Introduction

Amblyopia, which is the most common visual disorder among children, is a neurological disorder of binocular vision resulting from a disrupted visual experience during early visual development1. It affects visual acuity (VA), contrast sensitivity (CS), visual field, and stereoacuity2. VA and CS are two different but important parameters that are used to measure visual function. VA describes visual function under high contrast conditions3. CS testing, which characterizes the visual function under different spatial frequencies and different luminance, is another powerful technique to quantify the capability of the visual system4. However, CS is often neglected in clinical research. Compared with VA, CS can better describe the visual function in some cases5. Our research investigated the changes in CS among patients with ametropic or anisometropic amblyopia in achieving the corrected visual acuity (CVA) of 1.0.

Material and Methods

Participants

Patients with lower visual acuity than their normal peers were selected. All patients who cooperated with us in this study had no strabismus, no non-centric fixation, no keratopathy, and no cataracts or other diseases in their eyes. Fifty-seven cases (male 34, female 23; mean, 8.51 years; SD, 1.894 years) with ametropic and anisometropic amblyopia whose CVA had recovered to 1.0 after amblyopia therapy and 22 cases (male 17, female 5; mean, 8.23 years; SD, 1.541 years) with normal visual acuity of 1.0 were enrolled in this study between May 2011 and November 2012. Among the 57 cases with ametropic or anisometropic amblyopia, there were 34 cases (the spherical equivalent is 4.24 ± 2.130DS) with middle or high hyperopia of both eyes and 23 cases (the spherical equivalent of the amblyopic eye is 4.32 ± 2.175 DS, and the spherical equivalent of the non-amblyopic eye is 2.00 ± 1.663 DS) with anisometropia.

All patients were divided into three groups: the ametropic amblyopic group (MA), anisometropic amblyopic group (NA), and the normal control group (NC). The difference in spherical power and the astigmatic power between the two eyes was at least 1.5D and 1.0D, respectively, in the anisometropic amblyopic group.

Ethical approval and informed consent

Research protocol was approved by the Ethics Committee of Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology. The informed consents about the follow-up examination were obtained from all the patients. All experiments were performed in accordance with the Declaration of Helsinki.

Measurements of visual acuity

The measurements of visual acuity employed the international standard vision acuity chart (E chart, Shengde Company, China), the background luminance of which was 300 cd/m2 by a LCD monitor. The 1.0 lines of the chart were at the same height as the eyes. The chart was 5 meters away from the subject.

Measurements of contrast sensitivity

The refractive error of all patients with amblyopia was corrected by wearing glasses. The sinusoidal gratings were used as the contrast sensitivity testing standards to conduct the Functional Acuity Contrast Test (OPTEC® 6500 visual function tester (Stereo Company, U.S.)). The brightness values were 85 cd/m2 (light) and 3 cd/m2 (dark), respectively. The spatial frequencies of the sinusoidal grating were 1.5 cpd, 3.0 cpd, 6.0 cpd, 12.0 cpd, and 18.0 cpd. The right and left eyes were tested, separately. The test was carried out in the following steps: The remote switch was turned on to simulate the test distance of 6 m. Then, the Day button was pressed to test the bright mode using the 5 # to 9 # test pictures. Finally, the Night button was pressed for the dark mode test. If the answer was correct, the next test picture was presented; if not, the last correct answer was recorded6.

Statistical analyses

The data were analysed using a paired t-test by the Statistical Package for Social Sciences (SPSS) software (Version 17.0). P-values less than 0.05 were considered statistically significant.

Results

Table 1 and Fig. 1 show the contrast sensitivity of the three groups under photopic and scotopic conditions.

Table 1 The contrast sensitivity of the three groups.
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Figure 1: hows the contrast sensitivity of the three groups under the photopic conditions and the scotopic conditions, respectively.
figure 1

Error bars show 95%CIs. P-value of <0.05 was considered statistically significant.

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The differences between the ametropic amblyopic group and the normal control group

Table 2 shows that the two groups exhibited significant differences at 18 cpd under both the photopic and the scotopic conditions (P = 0.001, 0.025).

