<Tr> <Th> eMedicine </Th> <Td> article / 827104 article / 82926 </Td> </Tr> <Tr> <Td_colspan="2"> (edit on Wikidata) </Td> </Tr> <P> A subungual hematoma is a collection of blood (hematoma) underneath a toenail or fingernail (black toenail). It can be extremely painful for an injury of its size, although otherwise it is not a serious medical condition . </P> <P> Runner's toe is a common condition seen in runners caused by downward pressure or horizontal separation of nail plate from the nail bed . This repetitive traumatic injury leads to bleeding and pooling of blood underneath the nail plate . Clinically, it is characterized by reddish - black discoloration of the toe nail . The nail plate may also become thicker and more brittle as a result of the injury (onychochauxis). Fortunately, the deformed nail plate will gradually grow out and be replaced by new, normal - appearing nail plate in several months time . Infrequently, the toe may become painful and require surgical drainage . </P>

Where does the term dead as a door nail come from