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ARTICLES |
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Year : 1983 | Volume
: 31
| Issue : 3 | Page : 112-114 |
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Assessment of macular function in anterior segment surgery
VK Raju, George W Weinstein, Lakshikumar Pillai
Deptt. of Ophthal., West Virginia University Medical Centre Morgantown, W. Va. 26506, USA
Correspondence Address: V K Raju Deptt. of Ophthal., West Virginia University Medical Centre Morgantown, W. Va. 26506 USA
Source of Support: None, Conflict of Interest: None | Check |
PMID: 6676192
How to cite this article: Raju V K, Weinstein GW, Pillai L. Assessment of macular function in anterior segment surgery. Indian J Ophthalmol 1983;31:112-4 |
By using accepted techniques it is relatively simple for a trained ophthalmologist to determine retinal, including macular, function in normal eyes. However, if any degree of opacity exists in the ocular media, the ophthalmologist has difficulty in gaining an unobstructed view of the fundus. These opacities may stem from a variety of conditions including cataracts, vitreous hemorrhage, and opaque cornea.
Alternative methods for assessing retinal function, especially when the ocular media are opaque, include two point discrimination, Maddox rod, light projection, Purkinje's entoptic phenomenon, and pupillary response. At best each of the foregoing provides only a rough assessment of retinal function.
The authors have found that all these tests being subjective, do not provide the degree of accuracy needed, especially during preoperative examinations of patients with opaque ocular media scheduled for anterior segment surgery.
Experience has proven that the objective tests for assessing retinal function i.e. visually evoked potential (VEP) and electro. retinogram (ERG)] are considerably more reliable. This paper will address itself principally to the high degree of success the authors encountred by using VEP and ultrasonography in the evaluation of nineteen (19) patients' eyes for either corneal grafts and;'or cataract extractions.
Investigation and use of two different methods for assessing retinal function (visually evoked potential (VEP) and electroretinogram (ERG) revealed that the VEP method has two major advantages:
1. The VEP tests more of the visual pathway, from the retina to the occipital cortex. The ERG gives information about the photoreceptors and the inner nuclear layers only.
2. Using the VEP on eyes with opaque media results in a response which is dominated by input from macular and paramacular areas. Therefore, the VEP might be a good predictor of postoperative macular function. This is in contrast to the ERG method, which provides no information about the function of the macula.
Material and Methods | | |
In this on going study, 19 eyes with opacities of the ocular media due to corneal opacities and/or cataracts were studied prior to and after appropriate surgical procedure using visual inspection of the VEP. VEP was recorded by Grass P522 Photostimulator flashing mes/rec. Also preoperative ultrasonography was done to exclude retinal detachment. Visual acuity with best correction was highly predictable, both in subjects who obtained a rood visual acuity and in those who did not. In this regard. the VEP, administrated through opacities of the ocular media, serves as an objective test for best achievable visual acuity.
The results of visual analysis for subjects in the present study are shown in table. Good visual acuity was chosen as 20/50 or better with best correction. Bad visual acuity was worse than 20150. There was agreement between interpretation of the visually evoked potential and postop vision in 14 (73%) out of 19 eye (normal VEP with "good" visual acuity, or abnorma VEP with "bad" visual acuity). This is an ongoing study with more results to be analysed.
Discussion | | |
Many authors believe that absence of VEP is a poor prognostic sign, but none address the question of how many people will have vision "adequate for their needs", but with nonrecordable VEP.
One of us (G.W.) has recorded the steady state of VEP produced by a Grass P522 photostimulator flashing ten times per second. The normal wave form showed a large rhythmic response at l0Hz and also a smaller peak at the second harmonic. This small peak represents macular function and is absent in eyes with poor visual acuity.
Rubin and Dawson used a trans-scleral illuminator to stimulate VEP in patients with corneal or lens opacities. They inspected the wave forms obtained and made a subjective prediction of the expected postoperative acuity. It was found that patients could be separated with some reliability into those in whom final acuity was better than 20/200 and those in whom it was worse. Finer graduations of acuity prediction were impossible.
Thompson and Harding recorded the transient VEP produced by flash stimulation in 20 patients with dense unilateral cataracts. They used a quantitative approach to predict acuity and found that the VEP could provide a statistically significant separation of those patients with vision of 20/40 or better from those with vision of 20/80 or worse. An important aspect of this study is that the response of the normal fellow eye is necessary for an accurate prediction. Because of the large intersubject variability of the VEP, one cannot predict post-operative functions when testing one eye alone, and also VEP cannot be used on patients with bilateral opacities of the media.
In a different approach, Arden & Sheorey [4] produced VEP's by oscillating laser interference fringes on the retina. The authors claimed that their method is more useful in case of cataract or corneal scar than a simple flash; since red light produced by the laser and the use of a pattern tend to enhance the macular component of the VEP.
More experience with the VEP is needed before we know the best technique to use and before we can decide what role the VEP should play in preoperative evaluation of the macula. However, there is little doubt that the information gained from the VEP will have significant value in most patients. Some false positive may be expected with VER testing but false negatives should be very rare.[5]
Summary | | |
VEP is an indirect, though reliable, index of macular function and optic nerve integrity. With the development of less expensive signal averages and television display stimuli, it is rapidly becoming an accessible, easily measured potential in most clinics. fn this preliminary report our results suggest that VEP administered through opacities of the ocular media, serves as an objective test for best achievable visual acuity.
References | | |
1. | Weinstein, G.W., Trans. Am. Ophthalmol. Soc., 75: 627, 1977. |
2. | Rubin M.L. and Dawson, W.W. Invest. Ophihalmol. Visual Sci., 17: 71-74, 1978. |
3. | Thompson, C.R.S., and Handing, G.F.A. Doe. Ophthalmol. Proc. Ser., 15: 193, 1978. |
4. | Arden, G.B. and Sheorey, U.B. The Assessment of Visual Function in Patients with Opacities: A New Evoked-Potential Method using Laser Interferometer, in J.E. DeSmedt (Ed) Visual Evoked Potential in Man: New Developments. Oxford: Clarendon 1977, pp 381-394. |
5. | Skalka, H.W., Ophth. Surg., 12: 642-645, 1981. |
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