Slowed Dark Adaptation in Early AMD: Dual Stimulus Reveals Scotopic and Photopic Abnormalities

Abstract

PURPOSE. The recovery of visual sensitivity after a photobleach in early AMD is slowed in rods but cones also may be abnormal. The purpose of this article was to test different stimulus locations to investigate cone function and its relation to rod abnormalities. METHODS.Stimuli were presented at two locations, 3.08 and 5.58, in the inferior visual field. Post photobleach dark adaptation (DA) curves from 50 early-AMD patients were compared with those from 15 healthy controls of similar age. Curves were characterized in terms of four parameters: ct, cone threshold; a, the transition point from cone to rod function; S2, the slope of the second rod-mediated component; and b, the transition from the second to the third rodmediated component.RESULTS. There were strong location effects for the healthy group and the AMD group. Cone threshold was higher for the outer compared with the inner stimulus (P ¼ 0.001), S2 was steeper for outer compared with inner (P < 0.001), a was shorter for outer (P ¼ 0.004), and b was shorter for outer than inner (P ¼ 0.002). The high variance in the patient data, particularly for a and b, explained the absence of a group*location interaction in the statistics.CONCLUSIONS. The data provide a novel perspective on abnormal cone-and rod-sensitivity recovery in early dry AMD. The comparison of pairs of DA curves from different locations highlights the involvement of cones in the underlying pathology of AMD. Dynamic measures of visual function are particularly sensitive to early AMD.

Cones in ageing and harsh environments: the neural economy hypothesis

“…By studying mesopic vision to include both rod and cone responses, it becomes clear that rods play a role in colour vision, and not just light adaptation. The differing characteristics of rods vs cones for light level, wavelength, retinal eccentricity, and temporal and spatial aspects of vision result in functional differences between controls and patients with AMD for scotopic, mesopic, and photopic mechanisms, along with adaptation, colour contrast, acuity, etc …”

“…The differing characteristics of rods vs cones for light level, wavelength, retinal eccentricity, and temporal and spatial aspects of vision result in functional differences between controls and patients with AMD for scotopic, mesopic, and photopic mechanisms, along with adaptation, colour contrast, acuity, etc. 95,96 In addition to retinal remodelling that occurs with loss of neurons, 84 there is the potential for binocular summation or inhibition, with differences due to binocular disparity, spatial frequency, and eccentricity. 97 In patients with geographic atrophy and AMD, potential vision is not accurately assessed by limiting measurements only to initial ones.…”

The Light and the Dark of Early and Intermediate AMD: Cone- and Rod-Mediated Changes Are Linked to Fundus Photograph and FAF Abnormalities

“…DA was measured psychophysically using a PC-based system as previously described. 39,40,[54][55][56][57] The test was conducted monocularly in the study eye. The subjects wore their best optical correction for the test distance (90 cm), and the fellow eye was covered using an eye patch.…”

“…There is a sound physiologic basis for this. 49 Such techniques were used in the investigation of dry AMD by Jackson et al, 50 Murray et al, 51 and, more recently, Tahir et al, 40 who confirmed the existence of both photopic and scotopic abnormalities in early AMD.…”

“…33,34 The ratio of cones to rods in the macula is approximately 9:1, 35 and it is therefore likely that, by the time patients notice changes in their photopic vision, they will have lost many millions of rods. 34,[36][37][38][39][40] Classically, the dynamics of cone and rod function are assessed by exposing the eye to a photobleach and plotting the recovery of sensitivity against time. As has been known for many years, [41][42][43] this sensitivity recovery function, called the dark adaptation (DA) curve, is composed of cone and rod components.…”