Documenta Ophthalmologica 69:399-405 (1988) 9 Kluwer Academic Publishers - Printed in the Netherlands

V C T S chart evaluation as a screening test P.C. M A U D G A L , l R.W.O. STOUT & A.TH.M. VAN BALEN Free University Hospital, Department of Ophthalmology, Amsterdam, The Netherlands; 1 U.Z. St. RaphaYl, Kapueijnenvoer 7, B-3000 Leuven, Belgium Accepted 6 May 1988

Key words: VCTS chart, contrast sensitivity Abstract. Contrast sensitivity curves of 211 randomly selected patients were drawn using the VCTS chart. Depending on the type of curve obtained, patients were grouped in different diagnostic categories following the instructions supplied with the chart. Clinical examination revealed that the VCTS chart curves indicated a false diagnosis of cataract in 65.5% patients and in 54.2% patients a false diagnosis of glaucoma. In a number of additional patients, VCTS chart testing had failed to indicate the presence of cataract or glaucoma. Contrary to the claims of the manufacturer our results demonstrate that the VCTS chart has no value as a screening device in a clinical practice.

Introduction

In clinical practice, visual acuity is determined by the ability of a patient to read the smallest letters on a Snellen chart or one of the other standard charts on which letters or symbols diminish in size from line to line; thus progressively presenting a smaller visual angle to the eye. As the letters on these charts are black on a white background, the chart offers a maximum possible contrast. The charts are designed in such a manner that the measured visual acuity indicates the ability of the subject to resolve fine spatial details. However, in the real world one needs to perceive and resolve objects of different sizes, shapes and contours under varying contrast, where as visual resolution depends not only upon the size of the objects but also on their brightness in comparison to their background [1, 2]. It is common experience that some patients continue to complain about their poor vision, even though they possess good visual acuity by standard tests in the examination room. This visual deficit can be determined by measuring spatial contrast sensitivity functions, as been shown in cases of corneal oedema [3], cataract [4], ocular hypertension [5,6], glaucoma [5,7,8], optic neuritis [2], multiple sclerosis [9-11] and several other conditions. Modern contrast sensitivity tests consist of computer generated sinusoidal grating patterns, i.e. repetitive black and white bars of equal width, without sharp borders, under a constant luminance. Each adjacent dark and white

400 bar together form one cycle of grating pattern. Spatial frequency of the pattern is defined as the number of cycles subtending one degree of visual angle on the observer's retina [12], and the contrast of the grating pattern is defined as the luminance difference between light and dark bars divided by the sum of their luminance [2]. Contrast sensitivity is measured by presenting a number of sinusoidal gratings ranging from the finest detectable at high contrast to coarse patterns. By plotting the contrast sensitivity over the range of spatial frequencies a contrast sensitivity curve is obtained. Comparison of the contrast sensitivity curves of the test subject to those of normal individuals, contrast sensitivity deficit in the former can be determined. However, the equipment required to generate sinusoidal gratings is expensive, bulky and the test is rather lengthy, which preclude the use of contrast sensitivity measurements in a routine practice. Arden and Jacobson [13] solved many of these difficulties by printing sheets of gratings, which can be shown to the observer one by one. More recently, Ginsberg [14] has designed a chart of different gratings that is easy to use in routine. The chart, called VCTS, is marketed by VISTECH Consultants Inc., to be used for screening of patients for a variety of ocular disorders, i.e. cataract, glaucoma, amblyopia, pituitary adenoma, multiple sclerosis, and as an aid in evaluation of contact lens fitting/lens damage, low vision, (improved) refraction and patient or family member education [15]. We have evaluated the VCTS chart as a screening test for the predictability of diagnosis in our outdoor patients.

