The Development and Assessment of Vision

The normal visual development occurs from birth up to the age of 10-12 years, and requires a
normal visual system. Research during the last decades has emphasized the importance of
adequate visual stimulation during the first months and years of life. It is therefore of great
importance to identify children with subnormal vision early in life to start treatment and
habilitation. A prerequisite is that reliable testing methods are available and can reveal visual
defects as early as possible. The visual acuity testing methods in children can be divided into
3 subgroups according to the types of stimulus used; detection, resolution and recognition
acuity. Different acuity values are usually obtained with the different tests. Contrast
sensitivity measurements can reveal changes and loss of visual function that is not detected
when testing visual acuity. Sweep-VEP could become a valuable test to assess vision in
young children in a clinical setting.
Normal visual development
What can a newborn child see and how does the vision develop? A newborn child can hear
quite well, but the vision is subnormal and has not developed much. A newborn baby can
fixate a light, and from birth up to 4 weeks the baby starts to fixate and follow objects
presented closer than 1 meter. At about 2-3 months the child can fixate and follow an object at
2-3 meters. Eye contact is usually said to be developed at 4-7 weeks, and at 8-12 weeks the
child starts to play with its own fingers in front of the eyes.
1 6
The visual acuity, recorded as Snellen decimal acuity, is approximately 0.01-0.05 in a
newborn child, 0.1 at 6-12 months, 0.25 at 2 years, 0.5-0.65 at 3 years, 0.8 at 4 years and 1.0
at 6 years. Not until the age of 10 -12 years the visual acuity is fully developed,
approximately 1.3-1.6. However, the acuity values depend to a great extent on the methods
used in the examination and this issue will be discussed in more detail later in this chapter.
Development of the visual pathways
The subnormal vision in young children is due to the immaturity of the eye and the whole
visual system-the retina, optic nerve, chiasma, lateral geniculate body, optical radiation, and
visual cortex. At birth the axial length of the human eye is about 17 mm (or 3/4 of adult size),
and by 13 years of age the eye has reached adult size, and an axial length of 23 mm. The
fovea is immature at birth. In the fetal eye cup it appears as an elevation formed by the
ganglion cells. The ganglion cells migrate peripherally during the first 25 weeks of fetal life to
form the foveal pit. The retina and especially the fovea will reach full maturity at about 4
years of age. The main change is in the density and spread of cones in the retina during this
period. The myelination of the optic nerves begins in the lateral geniculate nucleus, reaches
the orbital part of the optic nerve at full term, and continues towards the retina over the
following 2 years. The maturity of the lateral geniculate body and the visual cortex also take
place during the first 4 years of life. Studies of vision in monkeys, with a visual system very
similar to that of the human, have shown that the visual acuity during the first years is much
lower than what could be expected from the structure of the retinal and the neuronal elements.
However, at higher ages it correlates quite well to morphological parameters. The
development of the still higher visual acuity in adulthood is mainly due to the maturity of the
higher visual systems in the brain beyond the visual cortex.
The sensitive period
The plastic period (also called the sensitive period or the critical period) of the visual
development is the time during which the visual acuity is developing and can be modified.
The Nobel laureates of 1981, Professor Torsten Wiesel (honorary president of the Sigvard and
Marianne Bernadotte Research Foundation for Children Eye Care) and professor David
Hubel, already in 1963 established the norms for the sensitive period in cats and monkeys.
The period for cats is up to 16 weeks and in monkeys up to 30 weeks. In the human the
1 7
sensitive period proceeds up to 8-10 years of age with the most critical part during the first 2-
3 months.
Abnormal visual development
The normal visual development requires a normal visual system. Different eye diseases such
as congenital cataract, strabismus, refractive errors (especially astigmatism and high
hyperopia), ptosis, lid hemiangioma etc. will prevent clear vision and retard visual
development. Amblyopia (amblyos= lazy, opia= vision) is a common visual sequelae to such
eye disorders. It is defined as a unilateral or bilateral decrease of visual acuity caused by
pattern vision deprivation or abnormal binocular interaction, where no signs of organic
disease can be detected by physical examination of the eye and which in appropriate cases is
reversible be therapeutic measures (see also chapter VI).
Amblyopia can be successfully treated up to approximately the age of 8 years, provided the
obstacle to normal form vision is removed, correct optical correction is provided, and
occlusion of the good eye is introduced to train the amblyopic eye. An alternative treatment to
occlusion is to use cycloplegic eye drops usually atropine in the good eye, a method that has
attracted renewed popularity in the United States, despite the disadvantages with the different
side effects. A contact lens to decrease the vision in the good eye can be used if other methods
fail. Improved vision in amblyopia with pharmacological treatment such as levodopa and
citicoline has also been reported (Leguire et al. 1993, Campos & Fresina 2006).
In congenital cataract the opaque crystalline lens should be surgically removed before the age
of 2 -3 months to reach good visual results (see also Chapter VI).
Methods to assess visual acuity
Visual acuity testing in young children in a clinical setting can be divided into three subtypes
according to the type of stimulus used. In detection acuity the stimulus, usually white balls in
various sizes, should be detected or distinguished from the black background. In resolution
acuity, the stimulus pattern, a black and white grating, should be resolved. In recognition
acuity the stimulus, a letter or symbol should be recognized by the subject and identified by
matching or naming.
