Cerebral Palsy and Visual Impairment in Children: Experience of Collaborative Practice in Scotland
Chapter 6 Assessment of Visual Function by Andrew Blaikie
Assessment of Visual Function
There are many different aspects of visual function that can be assessed in a child. The main aspects of visual function that may be impaired and that are possible to assess in children with cerebral palsy include:
- refractive errors and near focusing skills;
- visual acuity;
- contrast sensitivity;
- visual field;
- fast and slow eye movements.
The visual function of children with cerebral palsy can vary from one day to another. A child’s visual function may be poorer if they are tired or feeling ill. They may only cooperate with formal assessment for short periods of time and may tire quickly. Often only segments of a full assessment can be performed at one time. It is probably most meaningful to assess a child in a place familiar to them (school or home) when they are in a good mood, in the morning and not ill. Assessment is not a brief single event. It is a careful regular process of observation and formal testing over a long period of time involving many people. To gain an accurate and realistic idea of a child’s visual function requires careful listening and observation, time and patience.
Asking questions is the best way to start an assessment
Before 'testing' visual function it is usually very helpful to ask parents, teachers and other carers about a child’s vision. This may identify more complex types of cerebral visual impairment that might not be revealed by formal testing. Professor Gordon Dutton has developed a useful structured strategy for questioning parents and carers. This is described in an abridged version below.
A structured clinical history-taking strategy
These questions were published in a research paper (Houliston, Taguri, Dutton, Hajivassiliou & Young, 1999). Some questions have been altered and shortened for clarity and sake of space. For the following questions, which number best describes how your child responds in the following situations? Never 1 Occasionally 2 Most of the Time 3 Always 4
- Does your child recognise you before you speak?
- Does your child recognise other family members?
- Does your child recognise friends?
- Does your child recognise people from photographs?
- Can your child identify him/herself from photographs?
- Can your child recognise shapes?
- Can your child recognise objects?
- Can your child name colours?
- Can your child match colours?
- Can your child find his/her way around the house?
- How often does he/she ask for directions around the home?
- Does he/she lose objects around the house?
- Can your child find his/her way around new surroundings?
- How often does he/she ask for directions in new surroundings?
- Does he/she have difficulty reaching out for and grasping objects?
- Does your child have difficulty distinguishing a line from a step?
- Is your child able to see moving objects or are they seen only when they are stationary? (eg: pets, traffic, rolling ball)
- Does your child have difficulty seeing objects when he/she is moving quickly him/herself?
- Can your child find objects on a patterned carpet?
- Can your child find objects in complex pictures?
- Does your child eat food from only one part of the plate and ignore the rest?
- Does he/she misjudge going through doorways or along corridors?
Using this strategy an idea for a child’s strengths and weaknesses can be created. Once a ´feel═ for a child’s visual function has been formed, more formal observation and testing can be pursued. It is important that any assessment is performed with the child wearing his usual glasses if they have been prescribed.
Refractive error and near focusing skills
When most children are born their eyes tend to be either short or long sighted. Short and long sight are both types of ´refractive error═. Children with refractive error usually need glasses to see clearly and comfortably. Over the first few years of life these refractive errors become less and less until by the age of 5 years very few children continue to have any refractive error. Refractive errors disappear by an active process called emmetropisation. Emmetropisation depends on otherwise normal eyes and brain. If there is a problem with either the eyes or brain then this process is interrupted leading to more short and long sight refractive errors in childhood. In children with cerebral palsy emmetropisation (and loss of refractive errors) is less likely to be successful. This means that most children with cerebral palsy have a refractive error and are likely to benefit from wearing glasses. Any child with cerebral palsy should be tested by an optometrist or ophthalmologist to see if they have a refractive error and then prescribed glasses if it is thought appropriate.
Many children with cerebral palsy may not initially tolerate wearing glasses. This is likely to be because their visual system is not able to comfortably cope with a clear and in focus visual world. They are likely to be overwhelmed by the initial experience. Through a structured programme of progressive wear many children can learn to benefit from and enjoy wearing glasses.
Near focusing skills
The eye’s ability to dynamically increase focusing power to allow clear viewing of near objects is called Accommodation. This ability to focus for near objects declines throughout life and consequently most people of middle age need reading glasses to maintain near focusing. Many children with cerebral palsy similarly under accommodate. It is recognised that up to 80% of children with cerebral palsy (and also Down’s syndrome) have difficulty with focusing for near objects even in childhood. This leads to a blurred retinal image. This will inevitably negatively impact upon learning potential during structured lessons in school but also incidental learning at home and at play.
