Retinopathy of prematurity

Retinopathy of prematurity is when the retinal blood vessels don’t develop normally in babies that are born earlier than their expected date of delivery. 

What is retinopathy of prematurity (ROP)?

The retina is a delicate tissue made up of light-sensitive cells that lines the inside of the back of our eye. The retina converts light that we see into electrical signals that travel along the optic nerve to the brain. The brain interprets these signals so we can “see”. The retina is supplied with blood by a delicate network of blood vessels on its surface and by a layer of blood vessels underneath the retina.
 
Our retina needs a good blood supply to work properly. As a baby develops in the womb, the blood supply for the retina starts to develop around 16 weeks into pregnancy and only finishes around 40 weeks when a full-term baby is born. At full term, a baby's blood vessels are fully developed and can keep the baby's retina healthy.
 
If a baby is born too early then their retinal blood vessels don’t develop completely which can cause problems. Abnormal blood vessels may grow out of the retina and cause scar tissue to form, leading to detachment of the retina which can cause blindness. This damage is known as retinopathy of prematurity and if not monitored or treated at the correct time, can cause vision loss.
 

What causes ROP? 

The reasons why babies develop ROP are complicated, but ROP is more likely to occur in babies that:
  • are born very prematurely – particularly those born before the 28th week of pregnancy 
  • have a low birth weight of less than 1,500 grams
  • required oxygen treatment soon after they were born. 
Often premature babies need additional oxygen which is necessary for the baby's survival but this can increase the risk of developing ROP. 
 
Not all premature babies that need oxygen develop ROP. Paediatricians (hospital consultants that specialise in caring for children) monitor levels of oxygen a baby needs very carefully to ensure that enough oxygen is given for the baby’s survival while also taking into account the effects this may have on the baby's eye. Checks for ROP are a regular part of care for all premature babies born before 32 weeks or babies born weighing less than 1,500g (3lb).
 

How does ROP affect the eye? 

ROP mainly affects the retinal blood vessels. When the retinal blood vessels haven’t developed completely, the retina isn’t receiving enough oxygen so tries to grow new vessels. However, these new blood vessels grow abnormally and are fragile, can cause scarring and damage to the retina. The most severe forms of ROP are very rare but if left untreated, can lead to retinal detachment and blindness.  
 
There are a few ways of describing the amount of ROP present in the eyes, which includes its location, extent and stage. 

Location

The location on the retina is divided into three zones where zone 1 is the innermost and zone 3 is the outermost. The zones are centred around the optic nerve. This is because the blood vessels tend to grow from the area where the optic nerve leaves the eye called the optic disc.

Extent

The extent of ROP is described using clock hours. This refers to how much of the retina is affected by ROP. If you imagine the retina being divided up like a clock face: if there is ROP present from the bottom round to the left hand side, this would be equivalent to three clock hours. 

Stage

Broadly speaking there are five stages of ROP. These stages describe what the ophthalmologist (hospital eye doctor) sees when they look into a baby's eyes.  
 
Stage 1: This is where the junction of the central part of the retina that already has blood vessels and the peripheral part of the retina where blood vessels have not yet grown is marked by a demarcation line or a flat white line. Babies with stage 1 ROP usually recover without needing any treatment.  
 
Stage 2: In stage 2, the demarcation line is raised and is more like a ridge. Babies with stage 2 ROP may recover without needing any treatment as well. These babies may require more regular check-ups to monitor progress.
 
Stage 3: Stage 3 is where new blood vessels start to grow out of the ridge. These blood vessels are very weak and they will cause vision problems if they start to scar. If there is also enlargement and tortuosity of the main normal blood vessels of the retina, this is known as “plus disease”. Stage 3 ROP with plus disease usually requires treatment.
 
Stage 4: In stage 4, the fragile new blood vessels that grow in stage 3 can contract (shrink) causing scarring which can pull the retina away from the back of the eye (partial retinal detachment).
 
Stage 5: Stage 5 is where the retina can become totally detached from the back of the eye (total retinal detachment). This would lead to permanent loss of vision.
 
Most babies with ROP only develop stage 1 and stage 2 with only a small number developing stage 3 and needing treatment.