Intramuscular hematopoiesis in hypotonic infants with type 1 muscle fiber dysmaturation.

Three term infants had severe hypotonia, a maturation defect of type 1 muscle fibers, and extramedullary hematopoiesis (EMH) in quadriceps muscle specimens obtained within one month of birth. Although predominantly myelopoietic, signs of inflammatory myopathy were absent. One patient had congenital myopathy with maturation arrest of type 1 fibers, another had transient maturation delay of type 1 fibers, and the third patient was subsequently classified as having spinomuscular atrophy. Extramedullary hematopoiesis was demonstrated in normal muscle obtained from young fetuses, but not in muscle obtained at autopsy from infants representing the third trimester and first postnatal month, or in muscle biopsy specimens from 15 other hypotonic infants with type 1 fiber size disproportion. We conclude that EMH in muscle of hypotonic infants is an abnormal persistence of a fetal state that is associated with delayed muscle maturation with diverse origins. Extramedullary hematopoiesis in muscle may indicate hypoxia, but signs of perinatal asphyxia in these babies were inconclusive. Low blood flow due to inactivity or an unidentified product of immature muscle may promote intramuscular EMH, but there is no evidence to suggest that myelopoiesis is injurious to muscle fibers. Intramuscular EMH should be distinguished from inflammation.