Vision Testing and Screening in Young Children

Childhood vision development¹

Early visual development is in interesting area sitting on the border between ophthalmology and psychology with an element of mystery thrown in. Early development is largely marked by qualitative milestones but from about 12 months onwards, quantitative measures can be made, the accuracy of which increase with the child's age:

• 0-1month - baby turns to diffuse light and shows steady fixation. There is irregular horizontal pursuit and 'brightening' (the eyes widen and other movements are stopped when shown an interesting visual stimulus).
• 2-3 months - there is visually directed reaching and the baby starts to be more proficient in accommodating. By now, the most rapid visual development will have occurred and further changes are more progressive.
• 3-5 months - there is blinking to threatening stimuli and the baby starts to mimic facial expressions. Objects are examined in more detail.
• 6-12 months - the baby's vision rapidly approaches normal adult acuity and vision motivates and monitors movement towards a desired object. By a year old, near and distant acuities are good. There may be some mild hypermetropia but there is ability to focus, accommodate and the child has depth perception. They can discriminate between simple geometric forms, scribble with a crayon and are visually interested in pictures.
• By 2 years - myelinisation of the optic nerve is completed. All optical skills are smooth and well coordinated and acuity is now normal.
• By 3 years - retinal tissue is now mature.

Epidemiology²

Serious visual loss in childhood is uncommon; there is an incidence of 6 per 10,000 children in the UK each year developing severe visual impairment or becoming blind by their 16th birthday and ± 12 more becoming visually impaired (worse than 6/18). Therefore there are at least 4 newly visually impaired children each day in the UK. The prevalence of visually impaired or blind children is 20 per 10,000.

Children with milder visual loss, unilateral visual problems or eye disease without visual consequences considerably outnumber those with more serious disorders. However in the long-term, they require more of the disproportionately small resources available to them than the minority with more serious problems.

World-wide, vitamin A deficiency, trachoma, and other infections are major causes of blindness in children.

Screening for ophthalmic problems³

Screening is aimed at the primary prevention of visually impairing disease (e.g. cicatricial retinopathy of prematurity) as well as reducing the impact of already established disease (e.g. early detection and treatment of congenital cataracts). The beneficial effects of early identification of vision problems are far reaching as childhood visual impairment or loss can have a significant developmental, emotional and social impact.⁴ The majority of children with severe visual impairment (6/60 or worse) have additional sensory, motor or learning impairments ± chronic disease. More 'minor' problems such as an undiagnosed refractive error can be at the root of a cascade of events defining a child's future e.g. poor performance at school may contribute to the child's self confidence and their career.

Conditions that are screened for broadly fall into one of four (overlapping) categories:

• Neonatal conditions e.g. retinopathy of prematurity, retinoblastoma, cataracts - some of these conditions (e.g. retinoblastoma) need treatment irrespective of visual outcome whereas others need treatment primarily because of the visual outcome e.g. cataract.
• Risk factors for amblyopia e.g. cataract, retinoblastoma - identification of amblyopia in the pre-school age is associated with a good treatment prospects but beyond seven or eight years old, a decrease in cortical plasticity limits the outcome.
• Refractive errors - anisometropia (difference in refractive error between the two eyes) is the commonest cause of non-strabismic amblyopia and usually develops between two and four years old.
• Colour vision abnormalities - these are often not picked up until later but they are important in that there is evidence to suggest that this affects learning and it excludes individuals from certain jobs (e.g. electrician, train driver).

For more information the more common conditions screened for, please see our records on amblyopia, squint problems, refraction and refractive errors, colour vision and its disorders.

Overview of the screening process

Preschool screening is well established but the role of vision screening after school entry is controversial. Some areas offer a routine test of visual acuity and colour vision in older children and teenagers. There is debate whether these are worthwhile screening tests, and whether they should be included in the core programme of health surveillance for all children. However:

• Always take parental concerns about vision seriously and if in doubt refer to appropriate service (orthoptist, optometrist or ophthalmologist).
• A visual assessment should be included for any child being assessed for educational under-achievement or learning disability including dyslexia. If they cannot see the blackboard or the book they cannot learn. This may even present as a behaviour disorder.
• Remember, eye checks by optometrists are free to children under 16 (under 19 if in full time education). Encourage parents of older children and teenagers to use this service, particularly if there is a family history or myopia.

