This cross-sectional study from a South Indian glaucoma population demonstrates a consistent pattern of worsening of functioning (poorer GAL-10 scores) with increasing amounts of VF loss across the disease spectrum. This is consistent with outcomes reported by similar such studies in the Western glaucoma populations.
16,42–44 Patients with severe VF loss reported significantly worse functioning due to glaucoma. However, those with mild and moderate VF loss also reported functioning difficulties, providing evidence of the impact of VF loss and of a dose–response relationship: increasing severity of VF loss in the eye with better MD was associated consistently and independently with increasing activity limitation (i.e., lower negative logit scores on the GAL-10). Participants with severe VF loss had reductions in GAL-10 score of 72% (as compared with mild VF loss) and 61% (as compared with moderate VF loss). These findings are consistent with those of Nelson et al. who developed and used the GQL-15.
16
Our results are in accordance with those from developed countries that have demonstrated the deleterious effects of VF loss on vision-specific functioning in glaucoma.
16,43–48 For example, the population-based Los Angeles Latino Eye Study (LALES) used the National Eye Institute-Visual Function Questionnaire (NEI-VFQ) and reported that greater VF loss in patients with OAG impacts on the vision-related QOL.
47 Parrish et al. demonstrated that perceived difficulty with driving increases with worsening VF damage in the better eye of glaucoma patients.
49 Using the same GQL-15 as in the present study, Goldberg et al. reported that the GQL-15 scores differ significantly among patients with mild, moderate, and severe glaucoma, demonstrating a trend of poorer functioning with increasing severity in an Australian clinic-based sample.
44 In a recent study of a Singapore hospital-based glaucoma sample that also used the GQL-15, Wang et al. demonstrated a significant and independent association between the severity of bilateral glaucoma and functioning.
26 They reported that the deterioration in functioning is clinically significantly worse in those with bilateral moderate/severe glaucoma. Taken together, these findings suggest that glaucoma impacts functioning transcending all barriers such as ethnicity, race, and culture.
Until now, all four studies that used the GQL-15 had been conducted in Western populations (United Kingdom, Australia, Germany, and Singapore).
16,25,26,44 Two of the four studies used the raw (Likert) scores from the GQL-15, but the recent two studies in the German and Singaporean populations have performed Rasch analysis of the GQL-15. Our results of the GAL-10 are unique given that there is a lack of literature regarding the use of glaucoma-specific questionnaires such as the GQL-15 from India.
While other studies have reported variables such as age
44 and a history of surgery for glaucoma to be related to visual functioning,
50 we failed to find such associations. In our study, the median duration since diagnosis of glaucoma was 6.5 years, and 58% were on combination therapy for glaucoma, so it is unlikely that our participants were unaware of their diagnosis/status of glaucoma (although this information was not collected by us). Nonetheless, the knowledge of the diagnosis and information about the status of glaucoma in patients under periodic follow-up could potentially influence their responses to items on the GAL-10.
51 The mean GAL-10 score in the present study was −0.81 ± 1.33 logits, indicating that our participants did experience activity limitation regardless of the duration of glaucoma. Therefore, the knowledge or history of glaucoma would have had a negligible impact (if any in terms of underreporting) on the responses to the GQL-15. Our belief is further supported by the LALES, which comprised 75% newly diagnosed glaucoma patients and 25% patients with a history of glaucoma, who were found to have differences in QOL scores (using NEI-VFQ and Short-form 12 [SF-12]) by VF loss status despite adjusting or excluding those with history of glaucoma and/or treatment history from the analyses.
47 In the present study, using multivariate analysis, we found that patients in the middle-income group reported statistically worse functioning than those in the higher income group; this was not clinically significant. This finding is perhaps not surprising given their socioeconomic status; patients in the middle-income group could have had an advanced stage of the disease at presentation and may have lacked compliance and adherence to medications resulting in uncontrolled intraocular pressures. Other variables not included in our study, such as psychological factors (presence of depression, mood state, anxiety) and family support systems, could all have hypothetically affected responses to the GQL-15. These factors require investigation in future studies.
As mentioned in the Results section, other investigators who wish to use the GAL-10 can use either the Excel spreadsheets developed by us to obtain the interval-level scores for their raw data (if their sample is similar to that of present study) or perform Rasch analysis on their own data. In the latter case, it is likely that a new study of GQL-15 in a different population could result in another Rasch version of the questionnaire.
A number of studies that have used questionnaires have revealed aspects of vision that most influence patients' beliefs about their vision.
16,43,45,47,49,51–54 However, the relationship between vision and performance of daily activities is much more complex than can be gleaned from questionnaires on self-reported functioning. Aspects of vision include the ability to detect motion, recognize patterns (acuity), distinguish borders (contrast sensitivity), appreciate color, and notice objects in different parts of the VF.
