A case is reported of a 14-year old male MZ twin pair, with only one partner affected by situs viscerum inversus, whereby the condition is termed situs viscerum specularis. The high degree of mirror imaging is seen as an abnormal variation of the biological time of the cleavage giving rise to MZ twinning. Electrocardiographic findings are presented and the biological implications of the condition are discussed.

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High resolution tracings of atrial depolarization can be obtained by surface signal averaging technique. A study was conducted on 4 male and 4 female healthy MZ twin pairs aged 10.4 ± 1.3 yrs. Each subject underwent 3 or more recordings, at 10-days intervals. The essential reproducibility could be confirmed, with personal characteristics, as well as the reliability of the technique and the electrophysiological value of the spikes complex. In 3/8 pairs the cotwins showed a significant likeness of the atriogram, while in 5/8 pairs the tracings were not quite resembling between the twins and that is attributed to variations in the chronogenetic characteristics of MZ twins.

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