Prevalence and Comparison of Visual Acuity Charts in School Going Children with Low Vision in Garhwal Himalayan Region

Materials and Methods: Two groups of 40 each of the children age less than 15 years of age, 40 each in study and control group attending the out patients department were selected after fulfilling the inclusion criteria. All the children were tested with Snellen’s and Lea’s symbol chart. The response of each child with both type charts was recorded. Testing of both groups was done with and without correction.


Introduction
Vision or visual perception is the complex integration of light sense, form sense, contrast sense and colour sense. Visual acuity measurement involves the determination of a threshold level of vision. Visual acuity is represented as the reciprocal of the minimal angle of resolution (the smallest letters resolved) at a given distance at high contrast. Visual acuity testing especially in low vision depends on targets, distance, lighting, background, contrast, ocular condition, concentration and intellectual level of the patients. Selection of a test is dependent on ability to respond, developmental age and time of testing. Type of tests used, therefore, will modify responses elicited. This is true in children especially in those with low vision [1]. Different types of visual acuity charts which are using now a days to Snellens acuity chart, Lea

Basic Protocol
A standard protocol was used to collect and document all the details regarding the cases included in this study. Detailed information about the history and complaints of the patients were taken. This included the type of visual problems, duration of symptoms and any history of predisposing factors like refractive errors, family history, systemic disease etc.,

Data collection
Data includes demographic details of children (age, sex,), type of refractive error, and duration of symptoms, prior history of using glasses, surgery and visual acuity at presentation were collected. Complete ocular examination details of each patient such as visual acuity for distant vision (checked with Snellen's acuity chart and Lea's symbol chart), Slit lamp examination, fundus examination (done with +90D lens) and binocular indirect ophthalmoscopy, were recorded. Data regarding subjective correction were also noted. Visual acuity was measured before and after correction with Snellen's chart and Lea's symbol chart was also noted.

Procedure of testing in Snellen's chart
For testing distant visual acuity, the patient was seated at a distance of 6 m from the Snellen's chart, so that the rays of light are practically parallel and the patient exerts minimal accommodation. The charts were properly illuminated not less than 20 foot-candles. The patient was asked to read the chart with each eye separately and the visual acuity was recorded as a frication, the numerator being the distance of the patient from the letters and the denominator being the smallest letters accurately read. When the patient was able to read up to 6 m the visual acuity was recorded as 6/6, which is normal. Similarly, depending upon the smallest line that the patient can read from the distance of 6m, his or her vision was recorded as 6/9, 6/12, 6/18, 6/24, 6/36, 6/60. If one cannot see the top line from 6m, he or she was asked to slowly walk towards the chart till one can read the top line. Depending upon the distance at which one can read the top line; the vision was record as 5/60, 4/60, 3/60, 2/60, 1/60.
If the patient was unable to read the top line even from 1m, he or she was asked to count fingers of the examiner. His or her vision was recorded as CF-3, CF-2, CF-1, or CF close to face, depending upon the distance at which the patient was able to count fingers. When the patient failed to count fingers, the examiner moved his or her hand close to the patient's face. If one can appreciate the hand movements, the examiner noted whether the patient can perceive light or not, if yes, vision was recorded as projection of light positive and, if not, it was recorded as projection of light negative.

Procedure of testing in Lea's chart
The child was allowed to stand or sit at a table with the response card in front, eyes at a 10-foot distance from the chart. The child must be conditioned to match symbols by pointing to the same symbol on the response card as was being shown with a flash card or pointed to on the chart. Begin screening with one person holding an occluder over the child's left eye, another person pointed to the symbols on the Lea wall chart. The screener pointed to the symbols must be careful not to cover the box around the symbols as this could affect the results of the test.

Results
This prospective and comparative study was conducted at the tertiary eye care hospital over a period of one year between May 2012 to April 2013. 160 eyes of 80 patients with refractive errors were included in this study.

b) After correction:
In the control group of 40 children had visual acuity was more than 6/18 in 40 (100%) children in both Snellen's and Lea's chart (Table 5). There was different in visual acuity with different charts in 22.5% of cases before correction. Visual acuity did not differ between charts, in all cases after correction.

b) After correction:
In the study group, out of 40 children, the visual acuity was more than 6/18 in (17) 41.3% of cases in Snellen's chart and (30) 75.8% of cases in Lea's chart. Visual acuity was less than 6/18 in 23 (58.7%) children in Snellen's chart and 10 (24.2%) children in Lea's chart ( Table 6).

Discussion
This prospective and comparative study was conducted in the tertiary eye care hospital from May 2014 to February 2015. 160 eyes of 80 patients with refractive errors were included in this study. Niroula et al [2] reported that distribution of myopia was found to be higher (4.05%) than the hyperopia (1.24%) and astigmatism (1.14%). Niroula et al [2] in the present study, myopia was in 24 (60%), hypermetropia in 2 (5%) and astigmatism in 14 (35%) children. In study group out of 40 children we found to be myopia in 24 (60%), hypermetropia in 10 (25%) and astigmatism in 6 (15%) children. Tong et al reported bearing in mind that the visual acuity measurements were performed by two different groups of professionals, visual acuity screening using the ETDRS method appears to be more accurate than the simplified charts for the detection of myopia or any refractive errors in children. Tong et al [3] In present study Lea's symbol chart was found to be better than Snellen's chart by measuring the accurate visual acuity among the normal as well as the low vision children. Dobson et al [4] reported correlation between visual acuity results obtained with the two charts was high. There was no difference in absolute inter-eye acuity difference measured with the two acuity charts. However, on average, Lea's Symbols acuity scores were one log MAR line better than Bailey-Lovie acuity scores, and this difference increased with worse visual acuity. Cyert L et al [5], Lueder G [6] In present study also there was no much difference in normal children comparatively than low vision children.     be used to predict refractive errors in school children in Singapore in a sensitive and specific manner using a referral criterion of worse than or equal to 0.28 logarithm of the minimum angle of resolution. Dobson et al [8] reported in the comparitive visual acuity study of young children, in whom the primary source of reduced visual acuity was astigmatism-related amblyopia, the Lea's symbols chart produced visual acuity scores that were about 0.5 lines better than visual acuity scores obtained with ETDRS charts. Dobson et al [8] Similarly, in the present study also Lea's symbol visual chart produced visual acuity scores better than the Snellen's chart especially in low vision children. Uzma et al [9] found out that myopia was present in almost 51% of the study population, she stressed on provision of health education, periodic visual screening programs, and primary eye care by trained health care personnel in the elementary schools will prevent the prevalence of refractive errors and common ocular diseases in school children. Gupta et al [10] found out that refractive errors were the most common ocular disorders among school children of age 6-16 years.

Tong et al [7] studied modified ETDRS visual acuity chart can
Dandona et al [11] in his study stated that the prevalence of uncorrected refractive error, especially myopia, was higher in urban children. He further emphasized on the eye screening of school children. However, the approach used may be different for urban and rural school children [11,12].

Conclusion
Visual performance depends not only on size and distance, but also on contrast, illumination, and arrangement of targets. These factors, affected in low vision, may modify responses during testing, thus giving high false negatives. Use of appropriate charts is, therefore, mandatory while assessing children with low vision. This study also concluded myopia and astigmatism is the commonest refractive error among school going children. Lea's chart is easier to measure visual acuity in low vision school going children than the Snellen's chart. It was observed that the visual acuity differed with different charts, in low vision cases, before and after correction. The difference is dependent on the individual's absolute level of visual acuity. Visual acuity is easy to measure in children by Lea's chart.