A clear image with normal accommodation
A blurred image with impaired accommodation
A great deal of detail can be lost from a picture and text if near focusing (accommodation) is impaired. This can make an important impact upon a child’s ability to learn.
Many eye-care practitioners in North America and Scandinavia routinely prescribe bifocals for children with impairment of accommodation. It is recommended that children with cerebral palsy are tested for near focusing by "dynamic" retinoscopy. If impaired accommodation is noted then children should be prescribed bi- or multifocal glasses to help with near focusing. Glasses like these are likely to make a positive impact upon a child's ability to learn and improve their quality of life.
This is the most commonly measured aspect of vision. There are many definitions. Visual acuity is often described as the 'threshold for recognition of high contrast materials'. This means the smallest black object on a white background (or vice versa) that a child can see, and actually say what it is (recognise), when presented in good lighting. There are three main ways of assessing visual acuity:
- observation of a child playing and asking questions (taking a history);
- preferential looking tests;
- object recognition and matching.
Different means of assessment are used depending on a child’s developmental age, level of vision and ability to co-operate and communicate.
Observation of a child playing and asking questions
By carefully watching a child at play it is possible to make a reasonably accurate estimate of their visual acuity. It is often possible to make an assessment of many other aspects of visual function as well. The size of toys that a child locates and handles can give clues to their visual function. By introducing small objects of known size such as Smarties, sweeteners or 100s and 1000s cake toppings it is possible to make more confident assessments of visual function. Useful information can also be gained by asking parents, main carers and teachers about a child’s behaviour when playing. This can be just as valuable as spending time observing a child.
Preferential looking tests
Other tests are more formal such as forced choice preferential looking tests. These tests are based on the idea that a child, when presented with two different patterns, will fixate on a pattern that is coarse enough for them to see rather than an equivalently bright but plain pattern.
The Cardiff Card test uses outlines of objects such as a house, car or duck that will interest a child. The pictures are positioned either at the top or the bottom of a plain grey card. There are eleven visual acuity levels, with three different cards with different images at each level.
The examiner presents the cards, beginning with the largest picture, at a distance of either 1 metre or 50cm. The first card is presented at the patient’s eye level and the examiner watches the child’s eye movement, whether up or down, to estimate the direction of gaze. A mental note is made of this direction and then the second card is presented. Again the eye movement is observed. The examiner then checks the cards to see if both estimations are correct. If so, the next set of cards is presented in the same manner. If a wrong estimation of picture position is made or no definite fixation is observed, then the previous set of cards is again presented, using all three cards. The end point is found when two of the three cards are consistently seen correctly. This test is suitable for children of developmental age of 1 to 3 years.
Some children, especially those with cerebral palsy, have difficulty making accurate voluntary eye movements. This can sometimes make interpretation of preferential looking tests very difficult. Some children with cerebral palsy may also find it difficult to make sense of pictures so find it difficult to visually fix on a picture.
Other tests simply use black and white stripes called gratings. These tests are suitable for even younger children. These tests however really only assess whether a child can detect the pattern rather than necessarily recognise anything. Grating tests may not predict functional recognition visual acuity when a child is older. Examples of such tests include the Teller cards, Keeler Cards and Precision Vision grating paddles which are shown below.
Figure 6.3 Precision Vision grating paddles
Object recognition and matching
There are many Object recognition and matching tests. Some of the more common ones include:
- Kay’s pictures
- Lea Symbols
- Illiterate Es test
- Sheridan-Gardner test
- Snellen test type
All these tests require a means of communication with the child even if this is only pointing with his eyes. Useful recognition and matching tests are the Lea Symbols, examples of which are shown below.
Figure 6.4 Lea Symbols
Lea Symbols are suitable for most developmental age ranges and are available in many formats. Matching card games can be played with different sized symbols to interest and engage children. As the symbols become smaller they resolve into each other: this means they begin to look like each other. Because of this children are more likely to try to make guesses as they reach their threshold visual acuity rather than just give up and lose confidence and interest. The symbols are also available in a 3-dimensional format which can be easier for some children to make sense of.
This is a measure of how well a child can see different levels of grey against a white background. Contrast sensitivity is important in:
- communication, such as recognising facial expressions;
- orientation and mobility, such as identifying kerbs and steps;
- accessing information, such as reading letters and numbers;
- activities of daily living, such as eating food from a plate.
There are many different ways to assess contrast sensitivity. A useful way to do so in children with cerebral palsy is by using Hiding Heidi.