Table 2 The differences between the ametropic amblyopic group (MA) and the normal control group (NC).
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The differences between the amblyopic eyes (AE) of the anisometropic amblyopic group and the normal control group

Table 3 shows significant differences at 18 cpd under the photopic conditions (P = 0.005) and 6 cpd, 12 cpd,and 18 cpd under the scotopic conditions (P = 0.008, <0.001, 0.004).

Table 3 The differences between the amblyopic eyes (AE) of the anisometropic amblyopic group (NA) and the normal control group (NC).
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The differences between the non-amblyopic eyes (nAE) of the anisometropic amblyopic group and the normal control group

Table 4 shows no differences between the groups under both photopic and scotopic conditions at all spatial frequencies.

Table 4 The differences between the non-amblyopic eyes (nAE) of the anisometropic amblyopic group (NA) and the normal control group (NC).
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The differences between the non-amblyopic eyes and the corresponding amblyopic eyes in the anisometropic amblyopic group

Table 5 shows no differences between the non-amblyopic eyes and the corresponding amblyopic eyes in the anisometropic amblyopic group under photopic conditions. Differences were shown at 6 cpd, 12 cpd, and 18 cpd under scotopic conditions (P = 0.025, 0.045, 0.019).

Table 5 The differences between the non-amblyopic eyes and the corresponding amblyopic eyes in the anisometropic amblyopic group (NA).
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Discussion

CS is an important parameter in detecting visual dysfunction. There are two pathways from the retina to cortex: the parvocellular and the magnocellular pathways7. Both of them could transform and distribute information to the visual cortex (V1)8. The parvocellular system is selectively sensitive to middle to high spatial frequencies (low temporal frequencies); however, the magnocellular system is sensitive to a very broad temporal frequency range (low spatial frequencies)9. The contrast-sensitivity function (CSF) is controlled by the spatiotemporal characteristics of the visual pathway. The magnocellular system forms the basis of the achromatic CSF and dominates close-to-threshold detection. When the magnocellular system saturates, the parvocellular system dominates the higher contrast detection10.

Amblyopia is a disease with visual abnormality, such as visual acuity, binocular function, and contrast sensitivity11. Amblyopia showed decreased contrast sensitivity under mesopic conditions and at intermediate spatial frequencies12. The visual acuity was significantly better correlated with improved CS during amblyopia treatment13. Despite the significant correlation between VA and CS, VA cannot predict CS14. VA is a measure of the resolution under high contrast. CS is a measure of the ability to distinguish the visual markers under different levels of contrast. The latter is more adaptively used in daily life. Unlike other studies that focused on the lower visual acuity of patients with ametropic amblyopia or anisometropic amblyopia, our study focused on the contrast sensitivity of those whose visual acuity had recovered to 1.0.

Our study demonstrated that different types of amblyopia still had contrast sensitivity dysfunction at different spatial frequencies, even when the CVA had recovered to 1.0. The CS dysfunction of the ametropic amblyopia emerged at a high spatial frequency. The CS dysfunction of the anisometropic amblyopia was observed at middle and high spatial frequencies, particularly under scotopic conditions. This result indicated that the recovery of the visual function was different from the low spatial frequency to the high spatial frequency during amblyopia treatment, and the parvocellular system remained uncured, even when the CVA recovered to 1.0.

Compared with photopic visual function mediated by the cone cells, mesopic visual function is more complex because it requires that the rod cells and the cone cells interact with each other properly15. This might be why the CS under scotopic conditions is more difficult to recover.

Although the visual acuity of patients with amblyopia had recovered to 1.0, their contrast sensitivity did not return to the normal range simultaneously. In other words, these patients had lower resolution capacities than did those with normal contrast sensitivity, especially under a dark environment. Therefore, we suggest that researchers focus on the improvement of visual acuity and the development of contrast sensitivity during the amblyopia treatment. Perhaps future studies should be directed toward evaluating the impact of reduced contrast sensitivity on the quality of life of patients with a history of amblyopia treatment in whom visual acuity was successfully restored to 1.0. Our findings have enriched the body of knowledge about amblyopia treatment in clinical work.

Additional Information

How to cite this article: Wang, G. et al. An Assessment of the Contrast Sensitivity in Patients with Ametropic and Anisometropic Amblyopia in Achieving the Corrected Visual Acuity of 1.0. Sci. Rep. 7, 42043; doi: 10.1038/srep42043 (2017).

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