A New Parameter for the Assessment of Distance Visual Capacity: the Critical Visual Acuity (CVA)

In clinical practice, the conventional examination of the VA does not take into account the patient's reading speed, and does not reflect the requirements of everyday life (e.g. driving, watching subtitles on TV), in which each individual has limited time to react. In addition, the VA measured with the conventional method differs significantly among the different examiners, since there is no standard time limit for the patient to read the chart symbols.

However, the consideration of the speed for the recognition of the VA charts' symbols leads to the more accurate and comprehensive assessment of the visual capacity. Therefore, primary objective of this study is to develop and validate a new parameter for the more comprehensive evaluation of visual capacity: the critical visual acuity (CVA), and to determine normal values of population according to age. CVA takes into account the speed of recognition of the symbols of the distance visual charts.

Participants are recruited from the outpatient service of the Department of Ophthalmology of the University Hospital of Alexandroupolis in a consecutive-if-eligible basis. At first, each participant performs the conventional VA test monocularly. Then, he/she performs the CVA test with the same eye. Two different printed Landolt ring charts are used, the first one for VA and the second one for CVA assessment for all participants to avoid memorization. The VA and CVA are evaluated for all participants under the same environmental lighting conditions.

Regarding the VA and CVA examination procedure, the investigator masks the chart lines using a blank piece of paper and reveals one line each time. The investigator instructs the patient to read aloud the 5 letters of the Landolt chart, as quickly and accurately as possible, after hearing the words "Ready!… Go!". At the same time, a second examiner starts a stopwatch to record the reading time (in seconds, to the nearest 0.01 s) when the examiner fully reveals the line and the patient starts to read it. The first examiner counts the number of errors for each line and the second one enters the reading time for each line in a data file. The reading time of each line is compared with the automatically calculated average reading time of the larger lines minus 1.96 (for CVA) or 2.58 (for VA) times the standard deviation (SD) of the reading time of these lines.

Testing stops when the letters are too small for the examinee to discriminate and/or they are read with a speed lower than the maximum reading speed (MRS) of each examinee.

Inclusion criteria

• People aged 18 - 75 years

Exclusion criteria

• neurological diseases
• mental diseases
• mental retardation,
• difficulty reading
• previous intraocular surgery