|Year : 2017 | Volume
| Issue : 2 | Page : 91-96
Comparison of vertical cup-disc ratio and disc damage likelihood scale with respect to visual field global indices in primary open-angle glaucoma patients: A cross-sectional study
Smita Narayan, Reshmi Sreekumari
Department of Ophthalmology, Govt. Medical College, Thrissur, Kerala, India
|Date of Web Publication||10-Aug-2017|
Sreepury (H), Balussery P.O, Kozhikode - 673 612, Kerala
Source of Support: None, Conflict of Interest: None
Background: In primary open-angle glaucoma (POAG), structural changes in optic nerve head precede functional changes in visual field (VF). Hence, it is important to detect glaucoma early as well as to monitor changes in glaucomatous damage.
Aims: The aim of this study was to determine the correlation between each of vertical cup/disc ratio (VCDR) and the disc damage likelihood scale (DDLS) with global indices of Humphrey Field Analyzer II (HFA II).
Methods: A total of 142 eyes of 71 patients diagnosed with POAG who fulfilled the inclusion and exclusion criteria were examined to grade DDLS score VCDR. HFA II Swedish Interactive Threshold Algorithm Standard 24-2 VFs were obtained.
Statistical Analysis: The correlation of cup/disc (C/D) ratio with mean deviation (MD) and pattern standard deviation (PSD) was calculated by Spearman's correlation coefficient. Similar coefficients were obtained for DDLS also. Correlation values of selected variables were compared to find the difference in correlations.
Results: DDLS shows strong negative correlation with MD (− 0.725) and strong positive correlation (0.625) with PSD when compared to C/D ratio. However, the differences between these correlations were not statistically significant.
Conclusion: The DDLS shows stronger correlation with VF global indices than C/D ratio. Attention to disc diameter and rim width may increase the value of clinical optic disc examination
Keywords: Disc damage likelihood scale, global indices, primary open-angle glaucoma, vertical cup/disc ratio
|How to cite this article:|
Narayan S, Sreekumari R. Comparison of vertical cup-disc ratio and disc damage likelihood scale with respect to visual field global indices in primary open-angle glaucoma patients: A cross-sectional study. Kerala J Ophthalmol 2017;29:91-6
|How to cite this URL:|
Narayan S, Sreekumari R. Comparison of vertical cup-disc ratio and disc damage likelihood scale with respect to visual field global indices in primary open-angle glaucoma patients: A cross-sectional study. Kerala J Ophthalmol [serial online] 2017 [cited 2019 Jun 25];29:91-6. Available from: http://www.kjophthal.com/text.asp?2017/29/2/91/212761
| Introduction|| |
Glaucoma is a chronic progressive optic neuropathy characterized by loss of retinal nerve fiber layer (RNFL), recognized clinically as loss of neuroretinal rim (NRR) and visual field (VF) defects. Glaucoma burden estimates show that 4.5 million people have become bilaterally blind due to primary open-angle glaucoma (POAG) by 2010 and are expected to rise up to 5.9 million by 2020. It is already well known that there is a structure to function correlation between loss of NRR and VF changes in POAG. RNFL defects precede the development of detectable optic disc and VF changes.,,, About 40% of retinal ganglion cells will be damaged by the time the VF changes are first manifested., This highlights the importance of early detection of structural changes in NRR.
The clinical standard for detection of disease and its progression has been automated perimetry (e.g., Humphrey 24-2 VF) and clinical assessment of the optic nerve cup. However, once moderate VF loss occurs (in the range of 12– 15 dB mean deviation (MD) loss or more), perimetric test– retest variability rises substantially and limits a reliable determination of VF change. The limitations of reliability and reproducibility for VF measurement as the main parameter in the assessment of glaucoma damage inhibit optimal patient care and research into improved treatments.
Nowadays, many instruments such as Heidelberg retinal tomography (HRT), scanning laser polarimetry (SLP), optical coherence tomography (OCT), etc., have been designed to detect early structural changes, but each has a special flaw – a nonreproducible parameter which reduces its usefulness. For example, HRT needs the operator to manually outline the disc margin and the use of a reference plane in the calculation of many stereometric parameters. SLP instruments such as GDX are affected by anterior and posterior segment pathologies. OCT image quality is affected by ocular opacities and Stratus OCT cannot ensure that the measurements are obtained from the same location in baseline and follow-up scans. Hence, direct visualization of optic disc and recording the changes is the best possible way to detect early glaucomatous damage.
The most popular method of estimating disc changes is vertical cup/disc ratio (VCDR). However, the ability to detect glaucoma based on VCDR is limited due to its variability in normal population. There is significant intra- and inter-observer variability in estimating VCDR.,, It also does not take into account the size of optic disc, which is a significant factor in cup-to-disc ratio (CDR).
