IVH is bleeding into the ventricles (fluid-filled spaces) within the brain. All of us have two small, fluid-filled ventricles in the center of our brains. These ventricles manufacture cerebrospinal fluid. The fluid-filled space within those ventricles is called the intraventricular space. The areas just outside of those ventricles are the periventricular areas. Adjacent to the outer wall of the ventricle is the germinal matrix, an area of immature nerve cells and tender blood vessels. As the preterm baby matures, the germinal matrix tissues migrate out into the substance of the brain, and the germinal matrix gradually disappears.
The tender blood vessels within the germinal matrix can rupture and cause bleeding within the germinal matrix. This is called a germinal matrix hemorrhage or grade I intraventricular hemorrhage (IVH). The bleeding, if severe, can lead to bleeding within the ventricle itself, a grade II IVH. If there is a lot of bleeding, the ventricles can become enlarged and swollen by the blood, which is a grade III IVH. If the bleeding either involves or secondarily injures the periventricular brain tissue, it is a grade IV IVH or IVH with extension of the hemorrhage outside of the ventricular system into the brain substance.
The incidence of bleeding goes up with decreasing gestational age. The most severe bleeding (grades III and IV IVH) occurs in about 8 percent of infants with birth weights < 1500 grams (3lbs. 5 oz.). The causes of IVH are not clearly known, but it is thought that changes in blood pressure, pressure within the thorax and inability to clot the blood normally may contribute.