1 8
The psychologist Fantz already in the 60’s reported that infants will prefer to look at a
patterned stimulus when presented in a homogeneous field of vision. This behavioural method
was called the preferential looking (PL) test. It was further developed by Teller and coworkers
(1986) and brought into clinical practice. The test patterns are black and white gratings of
variable spatial frequencies presented on printed cards, called the Teller Acuity Cards.
We first performed a study in adults to evaluate the method in patients with strabismic
amblyopia. It was shown that grating acuity (resolution acuity) overestimates the acuity
values determined with recognition tests in patients with strabismic amblyopia (Rydberg
1997).
To see how the correlation between the methods were in children, the next study was carried
out in children with manifest strabismus, visual impairment due to organic eye diseases and
children with normal vision. An overestimation of visual acuity was also seen in these groups
of children, when grating acuity was compared with recognition acuity (Fig. 1). Hence,
similar results were obtained in the children and adults.
Figure 1. PL grating acuities plotted against the HVOT linear test in children with normal
vision (circles), visual impairment (triangles) and strabismic amblyopia (squares). Each data
point represents one subject, except for a few representing two subjects. The dashed line
represents the line of equality between the tests. Logarithmic scales are used on both axes
1 9
To see whether the PL test could be used as a screening test for amblyopia, the grating acuity
in the amblyopic eyes and the non-amblyopic eyes were compared. In 30% of the subjects
resolution acuity determined with PL was the same in both eyes, and the amblyopia would
consequently have been missed if the PL test had been used in visual assessment (Fig. 2).
Figure 2. PL-grating acuities in the amblyopic eyes, plotted against PL acuities in the nonamblyopic
eyes, in the adult patients with strabismic amblyopia (n=28). Each point represents
one patient. The dashed line represents the line of equality between the tests and the solid line
marks the limit for one octave difference between PL-acuity in normal and amblyopic eyes.
Linear scales are used on both axes
There are some Swedish studies on the development of recognition visual acuity using tests
with different kinds of optotypes. Lithander (1997) found a mean visual acuity of 0.48, with
the O-test (single symbols for near), in children 24-29 months old. With the KOLT test for
linear acuity, a visual acuity level of 0.55 was found in children aged 30-35 months, 0.67 in
children of 36-41 months and 0.77 in children 42-48 months old. Rydberg et al. (1998) found
a visual acuity of 0.67 with the LH-line test and 0.6 with the HVOT-test in children 29-41
months old and 0.99 with both tests at 48-83 months (median age 51 months). In a study by
Larsson et al. (2005), 81.6% of the control subjects had a recognition visual acuity of 1.33 or
more at the age of 10 years.
2 0
The crowding phenomenon
Crowding also called “separation difficulty” is the inability to discriminate letters and
symbols that are presented closely together, as optotypes in a line. Visual acuity determined
with single optotypes is usually much better than line acuity. Crowding phenomenon is
present to a small extent in normal eyes of young children. and is common in strabismic
amblyopia and also in children with cerebral visual impairment (Pike et al. 1994, Jacobson et
al. 1996).
Different tests assess different types of visual acuity
Different acuity values are often obtained in the same child with the different tests.
For example a child with visual impairment can sometimes detect a small pearl of 0.2 cm at a
distance of 3 meters. The grating acuity (resolution acuity) can be as high as 15 cycles per
degree corresponding to a Snellen visual acuity of 0.5. The recognition acuity with single
letters could be 0.3, whereas the recognition acuity with linear optotypes only is 0.1, due to
crowding. Detection and grating acuity do not correspond to Snellen acuity, and therefore
grating acuity should always be recorded in cycles per degree, and in measuring detection
acuity the size of the object and the distance used should always be recorded. The
discrepancies between the tests have to be kept in mind so that the acuity values are not
misinterpreted.
It is important to have reliable methods to detect subnormal vision at an early age to be able to
start the treatment and habilitation. However, there are still no tests available to get reliable
values when assessing vision in young children. It is not until the age of 3 ½ to 4 when
children can be tested with optotypes (symbols and letters) in a line that reliable responses can
be obtained (Rydberg et al. 1999).
Contrast sensitivity testing
Visual acuity represents only one aspect of visual capacity. Acuity is estimated as the smallest
stimulus that can be detected at a high contrast level. However, most objects in everyday
visual environment are larger than objects used in visual acuity testing and usually appear at a
lower contrast. Contrast is defined as the difference in luminance between adjacent areas in a
stimulus pattern. In determining the Contrast Sensitivity Function (CSF) gratings, symbols or
2 1
letters can be used at different spatial frequencies or sizes and at different contrast levels. A
bell shaped CSF curve is obtained with a peak of maximum sensitivity at 3-5 cycles per
degree (Fig 3). The cut-off spatial frequency is the highest spatial frequency that can be
resolved at maximum contrast and also represents the visual acuity value. This value is
normally 30-60 cycles per degree. There are many different tests to assess contrast sensitivity.
