The relationship between the Stiles-Crawford effect of the first kind (SCE-I) and myopia

Factors associated with reduced visual acuity in myopes with and without ocular pathologies after optical correction

PURPOSE

Considering that a certain proportion of high myopes have reduced visual acuity even after full optical correction, this study aimed to investigate the association between various refractive error components (sphere, cylinder and axis orientation) and reduced visual acuity in individuals with low to high myopia with and without pathologic myopia lesions.

METHODS

We analysed data from randomly selected eyes of 11,258 individuals with myopia (mean ± SD spherical equivalent (SE) -3.2 ± 2.9D; range: -0.5D to -21.5D). In total, 10,528 individuals had no pathologic myopia lesions. Sphere, cylinder and SE refraction were classified into mild, moderate and high categories. Astigmatism was defined as with-the-rule, against-the-rule or oblique based on the axis orientation. Reduced best-corrected visual acuity was defined as ≥0.18 logMAR. Logistic regression was performed to test factors associated with reduced visual acuity with and without pathologic myopia lesions.

RESULT

Overall, 6.4% (N = 720/11,258) of myopes had reduced best-corrected visual acuity. High sphere (≤-6.0D; Odd ratios [OR]: 16.1; 95% CI: 2.1-126.5), high cylinder (<-2.0 DC; OR: 2.5; 95% CI: 1.8-3.4), against-the-rule (OR: 1.5; 95% CI: 1.1-2.0) and oblique astigmatism (OR: 1.6; 95% CI: 1.2-2.1) were significantly (p ≤ 0.008) associated with reduced visual acuity in the absence of pathologic myopia lesions. Both moderate SE and high myopic SE were also associated with reduced visual acuity. In the presence of pathologic myopia lesions, tessellated fundus (OR: 6.9; 95% CI: 3.5-14.1), chorioretinal atrophy (OR: 7.7; 95% CI: 2.6-19.9) and choroidal neovascularisation (OR: 37.4; 95% CI: 3.3-419.3) were significantly (p ≤ 0.003) associated with reduced visual acuity.

CONCLUSION

Even after full optical correction, both refractive components and pathologic myopia lesions can independently cause reduced visual acuity, regardless of the degree of myopia.

Visual Axis and Stiles-Crawford Effect Peak Show a Positional Correlation in Normal Eyes: A Cohort Study

Purpose

The purpose of this study was to locate the visual axis and evaluate its correlation with the Stiles-Crawford effect (SCE) peak.

Methods

Ten young, healthy individuals (20 eyes) were enrolled. An optical system was developed to locate the visual axis and measure SCE. To locate the visual axis, 2 small laser spots at 450 nm and 680 nm were co-aligned and delivered to the retina. The participants were asked to move a translatable pinhole until these spots were perceived to overlap each other. The same system assessed SCE at 680 nm using a bipartite, 2-channel (reference and test) Maxwellian-view optical system. The peak positions were estimated using a two-dimensional Gaussian fitting function and correlated with the visual axis positions.

Results

Both the visual axis (x = 0.24 ± 0.35 mm, y = -0.16 ± 0.34 mm) and the SCE peak (x = 0.27 ± 0.35 mm, y = -0.15 ± 0.31 mm) showed intersubject variability among the cohort. The SCE peak positions were highly correlated in both the horizontal and vertical meridians to the visual axes (R2 = 0.98 and 0.96 for the x and y coordinates, respectively). Nine of the 10 participants demonstrated mirror symmetry for the coordinates of the visual axis and the SCE peak between the eyes (R2 = 0.71 for the visual axis and 0.76 for the SCE peak).

Conclusions

The visual axis and SCE peak locations varied among the participants; however, they were highly correlated with each other for each individual. These findings suggest a potential mechanism underlying the foveal cone photoreceptor alignment.

Subjective measurement of the Stiles-Crawford effect of the first kind with variation in accommodation

PURPOSE

To measure the Stiles-Crawford effect of the first kind (SCE-I), corresponding to central vision, with innovative technology to evaluate changes in the directionality and photoreceptor alignment with accommodation.

METHODS

A uniaxial Maxwellian system (spot size in pupil 0.5 mm diameter) was employed, incorporating a spatial light modulator to flicker at 2 Hz between two 2.3° fields corresponding to test (peripheral pupil) and reference (pupil centre) positions. Participants determined thresholds at 13 positions along the horizontal pupil meridian by indicating if the test field was brighter or dimmer than the reference field. Thresholds were determined by a staircase procedure after four reversals at each pupil location. After pupil dilation, seven emmetropes were tested at 0 D to 6 D accommodation stimulus levels in 2 D intervals. Data were fit by the Gaussian function, both when the fits were unforced or forced to pass through the sensitivity expected for the reference point. Directionality (ρ) and peak location values ( x max ) were determined for unforced and forced fits.

