The U wave is still an electrocardiographic deflection of enigmatic origin. Numerous hypotheses on its origin have been formulated, but to date none has been conclusively proven. Recently, a report described the first case of bifid (or notched) U waves. Until then this phenomenon has only been described in the T wave. This is the first report of double U waves—two separate deflections, ascribed to an accessory papillary muscle.

Hypothesis: The presence of a double U wave will be associated with an accessory papillary muscle (s).

Materials and methods: This is a retrospective analysis of 4729 patient files of patients who were evaluated at a cardiology practice. The 12-lead surface electrocardiogram was evaluated for the possible presence of a double U wave. In cases where a double U wave was found, the transthoracic echocardiogram was then scrutinized for the presence of an accessory papillary muscle.

Results: A total of 3 cases of a double U wave were found. In every case an accessory papillary muscle was clearly seen on the transthoracic echocardiogram.

Conclusion: A double U wave is a new variant of an old electrocardiographic deflection of enigmatic origin. This variant may be associated with an accessory papillary muscle.