A number of additional tests can be performed in the ophthalmologist’s
office. They are generally reserved for specific indications, as
described in the following sections.
Infants and young children obviously cannot perform subjective
visual acuity testing by asking them to read an eye chart. In many
cases, the behavioral methods of assessing vision discussed in Chapter 1 are adequate. However, these methods are not quantitative, and
more precise evaluation of visual acuity is sometimes desired. This
information may be useful in determining whether an intervention
is needed (cataract surgery, for example), or to monitor improvement
in vision while a patient is being treated.
Two useful methods for quantifying visual acuity in preverbal
children are described:
- Forced preferential looking tests.
These tests are based on the normal instinct for children to look
at interesting objects. In one form of this test, drawings are placed
on one end of a card, and the opposite end is blank (Figure 2–1A–C).
When the card is held in front of the infant, their attention will
naturally turn to the picture. The examiner watches the child’s
eyes from behind the card. If the child’s eyes consistently turn
and look in the direction of the picture, one infers that the infant
can see it. The cards come in a set with gradually smaller pictures.
As the size decreases the eye eventually cannot distinguish the
figure from the background. At this point, the infant will no longer
make consistent eye movements in the direction of the picture. The
size of the smallest identified picture is used as a measure of
acuity. The eyes are tested independently (Figure 2–2).
- Spatial-sweep visual-evoked potentials
(SSVEPs). Visual acuity in nonverbal children can be assessed
in a more sophisticated manner by measuring SSVEPs. In this test,
electrodes are placed on the occipital lobe and the child sits in
the parent’s lap. The infant watches a series of bar patterns
on a monitor (Figure 2–3). When the bars are large enough
to see, a visual impulse is created and this is transmitted from
the eye to the occipital lobe, where the scalp electrodes record
the activity. The bar width gradually decreases. A threshold is
reached at which the bars cannot be distinguished from the background,
and the cortical activity stops. This endpoint can be converted
into a measure of visual acuity.
Forced preferential looking testing cards. (A) A picture
is present on either the top or the bottom of the card. One of the
cards is held in front of the infant. If the child’s eyes
turn to the picture, this indicates the child is able to see it.
(B) and (C) ...