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Premature Baby
Premature Baby-Retinopathy of Prematurity.
 At about 16
weeks of pregnancy, the
retina, at the back of the inside of the eye, begins to develop
blood vessels which provide nourishment and
oxygen to the eye. Over time, this network of vessels gradually grows
forward, eventually covering the entire surface of the retina. The process
is usually complete at the end of term, or about 40 weeks
gestation.
Prematurity can
interfere with the development of these blood vessels. This condition is
called
retinopathy of prematurity (ROP) and it affects many premature babies.
As with other conditions, the risk of developing ROP increases with the
degree of prematurity. For example, about 80% of extremely premature baby,
those of less than 26 weeks
gestational age, have ROP whereas only about 15% of mildly premature
babies, more than 30 weeks gestational age, do. A premature baby is at highest risk
for ROP when born very prematurely or when the premature baby has a very low
birth weight of 1500 g or less.
Although most
cases are mild and treatable, ROP can permanently affect
vision. Depending on the severity, ROP may cause minor vision
impairments or, in more rare cases, blindness, especially if the condition
in premature babies
goes untreated.
 Blood
vessels generally grow from areas of high oxygenation, where the vessels
already exist, to areas of low oxygenation, where the vessels have not yet
formed. In this way, the retina is gradually and evenly covered in blood
vessels that grow from the back of the eye towards the front like a wave
approaching the shoreline.
The oxygen
level of the premature baby’s environment is an important factor for the progress of
normal blood vessel development. When the premature baby is in the
womb, the concentration of oxygen in its blood stream is kept relatively
steady and is lower than the levels that will occur after birth. In
comparison, after a premature baby is born, oxygen levels in the bloodstream
are kept in the desired range by adding oxygen to the air surrounding the
premature
baby; this helps the premature baby’s breathing. This rapid change in the
level of oxygen in the premature baby’s bloodstream has an important effect on the
regulation of blood vessel development. Moreover, if the premature baby is having
other complications that affect the regular flow of oxygenated blood, this
too can interfere with the normal growth and spreading of the blood vessels.
There are many
factors that contribute to the development of ROP. The knowledge that oxygen
levels have an effect came in the 1940s. At that time, many premature babies
were given supplemental oxygen whether they were having breathing problems
or not. At this time it was discovered that the premature babies who were given high
concentrations of oxygen had a much higher incidence of ROP than premature babies who
were not given oxygen. Although supplemental oxygen is often essential for a
premature baby’s
survival, especially those with the more severe form of lung and
breathing complications, the level of supplemental oxygen must be carefully
monitored and adjusted to keep oxygen levels in the bloodstream in the
desired range. Following this adjustment, the frequency of ROP has gone down
but not disappeared.
Stages of ROP
 ROP is graded,
or classified into stages of severity, from Stage 1, the mildest case, to
Stage 5, the most severe. At what stage ROP is graded depends not only on
the extent of the abnormal blood vessel growth but where in the eye the
growth has taken place.
Fortunately,
most cases of ROP are mild and resolve spontaneously. However, when ROP is
severe and does not resolve with maturation of the baby, if left untreated
these abnormal blood vessels can continue to grow on the surface of the
retina and result in formation of
fibrous tissue. This in turn can pull on the retina and may lead to
partial or full detachment of the retina.
All premature
babies are screened for ROP. If a problem is identified, screening will
continue at regular intervals. If treatment is deemed necessary,
surgery, usually with laser
photocoagulation or
cryotherapy, a kind of freezing, is performed.
Diagnosis of Retinopathy of Prematurity (ROP)
The
retina of the eye is like the film in a camera. It lines the inside
of the eye. At about 16 weeks of pregnancy, the retina begins to develop
blood vessels. The blood vessels generally grow from areas of high
oxygenation, where the vessels already exist, to areas of low
oxygenation, where the vessels have not yet formed. In this way, the
retina is gradually and evenly covered in blood vessels.
 Fluctuation
of
oxygen levels in the developing retina is an important factor in the
regulation of the growth of blood vessels. Moreover, if the premature baby is
having other complications that affect the regular flow of oxygenated
blood, this too can interfere with the normal growth and spreading of
the blood vessels. If the blood vessels grow abnormally as a result,
this is called
retinopathy of prematurity (ROP).
Since
development of retinal blood vessels is incomplete until about 35 to 37
weeks
gestation, the retina is vulnerable to injury. Abnormal blood
vessels can deliver either too much oxygen to the eye or too little.