This should include:²

• The red reflex: use an ophthalmoscope about 30cm from the infant's eyes. Dark spots in the red reflex can be due to cataracts, corneal abnormalities, or opacities in the vitreous. The red reflex may be absent with a dense cataract.
• Corneal light reflex to detect squint. Hold a penlight at arms length in front of child. When the child looks at the light, normally the light reflex is symmetrical and slightly nasal to the middle of each pupil.
• General inspection of the eyes may suggest other conditions. For example, one eye larger than the other may indicate glaucoma.
• Also at the 6-8 week examination, ask parents if they have any concerns about their child's vision.

A specialist examination is indicated in babies who:

• Have an abnormality detected in the above routine examinations.
• Have a known higher risk of visual disorders e.g. low birthweight babies at risk of retinopathy of prematurity, babies who have a close relative with an inheritable eye disorder like albinism or Usher's syndrome, babies with known hearing impairment. Congenital rubella syndrome is currently rare but may return with a falling uptake of the MMR vaccine.

Currently, many areas carry out a vision check on 4-5 year olds prior to school entry or as part of the school entry programme, the main aim being to detect amblyopia. This check is carried out by the school nurse or by an orthoptist, depending on local policy. Most 4 year olds can co-operate with a test of visual acuity; there are a number of different assessment methods appropriate for young children varying from picture or shape tests to matching tests and for those children able to, the Snellen chart. This said, there is increasing scepticism on the value of this test with systematic reviews showing little evidence to support its effectiveness.^5,6

There are not currently any national guidelines set out for the screening of these children. The main issue lies in detecting refractive errors. Unnoticed, these can cause a number of problems ranging from the specific problems (e.g. chronic headaches) to under achievement academically. This itself has a very broad impact on the child's future. We are still waiting for appropriate studies to assess the efficacy and impact of routine vision screening in this age group.

This problem is defined as being an inability to read at an expected level despite intelligence being within the normal range. Reading difficulties can affect up to 10% of the school age population and there may be an excess in boys. However this figure may be biased as boys with a reading problem are more likely to be reported than girls with a reading problem. Reading difficulties an be part of a wider spectrum of problems and may be associated with refractive errors or ocular motility disorders or, as in the majority of children, due to problems unrelated to the eye. Children with dyslexia tend to have an associated writing difficulty.

A manifest squint is when both eyes are not directed at the same fixation point. To prevent seeing double, the image from the 'squinting eye' is suppressed by the brain. Without treatment this can lead to amblyopia. About 4% of four year olds have a manifest squint. Risk factors include: first degree relative with a squint; preterm delivery; other neurodevelopmental disorders.

Most manifest squints are noticed by parents as an eye that 'turns' or is 'lazy' although some parents incorrectly think that a squint is normal in infants. Prominent epicanthic folds may mimic a squint (pseudosquint) but a manifest squint can be confirmed by:

• Checking for symmetry of eye movements in horizontal, lateral and oblique planes. Move a small object of interest for the child to follow with their eyes.
• Test of corneal light reflex.
• Cover test:
  • Attract the child's attention with a small object such as a finger puppet (not a penlight) about 30-50 cm in front of the child at eye level.
  • When their gaze is fixed on the object, cover each eye in turn.
  • Observe movement of the uncovered eye.
  • In normal eyes, the eye not covered should not move when the other eye is covered.
  • In a child with a manifest squint, as the 'good' eye is covered you can see the 'bad' eye move to take up the fixation on the object. When the 'bad' eye is covered, there is no movement of the 'good' eye which remains fixed on the object.

Refer to an orthoptist if you suspect a squint; this can be confirmed by a prism test.

Treatment aims to prevent, or minimise the severity of, amblyopia. Usually it is with occlusion of the good eye to force the squinting eye to fixate. If the cause of the squint is a refractive error, correction with glasses may be all that is required. Surgery may be needed to help with alignment and for cosmetic reasons.

In an eye with a latent squint, the deviation of the eye only occurs when it is covered and not used for seeing. When both eyes are fixed on an object, when an eye with a latent squint is covered it moves medially or laterally even though the other eye left uncovered remains fixed on the object. The relevance of a latent squint is not clear.