Some tests include patterns of different contrast values but only at one spatial frequency.
Other tests use patterns at only one contrast level but with many spatial frequencies (Rydberg
et al.1997). For a description of the whole CSF, different spatial frequencies at different
contrast sensitivity levels should be tested. The normal contrast sensitivity function in young
children is lower than in adults and the function increases with age, as shown in figure 3.
Contrast sensitivity measurements may reveal changes and loss of visual function that is not
detected when testing visual acuity.
Figure 3. CSF for normal adult subjects (thick solid line) and in comparison with CSF for
children at different ages (dashed lines) and older subjects (thin solid lines). (From
Abrahamsson & Sjöstrand 1992)
2 2
Sweep- VEP (Visual Evoked Potentials)
An electrophysiological method was introduced for objective assessment of vision in children
by Norcia & Tyler (1985). In this so called sweep-VEP method, the test patterns are
sinusoidal gratings and the visual evoked potentials (VEP) to the pattern stimulation are
recorded with surface electrodes placed on the occipital part of the scalp. The spatial
frequency of the grating changes gradually in a sweep from 32 to 2 cycles per degree. The
pattern is presented in an on-off or a pattern reversal mode, with such a high temporal
frequency that a “steady state” response is obtained. The VEP amplitude is plotted against
spatial frequency of the stimulus, and the visual acuity value can be extrapolated from the
curve.
Sweep-VEP could be a suitable method to assess vision in young children. We have tested
fifty normal subjects age 6 months-50 years (Rydberg et al. 2008). Sweep-VEP gave mostly
reliable results in adults. Good test-retest reliability was obtained in 5 adult patients who were
tested twice. In the younger children the results were more variable and sometimes difficult to
interpret. One possible explanation is poor fixation of the stimulus pattern. If fixation could be
controlled, sweep-VEP could become a valuable test to assess vision in young children in a
clinical setting. More research will be performed in this area in the future.
Conclusions
Reliable visual acuity measurements in children can be obtained at about the age of 3 ½-4
years, when visual acuity can be assessed with a recognition test using letters or symbols in a
line. Detection and resolution acuities overestimates the acuity values and different acuity
values are usually obtained with the different tests. The existing methods for clinical testing
of children under the age of 2 years are limited. Sweep-VEP could become a suitable test to
assess vision in young children.
References
Abrahamson M & Sjöstrand J (1992): Kontrastkänslighet och bländning. Nordisk medicin.
107:316-318.
Campos EC & Fresina M (2006): Medical treatment of amblyopia: present state and
perspectives. Strabismus, 14:71-73.
2 3
Jacobson L, Ek U, Fernell E, Flodmark O & Broberger U (1996): Visual impairment in
preterm children wih perivenricular leukomalacia-Visual cognitive and neuro-pediatric
characteristics related to cerebral imaging. Developmental Medicine and Child Neurology,
38:724-735.
Larsson E, Rydberg A & Holmström G (2005): A population-based study of the visual
outcome in 10-year-old preterm and full-term children. Archives of Ophthalmology, 123:825-
32.
Leguire LE, Walson PD, Rogers GL, Bremer DL & Mc Gregor ML (1993): Longitudinal
study of levodopa/carbidopa for childhood amblyopia. Journal of Pediatric Ophthamology
and Strabismus 30:354-60.
Lithander J (1997): Visual development in healthy eyes from 2 months to four years of age.
Acta Ophthalmologica Scandinavica, 75:275-276.
Norcia AM & Tyler CW (1985): Spatial frequency sweep VEP: visual acuity during the first
year of life. Vision Research, 25:1399-1408.
Pike MG, Holmström G, de Vris LS, Pennock LM, Drew KJ, Sonksen PM & Dubowitz LMS
(1994): Patterns of visual impairment associated with lesions of the preterm infant brain.
Developmental Medicine and Child Neurology, 36:849-862.
Rydberg A (1997): Assessment of visual acuity in adult patients with strabismic amblyopia: A
comparison between preferential looking and different acuity charts. Acta Ophthalmologica
Scandinavica, 75:611-617.
Rydberg A, Han Y & Lennerstrand G (1997): A comparison between different contrast
sensitivity tests for the detection of amblyopia. Strabismus, 5:167-184.
Rydberg A & Ericson B (1998): Evaluation of methods for assessing visual function in
children 1 1/2 years of age with normal and subnormal vision. Journal of Pediatric
Ophthalmology and Strabismus, 35:312-319.
Rydberg A, Ericson B, Lennerstrand G, Jacobson L, Lindstedt E (1999). Assessment of visual
acuity in children aged 1 1/2 – 6 years, with normal vision, visual impairment and strabismus.
Strabismus, 7 (1): 1-24.
2 4
Rydberg A, Bengtsson M & Andersson T (2008): Sweep-VEP, a method to assess vision in
children. 32nd Meeting of the European Strabismological Assoociation, Abstract P12:55.
Teller DY, Mc Donald MA, Preston K, Sebris SL & Dobson V (1986): Assessment of visual
acuity in infants and children. The acuity card procedure. Developmental Medicine and Child
Neurology, 28:779-789.