RESULTS

Regression slopes for ρ as a function of accommodation stimulus were not significant. There was a tendency for x max to shift temporally with increasing accommodation across the 6 D stimulus range. This was not significant for regression fitting (-0.059 mm/D, R²  = 0.06, p = 0.20), but a paired t-test for 0 and 6 D stimuli showed a weakly significant change of 0.62 mm (p = 0.05). The differences between the two fitting approaches were small and non-significant.

CONCLUSIONS

Directionality did not change with accommodation, but the pupil peak location showed a significant temporal shift of approximately 0.62 mm with 6 D accommodation stimulus. It is possible that substantial changes in the directionality and a shift in the direction of peak location might occur at very high levels of accommodation.

Subjective measurement of the Stiles-Crawford effect with different field sizes

The Stiles-Crawford effect of the first kind (SCE) is the phenomenon in which light entering the eye near the center of the pupil appears brighter than light entering near the edge. Previous investigations have found an increase in the directionality (steepness) of the effect as the testing location moves from the center of the visual field to parafoveal positions, but the effect of central field size has not been considered. The influence of field size on the SCE was investigated using a uniaxial Maxwellian system in which stimulus presentation was controlled by an active-matrix liquid crystal display. SCE directionality increased as field size increased from 0.5° to 4.7° diameter, although this was noted in four mild myopes and not in two emmetropes. The change with field size was supported by a geometric optics absorption model.

Changes in forward light scatter parameters as a function of refractive error in young adults

Background/aims

Some aspects of visual performance worsen with increasing myopia. Whilst the underlying causes are not always clear, reduction in retinal image quality is often attributed to structural changes in the posterior myopic eye. Forward light scatter, originating principally from the cornea and lens, is known to produce veiling glare which subsequently reduces retinal image contrast. It is therefore of interest to investigate whether forward light scatter varies with refractive error.

Methods

Thirteen young-adult subjects (18–25 years), with mean spherical errors (MSE ± sd, D) RE, − 1.69 ± 2.02 (range 0.38 to − 4.75); LE, − 1.91 ± 1.94 (range 0.50 to − 4.63) underwent binocular assessment of forward light scatter using the AVOT light scatter test. Five glare annuli, with effective eccentricities ranging from 2 to 10°, were used to estimate parameters, k and n, which define the light scatter function of the eye. These were then used to calculate the area under the light scatter function (k′) and the total volume of light scatter (k″).

Results

Significant correlation was found between increasing myopia and k′ values (RE, p < 0.05; r = 0.64; LE, p < 0.05, r = 0.66). Neither the ‘volume’ of light scatter (k″), the parameter, n, which controls the angular distribution of light scatter, or the straylight parameter constant, k, were significantly correlated with refractive error (p > 0.05 for both eyes). Axial length was also not correlated with any of the light scatter parameters measured.

Conclusion

The preliminary data from this study provide evidence that some light scatter parameters may be correlated with refractive error. Further studies are needed to characterize how changes in the anterior media of the eye, and inclusion of a wider range of refractive errors, may affect forward light scatter.

The Effects of Severe Myopia on the Properties of Sampling Units in Peripheral Retina

SIGNIFICANCE

Poor peripheral visual acuity in myopia may reflect, in part, photoreceptor misalignment with the exit pupil of the eye. We speculate that if such misalignment causes sufficient visual deprivation and/or disrupts retinal feedback processes, it may influence eye growth itself.

PURPOSE

It is known that myopic eyes have a reduced peripheral resolution acuity relative to emmetropic eyes, though it remains unclear how mechanical stretching of the retina in myopia impacts on peripheral visual performance. Our aim was to determine how retinal stretching affects the properties of sampling units in peripheral vision.

METHODS

Three-dimensional magnetic resonance imaging provided a depiction in vivo of ocular shape, allowing the inter-eye ratio of retinal image surface areas and the relative alignment of surfaces to be determined in our observer, who was unique in having severe myopia in the right eye (~21 D) but only modest myopia in the left (~3 D). Visual performance was assessed for the detection and direction discrimination of drifting sinusoids positioned 40° in the temporal retina. Applying the sampling theorem to our measures, we estimated the density and cut-off frequency of the underlying sampling units.

RESULTS

The retinal image surface area of the right eye was 40% larger than that of the left and was rotated 8.9° anticlockwise relative to the left eye's image surface. In agreement with a linear stretch model of myopia, the sampling density of the right eye was reduced by approximately the same ratio as that predicted from the inter-eye MRI data, namely, 1.18. However, the cut-off frequency (cycles/mm) of the right eye was approximately half that of the left, a reduction that cannot be explained solely by a linear areal expansion of retinal sampling units.

