【摘要】目的： 用弥散滤片模拟瞳孔及屈光间质的变化，以比较这些变化对Humphrey视野计(HFA)及倍频视野计（FDT）的影响。方法 ：测试15位2339岁正常人的左眼：(1) Snellen视力表；(2) PelliRobson(PR)对比敏感度视力表；(3)白视标白背景自动视野计检查，HFA Model 630, 采用302程序，刺激视标为III; (4) FDT检查, 采用N30程序。每一只眼均经无滤片，1，2，3，4，5 个滤片进行测试，测试顺序为随机。结果： Snellen 视力表：1个滤片只对2名受试者有影响，仅丢失1行；2个滤片对所有受试者影响甚微，对2名受试者无影响（9名丢失1行，4名丢失2行）；3个滤片对所有受试者效果一样，均丢失3行；4个滤片可致5名受试者丢失4行，5名受试者丢失5行，3名受试者丢失6行，2名受试者丢失7行；5个滤片可致所有受试者丢失全部9行。PR 视力表：3个滤片便可致13名受试者看不到所有字母，2名受试者仅看到1组字母。HFA和FDT的平均偏差值（MD）均随滤片增加而降低，HFA和FDT的图形标准差值（PSD）较小且非常一致。结论：以上结果表明，由弥散滤片所致的照明降低，光散射及瞳孔缩小对HFA及FDT均有明显影响，其视野缺损比较弥散均匀，而屈光间质混浊对FDT的影响比对HFA的影响要大。各组滤片的PSD值比较小且非常一致。
【关键词】 屈光间质；青光眼；视野计；平均偏差；图形标准差
Correspondence to: JiaQuan Shen. Department of Ophthalmology, Shandong Provincial Hospital, Shandong University, Jinan 250021, Shandong Province, China. [email protected]
Abstract AIM: To simulate the effect of changes of pupil size and optical media with diffusing filters on visual acuity, and to compare these effects on Humphrey field analyser(HFA) and frequency doubling technology (FDT) perimetry. METHODS: The left eyes of 15 normal subjects aged 2339 years underwent: 1) Snellen visual acuity and PelliRobson(PR) chart testing; 2) whiteonwhite automatic perimetry of HFA model 630 using the 302 programme and size III stimulus; 3) FDT perimetry using N30 programme. For each procedure, no filter, 1, 2, 3, 4 and 5 diffusing filters were mounted in random order in front of the eye to be tested.RESULTS: For Snellen visual acuity, 1 filter had no effect on all but 2 subjects who lost 1 line; 2 filters had little effect on all subjects and had no effect on 2 subjects (9 subjects lost 1 line and 4 lost 2 lines); 3 filters had the same effect on all subjects who lost 3 lines; 4 filters caused 5 subjects to lose 4 lines, 5 subjects to lose 5 lines, 3 subjects to lose 6 lines and 2 subjects to lose 7 lines; 5 filters caused 9 lines loss in all the subjects. For PR chart , 3 filters usage caused 13 subjects to see none of the groups of letters and while 2 subjects could see only 1 group of letters. Both mean deviation(MD) of HFA and FDT were reduced with increased numbers of filters. For HFA, 5 filters caused nearly absolute scotomas, while for FDT, 3 filters caused the same result. The pattern standard deviation(PSD) for both HFA and FDT was uniformly consistent; the shortterm fluctuation(SF) and corrected pattern standard deviation(CPSD) were all uniformly consistent.CONCLUSION: These findings showed that luminance reduction and light scattering caused by diffusing filters have marked effect on whiteonwhite and frequency doubling perimetry; the visual field loss of HFA and FDT was relatively diffuse, which could be effectively reduced through the use of pattern deviation analysis in separating visual field loss caused by glaucoma from that caused by cataract; the effect of media opacities on FDT was greater than it on HFA.
KEYWORDS: optical media; glaucoma; perimetry; mean deviation; pattern standard deviation
INTRODUCTION
Recent reports have suggested that substantial ganglion cell losses occur in glaucoma patients before whiteonwhite visual field defects are demonstrable, and that there is preferential loss of large ganglion cells[1]. Frequency doubling perimetry has shown promising results in detecting early glaucoma, since it selectively targets the magnocellular system[2]. However, the influence of developing cataract is commonly seen in older people where glaucoma is also frequently found[3]. It has been demonstrated that cataract has a profound effect on the visual field, primarily resulting in a general reduction in sensitivity across the visual field[4]. Cataract is thought to cause visual degradation by three mechanisms: light scattering; image blur; and decreased illumination. Of these, veiling glare from light scattering is thought to have the greatest effect on the subjective quality of the visual image as well as the perimetric threshold[5]. The differentiation of visual field changes caused by glaucoma from that caused by cataract still remains a problem, though much work has been done on this subject[6]. This study aimed to simulate the effect of optical media changes using diffusing filters in normal subjects on visual acuity, and to compare these effects on HFA and FDT perimetry.
