S.A. Patney: InteRyc Volume 4, Oct., Nov. & Dec. 1999 (P.8-11)

STRABISMUS SUMMARY SERIES

NOTE: Only the basic points are covered in this series but they are useful to the general/pediatric ophthalmologist and the aspiring strabismologist. So far I have not received any questions and that means one of the two things, either everything is crystal clear and written beautifully which I doubt. And second possibility is that there isn't enough interest in the subject. Please let me have the benefit of your feedback.

Visual acuity tests for infants continued:

(The examination of visual acuity is continued from Strabismus Summary - part 1. The first of the tests in infants (preverbal children), the fixation preference test has already been discussed. Now we go on to the other tests. As already mentioned in volume 3 of the InteRyc 1999, only the fixation preference test is widely used clinically. Other tests are mostly used for research or in better-equipped ocular motility clinics. However, the basics should be known to every ophthalmologist)

(1) The fixation preference test-variations

(The test was described in Strabismus summary part 1). It is only reliable if the deviation is more than 10 prism diopters. If it is less, the following tests can be useful.
(1)-a. 10 PD vertical prism test can help by creating a vertical diplopia. If there is equal visual acuity, equal preference for fixation with either eye will be observed.

(1)-b. 25 PD horizontal prism test can also be performed if the deviation is small or absent. The eye behind the 25 PD base out prism turns in and its visual acuity is decreased in that position. Result is a preference for fixation with the other eye. The prism is then switched over to the other eye. If the eyes can take up the fixation alternately the visual acuity is approximately equal.

(2) Grating acuity test

These tests include assessment of visual acuity using Teller acuity cards and the forced preferential looking (FPL) technique. The former can be used as a routine office/clinic procedure. These cards have parallel black and white stripes of various thicknesses (spatial frequencies). They are used at 38-cm fixation distance. Resolution acuity is measured with these cards in cycles per degree. The various cards with broader to narrower stripes are shown to the infant who sits in the parent's lap and its head movements are observed. The card with the narrowest stripes, which attracts the baby's attention, is made note of. Charts with Snellen acuity (recognition acuity) equivalents are available.
The visual acuity as estimated by this method varies from 6/240 at birth, to 6/60 at 3 months of age according to various studies. The visual acuity reaches 6/6 at 3 years of age*.
The FPL technique depends on the principle that an infant prefers to look at a pattern stimulus rather than at a homogenous one (object/surface) as long as the patterned stimulus is above the threshold of the visual acuity of the infant (figure 1).

Suitable age for these tests is upto four months of age as older infants are more liable to be distracted by the surroundings.

Advantage of these techniques:

These techniques are useful for keeping track of visual development and monitoring of the visual acuity of the occluded eye during treatment of amblyopia.

(3) Optokinetic nystagmus (OKN)

OKN has been used in research for many years for estimating the visual acuity of infants. This is yet another objective method.
Nystagmus is produced by showing the infant moving black and white stripes. The narrowest stripe that still elicits nystagmus is the measure of visual acuity. A handy version of the equipment is available in the form of a hand-held drum with vertical black and white stripes (figure 2). The drum is rotated to induce nystagmus.
The visual acuity by this method is stated to be 6/120 at birth according to most studies. It increases pretty quickly during the first few months after birth.

(4) Visually Evoked Potentials (VEP)

Visual acuity in infants is also estimated using different types of stimuli, cortical potentials and recording methods. The stimuli used more often are square wave gratings or checkerboard patterns. The visual acuity by this method has been found to be approximately 6/120 at 1-month age and almost normal adult acuity at 6 months of age according to various studies.
This technique is now proved to give fairly accurate readings of visual acuity but it needs expensive equipment and trained technician.

Figure 2, shows the hand-held model of the Optokinetic drum. It is held at 1 foot from the infant and rotated. The visual acuity is indicated by the smallest stripe that can produce nystagmus (by this method 20/200 in a 5 ½ months old).

Comparison of the above tests: Summing up:

· The fixation preference test is by far the most commonly used test as it is simple and inexpensive, not requiring any equipment. Moreover, it gives fairly accurate results if carried out properly.
· Grating acuity methods require a co-operative child and a well-trained and expert observer. Moreover, grating acuity is always better than recognition acuity (as in Snellen test), particularly in cases of amblyopia. The Snellen equivalents are only approximate.

(5) Visually Evoked Responses (VER)

The pattern visual evoked response (VER) has been used for estimating the visual acuity in infants. Its use is an important electrophysiological technique for this purpose. The VER is a gross response that reflects the visual input from the photoreceptors in the retina to the occipital cortex, foveal fibres contributing the most.
Most of the studies on this subject indicate that that visual acuity in an infant develops rapidly so that it reaches the 6/6 (20/20) mark in a 6 months old infant.
A comparison of the VER of the two eyes can help in the early diagnosis of amblyopia. It can also help in monitoring the occlusion therapy.

NOTE: All these tests give only approximate results. They are not as accurate as methods using recognition acuity, e.g., Snellen acuity.