Why might PVL lead to CVI?

Why might PVL lead to Visual Impairment?

The periventricular areas of the brain help carry visual information from the eyes to the special vision parts of the brain. Scarring in this area can slow or block passage of information. Children with PVL may then have difficulties understanding what visual information means. This can lead to a type of visual impairment known as Cortical (or Cerebral) Visual Impairment.

What is Cortical Visual Impairment?

Cortical Visual Impairment (CVI) is a condition where some of the special ‘vision’ parts of the brain and its connections are damaged. This causes visual impairment even though the eyes are usually normal and probably working. Often children with CVI actually have good visual acuity but can not ‘make sense’ of what they see. In most cases, once the damage has happened it does not get worse. As children grow older the visual difficulties they experience may slowly improve.

How is the diagnosis made?

If a child is suspected to have visual impairment an assessment can be organised. Sometimes it is the parents who notice (by the way their child acts) that their child’s vision is impaired. If they discuss this with their Family Doctor an assessment can be arranged. In most children damage to the brain may already have been diagnosed. The doctors looking after the child may then also suspect poor vision.

CVI can be diagnosed in a child who has:

Visual Difficulty
Damage to the ‘vision’ parts of the brain
But apparently normal eyes

A head scan will usually confirm damage to the brain. There are other special tests that can be done. These tests measure signals from the ‘vision’ parts of the brain when a child is shown patterns on a screen. Sticky patches are placed on the back of the head. The sticky patches are attached to wires that lead to a machine. The machine records the electrical signals made by the brain. The record of the signals will help the doctors decide what the matter is. If the signals are reduced in size or slow then CVI is more likely. This test is called a Visual Evoked Potential (VEP).

Often the best way to find out if a child has Cortical Visual Impairment is by asking questions. An eye doctor can find out from the parents and teachers of the child what kind of problems they seem to be having. The questions are based on the visual difficulties that commonly occur in children with CVI. If the child has difficulties that are typical of the condition then they are very likely to have CVI.

Children with CVI can have problems with:

Getting Around
Recognising Objects
Focusing for near objects
Fast eye movements
Visual Field Loss

The ‘dorsal stream’ helps children get around safely and quickly

The many different ‘vision’ parts of the brain combine together to make two visual ‘systems’.

One system helps the child to get around safely and quickly. It also helps the child pick objects up and avoid bumping into things and falling over. This visual system that tells the body how to get around is called the ‘dorsal stream’. It is called a ‘stream’ because it is a flow of information about the visual world from one place to another like water flowing in a stream. ‘Dorsal’ describes the part of the brain where the system is.

When the dorsal stream is damaged it is difficult to know precisely where things are in three dimensions. It can be difficult to:

Use stairs without falling.
Step onto pavements without tripping.
Reach forward and grab a cup or handle.

Damage to the dorsal stream can also make it difficult to see a lot of different things at the same time. This means it can be difficult to find a toy on a patterned carpet or to see something that is pointed out in the distance amongst other things.

The ‘ventral stream’ helps a child recognise objects

The other system, called the ‘ventral stream’ helps us to recognise faces, objects and places. Damage to this system leads to problems:

Recognising familiar faces.
Knowing what common everyday objects are.
Losing the way in places that should be well known to the child.

There are a number of other problems that can occur in children with CVI who still appear to have good vision. These include:

Difficulty remembering things they have seen.
Difficulty imagining ‘seeing’ things in their minds.
Some children’s vision can become ‘tired’ more quickly than others. This means that their ability to see can vary from one time to another.
Difficulty reading. This can be due to lots of different reasons.

Children with CVI can have difficulty focusing when looking at near objects

The focusing power of the eye needs to increase when looking at a close object. In children with CVI the focusing power can be reduced. It can also become tired more easily. This is the usual situation for most adults when they become 40 or 50 years old. When this happens many adults need reading glasses. Some children with CVI may also benefit from reading glasses for the same reason.

Children with CVI may have difficulty making fast eye movements

‘Fast’ eye movements are called saccades. We use saccade eye movements to quickly change the direction that our eyes are looking. This helps us look at something that has suddenly changed position. This is so the eyes can follow and fix accurately on a fast moving object. The eyes can then give clear signals to the brain to make clear vision.

Fast eye movements are also important for reading. They help us to quickly move our eyes across the page of a reading book. Saccades are important in many other visual tasks.

Children with CVI may have difficulty making fast eye movements. These children may tend to make quick head turns when looking around a room or reading, rather than making fast eye movements. There are a number of things that can be done to help children deal with this difficulty.

What is Visual Field Loss?

Visual field is the medical word for the full area that we can see: our visual world. If an area of our visual world is blurred or missing with the rest clear then visual field loss is present. It is due to damage to some of the special vision parts of the brain. The relationship between brain damage and visual field loss is all opposite to what you might think.

The right side of the brain is responsible for seeing the left side of the visual world. The left side of the brain sees the right side of the visual world. If the right side of the brain is damaged the left side of the visual world may not be seen. In the same way the upper part of the back of the brain is responsible for seeing the lower part of the visual world. A child with damage in this area will not see the ground when looking straight ahead. The child may then tend to trip over things.