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Background: It is of interest to examine paracentral functional deficits in early age-related maculopathy (ARM), as histopathological studies indicate that this is where the earliest changes occur. The purpose of this study is to detect the sensory functional deficits at chosen retinal areas around the fovea and at the fovea itself in patients with early age-related maculopathy and to determine the type of functional losses that are more pronounced in early ARM.
Methods: Ten participants with early ARM and 10 age-matched controls took part. Crowded and uncrowded visual acuity and static and transient contrast sensitivity were measured in the same selected eye of each participant at eight predetermined retinal locations plus the fovea in patients with early ARM and controls. All measurements were made using computer-generated targets.
Results: A significant difference between the controls and subjects with ARM was found in low spatial frequency static contrast sensitivity (p = 0.05) but not for transient contrast sensitivity (p = 0.586). Visual acuity (uncrowded VA and crowded VA) showed a borderline difference (p = 0.072 and p = 0.084, respectively). Compared to controls, there was no evidence of increased contour interaction effects in early ARM (p = 0.595).
Conclusion: The subjects with very early ARM showed significant loss of low spatial frequency static contrast sensitivity before the loss of high contrast VA, indicating that static contrast sensitivity may be one of the earliest functional losses in early ARM and this loss was found to extend across the central 10 degrees of the retina.
Age-related maculopathy (ARM) is the leading cause of severe visual impairment and irreversible visual loss in the elderly in developed countries.1–5 It is a degenerative retinal disease which is estimated to affect approximately 25 to 30 million people over the age of 50 globally.5,6 It is important to understand the functional changes that people with ARM experience. Visual acuity and contrast sensitivity have been studied extensively in this population, patients with ARM usually experiencing gradual and significant loss of VA and CS.7–12 These studies measured VA and CS with natural viewing, that is, at the fovea or the preferred retinal locus (PRL) in the case of people with central scotomata. There is evidence that histopathalogical13 and fundus changes in ARM, such as drusen and RPE pigmentary changes, start eccentrically. Knudston and colleagues14 found that drusen larger than 125 µm, soft indistinct drusen, reticular drusen and RPE depigmentation were most likely to develop outside the central 500 µm (approximately 1.5 degrees). In particular, drusen greater than 125 µm, soft indistinct and reticular drusen between 1.5 and 10 degrees had greatest cumulative incidence14 and these types of drusen are most predictive of progression to age-related macular degeneration (ARMD).15 Wang and associates16 found that the total percentage of area covered by drusen was greatest within 500 µm of the fovea. Knudston and colleagues14 found that eyes that developed neovascular or geographic ARMD were more likely to have early lesions between 1.5 and 5 degrees, although Wang and co-workers15 found that total drusen within the central 1.5 degrees followed by the 1.5 to 5 degrees ring were greater risk factors for progression. There are several studies that give evidence of early paracentral functional loss. These include forms of perimetry17–21 or sensitivity to distortion, such as the clinically-used Amsler chart, the Macular Computerized Psychophysical Test22 and shape discrimination.23 Brown and Lovie-Kitchin showed paracentral deficits of contrast sensitivity24 and measured temporal modulation sensitivity.25 Multi-focal ERG has also been used to study paracentral function.26–28
Some studies have specifically investigated cone or rod function. Brown and colleagues29 showed elevated thresholds for cone dark adaptation in paracentral retina, while Phipps, Guymer and Vingrys30 showed delayed cone recovery. Owsley and associates18 found more frequent losses for dark-adapted, rod mediated sensitivity than cone losses and these were most severe at paracentral locations and in a later study, found disturbances in rod-mediated rather than cone-mediated adaptation in the parafovea.31 Feigle and co-workers28 used mfERG to demonstrate delayed rod, but not cone, responses in early dry ARM.
The transient/sustained dichotomy starts in the bipolar cells of the retina32 and there is evidence that photoreceptor degeneration can lead to changes to synaptic connections with the bipolar cells.33 Therefore, it is of value to compare losses for static versus temporally modulated stimuli. Some studies have claimed that static CS is significantly lower in early ARM than age-matched controls.24,28,34–36 Others showed that there is a loss of alternating or transient CS in early ARM.10,30,35,37,38 Yet other studies have demonstrated that subtle deficits of CS due to widespread disease can be detected in patients with ARM before VA loss appears.35,36,38–40 CS measurement is believed to be useful for the evaluation of visual function and functional change in patients with ARM20,41 and the low contrast targets are likely to be more sensitive to visual loss than high contrast VA targets. Therefore, we examined whether changes in paracentral CS (and VA) can be demonstrated before there is evidence of central VA loss, using both transient and static gratings.
The first purpose of this study was to examine the extent of functional losses in very early ARM and to determine whether functional losses can be demonstrated before loss of high contrast acuity. The second purpose was to measure functional aspects of vision at and around the fovea, to determine at which retinal regions the earliest losses occur and whether these mirror the fact that the earliest receptor losses often do not occur at the fovea but paracentrally. Third, as it has been suggested that contour interaction may be increased in ARM,11 we wanted to measure the effect of contour interaction (crowding) at paracentral locations. This will give us further understanding of the functional impairments experienced by people with early ARM.