Subungual hematoma (blood under the fingernail or toenail), a common childhood injury, is usually caused by a blow to the distal phalanx (eg, crush in a door jamb, stubbing one's toe). The blow causes bleeding of the nail matrix (nail bed) with resultant subungual hematoma formation. Patients complain of throbbing pain and blue-black discoloration under the nail as the hematoma progresses. Pain is relieved immediately for most patients with simple nail trephination.
The trephination of subungual hematomas with intact nail fold and minimal nail or digit disruption is discussed here. The evaluation and management of distal phalanx fractures and finger tip injuries are discussed elsewhere. (See "Distal phalanx fractures" and "Management of fingertip injuries".)
Subungual hematomas may be simple (ie, the nail and nail fold are intact) or accompanied by significant injuries to the nail fold and digit (eg, fingertip avulsion) [1,2]. The nail matrix is the tissue under the nail that permits nail growth and migration. Its longitudinal fibers anchor the dermis to the periosteum of the distal phalanx. The matrix begins 7 to 8 mm under the proximal fold, and its distal end is the white crescent called the lunula. Scarring of the matrix, as occurs with nail trauma, can disrupt nail growth and lead to nail deformity or permanent loss of the nail (figure 1).
The richly vascularized nail bed lies directly beneath the plate to provide adherence and support and is the basis of the characteristic pink color. The proximal and lateral nail folds surround the plate on three sides, and the cuticle, an outgrowth of the proximal fold, provides a seal between the fold and the nail plate (figure 1) .
When a subungual hematoma occurs, the source of pain is pressure in a contained space pressing against nerve fibers and not from the soft tissue injury or bony injury alone.