Osteogenesis imperfecta, a type of skeletal dysplasia, is a congenital connective tissue disorder characterized by fragile bones and low bone mass. There are seven distinct subtypes of osteogenesis imperfecta, ranging in severity, with only one subtype considered fatal. Osteogenesis imperfect type II, which accounts for the majority of all cases diagnosed both prenatally and postnatally, is lethal in the perinatal period. The incidence rate for osteogenesis imperfect type II is 1 in 62,000 live births.
Osteogenesis imperfecta type II is further broken down into three subtypes: type IIA, type IIB, and type IIC. Type IIA is characterized by fragile, crumbling femurs, bony formations on the ribs, small gestational size, and osteoporosis, or severely porous bones, in the face and skull. In Type IIB, the ribs are less affected, so the chest is more normally formed and the baby experiences less respiratory distress. Type IIB is considered the only form to have postnatal survivors, however, all three subtypes of osteogenesis imperfecta are fatal in the perinatal period. Type IIC is characterized by small, thin fractures in the leg bones and ribs, small gestational size, and severe weakening of the bones.
Osteogenesis imperfecta type II is predominately an isolated condition, caused by a new mutation in either the COL1A1 or COL1A2 gene. Infants diagnosed with type II have no family history of the condition. Osteogenesis imperfecta type II has a recurrence risk of 7%.