SUBJECTS AND METHODS
Fifteen normal subjects were prospectively recruited from staff at the Institute of Ophthalmology; 2 of the subjects had refractive errors of less than 1.00D sphere and less than 0.50D cylinder; 1 subject had 4.00D myopia fully corrected; the average age was 31.6 years (range 2339 years). Left eye only was tested in each case. Six uniocular goggles were made; the goggles had oval apertures 4.75cm×3.5cm in size and foam backing designed to minimize stray light that might influence retinal adaptation. Five of the goggles were fitted with 1, 2, 3, 4, and 5 diffusing filters respectively (made by ourselves, the eighth white diffusing LEE filter 252, Andover, Hampshire, England), and one had no filter. The two tests were performed using the left eye of each subject with each of the six goggles in two test sessions. All tests were performedwith the natural pupil throughout. Each subject underwent 12 tests and the order of testing was randomized.
Firstly, visual acuity was measured by a backlit Snellen chart, and the PelliRobson(PR) contrast test was performed for each goggle at random times. The PR chart consists of 16 groups of three letters, all of constant size, whose contrast declines by 0.15 log units per group from 100 % contrast for the first group to 0.56 % for the 16th [6]. Subjects were allowed to have dark adaptation for 20 minutes prior to testing.
Whiteonwhite perimetry was performed using programme 302 of Humphrey field analyser (HFA) model 630 with a size III stimulus and the full threshold strategy. The standard procedure of testing visual fields on HFA was used in this study. Frequency doubling technology (FDT) perimetry was performed using FDT visual field instrument and N30 full threshold programme[2]. The mean deviation (MD), pattern standard deviation (PSD), shortterm fluctuation (SF), and corrected pattern standard deviation (CPSD) values of both perimetry were compared.
RESULTS
The results of Snellen visual acuity and PR contrast fraction for each filter were shown in Table 1. All errors in the reliability were below 20% for all HFA tests. Test time ranged between 10 and 17 minutes for HFA, 3 and 5 minutes for FDT. For Snellen visual acuity test, 1 filter had no effect on all but 2 subjects who lost 1 line; 2 filters had little effect on all subjects and had no effect on 2 subjects (9 subjects lost 1 line and 4 lost 2 lines); interestingly, 3 filters had the same effect on all subjects; 4 filters caused 5 subjects to lose 4 lines, 5 subjects to lose 5 lines, 3 subjects to lose 6 line and 2 subjects to lose 7 lines; 5 filters caused 9 lines loss in all the subjects. For PR chart, diffusing filters had marked effect on it. Even 3 filters caused 7 subjects to see none of the groups of letters and 1 subject could see only 1 group of the letters.
Both MD of HFA and FDT were reduced with increased numbers of filters; for HFA, the central region always had the greater sensitivity for each test and 5 filters caused nearly absolute scotomas, while for FDT, 3 filters caused the similar results (Table 2, Figure 1). The PSD for both HFA and FDT was fairly uniform and lower, the SF and CPSD for HFA were also very consistent (Table 2, Figure 2).Table 2 Mean results for each test with different number of filters（略）
DISCUSSION
Media opacities, such as changes in the crystalline lens that can occur with aging, exert their effect through light scattering and also by decreasing retinal illumination[7], while diffusing filters can produce light scattering, which has previously been used to simulate the effect of reduced pupil size and lens changes on whiteonwhite perimetry[8,9]. In our study, we used diffusing filters to simulate the effect of light scattering and reduced illumination as well as decreased pupil size, to Table 1 Snellen visual acuity and PR contrast for different number of filterssimulate the changes of visual field due to cataract. Our study showed that the influence of diffusing filter on HFA and FDT was relatively diffuse and uniformly consistent. The central region of the visual field for HFA always had greater sensitivity than the peripheral region. These findings were in accord with the results of Yao and Flammer[10]. For FDT, 3 filters could cause similar visual field defect as 5 filters for HFA, which meant the effect of media opacities on FDT was greater than that on HFA. This suggests that for patients of cataract complicated with glaucoma, HFA is better than FDT. Heuer et al [8,11] performed static perimetry using different pieces of ground glass in front of the normal eyes, which showed diffuse depression of the visual field. They demonstrated fairly uniform reduction of threshold across the visual field. Wood et al [5,12] induced light scattering by using latex beads with cells in front of normal eyes, which also produced generalized depression of the visual field. All these results are in agreement with our study. Our studies showed that diffusing filters could cause homogenous visual loss which is usually caused by cataract and/or miosis, and seldom by glaucoma[13,14]. We feel that the PSD probability analysis might be helpful in separating visual field changes caused by glaucoma from that caused by cataract, because diffuse loss is more prevalent in cases in which the intraocular pressure (IOP) is high and localized loss is more prevalent when IOP is normal[15].
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