CONCLUSIONS

Poor peripheral acuity in severe myopia may be caused, at least in part, by receptoral misalignment with the exit pupil.

Directional light-capture efficiency of the foveal and parafoveal photoreceptors at different luminance levels: an experimental and analytical study

A gradual drop in visibility with obliquely incident light on retinal photoreceptors is namely described by the Stiles-Crawford effect of the first kind and characterized by a directionality parameter. Using a digital micromirror device in a uniaxial flicker system, here we report on variations of this effect with luminance levels, wavelengths within the visible and near-infrared spectrum and retinal regions ranging from the fovea to 7.5° parafoveal. Results show a consistent directionality in mesopic and photopic conditions. Higher directionality is measured for longer wavelengths, and a decrease with retinal eccentricity is observed. Results are discussed in relation to an absorption model for the visual pigments taking the outer-segment packing and thickness of the neural retina into account. Good correspondence is found without enforcing photoreceptor waveguiding.

Highlighting Directional Reflectance Properties of Retinal Substructures From D-OCT Images

Optical coherence tomography (OCT), which is routinely used in ophthalmology, enables transverse optical imaging of the retina and, hence, the identification of the different neuronal layers. Directional OCT (D-OCT) extends this technology by acquiring sets of images at different incidence angles of the light beam. In this way, reflectance properties of photoreceptor substructures are highlighted, enabling physicians to study their orientation, which is potentially an interesting biomarker for retinal diseases. Nevertheless, commercial OCT devices equipped to automate D-OCT acquisition do not yet exist, meaning that physicians manually deviate the light beam to acquire a set of D-OCT images sequentially. Therefore, the intensities in the stack of images are not directly comparable, and a normalization step is required before differential analysis. In this paper, we present advanced image processing methods to perform differential analysis of a set of D-OCT images and extract the angle-dependent retinal substructures. Our approach relies on a robust and accurate normalization algorithm followed by a classification that is spatially regularized. We also propose a robust color representation that facilitates interpretation of D-OCT data in general, by detecting and highlighting angle-dependent structures in healthy and diseased eyes. Experimental results show evidence of photoreceptor disarray in a variety of retinal diseases, demonstrating the potential medical interest of the approach.

Insight into high myopia and the macula

The incidence of myopia is constantly on the rise. Patients of high myopia and pathological myopia are young and can lose vision due to a number of degenerative changes occurring at the macula. With the emergence of new technologies such as swept-source optical coherence tomography (OCT) and OCT angiography, our understanding of macular pathology in myopia has improved significantly. New conditions such as myopic traction maculopathy have been defined. Early, noninvasive detection of myopic choroidal neovascularization and its differentiation from lacquer cracks is possible with a greater degree of certainty. We discuss the impact of these new exciting and promising technologies and management of macular pathology in myopia. Incorporation of OCT in the microscope has also improved macular surgery. New concepts such as fovea-sparing internal limiting membrane peeling have emerged. A review of literature and our experience in managing all these conditions are discussed.

Directionality of individual cone photoreceptors in the parafoveal region

The pointing direction of cone photoreceptors can be inferred from the Stiles-Crawford Effect of the First Kind (SCE-I) measurement. Healthy retinas have tightly packed cones with a SCE-I function peak either centered in the pupil or with a slight nasal bias. Various retinal pathologies can change the profile of the SCE-I function implying that the arrangement or the light capturing properties of the cone photoreceptors are affected. Measuring the SCE-I may reveal early signs of photoreceptor change before actual cell apoptosis occurs. In vivo retinal imaging with adaptive optics (AO) was used to measure the pointing direction of individual cones at eight retinal locations in four control human subjects. Retinal images were acquired by translating an aperture in the light delivery arm through 19 different locations across a subject's entrance pupil. Angular tuning properties of individual cones were calculated by fitting a Gaussian to the reflected intensity profile of each cone projected onto the pupil. Results were compared to those from an accepted psychophysical SCE-I measurement technique. The maximal difference in cone directionality of an ensemble of cones, ρ¯, between the major and minor axes of the Gaussian fit was 0.05 versus 0.29mm(-2) in one subject. All four subjects were found to have a mean nasal bias of 0.81mm with a standard deviation of ±0.30mm in the peak position at all retinal locations with mean ρ¯ value decreasing by 23% with increasing retinal eccentricity. Results show that cones in the parafoveal region converge towards the center of the pupillary aperture, confirming the anterior pointing alignment hypothesis.

