A Study on Frequency of Visual Field Testing in Glaucoma
A Study on Frequency of Visual Field Testing in Glaucoma
This multi-centre cross-sectional study shows that no patients received the frequency of VF testing recommended by Chauhan et al and the EGS guidelines (six VFs in the first 2 years). Typically, patients had two to three VF tests in the first 2 years following diagnosis. The median time for six VF tests to be undertaken was more than 4 years. Frequent VF tests after initial diagnosis help to detect VF changes, and could determine the speed of disease progression in each individual patient, which subsequently allows the management to be appropriately tailored. It is important to note that the speed (rate) of VF progression varies widely between patients and timely detection of progression requires accurate and consistent measurement of VFs over years. Notably, the frequency of VF testing suggested by the EGS guidelines and Chauhan et al aims to determine, with good certainty, rapidly changing eyes (MD <−2 dB/year). Therefore, patients with slower progression will not be identified as quickly (at the same statistical power).
For monitoring patients with glaucoma, the clinician's aim is to try and maintain patients' visual function during their lifetime. Risk of visual impairment depends on the stage of disease at presentation, the life expectancy as well as the rate of visual deterioration. Clearly, patients with rapid VF progression are at greater risk of suffering blindness than patients with slow progression; hence, one method to identify those patients who require the most attention is to calculate their rates of VF loss over time. Significantly, recommendations of Chauhan et al do not incorporate information about a patient's level of VF damage at presentation, which is an equally important consideration when trying to ensure that a patient's visual function remains unaffected. Considering treatment and monitoring costs, it is imperative that resources are prioritised in favour of those patients who are in the greatest danger of suffering visual disability in their lifetime.
A significant finding of the current study was the observation that the value of VF test results for assigning future VF intervals to clinicians seems limited. Our results suggest that overall clinical impression is driven by the adequacy of IOP control rather than VF test findings. IOP control is a modifiable risk factor for disease progression and, therefore, it requires careful surveillance. Nevertheless, perimetry is the only direct method for monitoring functional change in patients. Therefore, it is surprising that no relationship was found between stability of VF defects, or the severity of disease, and the monitoring interval requested. While recommended monitoring intervals were considered as NICE compliant, VF intervals were variable for a given level of disease severity. The low frequency of VF testing in newly-diagnosed patients observed in this study, at first, seems at odds with the statistic that 87% of decisions on monitoring intervals were compliant with NICE guidance. There is insufficient evidence to conclude which guidelines, EGS or NICE, are likely to be most appropriate, but more research is necessary to establish the clinical-effectiveness and cost-effectiveness of the recommendations. Furthermore, unlike the EGS guidelines, the exact NICE monitoring interval depends on the disease severity and IOP relative to target IOP. So, for patients with stable disease and adequate IOP, annual monitoring is within NICE guidelines. Further, the intervals given by NICE are fairly wide, so that the permitted interval spans 2–6 or 6–12 months. An earlier single-centre study by Tatham and Murdoch also found that assigned intervals were in accordance with NICE guidance.
It is important to note that the current NICE guidelines have been unable to identify any evidence with sufficient quality about the role of VF testing in glaucoma monitoring; this study therefore provides important data on current UK practice of VF monitoring. These data are useful for healthcare planning for improving hospital resources. It is presumed that the current rate of VF testing is consistent with the VF capacity of hospital glaucoma outpatient services in terms of cost and staff restrictions. Increasing the number of VFs from the current practice to meet EGS guidelines would require an approximately threefold increase in the number of VFs for newly-diagnosed patients in the first 2 years, yet there is no evidence for whether the EGS guidelines are feasible with the resources available. For example, if resources are to be diverted, what is the detriment to other patients whose VF test frequencies would become even less frequent? This represents an important health service delivery research and health-economic question given the large number of patients being monitored. Some work on these issues is currently being undertaken. Furthermore, other studies have investigated other types of VF follow-up schemes which may be more efficient.
