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| January 2000 | Inside IOVS | Volume 41/1 |
| Noninvasive Testing: New Insights into Old Problems |
Albinism
Albinism has many ramifications for vision. In this study, Wildsoet et al. (p. 1) observed a wide range of refractive errors, including high myopia and high hyperopia, as well as high with-the-rule astigmatism. Mechanical factors are likely to underlie the refractive astigmatism because it always mirrored corneal astigmatism and was coupled to horizontal nystagmus. Surprisingly, the vertical meridian for hyperopes was always nearer emmetropia, raising the possibility of “meridional emmetropization,” as this meridian was also less affected by nystagmus-induced “smearing.” Myopes did not show this behavior, presumably due to operational constraints on emmetropization. A follow-up study encompassing different types of nystagmus is underway in search of more conclusive proof for meridional emmetropization.
Melanoma-Associated Retinopathy
Melanoma-associated retinopathy (MAR) causes night-blindness in some patients with metastatic cutaneous melanoma. Injection of human MAR IgG into the vitreous of rhesus monkey eyes was found to suppress the ERG ON-pathway responses of both dark- and light-adapted recordings. MAR IgG altered the ERG similarly to 2-amino-4-phosphonobutyric acid (APB), which is known to block activity of the depolarizing bipolar cells (DBCs) selectively. These observations by Lei et al. (p. 262) support the hypothesis that MAR is an autoimmune disease in which melanoma antibodies impair vision by cross-reacting with retinal neurons, presumably DBCs.
Macular Degeneration
Clinical trials of age-related macular degeneration (AMD) traditionally use cone-based vision tests (e.g. visual acuity) as outcome measures. Owsley et al. (p. 267) demonstrate that dark-adapted macula-wide psychophysical tests, emphasizing rod-mediated vision, may be as sensitive or more so to early stages of AMD. A parafoveal vulnerability to disease was also found with regional retinal measures of dark-adapted psychophysics. These observations by noninvasive assessment correlate with previous histopathological findings of greater rod loss than cone loss in the parafovea of AMD.
Glaucoma and Diabetes
Short-wavelength automated perimetry (SWAP) is capable of detecting early losses of visual function in a variety of diseases (e.g. glaucoma and diabetes). In the study by Remky et al. (p. 274), SWAP was performed in diabetic patients without clinically significant macular edema with normal visual acuity. Macular capillary density was obtained by measurements of the foveal avascular zone and the mean perifoveal intercapillary area. Reduced SWAP thresholds were related to decreased capillary density in contrast to visual acuity and conventional light sensitivity. The subtle deterioration of visual function as measured by SWAP may act as indicator of early ischemic diabetic maculopathy.
Retinopathy of Prematurity
Some children, whose mild retinopathy of prematurity (ROP) resolved in infancy, have deficits in dark-adapted visual sensitivity. Could the deficits be due to rod cell dysfunction? Hansen and Fulton (p. 320) measured rod increment threshold functions at parafoveal and peripheral sites and found all thresholds of ROP subjects were above normal. In some subjects, parafoveal thresholds were more elevated than peripheral thresholds, as their increment threshold functions were shifted "up and over," indicating the rods as the site of the sensitivity deficit. This suggests that even mild ROP is associated with long-lasting rod photoreceptor dysfunction.
Glaucoma, Aging
Glaucoma patients often show decreased visual sensitivity to moving targets. Such findings have been widely interpreted as indicating a deficit in the magnocellular (M) visual pathway. Willis and Anderson (p. 325) measured contrast sensitivity for motion stimuli, which is known to selectively stimulate the M pathway, in glaucoma patients, age-matched normals, and young normals, under both photopic and scotopic lighting conditions. Mean sensitivity for such stimuli declined with increasing age but was not significantly different between glaucoma and age-matched control groups. The results suggest that substantial neural loss specific for motion perception occurs with aging, and that sensitivity to motion targets per se may not be a useful indicator of neural integrity in glaucoma.
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