Photoreceptor misalignment accompanying a fibrous scar

Cone spacing and waveguide properties from cone directionality measurements

Reflectometric techniques estimate the directionality of the retinal cones by measuring the distribution of light at the pupil plane of light reflected off the bleached retina. The waveguide-scattering model of Marcos et al. [J. Opt. Soc. Am. A 15, 2012 (1998)] predicts that the shape of this intensity distribution is determined by both the waveguide properties of the cone photoreceptors and the topography of the cone mosaic (cone spacing). We have performed two types of cone directionality measurement. In the first type, cone directionality estimates are obtained by measuring the spatial distribution of light returning from the retina with a single-entry pupil position (single-entry measurements). In the second type, estimates are obtained by measuring the total amount of light guided back through the pupil as a function of entry pupil position (multiple-entry measurements). As predicted by the model, single-entry measurements provide narrower distributions than the multiple-entry measurements, since the former are affected by both waveguides and scattering and the latter are affected primarily by waveguides. Measurements at different retinal eccentricities and at two different wavelengths are consistent with the model. We show that the broader multiple-entry measurements are not accounted for by cone disarray. Results of multiple-entry measurements are closer to results from measurements of the psychophysical Stiles-Crawford effect (although still narrower), and the variation with retinal eccentricity and wavelength is similar. By combining single- and multiple-entry measurements, we can estimate cone spacing. The estimates at 0- and 2-deg retinal eccentricities are in good agreement with published anatomical data.

Local photoreceptor alignment measured with a scanning laser ophthalmoscope

The aim of this study was to develop a fast test for local photoreceptor alignment. Photoreceptor alignment is an important indicator of retinal integrity. Digitized images of fundus reflectance were obtained for 20-30 pupil entry positions with a custom built scanning laser ophthalmoscope (SLO). The data permitted the calculation of curve peakedness of the optical Stiles-Crawford effect (SCE) as a function of retinal location. We found that the peakedness is low in the central 0.5 deg, reaches a maximum at an eccentricity of 1-2 deg and gradually drops with increasing eccentricity. These data are in conformity with the anatomy of foveal cones. Additionally, the psychophysical SCE was measured with red light and an 8 deg stimulus. The mean peak position of the SCE in the pupil plane for both methods was similar, but the optical SCE was clearly steeper. The SLO provides a fast, reliable and objective way to determine local receptor alignment in the central retina.

A reflectometric technique for assessing photoreceptor alignment

Clinical studies of photoreceptor orientation are limited by the fact that psychophysical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. We have developed a novel instrument, the photoreceptor alignment reflectometer (PAR), that determines photoreceptor alignment by measuring the distribution in the pupil of light reflected by one retinal location. This determination is accomplished in a measurement time of 4 sec and requires minimal co-operation from the subject. The technique is not significantly affected by reflections at the limiting membrane, or by changes in entrance and exit pupil configuration, or by location of bleaching light entry. The PAR was used to measure the orientation of foveal photoreceptors, their directionality, and the ratio of directional to diffuse flux in 20 normal subjects ranging in age from 20 to 60 yr.

Parametric representation of Stiles-Crawford functions: normal variation of peak location and directionality

Evidence suggests that the psychophysically determined Stiles-Crawford effect of the first kind (SCE) reflects waveguide properties of human photoreceptors. The peak of the SCE data set is assumed to reflect the principal alignment tendencies, and the spread (e.g., rho value, the curvature or width at half-height) is assumed to reflect the directionality (i.e., interreceptor differences in alignment) of the population of photoreceptors being tested. As such, disruption of the normal SCE can be used and/or has been used (1) to document early natural history of retinal pathology involving the photoreceptors, (2) to provide a firm rationale for therapeutic intervention, and (3) to provide a method for monitoring therapies designed to alter the natural course of retinal-disease processes. We report large-sample norms for foveal SCE peak location and spread (horizontal peak location, nasal 0.51 +/- 0.72, horizontal rho value 0.047 +/- 0.013, vertical peak location, superior 0.20 +/- 0.64, vertical rho value 0.053 +/- 0.012), compare these norms with values determined in other laboratories, and discuss the various mathematical forms used for the empirical description of SCE data sets.

Scotopization and pseudoprotanomaly in blue-yellow/colour vision defects

With a routine clinical colour vision test battery we found scotopization in 32% of retinal diseases presenting with pseudoprotanomaly as sign of an acquired type III blue-yellow colour vision defect. In blue-yellow colour vision defects of retinal origin scotopization is a transient phenomenon, present in early stages of the disease, but it is not an obligatory finding. There is no evident relationship between visual acuity and scotopization.

Main types of bull's eye maculopathy. Functional classification

Bull's eye maculopathy is a non-specific reaction of the posterior pole of the eye. The concentric dispersion of pigment occurs when the disease interferes with the function of the retinal pigment epithelium. The site of the primary process may be the bipolar cell layer, the receptor cell layer or the retinal pigment epithelium. By means of electrodiagnostic techniques and color vision testing the site of the primary lesion as a rule can be retraced.

Foveal densitometry in central serous choroidopathy

We evaluated foveal cone photopigment kinetics by retinal densitometry in 14 patients (12 men and two women, ranging in age from 30 to 54 years) with central serous choroidopathy. At the initial examinations when 12 of the 14 patients showed active leaks, the measured two-way density of pigment was low and the time-course of regeneration was slow. At later examinations, we found low two-way densities with normal regeneration times in five patients without active leakage. Near normal two-way densities occurred only in three patients who had complete clinical recoveries.

Basic phenomena in acquired colour vision deficiency

Acquired colour vision defects are directly related to the fixation mode: blue-yellow defects in foveolar fixation, blue-yellow or red-green defects in eccentric fixation. The primary localization of a disease can be retraced from the degree of cone damage. Optic nerve diseases essentially lack signs of cone damage. Processes at the level of the choriocapillaris/retinal pigment epithelium induce a non-selective receptor impairment. There are minor signs of cone damage. In cone dystrophies there is selective cone damage. Scotopization indicates a relatively well-preserved rod function.

Colour vision as a diagnostic aid

The minimum requirements for a reliable study of (acquired) defects of colour vision have been formulated by Verriest. Taking these minimum requirements as a guide, about 200 patients were selected. For the differential diagnosis of disorders of the retina and the optic nerve the determination of the neutral zone and examination with the anomaloscope are important. This is true for both red-green and blue-yellow defects.

Visual function abnormalities in macular heterotopia caused by proliferative diabetic retinopathy

In seven patients with displacement of the macula in one eye caused by vitreoretinal traction from proliferative diabetic retinopathy, the macula was displaced toward the optic disk in all but one eye. The eyes with macular heterotopia showed (1) reduced visual acuity (ranging from 6/6 (20/20) to 6/24 (20/80), (2) reduced static perimetry sensitivity (particularly in the temporal field) with displacement of the peak sensitivity toward the blind spot, (3) loss of hue discrimination, and (4) metamorphopsia. In two eyes with macular heterotopia, Stiles-Crawford data provided evidence for photoreceptor disorientation; one eye had mixed orientation, and the other a displaced peak suggesting regular photoreceptor tilting. The proposed mechanisms of visual loss in eyes with macular heterotopia secondary to traction from proliferative diabetic retinopathy include detachment of the macula, disorientation of photoreceptors in the macula, and disturbance of normal neural connections within the retina caused by stretching of the retina.