BCVA is commonly used as an outcome measure in clinical studies and as a general measure of visual function. The need for an objective method of testing visual outcome, such as BCVA, in response to treatment is widely accepted. The importance of VFQ as a subjective method of vision assessment is also accepted, but it is acknowledged that BCVA has a low correlation with VFQ [14]. Microperimetry has been shown to correlate significantly with visual acuity (p = 0.0001) [18]. Our results now suggest that microperimetry could be a superior measure of visual outcome than visual acuity because it correlates better with VFQ.
Unlike microperimetry, visual acuity has no standardised measurement, as there are several different methods of examination in clinical use [4, 19, 20], making it difficult to directly compare results from different clinics and research centres. Visual acuity measures a person’s ability to discriminate between stimuli when presented on a highly contrasted background [4]. For routine visual acuity assessment in daily clinical practice, the Snellen Visual Acuity Chart or the ETDRS Chart is generally used [20].
Previous studies recommend the ETDRS method of visual acuity measurement in patients with AMD because it has better accuracy and reproducibility than Snellen, particularly in patients with advanced disease [4, 19, 21]. This is especially relevant as visual acuity may not be affected in patients with AMD until the disease has progressed into the late stage [2, 7, 10]. The ETDRS chart measures visual acuity from a distance of four metres, so a specially adapted room is required, which is not the case when using microperimetry. Moreover, the accuracy of the visual acuity assessment is often dependent on the competency level of the examiner, leading to inter-observer variability [20]. Unlike the visual acuity assessment, microperimetry is an automated functional test, meaning that the investigator does not run the same risk of acquiring unreliable data.
Microperimetry is a non-invasive procedure to assess macular sensitivity while the fundus is directly examined through live imaging [3]. Microperimetry enables clinicians to directly relate visual function to underlying fundus morphology, giving insight into the pathophysiology and natural history of retinal disease. Even in the presence of relatively good visual acuity, such as in the early stage of AMD, microperimetry can provide relevant information regarding macular dysfunction [5, 6]. Sugawara et al. have already shown a significant positive correlation (p = 0.0003) between macular sensitivity as measured by microperimetry and vision-related quality of life in patients with retinitis pigmentosa [22]; now our correlations reveal that macular sensitivity relates more closely to vision-related quality of life in patients with AMD than does the ETDRS measurement of visual acuity.
Microperimetry technology contains an eye-tracking system that automatically corrects the position of the stimulus when a patient changes their fixation. The Nidek MP-3 has an eye tracking system that automatically registers the position of the eye relative to anatomical landmarks twenty-five times per second [4]. Additionally, microperimetry has been shown to have high test-retest reliability even when visual acuity is poor and fixation is unstable and eccentric [23]. Microperimetry is therefore proven to be a useful tool in tracking disease progression when looking at treatment efficacy or performing a longitudinal study [2, 24].
The Nidek MP-3 microperimeter can provide an overall macular sensitivity by calculating the mean of all DLS points inside the region of the macula. Patients with AMD may show macular dysfunction that precedes noticeable vision loss [2, 24] and our results show that overall macular sensitivity is reduced in early and late AMD compared with eyes with healthy retina. Therefore, microperimetry may be a more sensitive screening tool for early disease than visual acuity.
In patients with AMD, DLS points can vary greatly in terms of their retinal sensitivity. This produces a wide range of results within the same macula: for example, a clinically significant difference in retinal sensitivity is found at the border of a scotoma. Analysis of retinal sensitivity at individual DLS points therefore allows for a more localised assessment of macular function [3, 5, 24] that can be helpful in the management of retinal pathology [7]. Macular subfield analysis can be used clinically in the management of patients with AMD to determine the impact of the disease on specific areas of the macula.
Although macular sensitivity is more closely aligned with vision-related quality of life than the ETDRS method of visual acuity testing, microperimetry is not without limitations. The investigation can be time-consuming and it requires good patient cooperation. Microperimetry in patients with AMD with unstable fixation can be even more time-consuming as in order for the investigation to proceed to eventual completion, eye movements must be either corrected automatically by the microperimeter or manually by the technician. Furthermore, microperimetry equipment comes at a cost to the healthcare provider [25], although we might reasonably expect the apparatus to become less expensive over time.
We chose to correlate macular subfields with the near-distance activities sub-score of the VFQ because the inability of patients with AMD to maintain steady fixation is strongly associated with slower reading [26]. This particular aspect of visual function can affect quality of life in patients with AMD [27].
In both AMD sub-groups, we observed that the nasal macula strongly correlated with VFQ composite and near activities scores. A person with healthy retina would normally use their fovea to perform near-distance activities, but patients with macular dysfunction typically recruit a parafoveal region of the macula as their preferred retinal locus for fixating and scanning text [8, 9, 28]. The strong correlation between the nasal macular sensitivity and the VFQ near activities sub-score in particular suggests that the nasal outer and inner sub-regions are preferred retinal loci in patients with early and late AMD respectively. This is suggestive of a pattern of disease-mediated photoreceptor loss, but further studies are required to investigate the possible clinical implications of this finding.
There were no statistically significant correlations observed in the group without vitreoretinal disease. This may be due to a ceiling-effect caused by a narrow range of retinal sensitivities among those without vitreoretinal disease. Indeed, a similar trend was observed by Barboni et al. in the control group of their study [24].
A strength of this study is that we divided patients by stage: those with either neovascular AMD and/or geographic atrophy were allocated one sub-group, while those with non-neovascular AMD without geographic atrophy were allocated another. A limitation of this study is that we did not further divide the patients according to disease severity. Future studies could repeat our experiment with the addition of an intermediate AMD sub-group.