Transthoracic two-dimensional M-mode and Doppler echocardiography has made substantial contribution and revealed itself as a useful non-invasive tool for differentiating the extreme phenotypes of physiological athlete's heart from cardiac pathology, particularly HCM, which manifests with both structural alterations and malfunctioning (Maron, 2005; Pelliccia et al., 2002; Rajiv et al., 2004; Sharma, 2003; Whyte et al., 2004). Given the limitations (e.g. expensive procedures) of other diagnostic techniques, transthoracic echocardiography remains the principal means in distinguishing between adaptive and maladaptive cardiac hypertrophy. In the majority of countries, routine echocardiographic testing is, however, not mandatory for athletic population, and is performed primarily when sportsmen are directed due to abnormal screening results (e.g. ECG). This can also happen when a physician, a coach, parents and/or athletes themselves show a desire to undertake the procedure; or when they are recruited into scientific study as volunteers.

Whereas the increase in heart mass due to exercise training usually is not so profound (in fact, not always present) as in HCM patients, a fair proportion of athletes still possesses a myocardial structure similar to that of patients with mild phenotypic expression of HCM (Maron, 2003; 2005; Whyte et al., 2004). The discrimination between benign athlete's heart and pathological cardiac hypertrophy is obviously important: in addition to the undisputed significance to the individual, the differentiation and subsequent measures are also important because the unnecessary furor drawn by a sports-related sudden cardiac death following a serious condition overlooked may serve as a deterrent from exercise for the general population. Furthermore, familial evaluation should follow the established cardiomyopathy to detect and treat affected relatives. So, what are the criteria behind this differential diagnosis and to what extent can echocardiography be useful in this dilemma?

First of all, it is important to delineate the extent and type of a cardiac hypertrophic response to athletic conditioning. Concentric type of myocardial hypertrophy is characterized as an increase in LV mass with an augmented relative wall thickness (RWT, which is the ratio of the sum of posterior wall thickness and interventricular septum thickness to the LV diameter) above arbitrary 0.42 or 0.45 value, whereas eccentric hypertrophy is an increased LV mass without augmented RWT (Haykowsky et al., 2002). It is usually both processes of chamber dilation and wall thickening that occur during long-term adaptation to serious athletic training, that's why the term 'symmetric' remodelling (De Castro et al., 2006).

Large-sample studies have shown that a significant proportion of endurance athletes exceed the 'normal upper limit' (55 mm) for LV end-diastolic internal diameter (Pelliccia et al., 1991; Spirito et al., 1994; Urhausen et al., 1997). Perhaps the most striking results were obtained by the authors who detected that more than one-half of small-body-size male 100-km runners had LV diameter in excess of 60 mm (Nagashima et al., 2003). In addition, 33 (13%) of their subjects showed LV dilation with LV diameter of 70 to 75 mm. LV diameter was over 55 mm in 55% of male rowers (Pelliccia and Maron, 1997). Among high-level athletes from different sports, LV diameter was measured to be 60 to 70 mm in 6% (Whyte et al., 2004) and in 14% (Pelliccia et al., 1991) of male Caucasians. LV systolic and diastolic function was normal or super-normal in all individual cases (Pelliccia et al., 1991). Others have reported maximum values of 67 mm in the rower (Urhausen and Kindermann, 1992), and 70 mm in the cyclist (Rost, 1997) with no signs or symptoms of pathology. These and similar situations represent examples of the extreme physiological adaptation to endurance conditioning.

In athletes with substantially dilated LV, the differentiation from dilated cardiomyopathy is relatively simple, and the latter condition can be eliminated simply by reporting well-tolerated athletic activity. Uncertainty may be completely dispelled if increased systolic function in response to physical exertion is observed (Firoozi et al., 2003).

The increase in RWT is rather frequently observed in response to aerobic conditioning (Palazzuoli et al., 2002; Venckunas et al., 2005, 2006), making this cardiac hypertrophic index higher in endurance athletes such as distance runners (normally about 0.40) than in healthy non-athletes (about 0.35) (Urhausen et al., 1997). However, when an individual possesses a truly concentric myocardial hypertrophy (RWT in excess of 0.45), the investigator should carry out more thorough evaluation rather than stop short at taking the measurements of heart structure (Pelliccia and Maron, 1997; Urhausen et al., 1997).

