Year : 1985 | Volume
: 33 | Issue : 5 | Page : 323--325
Correlation between dark adaptation and refractive errors
V Seetharaman, S Motwane, SM Sathe
Department of Ophthalmology, L.T.M. Medical College, Mumbai, India
Department of Ophthalmology, L.T.M. Medical College, Mumbai 22
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Seetharaman V, Motwane S, Sathe S M. Correlation between dark adaptation and refractive errors.Indian J Ophthalmol 1985;33:323-325
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Seetharaman V, Motwane S, Sathe S M. Correlation between dark adaptation and refractive errors. Indian J Ophthalmol [serial online] 1985 [cited 2021 Apr 16 ];33:323-325
Available from: https://www.ijo.in/text.asp?1985/33/5/323/30741
Dark adaptation examination has come to be regarded as a scientific curiosity with limited practical bearing due to the complicated nature of the apparatus and the lengthy procedure deserving the expenditure of a lot of time. Much work done on this subject suffer from serious defect of insufficiency of controlling the various factors influencing the results. A lot of valuable information can be obtained from this test in a variety of ocular conditions. Dark adaptation tests are clinically applicable in more cases now with the availability of the standardised Goldmann's Dark Adaptometer. A short test of dark adaptation requiring only one third of the usual time is also possible. This should be extremely useful in screening large groups of subjects(2).
The present study was conducted to note its relation of dark adaptation in cases with refractive errors.
MATERIALS AND METHODS
A total of 55 cases were screened, which included 44 cases with refractive errors. The rest of the 11 cases were emmetropes, kept as control. The cases included members of both sexes and of all age group. Each case was subjected to a routine series of investigations comprising of visual acuity testing followed by accurate refraction. A thorough external, and fundus examination including indirect ophthalmoscopy was done in all the cases. The cases were then grouped as Myopes, Hypermetropes, and Emmetropes based on refraction. 3 cases in the present study bad Retinitis Pigmentosa associated with myopia. 2 cases of Vitamin A deficiency with normal acuity of vision were also included in the study. Cases with anisometropia and astigmatism were excluded from this study. All the cases were examined with the standard Goldmann Weeker's Adaptometer. The test was carried out in a darkroom after dusk. Illumination of the Adaptometer and the test field were maintained constant, and the absolute threshold estimation was done in all the cases.
The following observations were made. The average dark adaptation time was 25 minutes in the Emmetropes, the cone saturation occuring after 7 minutes; followed by rod saturation and the retinal sensitivity changing by 4.5 log units [Figure 1]
In mild and moderate myopes with normal fundus the dark adaptation was almost normal. In high myopes with healthy retina, the dark adaptation was seen to be prolonged, but with normal retinal sensitivity. In myopes with retinal degeneration, the adaptation time is prolonged with a reduced retinal sensitivity. [Figure 2][Figure 3]
In mild and moderate Hypermetropes, the adaptation time was normal, while an earlier adaptation was seen in aphakic patients. All the Hypermetropes in the present study had normal fundus appearance. In Retinitis Pigmentosa, and Vitamin A deficiency the adaptation was shortened. In Retinitis pigmentosa, the adaptation was seen to be monophasic, without a rod adaptation component.
Dark adaptation represents the potential of the retinal photoreceptors. The increase in visual sensitivity which occurs in darkness is termed Dark Adaptation. If a person remains in darkness for a long time, the visual receptors become more sensitive due to certain biochemical changes occuring in the photoreceptors, all the Opsins and the Retinal get converted into light sensitive pigments. Moreover large amounts of Vitamin A get converted into Retinal. Between the limits of maximal dark adaptaion and maximal light adaptation, the eye can change it's sensitivity (3). The process of dark adaptation requires half an hour or longer (4). There are two components of the dark adaptation curve, the first drop in visual threshold is due to adaptation of the cones. In the peripheral retina, a further drop occurs due to the adaptation of the rods. Thus the typical dark adaptation curve is biphasic. The course of dark adaptation is influenced by two parameters namely the state of the retinal photoreceptors and the photochemical reaction occuring in them. Thus in the conditions affecting these, the dark adaptation is altered. In Emmetropes, the darkadaptation is normal. In Myopes with healthy fundus also it is normal, as the parameters are unaffected. In high Myopes with healthy retina the adaptation is prolonged probably due to delayed biochemical reactions in the photoreceptors but as they are intact, the retinal sensitivity is unaffected.
In mild and moderate Hypermetropes, the adaptation is normal, In aphakic patients, the adaptation is shortened, probably due to poor peripheral vision.
In subjects with Retinitis Pigmentosa, the adaptation is shortened and monophasic due to the poor function of the rod component. In view of the role of Vitamin A in thes ynthesis of Rhodopsin and lodopsin, it is not surprising that avitaminosis A produces visual abnormalities. Prolonged deficiency is associated with anatomic changes in rods and cones followed by degeneration of the neural layers of the retina. As dark adaptation depends on regeneration of retinal photoreceptors, in subjects with Vitamin A deficiency, the adaptation occurs more slowly and the sensitivity is less.
|1||Rushton, W.A.H., 1981, Adler's physiology p. 661 ed. R. A. Moses C.V. Mosby Co. London.|