A newly introduced method of detecting disc changes is disc damage likelihood scale (DDLS) [Table 1]. This is useful in the diagnosis of glaucoma and to categorize the severity of glaucoma. It is particularly useful in monitoring the course of glaucoma while VCDR is of little value in this aspect. However, it is time-consuming and difficult to master; hence, many clinicians are not using it.
MD and pattern standard deviation (PSD) are the global indices which gives a numerical quantification of VF loss in decibels. Early structural changes can be identified by the depression which they produce in actual threshold values of MD and PSD.
The purpose of this study is to compare the correlation of DDLS and VCDR with respect to MD and PSD in glaucomatous eyes. Our aim is to detect early structural changes, which has not yet produced a field defect, but are very likely to do so in the near future.
| Methods|| |
The study was performed after getting informed written consent from the patients and approval by the Institutional Review Board. This cross-sectional study was conducted from June 2014 to May 2015. The inclusion criteria was all diagnosed cases of POAG with best-corrected visual acuity >6/60 in the worse eye and age >30 years. Subjects who had spherical errors >5D, cylindrical errors >2.5D, concomitant ocular disease with raised intraocular pressure, cloudy media which impairs fundus examination, history of ocular trauma, and history of intraocular surgery were excluded from the study.
A sample size of 71 was obtained using the G*Power (Kiel, Germany) software with the following formula assuming a power of 90%
N = sample size; r = Pearson's correlation coefficient (0.4).
Zα = alpha error (5% i.e., 1.96); Zβ = beta error (10% i.e., 1.282).
We defined POAG as patients having an open angle on gonioscopy and glaucomatous optic nerve damage (generalized enlargement of cup >0.5 or asymmetry of cup >0.2 or focal enlargement of cup with the presence of typical thinning in the NRR or notching). Detailed history taking and slit-lamp examination were done for all patients to rule out secondary causes of glaucoma. Intraocular pressure was measured using Goldmann applanation tonometry. One examiner performed the fundus examination to assess the VCDR and DDLS staging using slit-lamp biomicroscopy (Haag-Streit slit lamp and Volk 90D lens). Disc size and NRR thickness in various quadrants of the disc were measured using the same. Stereo photographs of the disc were also taken. The other examiner tested the VF on Humphrey Field Analyzer II (HFA II) (SITA) standard using 24-2 program within 1 month of enrollment. Three separate tests were conducted for each eye. Global indices (MD, PSD) of the third field test were taken for the study purpose. The examiner who assessed the optic nerve head was unaware of the VF status.
Data were coded and entered into Microsoft Excel. It was then analyzed using Microsoft Excel. Spearman's correlation coefficient was used to find out the correlation between various study variables. Correlation values of selected variables were compared. Z-test was performed to find the difference in correlations.
| Results|| |
Our study included 142 eyes of 71 POAG patients. This included 32 males and 39 females. The mean age of patients was 57.87 years (SD = 8.56). The mean AT was 14.69+/− 3.826. 126 eyes had a disc size 1.5– 2 mm. The mean visual field index (VFI) was 76.06 ± 24.164. 75% of eyes satisfied 3 Anderson's criteria and 25% eyes satisfied 2 Anderson's criteria. 57 eyes had MD >− 6, 36 eyes − 6 to − 12 and 49 eyes <− 12. 53 eyes had VCDR 0.5– 0.7 and 65 eyes >0.7 [Figure 1]. 108 eyes had DDLS stage 0– 3, 25 eyes stage 4 and 5 and only 9 eyes stage 6 and 7 [Figure 2].
|Figure 2: Bar chart showing distribution of disc damage likelihood scale stages|
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Spearman's correlation coefficient was calculated for the following combinations: VCDR versus MD, VCDR versus PSD, DDLS versus MD, DDLS versus PSD, VCDR versus VFI, and DDLS versus VFI. [Table 2] shows strength of correlation between different parameters.
Correlation values of selected variables (MD, PSD, and VFI) with VCDR and DDLS were compared. Z-test was performed to find the difference in correlations. This showed no significant difference between correlations. [Table 3] shows summary of P values showing difference between correlations.
Scatter plots were obtained for the relation of DDLS and VCDR with field MD and PSD. There was a strong inverse correlation between DDLS stage and MD and between VCDR and MD. A positive correlation was found between DDLS stage and PSD and also between VCDR and PSD [Figure 3],[Figure 4],[Figure 5],[Figure 6].
|Figure 4: Relation of pattern standard deviation and vertical cup/disc ratio|
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|Figure 6: Relation of pattern standard deviation and disc damage likelihood scale|
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| Discussion|| |
In our study, we are analyzing which technique of quantifying disc changes, i.e., VCDR or DDLS correlates better with the global indices of HFA II of the given patient with POAG. Previous studies have already demonstrated that, as the glaucomatous damage progress, the area of the NRR progressively diminishes and its form continuously changes.,, The glaucomatous NRR loss takes place in a sequence beginning in the inferotemporal region and progressing to the superotemporal, temporal horizontal, inferior nasal, and finally superior nasal sectors.
