“…In total, 14 studies reported data concerning the frequency of UMDs [49,50,57,59,60,66,70,74,83,84,88,93,95,98]. Three studies [83,84,93] used an established UMD definition, seven [49,50,59,60,70,74,95] developed their own and four [57,66,88,98] did not report any definition.…”