Prematurity and Vision
Friday 3 March 2006
Visual Problems in Preterm Children
Dr Mary MacRae
Specialist Registrar
Princess Alexandra Eye Pavilion
Measurement of Vision
What do we look for in the Clinic?
- Visual Acuity
- Ocular Alignment
- Refraction
- Ocular Examination
- (Visual Field)
Visual Acuity
What do we mean by visual acuity?
- Measurement of ability to read black text on a white (sometimes illuminated) background at a defined distance.
- Usually use Snellen chart at 6m in older children and adults.
- In younger children use matching cards.
- In very young use preferential looking tests.
Distance Acuity
Result expressed as a fraction
| Snellen acuity = | distance from patient to chart |
| distance at which a normal eye can read a given line |
Healthy individuals 6/6
Driving standard between 6/9 and 6/12
Near Vision
Measured by asking child to read text of decreasing sizes from a standardised chart and given an "n" value
Text in school books
- 5 yrs n=36
- 8 years n=12
Ocular Alignment
- Squint (strabismus) more common in preterm children
- May see eye turn out (exotropia) or in (esotropia)
- May be clue to other problems with visual function (Periventricular Leucomalacia)
Refractive Error
"Test for Glasses"
- Myopia – point of focus too close; may see better for near than distance, often seen after treatment for ROP.
- Hypermetropia – point of focus too far away; greater focusing effort required for near than distance.
- Astigmatism - eye "rugby-ball" shaped.
Preterm children have increased incidence of any of above
Refractive error may result in amblyopia (lazy eye) if uncorrected and is often associated with squint.
Ocular Examination
Look at fundus through dilated pupils for abnormalities of
- Optic nerve
- Macula
- Periphery
Photos
Visual Field
Peripheral or "side vision"
3D space in shape of a half sphere
Measurement of Visual Field
In older children and adults field analyser used.
Child asked to fix on a central target whilst a light approaches centre from periphery.
Buzzer pressed when light appreciated in field of vision.
Usually plotted as 2D representation on a flat piece of paper
In younger children toys brought into field of vision from behind.
Examiner notes when eyes move towards object.
Average visual fields
- one eye open: 1500 horizontal, 1200 vertical
- both eyes open: 1800 horizontal
- Binocular (ie depth vision)1200
Retinopathy of Prematurity (ROP)
- Blinding condition when severe disease not treated.
- Abnormal new blood vessels grow in eye and cause retinal detachment.
- At risk infants screened
born at 31 weeks gestation or earlier
weighing less than or equal to 1500g
Visual Acuity in Prematurity
- Premature children without ROP generally have normal or only slightly reduced visual acuity.
- Risk of moderate visual loss increased with ROP, whether treated or untreated.
- Severe visual loss and blindness rarely seen now since introduction of treatment for ROP (cryotherapy then laser).
Visual Field in ROP
- ROP and its treatment with cryotherapy (freezing) reduces visual field.
- Child often unaware of this deficit.
- Reports of extent of field loss vary.
Cerebral Visual Impairmenrt
- Insult to the brain between 24-34 weeks of gestation may result in periventricular leucomalacia.
- Loss of white matter in posterior, superior brain adjacent to lateral ventricles.
- Associated with periventricular haemorrhage.
- Can result in spectrum of disability from mild learning disability to severe physical and mental handicap.
- Visual problems can also vary from mild (when they may go undiagnosed) to severe.
Interruption of normal flow of information between visual cortex where "picture" is initially gathered to areas of the brain which are required for filtering and processing of this information.
Dorsal Stream
- Occipital cortex to posterior parietal and frontal lobes.
- Filtering out information from visual scene, giving selective attention to specific items.
- Planning and executing movement within that scene.
Ventral Stream
- Visual cortex to medial temporal lobe.
- Responsible for visual memory – ability to recognise familiar faces, recall routes.
- Difficulty with movement through space due to impaired depth perception especially in lower visual field.
- Diffuculty with locating toys on patterned carpet or in crowded toybox.
Crowding
"Crowding" seen in premature children where they are able to read words or letters when they are spread out but not when they are close together.
As age of a child increases, educational text becomes smaller and crowding can become a problem. Poor visual recognition in crowd of faces, eg; mothers at school pick-up time.
Summary
Health professionals need to recognise visual problems through careful history taking and examination.
Identification of difficulties experienced by children allows parents and teachers to introduce strategies for coping with such problems.