Frank Thorn

A Dynamic Relationship between Myopia and Blur-driven Accommodation in School-aged Children

Previously we reported that recently myopic children accommodated insufficiently to blur induced by negative lenses. The purpose of the present study was to relate changes in blur-driven accommodation to myopia development in children. Refractive errors and the accommodation response function (ARF) were measured in 23 myopic and 40 emmetropic children on two occasions separated by periods ranging from 6 to 12 months. Repeated measures of accommodation were made with a Canon R-1 autorefractor while negative lenses of increasing power were placed in front of the childs right eye viewing 20/100 letters at 4 m. Concomitant changes in refractive error and in accommodative function over periods of 6-12 months were found to be highly correlated in myopes (r = 0.77) but not in emmetropes (r = 0.09).

Wavefront aberrations in eyes of emmetropic and moderately myopic school children and young adults

Wavefront aberrations were measured using a psychophysical ray-tracing technique in both eyes of 316 emmetropic and moderately myopic school children and young adults. Myopic subjects were found to have greater mean root mean square (RMS) value of wavefront aberrations than emmetropic subjects. Emmetropic adults had the smallest mean RMS, which remained smaller than the values for myopic adults and children and for emmetropic children both when second order Zernike aberrations (astigmatism) and third order Zernike aberrations were removed. Twenty percent of myopic adults had RMS values greater than values for all of the emmetropic adults, with significantly greater values for Zernike aberrations from second to seventh orders. High amounts of wavefront aberrations, which degrade the retinal image, may play a role in the development of myopia.

Astigmatism and the development of myopia in children

While it is now established that astigmatism is more prevalent in infants and young children than in the adult population, little is known about the functional significance of this astigmatism, especially its role, if any, in emmetropization and the development of myopia. Manifest refractions (mean of 16 per subject) were obtained from 245 subjects starting in the first year, with 6-23 years of regular follow-up. Results showed that infantile astigmatism is associated with increased astigmatism and myopia during the school years. Two possible mechanisms underlying this association are discussed: (1) infantile astigmatism disrupts focusing mechanisms; and (2) ocular growth induces astigmatism and myopia.

Wave-front aberrations in the anterior corneal surface and the whole eye

In order to investigate the sources of wave-front aberrations in the human eye, we have measured the aberrations of the anterior cornea and the whole eye using a topographic system and a psychophysical wave-front sensor. We have also calculated the aberrations for the internal optics of both eyes of 45 young subjects (aged 9 to 29 years). The mean rms for the anterior cornea was similar to that for the internal optics and thewhole eye when astigmatism was included, but less than that for both the internal optics and the whole eye with astigmatism removed. For eyes with low whole-eye rms values, mean rms for the anterior cornea was greater than that for the whole eye, suggesting that the anterior corneal aberration is partially compensated by the internal optics of the eye to produce the low whole-eye rms. For eyes with larger whole-eye rms values, the rms values for both the anterior cornea and the internal optics were less than that for the whole eye. Thus the aberrations for the two elements tend to be primarily additive. This pattern exists whether or not astigmatism was included in the wave-front aberration rms. For individual Zernike terms, astigmatism and spherical aberration in the anterior cornea were partially compensated by internal optics, while some other Zernike terms showed addition between the anterior cornea and internal optics. Individual eyes show different combinations of compensation and addition across different Zernike terms. Our data suggest that the reported loss of internal compensation for anterior corneal aberrations in elderly eyes with large whole-eye aberrations [J. Opt. Soc. Am. A 19, 137 (2002)] may also occur in young eyes.

Response AC/A ratios are elevated in myopic children.

In children little is known about the relationship between the AC/A ratio and the development of myopia, although they have been linked in adults. The purpose of this study was to investigate the interaction between accommodation and convergence and its relationship to refractive errors in children. Accommodation was measured for the right eye using the Canon R‐1 autorefractor, and concomitant changes in vergence were assessed using a Maddox rod and a Risley prism before the left eye. Thirty‐three myopic and 68 emmetropic children were tested wearing best subjective correction while looking at a distant (4.0 m) letter array and a near (0.33 m) one through additional plus and minus lenses. Lens‐induced and distance‐induced response AC/A ratios were calculated from the data. Both types of AC/A ratios are elevated in myopic children, who show reduced accommodation and enhanced accommodative convergence. Myopic children with esophoria underaccommodate at near. This suggests that a child who is esophoric must relax accommodation to reduce accommodative convergence and maintain single binocular vision. The reduction in accommodation could produce blur during near work, which could induce myopia as in animal models.

Effects of dioptric blur on Snellen and grating acuity.

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of “spurious resolution” in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patients refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.