Hypoxemia, an insufficient oxygen supply in the blood, leads to
local tissue damage called ischemia, or scarring. Hyperoxemia, which
is too much oxygen, reduces the normal
stimulus for development of the blood vessels; this will later
result in loss of some of the developing blood vessels and ultimately
lead to ischemia.
How ROP is diagnosed
All
premature babies who fall within a screening protocol defined as birth
weight less than 1500 g and
gestational age of below 30 weeks are routinely examined for ROP.
These premature babies will likely be examined initially at four to six
weeks after birth. An eye doctor will use drops to
dilate the pupils, which will allow for a better view of the inside
of the eye.
Depending
on the amount of abnormal blood vessel development, the premature baby’s condition
will be graded and further examinations will be conducted every one to
two weeks, depending on a variety of factors. These factors include the
severity and location in the eye of ROP, and the rate of progression of
blood vessel formation, called vascularization. In the majority of
cases, even when ROP develops, it will resolve spontaneously with
minimal effect on
vision. However, a small percentage of premature babies screened for ROP,
approximately 10%, will progress to the extent that it is no longer safe
to wait for
spontaneous resolution. For these babies, treatment will be offered
to reverse the progression of ROP.
Stages of ROP
The
severity of ROP is assessed by the area of retina that is involved, also
referred to as the zone of vascularization, and the extent of the
abnormal growth. The lower the number for the zone, the greater the area
of the retina affected. The stages, or extent of the growth of abnormal
blood vessels, are defined as follows.
 Stage 1 ROP
In Stage 1
ROP, there is a thin line between the area with blood vessels and the
area where blood vessels have not grown yet. At this stage, the vessels
may grow normally on their own, but the condition must be watched.
Stage 2 ROP
In Stage 2
ROP, the line between the areas with and without blood vessels widens
and thickens into a ridge. At this stage, the condition may still
resolve, however, it may also progress to stage 3 ROP.
Stage 3 ROP
In Stage 3
ROP, new blood vessels begin to grow along the ridge and extend into the
clear gel that fills the eye, called the
vitreous body. These blood vessels can bleed and form
scar tissue.
Stage 4A
ROP
In Stage 4
ROP, the abnormal blood vessels and scar tissue pull on the retina,
partially detaching it. In stage 4A, the centre of vision, called the
fovea, is not involved.
Stage 4B
ROP
In Stage 4B
ROP, the retina is still only partially detached, but the fovea is
affected usually leaving both the centre and peripheral vision impaired
to some degree.
Stage 5 ROP
In Stage 5
ROP, the retina is completely detached, severely affecting vision.
Treatment of Retinopathy of Prematurity (ROP)
 Retinopathy of prematurity (ROP) is an abnormal growth of
blood vessels in the eye that can interfere with
vision. Although usually mild, in its most severe form, ROP can
cause a partial or complete detachment of the
retina in premature babies. Treatment of ROP depends on the extent of the condition
and its location in the eye. Since they are known to be at risk, all
at risk premature babies are examined for ROP until the blood
vessels reach the peripheral, or outer parts of the retina. If
assessment shows any indication of ROP, assessment of the retina
will continue at regular intervals. In a small number of cases,
growth of normal vessels is prevented by the growth of abnormal
vessel which may require treatment.
Surgical treatment of
ROP
Treatment of ROP consists of laser
surgery or, less frequently,
cryotherapy. The purpose of these surgical procedures is the
same, in that they aim to stop the growth of abnormal blood vessels.
Laser
photocoagulation is done using a laser to remove parts of the
retina which have not yet developed blood vessels. Cryotherapy uses
freezing to accomplish the same goals.
Cases
that do not resolve themselves and show signs of progression are
usually treated by
laser therapy to destroy the peripheral avascular retina, the
area of the retina that has not developed abnormal blood vessels
which is short of
oxygen. This leads to the regression of abnormal blood vessels
and allows normal vessels to grow.
It is
possible for the disease process to continue in spite of treatment
with laser or cryotherapy. In these cases, partial or full
detachment of the retina may develop and in some cases this will
require surgery. The severity of the detachment will be the major
determining factor for long-term vision. Although surgical
procedures can sometimes successfully reattach a retina, most
premature babies will have some kind of visual impairment. Those few
premature babies
with a fully detached retina will likely be blind or nearly blind in
that eye. In many cases, premature babies with a partial detachment will go on
to have useful, though not perfect, vision in that eye.
Reviewed by Nasrin Najm-Tehrani, MSc, MD, MB,
BCh, FRCS Ed (Ophthalmology)
Ophthalmologist, Ophthalmology and Vision Sciences
The Hospital for Sick Children
Assistant Professor, Ophthalmology and Vision Sciences
University of Toronto
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