Figure 6.5 Hiding Heidi Contrast Sensitivity Test
This test is also based on the idea that a child, when presented with two different patterns, will look at a pattern that he can see rather than a plain white card. A simple line diagram of Heidi’s face is presented at the same time as a white card. Gradually paler faces are shown to the child until he no longer has a preference as to which card he looks at. By sequentially presenting increasingly paler faces and observing the child’s responses, an idea of a child’s ability to detect subtle shades of grey is gained.
This is the area over which a child can see and then notice and attend to an object when holding his head and eyes still. The visual field is oval in shape. It is wider (180 degrees) than it is high (140 degrees). It extends further downwards (80 degrees) from the centre (fixation) than it does upwards (60 degrees) from fixation. This is because the brow above the eye tends to be more prominent than the cheek below.
Figure 6.6 A normal full visual field with the four quadrants named and the approximate size of each quadrant described. The large black dot represents the centre of the field or fixation.
It is possible to get an idea of the size of your own visual field by stretching your arms out and waggling your fingers at the sides while looking straight ahead. Most people can actually see a little behind themselves horizontally but will notice that their field is more restricted above by their brow than below by their cheeks. Visual acuity is poor at the edges (the periphery) of the visual field. It is much more difficult to tell how many fingers are being held up at the edges of your own visual field than in the centre. It is however very easy to notice something moving at the edges of your visual field: try waggling your fingers.
Doctors think of the visual field, and examine it, as four quarters or quadrants. This is because a different part of the brain ´sees═ each different quadrant. Different parts of the brain see different parts of the visual field from what you might think. For example: the left side of the visual field is seen by the right side of the brain, the right side of the visual field is seen by the left side of the brain, the lower part of the visual field is seen by the upper parts of the brain and the upper parts of the visual field are seen by the lower parts of the brain.
Many children with cerebral palsy may have difficulty seeing in some part of their visual field, eg: children with hemiplegia affecting the right side of their body may have difficulty seeing objects in their right visual field. This is known as visual field loss. Knowledge of the presence of visual field loss may help explain:
- Problems with mobility: eg: tripping over things on the floor or always bumping into things on only one side
- Abnormal head posture (AHP): eg: a child may tilt or turn their head to move the seeing part of their visual field into a more useful position. Children may also angle books in the opposite direction to their head tilt to make best use of their vision.
- Poor reading skills despite good visual acuity: Children with visual field loss on the right side will be forced to read into their poorly seeing field. This will make finding the next word on a line very difficult. A child with mainly left sided visual field loss will have difficulty finding the start of the next line below after finishing the end of the line above. Because of these difficulties a child with good visual acuity but visual field loss may have difficulty reading.
- Problems locating objects: Children with right sided hemiplegia consistently pick up objects in one area in front of them, eg: on their left side and ignore objects on the right side.
Knowledge of the presence of visual field loss in a child will also help a teacher decide the best place for a child to sit in the classroom.
Assessment of visual field loss
As with other aspects of visual function watching a child playing and asking parents, main carers and teachers questions will often help identify whether a child might be suffering from visual field loss or not. A head turn to the right can suggest visual field loss to the same side. If a child has a movement impairment down one side of their body then visual field loss, if it is present, is likely to also be on the same side as the movement impairment. Clearly if a child always bumps into things on one side, or consistently fails to notice toys placed to one side, then this is likely to be due to visual field loss affecting that side.
Children with spastic diplegia and quadriplegia can characteristically suffer from lower visual field loss. Children with lower field loss may not notice things near to them on table tops. They may trip unexpectedly over toys that have not been tidied away. Lower field loss can also make going down stairs difficult.
Formally trying to assess visual field function is challenging and possibly unnecessary if a good history of symptoms consistent with visual field loss is given. If there is uncertainty from the history, or a child has a condition where visual field loss is especially likely, then testing can be helpful.
There is no single definitive way to assess a child’s visual field. Familiarity and experience of the responses of lots of children with a full and normal visual field is useful when trying to assess and interpret the responses of a child with an abnormal field of vision. The task is made more challenging with children who may have physical and intellectual impairment with additional communication difficulties.
One way of assessing visual fields is by confrontation. This usually requires two examiners although a third can be helpful. Examiner 1 sits directly in front of the child and engages their attention by talking to them and showing them an object of interest. Examiner 2 quietly begins to move a target in from behind the child. Examiner 1 then removes the central target of interest and observes the child’s eye and head movements. At the same time Examiner 2 continues to slowly and progressively move the target in towards the centre of the child’s field (fixation) along a diagonal line through each of the child’s four ´quadrants═.
Figure 6.7 The set up for examining a child’s visual field to confrontation using two examiners
Examiner 1 carefully observes when the child first notices the target. The child’s responses in the four quadrants are compared. Examiner 2 tries to make sure the target is moved at an even speed along a diagonal path through the middle of each quadrant.