55 Given the results of previous studies using questionnaires in glaucoma patients, some correlation between clinical measures of vision and the GAL-10 score would be expected in our sample. The relationship between activity limitation and all clinical measures of vision used in the present study was equally strong. Nonetheless, the role of contrast sensitivity—the not-so-commonly recorded clinical measure—deserves mention. We found VA and contrast sensitivity to be moderately correlated with each other (
r = −0.63,
P < 0.0001). Correlations in the range of 0.5 to 0.6 with VA have been reported previously,
56 so our findings are as expected. This relationship perhaps suggests that contrast sensitivity is less likely to provide any new information at higher spatial frequencies as has also been demonstrated in the work by Elliott and Hurst.
57 However, Pelli-Robson contrast sensitivity provides information concerning low to medium spatial frequencies only, and this is the region that is most closely associated with activity limitation in glaucoma patients for tasks such as reading, face recognition, and mobility.
16 Therefore, in accordance with other investigators,
58,59 we believe that the Pelli-Robson contrast sensitivity test is likely to provide important information about the difficulties faced by glaucoma patients in performing daily activities, especially in those with near-normal VA.
While the pattern of these correlations was in the expected direction, all of these relationships possessed considerable scatter (
Fig. 4), which also appears to be consistent with several earlier studies.
16,43,60–63 In addition, the correlations were not significantly different in the better or worse eye for any of the clinical measures of vision. Other studies have also reported a lack of distinct difference in the correlation of visual functions of better or worse eye with self-reported visual functioning.
42,43,45 In our study, the worse eye acuity followed by better MD showed the strongest correlation with the GAL-10 score. This finding is similar to that by Jampel et al., albeit using a different questionnaire, the NEI-VFQ, in an American population.
60 As proposed by Jampel et al., the reason that worse eye acuity and better MD relate more closely to patient's perspective could be due to the difference in the psychophysical measurement of VA and VF, which reveals aspects of inter-eye performance that differ from VF testing.
60
The use of questionnaires in the assessment of activity limitation has pitfalls because they are, by their nature, subjective and are affected by various factors; for example, culture, language, education, social desirability, and so forth.
64–66 These nonvisual variables could be the reason for the scatter observed in patients' responses to questionnaires. Given the limitations of questionnaires, performance-based measures (PBMs), which involve testing what a person can and cannot do by actually observing the person attempting to perform specified tasks, have received increased attention.
67–69 Assessment of disability related to vision is one such PBM that has been developed recently in ophthalmology and has been shown to be a valid and reproducible method for assessing glaucoma patients.
61,68
The strengths of our study are the relatively large sample size and the inclusion of participants with mild to severe VF loss. The use of a glaucoma-specific questionnaire and Rasch analysis to validate it, so as to produce an estimated linear interval overall measure of activity limitation, is another significant strength of this study. To our knowledge, this is the first time that GQL-15 and its subscales have been subjected to Rasch analysis in a South Indian glaucoma population, and it has resulted in a valid unidimensional measure of activity limitation (GAL-10) in these patients. However, our study has some limitations. There was a male preponderance (67%), a higher proportion were literates (89%), and the majority were from urban areas (84%); however, this was perhaps expected given the tertiary eye care centre–based sample, so our sample may not be fully representative of the population, and the results cannot be generalized to glaucoma patients in a community setting. Furthermore, the cross-sectional design of our study has to be considered while interpreting the reduction in glaucoma functioning (GAL-10 scores) across subgroups of participants. The reduced scores are based on comparison between participants categorized into different groups and do not necessarily indicate a longitudinal shift in the activity limitation with a change in the VA or VF or the status of glaucoma. We studied the VA, CS, and VF in better and worse eye separately, but not binocularly. One could argue for the need to include binocular data. We did not explore the potential effects of loss of color vision and other psychophysical measures, including disability glare, dark adaptation, and stereopsis on activity limitation. These psychophysical measures have been reported to be compromised in glaucoma.
16 The potential effects of these measures on activity limitation need further exploration in futures studies.
In conclusion, our study provided evidence of the impact of glaucoma on visual functioning (activity limitation) as measured by GAL-10 in glaucoma patients. With superior measurement properties, the GAL-10 can be used in place of the original GQL-15 as a summary index of activity limitation for glaucoma patients, and its brevity may make clinical application simpler. By using Rasch analysis to generate an overall score with interval characteristics for the GAL-10, our data showed a dose-related gradient relationship between the severity of VF loss and activity limitation in a South Indian glaucoma population. Nonetheless, the limitations of questionnaires, including the GAL-10, in the assessment of activity limitation should be considered when interpreting the results. Given the potential advantages of PBMs of visual function, for example, the Assessment of Disability Related to Vision (ADREV) PBMs are likely to play an important role in the assessment of impact of glaucoma on patients. Nonetheless, PBMs have only recently been introduced in ophthalmology, and their relationship with questionnaires and clinical measures of vision is still being studied. Until PBMs gain wide acceptance, the results of studies using reliable and valid questionnaires, such as the GAL-10 in our study, would offer clinicians a reasonably good understanding of activity limitation from the glaucoma patient's perspective and serve as a guide in referral to low vision rehabilitation services at the appropriate time.