Identification of Amblyopia

Identification of Amblyopia

 

          The diagnosis of amblyopia is not straightforward

because of the difficulties in testing vision

in small children and uncertainties about the

contribution made to any visual abnormality by

refractive error and ocular pathology.

Visual acuity measurements in infants and

children are confounded by the normal process

of visual development, including emmetropisation

of physiological refractive errors, the inability

of the child to report accurately what he

or she sees, and inattention.

           The normal adult human visual system is

capable of resolving targets of one minute of

arc. This is the basis of Snellen, and to an extent,

LogMAR, visual acuity tests. Other visual function

tests such as contrast sensitivity and

vernier acuity are not currently used for the

clinical diagnosis of amblyopia.

Like all biological parameters, there is a

range of normal functioning in the population

[16, 18, 31], which is affected by the frequency of

minor refractive errors. This range has been

difficult to determine, partly because of the imperviousness

of the commonly used Snellenbased

tests to statistical analysis. Nevertheless,

the range of normal acuity seems to be tight in

visually normal adults.

            The resolving power of the infant visual system

is not known precisely. It seems likely from

studies of retinal anatomy in infants, and also

from consideration of the developing infant

brain, that resolving power in infancy is less

than in adulthood. Behavioural studies in infants

using visual evoked potential measurements

and preferential looking show a normal

range which is wider and lower in younger children.

 

            These tests are limited by the problems of

reporting and attention, and may underestimate

true acuity.From around a developmental age of 3 years,

children are able to match letter optotypes.

Using Snellen based tests, it has been widely

assumed that if acuity is not 6/6 using these

measures then the resolving power of the system

is abnormal. However, the use of LogMAR

tests has enabled normal ranges to be calculated.

The mean visual acuity using these tests in

4-year-olds is around 0.1 LogMAR, with a normal

range, as measured by 2 standard deviations

from the mean, extending from 0.0 to 0.2

.  In other words, the visual system is not fully

developed at age 3 years. Further evidence for

lack of maturation is the crowding phenomenon.

Crowding was first described in relation to

amblyopia and is defined as the inability to

resolve an optotype which is surrounded by

other optotypes or bars, when that optotype is

capable of being resolved when presented in

isolation. In 3- to 5-year-olds, ranges of normal

visual acuity are lower on crowded than on uncrowded

tests.

            Crowding is also seen in cases of cerebral

visual impairment,whether due to neonatal encephalopathy,

meningitis or other causes. Here

it is described as the inability to pick single

objects out of complex scenes, when the single

object can be recognised in isolation. So crowding

seems to be a normal feature of the developing

visual system, which persists in amblyopia

and cerebral visual impairment

 

 

 

 

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