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FREQUENCY-DOUBLING TECHNOLOGY STAGING SYSTEM ACCURACY IN CLASSIFYING GLAUCOMATOUS DAMAGE SEVERITY
PAOLO BRUSINI and CLAUDIA TOSONI
Department of Ophthalmology, Santa Maria della Misericordia Hospital, Udine, Italy
Introduction
Frequency-doubling technology (FDT), which selectively analyzes My ganglion cells, is at present a widely used non-conventional visual field testing method to detect early glaucoma damage.1-8 The test is quick and easy to perform and the results are little affected by blur or pupil diameter. However, it may sometimes be difficult to interpret the results and to classify the severity of damage, especially in cases where only slight defects are present and when all points are abnormal. We recently designed a new staging system, which uses FDT mean defect (MD) and pattern standard deviation (PSD) values on a Cartesian diagram.9 This method divides defects into six stages of increasing severity, and supplies information on the type of functional damage.
The aim of this study is to evaluate the accuracy of this system in staging the severity of glaucomatous damage.
Material and methods
Seventy-six patients with chronic open-angle glaucoma at various stages of severity (mean age: 68.9 ± 11.4 years; range: 37-81 years) underwent standard automated perimetry (SAP, Humphrey 30-2 threshold test) and FDT (Welch-Allyn, Skaneateles Falls, NY, and Zeiss-Humphrey Systems, Dublin, CA), using the N-30 threshold test (19 areas tested within the 30° visual field). Only one randomly chosen eye was taken into consideration in the analysis of results. SAP tests were classified using both the glaucoma staging system (GSS)10 and the Hodapp et al. method.11
Structural damage was assessed both with optic disc stereoscopic examination, taking into account the C/D ratio, neuroretinal rim shape and amount of pallor, and with the GDx Nerve Fiber Analyzer (Laser Diagnostic Technologies, Inc., San Diego,
Address for correspondence: Paolo Brusini, MD, Via Pordenone 41, 33100 Udine, Italy. Email: brusini@libero.it
Perimetry Update 2002/2003, pp. 389–395
Proceedings of the XVth International Perimetric Society Meeting, Stratford-upon-Avon, England, June 26–29, 2002
edited by David B. Henson and Michael Wall
© 2004 Kugler Publications, The Hague, The Netherlands
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CA) considering GDx statistical indices and ‘the Number’. Finally, patients were subdivided into three groups (early, moderate, and advanced damage), according to a clinical classification based on both functional and structural damage, as follows:
•early damage: a. visual field test classified as Stage 1 of GSS or with an early defect according to the Hodapp et al. classification; b. cup/disc ratio <0.4, with no evident notching of the neuroretinal rim; and c. less than three GDx abnormal (p < 5%) statistical indices, or a Number <35;
•moderate damage: a. visual field test classified as Stage 2 or 3 of GSS or with a moderate defect according to Hodapp et al.; b. cup/disc ratio from 0.4 to 0.7, with localized notching of the neuroretinal rim or with some pallor; or c. 3 to 5 GDx abnormal (p < 5%) statistical indices and a Number between 35 and 70;
•advanced damage: a. visual field test classified as Stage 4 or 5 of GSS or with an advanced defect according to Hodapp et al.; b. cup/disc ratio >0.7, with wide loss of the neuroretinal rim and with a large area of pallor; or c. >5 GDx abnormal (p < 5%)
statistical indices, and a Number >70.
Then we measured the ability of the FDT staging system to classify the extent of glaucomatous damage. This new two-axis diagram uses the MD and PSD indices on the x and y axis, respectively (Fig. 1). Defects are classified into six stages (from stage 0, completely normal test, to stage 5, very advanced damage) and into three types (generalized, mixed, and localized).
Fig. 1. The FDT staging system.