Subjects, materials and methods VCTS chart

The contrast sensitivity chart consist of 5 rows of round printed patches of sinusoidal gratings, about 7 cm in diameter. The rows are marked A, B, C, D and E starting from above. Each successive row exhibits increasing spatial frequency, i.e. 189 3, 6, 12, 18 cycles/degree. Each row, from left to right, is marked from 1 to 9 and contains one sample patch and eight test patches having a progressively decreasing contrast in about 0.1 log unit steps. The gratings are either vertical or tilted 15~ to the left or right of the vertical in different patches. The contrast and orientation of the patches are randomized for each row. A sixth row of 4 patches at the bottom of the chart demonstrates the orientation of gratings to the patient and also contains a 'blank' patch, without gratings. The patient is instructed to read the chart showing this row. The chart is read from left to right, from a 3 meter distance, and in each

401 row the number of patches in which orientation of gratings is detected correctly is noted. These numbers are recorded on a small chart, supplied by the manufacturer to plot the contrast sensitivity curve for each eye separately. This curve is compared with the range of contrast sensitivity curves for a group of normal population plotted on the same chart. Following the instructions supplied by the manufacturer, the curves deviating from the normal contrast sensitivity values can be interpreted to predict the diagnosis of ocular problem.

Subjects A total of 211 patients with diverse ocular complaints who presented for consultation at our clinic were included in the study without preselection. The mean age of the patients was 53 years, varying from 12 years to 85 years. They included 122 females and 89 males. Visual contrast sensitivity was also tested in a control group of 30 healthy volunteers, 15 men and 15 women, without any history of eye disease. The age range in this group was from 22 to 63 years, with an average of 27 years.

Procedure Contrast sensitivity curves for each eye of the test person were drawn using the VCTS-chart, according to the instructions of the manufactures. The test persons were allowed to wear their spectacles or contact lenses. Reading of VCTS chart was followed by the visual acuity measurements by Landolt-C chart. Visual acuity and contrast sensitivity measurements were done by the same person. Subsequently, a thorough ocular examination of all subjects included in the study was performed by a different person to determine the diagnosis of ocular condition and to administer therapy. Additional tests like flouresceine angiography, visual fields, colour vision tests, etc. were done whenever necessary. Findings of the contrast sensitivity tests were not revealed to the person who performed clinical examination. Then, the records of both examiners were compared to determine whether the diagnosis predicted by using the VCTS chart could be confirmed by clinical examination.

Results

The contrast sensitivity curves of the healthy volunteers did not differ from the range of contrast sensitivity distribution for a normal population. Of the 211 patients, Table 1, contrast sensitivity curves indicated a diag-

402 Table 1. The number of patients with either cataract or glaucoma according to VCTS chart screening and clinical diagnosis.

Clinical diagnosis

Diagnosis from the VCTS chart contrast sensitivity curve Not specified*

Cataract

Glaucoma

Total

Not specified" Cataract Glaucoma

119 5 8

29 19 7

13 none 11

161 24 26

Total

132

55

24

211

* Data not analysed in detail.

nosis of glaucoma in 24 patients and cataract in 55 patients. The remaining 132 cases had either normal contrast sensitivity curves or they did not show any alterations characteristic for a specific eye disease. Of the 55 patients that should have had cataract according to the contrast sensitivity curves, clinical ocular examination confirmed the diagnosis in only 19 patients, i.e. 34.5% true positive detection by the VCTS chart test. The remaining 36 patients did not have cataract (65.5% false positive). Seven of these patients had actually glaucoma and the others had presented with different other ocular disordersl Clinical examination revealed cataract in 5 more patients who had escaped detection by the VCTS chart screening, so that the total number of patients who actually had cataract in our study was 24. This gives a false negative cataract predictability incidence of 20.8 %. Similarly, glaucoma was clinicially diagnosed in 11 out of 24 patients that were judged to have this disease by contrast sensitivity measurements, giving a true positive detection rate of 45.8%. Thirteen patients were falsely (54.2%) indicated to have glaucoma by the VCTS chart screening. None of these patients had cataract; they had other different ocular disorders. Clinical ophthalmological examination revealed glaucoma in 15 additional patients, who had escaped detection by the VCTS chart testing, bringing the actual number of glaucoma patients to 26, in this series. Thus VCTS chart screening gave a false negative detection of glaucoma in 57.7% cases. As the rate of true positive glaucoma and cataract detection was low, and the rates of both false positive and false negative estimations by the VCTS chart screening were high, we decided not to analyse the data of remaining patients in whom other ocular diseases were clinically diagnosed. The types of contrast sensitivity deficit observed in all patients are shown in table 2. Although the visual acuity measured by the Landolt-C chart corresponded to the visual acuity measured by Snellen chart by the clinician, we could not find any co-relation between the contrast sensitivity curves and the

403 Table 2. Type of abnormality observed in the contrast sensitivity. Diagnosis

Contrast sensitivity deficit None

Cataract Glaucoma Remaining Total

Total

Mid-frequency

Mid-High frequency

High frequency

4 7 97

9 13

20 9 44

1 7

108

22

8

211

24 26 161

visual acuity in our patients. |t was not possible to predict the visual acuity from the contrast sensitivity curves obtained from the VCTS chart.

