A subungual hematoma is a collection of blood (hematoma) underneath a toenail or fingernail. It can be extremely painful for an injury of its size, although otherwise it is not a serious medical condition. Nature
A laceration of the nail bed causes bleeding into the constricted area underneath the hard nail plate. The blood pools under the nail, giving a reddish, brownish, blueish, or grey/blackish discoloration. The blood puts pressure to the nailbed causing pain which can be throbbing in quality and disappears when the pressure on the nail bed is relieved.Subungual hematomas typically heal without incident, though infection may occur. The pressure of the blood blister may cause separation of nail plate from the nail bed (onycholysis), but the nail should not be pulled off, as this can cause scarring of the nailbed and deformed nails. Nail discolouration may last some months.The nail plate may also become thicker and more brittle as a result of the injury (onychochauxis). The deformed nail plate will gradually grow out and be replaced by new, normal-appearing nail plate in several months time. Infrequently, the nail may become painful and require surgical drainage. Causes
The condition is caused by a traumatic injury, such as slamming a finger in a door, or from sports activities, especially those involving sudden accelerations, such as soccer, basketball, and tennis,  or going downhill, such as running or hiking rugged terrain, and ill-fitting footwear. : 52, 135 Repeatedly thrusting the toes against a shoes toe box can cause a subungual hematoma called joggers toe, runners toe, or black toenail.
Therefore, treatment with phenobarbital is effective, generally with a decrease of at least 25% in serum bilirubin. In fact, this can be used, along with these other factors, to differentiate type I and II. Differential diagnosis
Neonatal jaundice may develop in the presence of sepsis, hypoxia, hypoglycemia, hypothyroidism, hypertrophic pyloric stenosis, galactosemia, fructosemia, etc. Hyperbilirubinemia of the unconjugated type may be caused by:

increased production
hemolysis (e.g., hemolytic disease of the newborn, hereditary spherocytosis, sickle cell disease)
ineffective erythropoiesis
massive tissue necrosis or large hematomas
decreased clearance
drug-induced
physiological neonatal jaundice and prematurity
liver diseases such as advanced hepatitis or cirrhosis
breast milk jaundice and Lucey–Driscoll syndrome
Crigler–Najjar syndrome and Gilbert syndromeIn Crigler–Najjar syndrome and Gilbert syndrome, routine liver function tests are normal, and hepatic histology usually is normal, too. No evidence for hemolysis is seen. Drug-induced cases typically regress after discontinuation of the substance. Physiological neonatal jaundice may peak at 85–170 μmol/L and decline to normal adult concentrations within two weeks. Prematurity results in higher levels. Treatment
Plasmapheresis and phototherapy are used for treatment. Liver transplant is curative. Research
A San Francisco-based company named Audentes Therapeutics is currently investigating the treatment of Crigler–Najjar syndrome with one of their gene replacement therapy products, AT342. Preliminary success has been found in early stages of a phase 1/2 clinical trial.One 10-year-old girl with Crigler–Najjar syndrome type I was successfully treated by liver cell transplantation.The homozygous Gunn rat, which lacks the enzyme uridine diphosphate glucuronyltransferase (UDPGT), is an animal model for the study of Crigler–Najjar syndrome.