Figure 6.8 The direction and angle of movement of a target through each quadrant of the visual field towards fixation (the central black dot)
If a child consistently notices the target later on one side than the other then a visual field defect is likely to be present in that area. The findings are more credible if they are also consistent with information gained from the history and from other physical findings such as an impaired movement on the same side.
Fast and slow eye movements
It is very difficult to assess eye movements. It is particularly challenging in children with cerebral palsy. It can be difficult to be confident about interpreting observations, as many children may be unable to make voluntary eye movements. This challenge can be compounded by limitations in communication.
Two important types of eye movement include:
- slow smooth pursuit;
- fast saccades.
Slow smooth pursuit movements let the eye and brain steadily and constantly fixate an object moving at less than 30 degrees per second. If an object moves more quickly than this then the eye will try to maintain fixation by making fast eye movement (saccadic) jumps. Fast eye movements are also used to change attention from one object to another. For instance if something is noticed in the peripheral visual field a fast eye movement is used by the eye and brain to quickly move the object of interest into the central visual field.
By using these two types of eye movements the eye can find and then keep an object of interest within the central visual field. This is where highest visual acuity and best colour vision is found. Normal vision is therefore dependent on both kinds of eye movements working normally. Without normal eye movements even if a child has good visual acuity, full visual fields and normal contrast sensitivity he will not be able to see well and enjoy normal visual function. Importantly reading is also likely to be difficult.
Children with cerebral palsy can have difficulty making accurate voluntary movements. They can in particular often have difficulty making fast saccadic eye movements. This is most common if the child has spastic diplegia or quadriplegia or athetosis.
Assessment of eye movements
As already discussed taking a history as part of the assessment is crucial. If a child gives a history of making ´head thrusts═ or other fast unusual head movements while reading or shifting attention from widely spaced objects, then an impairment of fast eye movements may be present.
As with other methods of assessing visual function there is no single definitive way to test eye movements.
Some general rules are however worth following:
- Any target must be big enough for the child to see.
- Usually it is helpful if the target is of some interest to the child (such as an internally lit toy) and not just a plain white target.
- Try to assess with a plain uncluttered background behind the examiner.
- Try to test movements to all nine cardinal positions of gaze (Figure 6.9).
- Try to test movements from central fixation outwards and then back in again to fixation.
- Try to avoid testing eye movements across the vertical or horizontal midlines.
- Try to provide the child with the best support required to enable him to gain the best head control and/or compare results in different positions, that is, lying supported as opposed to sitting up.
Figure 6.9 The nine cardinal positions of gaze
Assessment of slow smooth pursuit eye movements
To test slow eye movements a target must move slower than 30 degrees per second. This means that any target must take more than 3 seconds to travel across one half (90 degrees) of a child’s visual field. If a target moves more quickly than this then the eye and brain will try to use fast saccadic movements to catch up with the target.
Assessment of fast saccadic eye movements
To test fast eye movements it is important to try to force a child to quickly move fixation between two widely spaced targets. Try to use two different targets that are large enough for the child to see and that are also likely to interest him. Movements can be assessed horizontally or vertically.
Optokinetic nystagmus drum
This simple device can be used as a screening tool to quickly assess fast and slow eye movements. The drum is spun so as the stripes move less than 30 degrees per second. If the child’s visual acuity is not too low then he will follow the stripes using slow smooth pursuit eye movements. When the stripes disappear off one side of the drum then fast saccadic eye movements are used to find newly appearing stripes on the other side of the drum. By carefully observing eye movements while spinning the drum a quick impression of a child’s ability to move his eyes can be developed.
Figure 6.10 Assessment of fast and slow eye movements using an Optokinetic Nystagmus (OKN) drum
It can initially be very intimidating trying to assess vision in a child with cerebral palsy. At times it can seem as though no useful information is being gained. Through persistence and practice, skills in interpreting subtle clues and characteristic patterns of behaviour can be developed. By ´segmenting═ assessments and listening carefully, an overall ´picture═ of visual function can be gained. Once information on different aspects of visual function has been determined it should all be put together into a report. This report should be written in plain language and distributed to the multi-disciplinary team. This way a child can benefit from the assessment as all professionals who care for the child are better informed.
Houliston M J, Taguri A H, Dutton G N, Hajivassiliou C, and Young D G (1999) Evidence of cognitive visual problems in children with hydrocephalus: a structured clinical history-taking strategy. Developmental Medicine & Child Neurology, 41, (298-306).