This retrospective study has its limitations. It is assumed that the six units sampled are representative of national practice. Being a cross-sectional study, sampling bias is an inherent issue. Further, for each centre the patients sampled during the study week are assumed to be representative of the general population of COAG patients seen in that unit. The number of patients from Portsmouth was only four, which is not comparable with other participating centres. However, out of the 32 patients seen in Portsmouth during the study week, 28 patients did not meet the study inclusion criteria (17 had a diagnosis other than COAG and 11 had recent intraocular surgery). Although the purpose of our study was not to draw comparisons between participating hospitals, this unequal sampling across units could bias our results. It is also worth noting that patients requiring more intensive control are more likely to be included in the study, and so there are likely to be a higher proportion of patients with poor IOP control (and advanced glaucoma severity) than those with adequate IOP control in the study population. However, this is likely to make the sample representative of the distribution of patients (in terms of proportions with different IOP control and disease severity) seen in the clinic annually.
Data were collected on patients since initial diagnosis and the additional assumption has been made that clinical practice, in terms of monitoring intervals, has remained unchanged during this time. In addition, clinical note taking was often poor or incomplete; several patient records contained no comments on adequacy of IOP control, VF status or overall disease status. Inevitably with any retrospective analysis of patient records it is difficult to draw certain conclusions since notes are often made by multiple clinicians, with varying levels of expertise. Where available, the clinician's comments regarding optic disc and/or VF progression were assumed to be documented and an accurate representation of the patient's clinical state in each case.
In conclusion, this study shows that the number of VF tests performed in newly-diagnosed COAG patients in the first 2 years of follow-up is much lower than the recommended number suggested by current research and EGS guidelines. On average, less than one VF test was performed per year over the duration of follow-up in the patients captured by this cross-sectional study. Monitoring intervals for the majority of patients were in compliance with NICE glaucoma guidelines, although the importance of VF testing on monitoring intervals is limited. Further studies to determine the effect of VF testing intervals on patient outcomes and cost-effectiveness are required.
Discussion
This multi-centre cross-sectional study shows that no patients received the frequency of VF testing recommended by Chauhan et al and the EGS guidelines (six VFs in the first 2 years). Typically, patients had two to three VF tests in the first 2 years following diagnosis. The median time for six VF tests to be undertaken was more than 4 years. Frequent VF tests after initial diagnosis help to detect VF changes, and could determine the speed of disease progression in each individual patient, which subsequently allows the management to be appropriately tailored. It is important to note that the speed (rate) of VF progression varies widely between patients and timely detection of progression requires accurate and consistent measurement of VFs over years. Notably, the frequency of VF testing suggested by the EGS guidelines and Chauhan et al aims to determine, with good certainty, rapidly changing eyes (MD <−2 dB/year). Therefore, patients with slower progression will not be identified as quickly (at the same statistical power).
For monitoring patients with glaucoma, the clinician's aim is to try and maintain patients' visual function during their lifetime. Risk of visual impairment depends on the stage of disease at presentation, the life expectancy as well as the rate of visual deterioration. Clearly, patients with rapid VF progression are at greater risk of suffering blindness than patients with slow progression; hence, one method to identify those patients who require the most attention is to calculate their rates of VF loss over time. Significantly, recommendations of Chauhan et al do not incorporate information about a patient's level of VF damage at presentation, which is an equally important consideration when trying to ensure that a patient's visual function remains unaffected. Considering treatment and monitoring costs, it is imperative that resources are prioritised in favour of those patients who are in the greatest danger of suffering visual disability in their lifetime.