LV posterior wall thickness higher than 13-14 mm (values compatible with HCM), especially without chamber dilation, is rare in athletes and should not be regarded as an adaptation due to athletic training (Douglas et al., 1997; Fagard, 2003; Henriksen et al., 1996; Maron, 2005; Pelliccia et al., 1991; Pelliccia and Maron, 1997; Urhausen et al., 1997; Whyte et al., 2004). Sportswomen usually possess LV wall thicknesses below 11 mm (Maron, 2005). Maximum values reported in male (elite) athletes were 16 mm in rowers and cyclists (Pelliccia et al., 1991), and 19 mm in a road cyclist (Rodriguez Reguero et al., 1995). Thus, a higher percentage of professional male road cyclists, skiers, rowers, paddlers, and probably other athletes from sports where large muscle mass is involved into ('more isometric') training, are subjected to the differential diagnosis for HCM. Here the importance of proper differentiation comes into play in order to limit the rate of false positive (as well as false negative) diagnosis of HCM and subsequent unwarrantable disqualification from physical activity on the one hand, and prevent tragic events of a sudden cardiac death in a field on the other hand.

In athletes with LV wall thickness >13 mm, the following main echocardiographic criteria (Table 1) are suggested for making a distinction between the adaptive vs. pathological nature of cardiac hypertrophy:

Cardiac chambers are enlarged in athletes (Fagard et al., 1989; George et al., 1991; Henriksen et al., 1996; Rost, 1997; Venckunas et al., 2006), while in HCM the heart is hypertrophied asymmetrically with the LV chamber rarely increased in size (D'Andrea et al., 2006; Firoozi et al., 2003; Maron, 2002; 2003): its end-diastolic diameter is often below 50 mm in HCM, but frequently above 55 mm in elite male endurance athletes (Maron, 2005; Pelliccia et al., 1991). Marked left atrial enlargement is inherent to the HCM-affected heart (Firoozi et al., 2003; Maron, 2005), while in the athletic heart all cardiac chambers are enlarged proportionally (Barbier et al., 2006; George et al., 1991; Hauser et al., 1985; Pelliccia et al., 2005; Rost, 1997). Mitral valve systolic anterior motion is never present in athlete's heart (Maron, 2005).

Cardiac acoustic density, in contrast to cases of HCM, is normal in healthy athletes (Giorgi et al., 2000; Hildick-Smith and Shapiro, 2001; Lattanzi et al., 1992). In HCM, ultrasonic myocardial reflectivity is postulated to be due to increased collagen content and/or profound muscle fiber disarray (Frenneaux, 2004; Lattanzi et al., 1992). Incidentally, acoustic properties were also shown to be unchanged in patients with myocardial hypertrophy due to hypertension (Gigli et al., 1993), though the correct diagnosis in these individuals is usually easily performed by serial blood pressure measurements. In addition, strain rate analysis could be used to discern myocardial wall thickening due to athletic conditioning from cardiac hypertrophy due to hypertension (Saghir et al., 2007).

Interventricular septum thickness is usually above 16 mm in patients with HCM (Firoozi et al., 2003). In the absence of cardiac pathology, the interventricular septum to LV posterior wall thickness ratio is usually less than 1.3 (Douglas et al., 1997; Hildick-Smith and Shapiro, 2001). When interventricular septum motions, shape, thickness, and its ratio to LV posterior wall thickness together with global ejection fraction are normal, the diagnosis of HCM can almost certainly be denied (Douglas et al., 1997; Urhausen et al., 1997; Whyte et al., 2004).

Left ventricular diastolic function assessed by pulsed or tissue Doppler is preserved or even improved (especially during exercise but sometimes at rest as well) in athletes free of HCM or other heart diseases, be hypertrophy present or absent (Cardim et al., 2003; D'Andrea et al., 2006; Fagard et al., 1989; Hildick-Smith and Shapiro, 2001; Libonati, 2000; Maron, 2005; Pelliccia et al., 1991; Pelliccia and Maron, 1997; Rajiv et al., 2004; Urhausen et al., 1997). Diastolic LV function is often impaired in patients and their resting E/A ratio, still a most frequently used index of diastolic function, may be ~1 or lower.

In addition, coronary vascular architecture and flow reserve as possible criteria of the nature of the cardiac enlargement have also been suggested (Hildick-Smith and Shapiro, 2001; Indermuhle et al., 2006), and contrast echocardiography (Indermuhle et al., 2006), as well as myocardial Doppler imaging (D'Andrea et al., 2006; Rajiv et al., 2004) and three-dimensional echocardiography (De Castro et al., 2006, 2007) have recently emerged as valuable tools for the differentiation discussed.