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Table 1. Cross-tabulation (FDT staging system versus clinical classification)
FDT staging system |
Early |
Moderate |
Advanced |
|
damage |
damage |
damage |
|
|
|
|
Stage 0 |
10 |
0 |
0 |
Stage 1 |
18 |
2 |
1 |
Stage 2 |
5 |
4 |
0 |
Stage 3 |
0 |
9 |
2 |
Stage 4 |
0 |
4 |
11 |
Stage 5 |
0 |
0 |
10 |
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Results
According to our clinical classification, glaucomatous damage was distributed as follows: a. early damage: 32 cases; b. moderate damage: 20 cases; and c. advanced damage: 24 cases.
The different methods used to stage the damage generally correlated well. Only GDx results showed some disparities, either underestimating or overestimating the severity of glaucomatous loss in a few cases. An anomalous corneal birefringence not perfectly corrected by the integrated compensation system could be a partial explanation for these disparities.
A statistically significant correlation was found between the FDT staging system classification and the clinical classification of the severity of the damage (p < 0.001, Pearson chi-square test).
The relationship between these two classifications is shown in Table 1.
The correlation was almost perfect in advanced cases, but was also very satisfactory in early glaucoma (Figs. 2, 3, and 4).
Discussion
A standardized staging of glaucomatous functional damage severity would be very useful both in the research field and in day-to-day clinical practice. Using SAP, a number of methods have been proposed in the past, including the glaucoma staging system,10 Hodapp et al. classification,11 and AGIS classification.12
FDT is a relatively new testing method that has already been widely adopted. Various methods are available at present for staging the glaucomatous functional loss with FDT: a score obtained, taking both the depth of a defect and its location into consideration;13 an algorithm proposed by Sponsel et al.,14 based on analysis of the FDT probability map, and others. These methods usually work quite well, but are time consuming and require careful analysis of FDT data or repetitive calculations. Moreover, they can miss small, but significant peripheral defects or slight diffuse sensitivity depressions.
The FDT staging system, on the other hand, uses both MD and PSD indices (as does GSS) to classify the severity of a defect into six different stages. It is very fast and simple, as you only need to put two numbers on the chart. Moreover, as with GSS, this system is able to classify the defects into three types: generalized, mixed, and
Fig. 2. Right eye of a 66-year-old male with early open-angle glaucoma. The optic disc has early cupping at the superior pole, with a 0.3 C/D ratio. With GDx, the superior/nasal index only was outside normal limits, but the Number (= 56) indicated moderate damage. SAP (left) shows a relative, but definite nasal defect, classified as ‘localized defect Stage 1’ by GSS, and as ‘early damage’ by the Hodapp et al. classification. This case was clinically classified as having early damage. FDT (center) shows a nasal defect, classified as ‘Stage 1 mixed’ by the FDT staging system (right).
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accuracy system staging technology doubling-Frequency
Fig. 3. Moderate glaucomatous damage (0.7 C/D ratio with superior notching; GDx Number = 39) in a 60-year-old male with chronic glaucoma. SAP (left) shows an arcuate inferior defect (GSS ‘Stage 3 localized’; ‘moderate damage’ according to Hodapp et al.). FDT (center) shows an inferior and superior defect, classified as ‘Stage 3 localized’ by the FDT staging system (right).
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Fig. 4. Advanced damage in the right eye of a 75-year-old male with severe disc cupping with large inferior notching. GDx showed a severe atrophy of the nerve fiber layer especially in the inferior sector (the Number = 93). An absolute superior visual field defect, with a depression in sensitivity in the inferior hemifield is present (SAP, left). This defect is classified as ‘Stage 4 mixed’ by GSS and as ‘advanced damage’ according to Hodapp et al. With FDT, all points but one are significantly abnormal (center). The FDT staging system classifies this defect as ‘Stage 5 mixed’.
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localized. The chart area where these values intersect immediately shows you both the severity and type of the defect. Of course, this method, like the other systems, does not supply any information on the location or morphology of a defect. Only direct analysis of the FDT map can supply these data.
The results of this study confirm that FDT is able to quantify the glaucomatous damage, since it allows us accurately to grade the extent of damage, and can also supply an estimate of structural loss. However, this estimate of structural loss needs to be confirmed by further studies using a more accurate analysis of the optic nerve head. The FDT staging system may be used quickly and reliably to classify the extent of loss in glaucomatous patients, giving useful information on the type of defect.
References
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