(Vanderbilt Children's Hospital, 2009), so ROP
will generally be more severe the earlier the child
is born.
Pathology of ROP
42
There are two stages in the progression of
ROP. The first phase is the loss of blood vessels
to the retina, and the second phase involves the
abnormal, excessive growth of new blood vessels
(neovascularization). The normal progression of
the development of blood vessels begins at 4
months (about 16 weeks) of gestational age, beginning at the optic nerve, and completes at
around 8 months (36 weeks), reaching the ora
serrata. Thus, ROP generally progresses through
the first stage at about 30-32 weeks. The first
stage of ROP is when the area of the retina without blood supply due to the non-development of
vessels becomes hypoxic, or deprived of oxygen. The second phase of neovascularization occurs at about 32-34 weeks, triggered by increased metabolic activity in the infant and in response to the under-oxygenated area of the retina. This new, abnormal growth of blood vessels
can cause scarring, leakage, and tangles of vessels which can extend into the vitreous and lens
causing retinal detachment (Chen & Smith,
2007).
One theory for the cause of ROP is the exposure to oxygen after birth. In utero, the oxygen
levels the fetus is exposed to are 1/2 to 1/3 the
levels the infant will breathe in normal room air. If
oxygen therapy is used, the level increases even
more. Chen found that in mice models, once the
mice were exposed to increased oxygen levels,
normal vessel growth stopped and regressed. The resulting hypoxic area of the retina
induced new, abnormal vessel growth. The specific molecular causes and progressions for vessel growth have been found to be related to levels