A significant finding of the current study was the observation that the value of VF test results for assigning future VF intervals to clinicians seems limited. Our results suggest that overall clinical impression is driven by the adequacy of IOP control rather than VF test findings. IOP control is a modifiable risk factor for disease progression and, therefore, it requires careful surveillance. Nevertheless, perimetry is the only direct method for monitoring functional change in patients. Therefore, it is surprising that no relationship was found between stability of VF defects, or the severity of disease, and the monitoring interval requested. While recommended monitoring intervals were considered as NICE compliant, VF intervals were variable for a given level of disease severity. The low frequency of VF testing in newly-diagnosed patients observed in this study, at first, seems at odds with the statistic that 87% of decisions on monitoring intervals were compliant with NICE guidance. There is insufficient evidence to conclude which guidelines, EGS or NICE, are likely to be most appropriate, but more research is necessary to establish the clinical-effectiveness and cost-effectiveness of the recommendations. Furthermore, unlike the EGS guidelines, the exact NICE monitoring interval depends on the disease severity and IOP relative to target IOP. So, for patients with stable disease and adequate IOP, annual monitoring is within NICE guidelines. Further, the intervals given by NICE are fairly wide, so that the permitted interval spans 2–6 or 6–12 months. An earlier single-centre study by Tatham and Murdoch also found that assigned intervals were in accordance with NICE guidance.
It is important to note that the current NICE guidelines have been unable to identify any evidence with sufficient quality about the role of VF testing in glaucoma monitoring; this study therefore provides important data on current UK practice of VF monitoring. These data are useful for healthcare planning for improving hospital resources. It is presumed that the current rate of VF testing is consistent with the VF capacity of hospital glaucoma outpatient services in terms of cost and staff restrictions. Increasing the number of VFs from the current practice to meet EGS guidelines would require an approximately threefold increase in the number of VFs for newly-diagnosed patients in the first 2 years, yet there is no evidence for whether the EGS guidelines are feasible with the resources available. For example, if resources are to be diverted, what is the detriment to other patients whose VF test frequencies would become even less frequent? This represents an important health service delivery research and health-economic question given the large number of patients being monitored. Some work on these issues is currently being undertaken. Furthermore, other studies have investigated other types of VF follow-up schemes which may be more efficient.
This retrospective study has its limitations. It is assumed that the six units sampled are representative of national practice. Being a cross-sectional study, sampling bias is an inherent issue. Further, for each centre the patients sampled during the study week are assumed to be representative of the general population of COAG patients seen in that unit. The number of patients from Portsmouth was only four, which is not comparable with other participating centres. However, out of the 32 patients seen in Portsmouth during the study week, 28 patients did not meet the study inclusion criteria (17 had a diagnosis other than COAG and 11 had recent intraocular surgery). Although the purpose of our study was not to draw comparisons between participating hospitals, this unequal sampling across units could bias our results. It is also worth noting that patients requiring more intensive control are more likely to be included in the study, and so there are likely to be a higher proportion of patients with poor IOP control (and advanced glaucoma severity) than those with adequate IOP control in the study population. However, this is likely to make the sample representative of the distribution of patients (in terms of proportions with different IOP control and disease severity) seen in the clinic annually.
Data were collected on patients since initial diagnosis and the additional assumption has been made that clinical practice, in terms of monitoring intervals, has remained unchanged during this time. In addition, clinical note taking was often poor or incomplete; several patient records contained no comments on adequacy of IOP control, VF status or overall disease status. Inevitably with any retrospective analysis of patient records it is difficult to draw certain conclusions since notes are often made by multiple clinicians, with varying levels of expertise. Where available, the clinician's comments regarding optic disc and/or VF progression were assumed to be documented and an accurate representation of the patient's clinical state in each case.
In conclusion, this study shows that the number of VF tests performed in newly-diagnosed COAG patients in the first 2 years of follow-up is much lower than the recommended number suggested by current research and EGS guidelines. On average, less than one VF test was performed per year over the duration of follow-up in the patients captured by this cross-sectional study. Monitoring intervals for the majority of patients were in compliance with NICE glaucoma guidelines, although the importance of VF testing on monitoring intervals is limited. Further studies to determine the effect of VF testing intervals on patient outcomes and cost-effectiveness are required.
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