Chapter 2

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.

###### FIGURE 2–1

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. ...

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