|Year : 1984 | Volume
| Issue : 3 | Page : 139-141
Retinal sensitivity in astigmatism and meridional amblyopia
BS Goel, R Maheshwari, A Moiz
Institute of Ophthalmology A.M. U Aligarh, India
Institute of Ophthalmology, Aligarh Muslim University, Aligarh U.P: 202 001
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Goel B S, Maheshwari R, Moiz A. Retinal sensitivity in astigmatism and meridional amblyopia. Indian J Ophthalmol 1984;32:139-41
Meridional amblyopia has been known clinically for a very long time. Recently, however, it has been explored in detail as a result of laboratory experiments with cats who have undergone meridional visual-deprivation., It has been proved that persons born with a high degree of astigmatism, even when provided with correcting spectacles at a later date retain permanently poor differentiation in the astigmatic meridian while it may remain normal in other meridians,. Freeman investigated contrast sensitivities for gratings of various spatial frequencies and orientations in meridional amblyopia and found that the entire contrast sensitivity function is reduced for gratings oriented in the amblyopic meridian. In this context the aim of the present study is to measure the retinal sensitivity to light in various meridian in cases of astigmatism with and without amblyopia and to compare it with the values in amblyopia due to spherical ametropia and normal.
| Material and methods|| |
Present study deals with cases of astigmatism above ± 1D randomly selected among those attending refraction department, with or without amblyopia. Correcting glasses were prescribed after retinoscopy under mydriasis and post-mydriatic test and final improvement of visual acuity was assessed. These cases were investigated for detailed orthoptic check-up specially with regard to binocular functions, visual acuity in horizontal and vertical meridian with the help of specially designed Snellen's Chart and assessment of central visual fields for retinal sensitivity with the help of Friedman visual field analyser.
The instrument employs static target and 2, 3 or 4 simultaneous stimuli from and electronic flash lamp through a movable lever in 15 different positions, predetermined as per manufacturer's design. The sensitivity of various retinal points can thus be determined in log units of intensity of light by changing the various neutral density filters. The test is carried out with the spectacles for distance while occluding the other eye and the subject fixating the central white fixation target. After setting the filter density for the age, flash is fired in turn on each of the 15 positions. A lower filter setting is substituted for the points missed by the patient till he could appreciate the flash at that point. This suggested a poor threshold of the particular retinal point in comparison to normal. In this fashion and average of the retinal sensitivity of points falling on the meridian of the astigmatism and at right angles was worked in each case and mean obtained in each of the following groups
Group I Cases with visual = 36 eyes
acuity 6/18 or worse
Group II Cases with visual = 42 eyes acuity better than 6/18
Group III Cases of amblyopia = 16 eyes due to spherical anisometropia.
Group IV Normal cases without = 24 eyes any refractive error and astigmatism with normal vision
| Observation|| |
Table shows the retinal sensitivity in group I, in the amblyopic meridian and at right angles with and without inclusion of the points falling within 15° of the axis of the astigmatism and at right angles on each side, in group II in ametropic meridian and at right angles. It shows an average retinal sensitivity of 1.484±0.292 as compared to 1.612±0.228 at right angles to the amblyopic axis. The difference between the two is significant at 2% level (P<0.02). When average of the amblyopic meridian with 15° on each side inclusive and at right angles to amblyopic meridian with 15° on each side included were compared, it was seen that average were similar (1.546±0.242 and 1.596±0.235) with insignificant differences (P <0.05).
Retinal sensitivity in group II in the meridian of greater ametropia and in the meridian of lesser ametropia or emmetropia in simple astigmatism was 1.506±0.282 and 1.547±0.272 respectively in the two meridian. The statistical analysis showed insignificant difference between the two means (P >0.50).
[Table - 2] shows retinal sensitivity of the astigmatic axis in amblyopia compared with the same axis of the sound eye. It reveals an average of 1.469±0.253 and 1.75±0.239 respectively. The statistical analysis revealed a highly significant comparison of the means (P <.0001).
24 normal eyes were also investigated for retinal sensitivity in various meridian to check if there is any difference in the retinal sensitivity. The findings in each meridian were the same.
[Table - 3] shows the average values of retinal sensitivity in various groups. The overall retinal sensitivity was the minimum in group I, and a gradually increasing sensitivity in other groups. The retinal sensitivity in the astigmatic meridian was also lowered in group I than in group II.
| Discussion|| |
In an uncorrected astigmatism one focal line is defocussed than the other which may lead to amblyopia in the same meridian, this condition has been called as meridional amblyopia. As the generalised retinal sensitivity is reduced in cases of amblyopia, one would expect a reduced sensitivity in one meridian in cases with meridional amblyopia specially in cases of high astigmatism. The present investigation revealed a reduced retinal sensitivity in cases of amblyopia due to astigmatism in the axis of greater ametropia. Comparing with the normal eyes this reduction was highly significant. Similar finding of a reduction of contrast sensitivity have been reported in meridional amblyopia. Present study further revealed that there is considerable reduction in retinal sensitivity in meridional amblyopia in one meridian than the other (non amblyopic) at 2% level which also showed a reduction as compared to normal eyes at 0.001% significance level. Clinically these eases were observed to have a greater reduction of visual acuity in one axis than in the other. Thus these cases resembled those of meridional amblyopia. This would indicate that the depression of the functions begin in one meridian causing meridional amblyopia but later involving depression in other meridian causing amblyopia in general.
The present study gives a suggestion of depression of function in one meridian due to inadequate stimulus if uncorrected, but if amblyopia does not occur for example when astigmatism is corrected the depression of retinal functions was much less. This however, may be either due to an effect directly on the retinal receptors or in the-transmission to the central nervous system.
| Summary|| |
The present study deals with retinal sensitivity to light in 36 eyes of astigmatism with amblyopia, 42 eyes without amblyopia, 16 eyes of spherical anisometropia and 24 normal eyes. The study revealed reduced sensitivity in the astigmatic meridian in comparison with the other meridian in cases with meridional amblyopia. It was reduced more than in astigmatism without amblyopia, amblyopia due to anisometropia and normal eyes in that order.
| Acknowledgement|| |
The financial help for this study was provided by the Indian Council for Medical research on the Scheme "Aniseikonia, Amblyopia & Binocular Functions in Astigmatism."
| References|| |
Blakemore, C., and Cooper, G.F., 1970, 228: 477 478.
Hirsch, H.V.B., and spinelli, D.N., 1970, Science168:869-871.
Luedde, W.H., 1922, Amer. J. Ophthalmol. 5: 441 451.
Freemen. R.D., 1975. Invest Ophthalmol. 14: 78 81.
[Table - 1], [Table - 2], [Table - 3]