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.

If children are old enough to cooperate, computer-based visual field testing can be performed (automated perimetry). The patient’s head is positioned so that he looks into a large bowl-shaped machine, and he is asked to look straight ahead. The computer then generates a series of brief light flashes in the peripheral visual field. The patient pushes a button when he notices the light (Figure 2–4A and B). The computer tracks the responses and gradually dims the lights in each portion of the field until the patient can no longer see them, and these thresholds are recorded.

This type of testing requires a fair amount of cooperation and concentration. The examiner must monitor the patient’s fixation to be sure he is staring straight ahead, because the natural inclination for most people taking the test is to move the eyes toward the light targets. The computer randomly checks for false-negative and false-positive responses. False negatives are recorded when the patient fails to respond to bright light in the center of fixation. False positives occur when the patient indicates that he sees a dim light that is intentionally placed in the blind spot.

When the test is completed, the computer generates a printout of the results (Figure 2–5A and B). In addition to the graphical representation, a great deal of other statistical information regarding the patient’s performance is recorded, including whether the test appears to be abnormal and what specific defects are present. This information is stored, and can be statistically compared on future tests to monitor for changes.

For younger patients, visual fields may be measured manually using a Goldmann perimeter. The patient is seated in front of a white bowl, similar to that used for automated perimetry. The examiner monitors the patient’s fixation and projects light of various intensities and sizes in the peripheral visual field. The lights is slowly moved centrally until the patient indicates that they see it. Manual perimetry is less precise than automated perimetry, but it is easier for many younger patients to perform (Figure 2–6).