Optonet Vision Unit Documentation

Please note that this User Guide is being currently updated; new sections will be added shortly.

3.7.2.Symmetrical Optotype #

This optotype is recommended for VA measurement of preschoolers who can successfully match letters during a practice session. Children develop horizontal laterality later than vertical, so all the letters selected for this test are symmetrical with respect to the vertical midline, which avoids confusion in the smaller ones. Six symmetric letters are used: X, T, U, O, H, and V, whose difficulty is statistically similar.3 These letters are drawn on a 5×5 grid, using a stroke 1/5 of the size of the letter.



As with all paediatric acuity tests, an initial test distance of 3-4 meters elicits better cooperation from most preschoolers, thus reducing the time necessary to estimate the VA. Within the “Screen Calibration setup” menu there is a separate calibration section for the two paediatric tests (the “Symmetrical Optotype” and the “Broken Glasses Test”). This option allows calibration of the VA values for whatever test distance is desired.


Many young children are able to match letters before they will correctly name them, especially in the unfamiliar examination setting. Using a “matching card”, i.e., a sheet with the printed letters, and teaching the child to point to the same letter held or shown by the examiner, will quickly teach the task and identify which children are ready before testing begins. The child may eventually say the name of the letter, but this is not necessary for the test. “Matching cards” can be downloaded from the OVU documentation.



It is also often very helpful for the child to practice with their parents before the visual examination. To do this, parents can use the “matching card” together with printed sheets with several lines of optotypes (downloaded from the OVU documentation), which will allow them to practice at home. This training could also be carried out with a digital animation that it is included in Optonet Webpage: https://optonet.co.uk/practice-visual-acuity/

Thus, the child can practice indicating (pointing or telling) the same letter on the matching card as shown by the parent and will easily learn the dynamics of the visual acuity test. This practice can significantly improve the child’s cooperation and acuity measurement.

When the child comes to the examination, we usually have his attention and cooperation for a very limited time. To take advantage of their limited window of cooperation, start testing using isolated letters with contour interaction in successive, decreasing size, until the threshold is estimated.4



Once the threshold value is estimated, isolate a row of letters from a slightly larger size, and introduce the contour interaction bars, to make the test more sensitive for detection of amblyopia.



From this row we can increase or decrease the size of the letters. The final VA value will be set based on the total number of letters correctly identified using the crowded test format.

Percentile Curves

The “Symmetrical Letters” optotype is especially useful (both for the professional and for the parents) when monitoring how the child’s vision evolves through the “VA Percentile Curves”.

These curves come from a study carried out in 2008, in which VA was taken (with symmetric letters in isolated rows with contour interaction, similar to those included in the vision unit), to a population of about 3,000 patients aged 2 years, 9 months to 8 years of age.5,6 The OVU documentation includes the Measurement Protocol along with the curves for monocular and binocular VA.



We can give parents the printed percentiles chart in which we will record the VA values obtained in their child at different ages, to show the progress and evolution of their VA with age. Parents usually have percentile charts for monitoring children’s weight and height, so they are familiar with this type of curve.

The horizontal axis of the abscissa indicates the age of the child, in the left ordinate axis the VA in VAR notation and to the right the percentage of children representing each curve. The shaded area in gray corresponds to 10% of the VAs (the lowest). When a value falls in that area a complete visual examination is recommended.



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