Checking the visual acuity

Visual acuity is measured quantitatively by checking the smallest object that the examinee can distinguish at a given distance. For children who know the alphabet the objects are usually letters or numbers displayed in an eye chart, or alternatively drawings or paintings can be used.

Each eye is examined separately. The binocular (with both eyes open) measurement is not reliable and may give "normal" results,
when in fact one of the two eyes may have some sort of problem.
Special attention should be paid especially when measuring visual acuity in children, so when one eye is examined, the child wont peek through the other.

Since the measurement of visual acuity requires the cooperation of the child, it is possible that children that are not cooperative or are abstract, but have a normal vision to show worse outcomes from reality.

Measuring the visual acuity by using optotypes
Optotype tables consist of rows of symbols or letters that gradually get smaller as we go down. The background is usually white and the letters or symbols are black.

Many different tables have been proposed and used in recent years for the measurement of visual acuity as well as different ways for its recording.

Snellen’s table is still the most widely used despite its drawbacks. The traditional table contains 11 rows of Roman characters (C, D, E, F, L, N, O, P, T, Z), but unfortunately are not all in common with Greek alphabet, which makes its use problematic for Greeks.

 Alternative tables include:
The Rotating E’s often referred to children as "forks" (with the question "towards where are the forks facing, up, down, left or right? ") and
which is not only suitable for small children and illiterate, but have the additional advantage that they show diversity at the level
of difficulty that characterizes the letters, as in Snellen’s table (wherein T can be distinguished more easily than e.g. a D which resembles to an O).

The drawings Lea or Allen, which can be used in younger children and include drawings of familiar objects, instead of letters.
The results of measurement in visual acuity are expressed as a fraction having the distance from the test optotypes as the numerator and as
the denominator the distance from which a normal eye would recognize the same symbols.

So an eye that recognizes the symbols of the last row of the optotype at a distance of 6 meters, which is the normal distance, its visual
acuity is 6/6. If it can only see the larger letters normally read from eg 12 meters then the visual acuity is 6/12. In Greece and in
other European countries has prevailed to transforming the fraction into tenths has been prevailed, so we have a 10/10 vision (instead of 6/6), 5/10 vision (instead of 6/12), 1/10 vision (instead on 6/60) and so on.

Technology evolution has transformed traditional eye charts to electronic tables with latest achievement a special software that can be installed and used on any computer (fixed or laptop), without additional hardware and with minimal cost for use even in a pathological or pediatric clinic.

Measuring the visual acuity without using optotypes With children that are too young to measure their visual acuity with tables of letters or symbols we are forced to use other techniques, to estimate even roughly, their vision.

Initially we draw the attention of the child with a small object, such as the lens-pen or a small toy.

Next we move the object and see how well the child's eyes follows it, gradually increasing the distance.
Again, it is important that each eye is examined separately. If one eye has poorer vision and we cover the other one with the better vision, we may notice that the child is not interested anymore to follow the gaze of the object.
The problem with this technique is that it is not quantitative and gives no numerical results. So we cannot compare it to normal rates nor can we follow the course of the visual acuity of the child from visit to visit. However, it is a good way to detect vision problems in time, because if the diagnosis is delayed, there is a difficulty in treatment.

There are at least three techniques that can measure and quantify the visual acuity in children who are too young for us to use the optotype. And in these three techniques a pattern is presented (an even repeating pattern), e.g. white and black stripes or squares, from which we monitor the child's reaction. Then, the same pattern is presented in a smaller size, and this goes on until the child stops reacting. The size of the smallest pattern that causes the child to react shows its visual acuity.

The first method is based visual-moving nystagmus (a normal involuntary reflex) and is rarely used in clinical practice.

The second method uses Teller’s visual acuity cards and is based on the fact that children prefer to look at motifs with rows than blank regions.

These cards are presented to the child in such a way that the lines are on the one side (left or right) and the empty area on the other.

The examiner sees if the child's head is focused on theside of the card that the lines are. Next, he tries the next card which has thinner lines (and sometimes changes the position of the pattern left or right) and continues until the child stops turning his gaze to the right side, an indication that these lines are too thin to distinguish.

The third method is the measurement of evoked potentials and involves using electrodes placed on the child's head, who is only required to look at patterns on a screen.