Rapid measurement of individual cone photoreceptor pointing using focus diversity

A novel method is presented to rapidly measure the pointing direction of individual human cone photoreceptors using adaptive-optics (AO) retinal imaging. For a fixed entrance pupil position, the focal plane is rapidly modulated to image the guided light in various axial planes. For cones with different pointing directions, this focus diversity will cause a shift in their apparent position, allowing for their relative pointing to be determined. For four normal human subjects, retinal images were acquired, registered, and the positions of individual cones tracked throughout the dataset. Variation in cone tilt was 0.02 radians, agreeing with other objective measurements on the same subjects at the same retinal locations.

Measurement of the photoreceptor pointing in the living chick eye

The chick eye is used in the study of ocular growth and emmetropization; however optical aberrations in the lens and cornea limit the ability to visualize fine retinal structure in living eyes. These aberrations can be corrected using adaptive optics (AO) allowing for cellular level imaging in vivo. Here, this capability is extended to measure the angular tuning properties of individual photoreceptors. The left eyes from two White Leghorn chicks (Gallus gallus domesticus) labeled chick A and chick B, were imaged using an AO flood illuminated fundus camera. By translating the entrance pupil position, the same retinal location was illuminated with light of varying angles allowing for the measurement of individual photoreceptor pointing. At 30° nasal from the pecten tip, the pointing direction for both chicks was towards the pupil center with a narrow distribution. These particular chicks were found to have a temporal (T) and inferior (I) bias in the alignment with peak positions of (0.81 T, 0.23 I) and (0.57 T, 0.18 I) mm from the pupil center for chicks A and B respectively. The rho, ρ, values for the major, ρL, and minor, ρs, axes were 0.14 and 0.17mm(-2) for chick A and 0.09 and 0.20mm(-2) for chick B. The small disarray in the alignment of the chick photoreceptors implies that the photoreceptors are aligned to optimize the light entering the eye through the central portion of the pupil aperture. The ability to measure pointing properties of individual photoreceptors will have application in the study of eye growth and various retinal disorders.

Bifocal lens control of myopic progression in children

Bifocal spectacle lenses have been used as a strategy to slow myopic progression in children since the 1950s and perhaps earlier. The reported success of this strategy varies greatly, as does the design of studies reporting the outcomes of their use-from earlier retrospective analysis of records to later prospective clinical trials. Collectively, published data support the suggestion that bifocal lenses inhibit myopic development in children but only by a small amount and only in a subset of children. Possible reasons for the greatly varying outcomes include a lack of individualism of the treatment and failure to take the vergence system into account. This review summarises the results of bifocal and multifocal studies, describes how accommodation, convergence and their interaction are linked to myopic development and details how a bifocal treatment that takes this into account may be devised. Also discussed is whether alterations to peripheral retinal blur contribute to bifocal lens effects.

The association of wavefront aberration and accommodative lag in myopes

Accommodative lags, induced by a target at 33 cm (distance-induced condition) and by a -3.0 D lens (lens-induced condition), and wavefront aberrations were measured in 27 young myopic eyes. The accommodative lags and Strehl ratios derived from the wavefront aberrations in myopes were compared with those from 57 emmetropes. Accommodation was measured using a Canon R-1 autorefractor, while aberrations were measured using a psychophysical ray-tracing technique. In accord with previous results, larger accommodative lags were found for the myopes than the emmetropes in both the lens-induced and distance-induced conditions. The mean Strehl ratio was smaller in the myopes (0.079) than the emmetropes (0.091); this difference approached significance (p = 0.055). In addition, for myopes the accommodative lag was significantly correlated with the Strehl ratio in the lens-induced condition (r = -0.45, p < 0.02) and approached significance in the distance-induced condition (r = -0.35, p = 0.07). No significant correlations were found for emmetropes. Possible reasons to account for these results are discussed.

Evidence for transient forces/strains at the optic nerve head in myopia: repeated measurements of the Stiles-Crawford Effect of the First Kind (SCE-I) over time

Probable transient changes in photoreceptor alignments, inferred from the measurements of the Stiles-Crawford effect of the first kind (SCE - I), were demonstrated in myopic eyes with elongated axial length (regardless of the magnitude of refractive error) at three retinal locations; the fovea (point of fixation), and 22 degrees and 27 degrees in the nasal retina. The changes were much bigger at 22 degrees and 27 degrees in the nasal retina (which are located beyond the optic nerve head) than at the fovea. These transient effects were revealed by repeatedly testing the same retinal locations over a period of time. Time intervals between the subsequent/repeated measurements ranged from less than an hour to several months. In some locations, the changes were recorded in less than an hour. Collectively, these were very meaningful changes. Generally bigger effects were recorded in the horizontal SCE - I than in the vertical SCE - I.