The VCDR is the age-old method of assessing glaucomatous damage to optic disc. However, it has several shortcomings. It only indirectly examines the NRR tissue, concentrating on the width of the hole rather than the surrounding rim tissue that determines its border. Furthermore, the examiner is focused on the ratio in the vertical disc axis and may overlook focal thinning in an oblique axis. The VCDR does not take into consideration the optic disc size. Hence, large discs which are likely to have larger VCDR (but may have normal NRRs) are more likely to be classified as glaucomatous while small discs with small VCDR are more likely to be classified as normal, whether they actually have glaucoma or not.
The two major advantages of DDLS: first, it considers disc size and second, it focuses on how much NRR tissue is present. Consideration of disc size is central to the DDLS process. By categorizing discs as small, medium, or large, the expectation of rim thickness is adjusted. This reduces misclassification bias based on disc size. The examiner is forced to examine the rim throughout its circumference, documenting the area of greatest thinning.
DDLS has some limitations. It is theoretically possible that a patient with a static DDLS grade may have advancing damage, for example, if focal notching of the disc was followed by generalized atrophy. For this reason, patients should be monitored longitudinally with other modalities. It is also not easy to apply to tilted optic nerves or to those with sloped temporal rims. Furthermore, DDLS does not offer a system to document progression in >1 region or a new region of the optic disc until it is more severely involved than the originally documented area. The method requires some effort to learn and is best carried out with the table of stages at hand during slit-lamp evaluation of the fundus. While the cup/disc (C/D) ratio is ingrained as a clinical tool, the additional discipline of systematic DDLS grading adds value to clinical observation.
Our results demonstrate that using Spearman's correlation coefficient, DDLS shows a strong negative correlation with MD in VF (− 0.725) than VCDR with MD (− 0.639). DDLS also shows a strong positive correlation with PSD (0.643) when compared to VCDR with PSD (0.585). Correlation between DDLS and VFI is also strongly negative (− 0.705) than VCDR and VFI (− 0.624). All these results are statistically significant with P = 0.000, but the differences between these correlations were not statistically significant. Therefore, it is evident that both VCDR and DDLS are excellent tools for diagnosing glaucomatous disc changes. DDLS is better among the two as each of its correlation values is higher. It is again emphasized that the difference is not significant statistically.
A study was conducted by Danesh-Meyer et al. to evaluate the relative diagnostic strength of C/D ratio, DDLS, and HRT-II in patients with glaucoma, glaucoma suspects, and normal controls. The MD on Humphrey VF assessment using SITA standard was − 4.95 D (SD 5D). Clinical examination using DDLS had the best predictive power with an area under the receiver operating characteristics curve value of 0.95 when glaucoma patients and suspects were separated from borderline or normal.
A comparative study of two methods of optic disc evaluation in patients of glaucoma was conducted by Chandra et al. showed that the difference between correlations of C/D ratio and MD versus DDLS and MD were not statistically significant. Similarly, PSD did not show statistical significance. However, P value for the difference in the correlation coefficient r between C/D ratio with RNFL and DDLS with RNFL was statistically significant (P < 0.01) when correlation of C/D, DDLS with RNFL was considered. The disc diameter and rim width increase the value of clinical optic disc examination.
Bobrow also found a negative correlation (− 0.68) between DDLS and MD as was obtained in our study. Capriolidemonstrated that NRR area correlates more strongly with field damage than C/D ratio or cup volume.
In a study conducted by Kara-Jose et al., a strong positive correlation was found between DDLS and CDR (Spearman r = 0.82; P < 0.001). Weaker correlations were found between DDLS and VF MD index (r = − 0.51; P < 0.001).
Several limitations must be acknowledged in this study. Since this study was based on a smaller group, the results would require confirmation by further studies involving larger number of POAG patients. The sensitivity and specificity of the two testing methods, VCDR and DDLS, could not be calculated as the study population did not include normal people without POAG. Correlation of various stages of DDLS and VCDR with MD and PSD was not compared.
| Conclusion|| |
Both VCDR and DDLS show good correlation with MD and PSD in HFA II. However, DDLS shows greater statistically significant correlation. In DDLS, each grade is assigned a numerical value, and hence, this system can be used in research settings to determine severity or degree of progression. VCDR on the other hand is an excellent tool for clinical evaluation of optic disc.
We would like to thank Dr. K.C. Rajini, HoD Ophthalmology Govt. Medical College, Thrissur, Kerala, India.
Mrs. Rejani P.P, Assistant Profess, Medical Statistics, Govt. Medical College, Thrissur, Kerala, India.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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