Discussion

Evaluation of contrast sensitivity has demonstrated visual deficit in a variety of ocular conditions [3-11,16-18]. The reported data in literature pertain to patients having a specific eye disease. However, in the present study we have used the contrast sensitivity measurement as a screening test. The manufacturer of the VCTS chart, that we employed to measure contrast sensitivity, claims this test to be of diagnostic value in a clinical practice [15]. Following the instructions of the manufacturer [15], we attempted to classify 211 patients, after VCTS chart screening, into various disease groups. The data presented in this paper on cataract and glaucoma patients reveals very low rate of correct diagnosis based on the abnormality of contrast sensitivity curve. VCTS chart curves indicated the presence of cataract in 55 patients. However, true positive detection was found in only 34.5% of cataract patients as compared to 65.5% false positive indication of cataract. Seven patients, who actually had glaucoma, were wrongly included in the cataract groups on the basis of VCTS chart test. An additional 5 cataract patients had escaped detection by VCTS chart, giving a false negative detection rate of 20.8 % in a group of 24 cases wfio were found to have cataract upon clinical examination. The results in the case of glaucoma were equally unsatisfactory. VCTS chart testing indicated the presence of glaucoma in 24 cases, of whom 45.8 % really had glaucoma and 54.2% did not have glaucoma. Moreover, VCTS chart test did not indicate glaucoma in 15 additional patients, giving a very high false negative detection rate of 57.7% in a group of 26 actual glaucoma patients.

404 The above observations on glaucoma and cataract patients dissuaded us from analysing the data on remaining patients. Although, specific contrast sensitivity deficits, i.e. in low, middle or high spatial frequencies have been reported in different ocular disorders [12,15], very poor results using the VCTS chart in this study are difficult to explain. However, the set up of our investigation was different as we included patients at random in the study without prior clinical examination, whereas the data reported in literature pertains to subjects selected after the diagnosis is established by clinical examination. Moreover, we tested the patients only once, as would be the case if the VCTS chart is to be used for screening purpose, while experienced subjects, who are tested more than once, are likely to give different results [9], as they may misleadingly appear to be more sensitive at the highest spatial frequencies [19]. The limitations of the contrast sensitivity measurement with respect to its clinical application might have been partly responsible for the erroneous results. When applying any contrast sensitivity test for screening purpose, one is inclined to ask what types of contrast sensitivity abnormalities one would detect when several ocular disorders are present simultaneously, e.g. in a patient with a small refractive error, ocular hypertension and early senile cataract. Should the contrast sensitivity curve of another patient with established glaucoma, a small refractive error and an early senile cataract, necessarily be different. From the available published data the answer seems to be negative, as not all patients with a particular disease exhibit a consistent and unequivocal contrast sensitivity deficit. Only 63% of the ocular hypertension and glaucoma patients of Ross et al [6] had abnormal contrast sensivity curves. Regan et al [9] detected a loss of contrast sensitivity in 20 of 48 multiple sclerosis cases. In view of these observations, and our own findings reported here, contrast sensitivity measurement, at present, does not appear to be useful for screening patients in a clinical practice. Contrast sensitivity tests are, nevertheless, important adjunct investigational tools to detect 'hidden' visual loss in patients with good visual acuity, but their role in a clinical practice is not yet completely defined [20]. The results presented in this article refute the claims of the VCTS chart manufacturer, although we are unable to explain whether the failure of this test in our hands was due to the inferiority of the chart as a test device or due to the limitation of the technique itself. In any case, it will be imprudent to base clinical decisions on the contrast sensitivity curve obtained with a VCTS chart rather than on a carefull and thorough clinical ocular examination. Otherwise a device bought to 'build practice' may, to the contrary, ruin it.

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Address for offprints: Prof. Dr. A. Th.M. van Balen, Free University Hospital, Department of Ophthalmology, P.O. Box 7057, 1007 MB Amsterdam, the Netherlands.