical practice. New, nearly fully automated, tools of analysis (eg, harmonic phase, HARP [4]) are available, however, and will rapidly bring this technique into daily use.

Assessment of left ventricular wall thickness
Echocardiographic studies have shown that the vast majority of young athletes have a left ventricular wall thickness less than 12mm. Only 0.4% of males had a wall thickness greater than 12mm, and in no female was it greater than 11mm, prompting further investigation in a study of 720 elite adolescent athletes [5]. In another study, the number was higher, 2.5% of male elite athletes having a wall thickness greater than 12mm [6]. Most cases of hypertrophic cardiomyopathy exhibit a wall thickness of 20mm or more; however, some patients with hypertrophic cardiomyopathy may have only a mild hypertrophy – as little as 13mm [7–9] – thus creating some overlap with competitive athletes. Wall thickness alone may thus not suffice to differentiate between athletes and hypertrophic cardiomyopathy in individuals in whom the wall thickness is between 13 and 15mm. Suspicion of pathology should be greater if borderline thickness is found in athletes performing isometric activities (weight-lifting, etc) than when it is present in athletes performing endurance sports (rowing, cycling), because left ventricular wall thickness and left ventricular muscle mass are greatest in the latter [10–12]. However, such a thickening of the left ventricular wall is usually in parallel with significant left ventricular enlargement [13].
Training causes a similar increase in thickness of all myocardial segments, with the anterior septum taking the lead. Differences between different segments are usually less than 2mm and show a smooth transition. In patients with hypertrophic cardiomyopathy, similar thickening of the anterior septum can be observed; however, significant differences between different myocardial segments occur frequently, being most pronounced in patients with hypertrophic obstructive cardiomyopathy, but also in nonobstructive disease. Pathologic hypertrophy tends to be asymmetric and to show abrupt differences in wall thickness; furthermore, frequently, segments other than the anterior septum demonstrate the greatest thickness (Figure 3).

Cavity dimensions
End-diastolic cavity dimensions can give important information in cases with extreme values. A well-trained athlete is likely to have have an enlarged left ventricular end-diastolic diameter (>55mm), which needs to be discriminated from dilated cardiomyopathy (usually >60mm) [13]. In contrast, most patients with hypertrophic cardiomyopathy show a rather small diastolic cavity dimension of less than 45mm. In these patients, however, dilatation of the ventricle may occur over time, and reach similarly high values. Cavity dimensions between 45mm and 55mm, which are found in the majority of athletes, do not help in making the differential diagnosis.

Diastolic function
As a rule of thumb, athletes can be expected to have normal diastolic function, independent of their myocardial thickness. In contrast, most patients with hypertrophic cardiomyopathy have abnormal diastolic function. However, individuals with hypertrophic cardiomyopathy, in particular those with mild to moderate hypertrophy, are less likely to show abnormalities. Thus any abnormality in diastolic function confirms the diagnosis of pathology, whereas normal diastolic function does not exclude disease.
Diastolic function may be assessed from the transmitral flow pattern determined with Doppler echocardiography, with an inverted E/A ratio in diastolic dysfunction (A >E; early passive diastolic filling is decreased, late filling during atrial contraction is increased) [14].

Assessment of regional myocardial motion and cardiac rotation and relaxation
With classic imaging techniques, rotational and intramural components of motion are neglected. Studies using myocardial tagging have shown differences in early diastolic rotation (untwisting) between patients with different types of left ventricular hypertrophy. Diastolic untwisting is an important part of diastolic function: it is the most rapid motion component of the healthy heart. In athletes (rowers), early diastolic untwisting is enhanced and rotation velocity increased [15], but patients with hypertrophic cardiomyopathy show a prolongation of diastolic untwisting, with a reduction in rotation velocity. This finding has been related to differences in fiber orientation, with fiber disarray (Figure 2).
Similarly, regional differences in shortening and lengthening velocities have been observed by means of tissue Doppler echocardiography [16–20].

Assessment of myocardial fibrosis
Similar to the fibrotic alterations found in myocardial infarction, fibrotic changes [21] or severe fiber disarray with increased interstitial space have an enlarged distribution volume for magnetic resonance contrast agents. These areas show an enhancement in a strongly T1-weighted magnetic resonance scan. Lesions have been found in typical locations in patients with hypertrophic cardiomyopathy, and the amount of enhanced areas correlates with the patient's prognosis [22] (Figure 3). The likelihood of a cardiac event was significantly greater in patients with larger fibrotic areas (28.5% of left ventricular muscle mass) than in patients with only mild (8.7%) or no fibrosis.

Family screening
Because of the genetic transmission of hypertrophic cardiomyopathy, family screening may help to establish the diagnosis in borderline cases. The presence of hypertrophic cardiomyopathy in a family member confirms the diagnosis; however, its absence does not exclude it, because the genetic disorder shows incomplete penetration [23]. Potentially, screening could also be performed by DNA analysis; however, the heterogeneity of abnormalities makes this task similarly difficult [23].

Deconditioning
A final step in the diagnosis is the need to refrain from training for several months, which will allow physiologic hypertrophy to normalize, whereas hypertrophic cardiomyopathy will remain unchanged despite the interruption to training [24,25]. Serial examinations should be performed with magnetic resonance rather than echocardiography because of its greater reproducibility and, thus, ability to detect changes early and allow the patient to resume training again as soon as possible.

Conclusions
The combination of several imaging parameters may prove the presence of pathologic hypertrophy. If no abnormalities are found, the likelihood of cardiomyopathy is low; however, in a few patients deconditioning may be required to enable the final diagnosis to be made. In general, echocardiographic techniques are sufficient; in difficult cases, magnetic resonance imaging may yield additional and valuable information. ?

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REFERENCES

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