Characteristics of Accommodative Behavior During Sustained Reading in Emmetropes and Myopes

Accommodation has long been suspected to be involved in the development of myopia because near work, particularly reading, is known to be a risk factor. In this study, we measured several dynamic characteristics of accommodative behavior during extended periods of reading under close-to-natural conditions in 20 young emmetropic and stable myopic subjects. Accommodative responses, errors, and variability (including power spectrum analysis) were analyzed and related to accommodative demand and subject refractive error. All accommodative behaviors showed large inter-subject variability at all of the reading demands. Accommodative lags and variability significantly increased with closer demands for all subjects (ANOVA, p<0.05). Myopes had significantly greater variability in their accommodation responses compared to emmetropes (ANOVA, p<0.05) and had larger accommodative lags at further reading distances (unpaired t test p<0.05). Power spectrum analysis showed a significant increase in the power of accommodative microfluctuations with closer demands (ANOVA, p<0.05) and with increasing myopia at the closest reading demand (ANOVA, p<0.01). The difference in the stability of the accommodative behavior between individuals with different refractive states suggests a possible relationship between variability in accommodation and the development of myopia.

Myopia and ambient night-time lighting.

Quinn et al. report a strong association between myopia in children and their exposure to night-time lighting during their first two years. We have been unable to confirm this surprising result, but we find that myopic parents are more likely to employ night-time lighting aids for their children. Moreover, there is an association between myopia in parents and their children.

Development of axial ocular dimensions following eyelid suture in the cat.

Eyelid suture has been shown to produce anatomical changes in the lateral geniculate nucleus (LGN) (Wiesel and Hubel, 1963a; Guillery, 1973), physiological abnormalities in both the LGN and striate cortex (Wiesel and Hubel, 1963h 1%5; Hoffman and Sherman, 1974), and behavioral deficits (Dews and Wiesel, 1970; Chow and Stewart, 1972). Changes in the structure of the eye due to eyelid suture were not reported in any studies prior to 1976. Recently, several investigators have reportal that prolonged lid suture in young subjects kads to myopia and an increase in the eye’s axial length in monkeys (Wiesel and Raviola, 1977) and tree shrews (Sherman, Norton and Casagrande, 1977) and noncomeal myopia in cats (Wilson and Sherman, 1977). Since most of the visual deprivation experimentation involving eyelid suturing has been performed with kittens, it is crucial to ascertain if eyelid suturing causes an elongation of the posterior segment of the kitten’s eyeball. Thus, we have measured the axial dimensions of the various components of the eyeballs of kittens which have experienced prolonged eyelid suture. Six cats had monocular lid sutures as kittens prior to eye opening and were so maintained for various intervals up to 28 months before ocular measures were taken. Prior to measurement the cats were anesthetized with 4Omg/kg of ketamine hydrochloride (Vetelar, Parke-Davis) and the palpebral fissure of the sutured eye re-established. One drop of cyclopentolate hydrochloride (Cyclogyl, 1%. SchMelin) was then placed in each eye as a cycloplegic and one drop of phenylephrine hydrochloride (Neosynephrine, loo/ Winthrop) added to retract the nictitating membrane. An ultrasonographic A-scan was made along the geometric axis of each eye with an ocular-reflectoscope (Automation Industries, lnc). Alignment with the geometric axis of the eye was ascertained by positioning the ultrasonic probe for simultaneous maximization of the echoes from the cornea, anterior and posterior lens, and retina. These echoes are greatest when the probe is directed perpendicular to these surfaces. The horizontal and vertical meridians of the anterior corneal curvature were measured with a Bausch & Lomb Keratometer. Eight normal controls were similarly tested. The details of the apparatus and technique are described elsewhere (Thorn, Gollender and Erickson, 1976). In a second experiment, the axial dimensions of a litter of five kittens were measured periodically between 11 and 69 days of age. Two of the kittens had opposing monocular sutures, one had binocular sutures, and the remaining two were unsutured controls. The measurements were made through the closed eyelid with the kittens facing upward. The probe was coupled to the eyelid by a pool of water which was held in place by a petrokum jelly dam at the apex of the eye. When the palpebral fissure was restored in the two monocular kittens at 69 days of age, a comparison was made with four normal kittens whose ages were 45, 69.69, and 74 days. Two other monocular eyelid sutured kittens were evaluated at 140 and 272 days. The axial length of the cat’s eye was significantly altered by prolonged eyelid suture. This was due to changes in the axial dimensions of both the aqueous and vitreous chambers. The thickness of the lens was not significantly affected by lid suture. Since the axial length changes were not systematic (i.e. did not always increase or always decrease), the changes induced by eyelid suture had to be studied by comparing the axial dimension differences between the two eyes of individual cats. When the axial length of the shorter eye was compared to that of the longer eye in the eight normal adult cats, the average difference between the eyes was only l.vA with half having the right eye longer (Fig. 1). Since most of this difference could be explained by measurement error (a = *O.P,/,), we must conclude that the axial lengths of the two eyes of normal cats were virtually identical. When this same comparison was made in monocularly sutured adults, the average difference between the eyes was 3.1%. That is, the axial length of the lid sutured eye

Development of spatial contrast sensitivity from infancy to adulthood : Psychophysical data

Purpose. This study examined changes in contrast sensitivity, the location of the peak of the contrast sensitivity function (CSF), and the shape of the function from infancy to adulthood. Methods. Contrast thresholds were obtained using behavioral methods, preferential looking for infants, and operant techniques for older children and adults, with the same stimuli for all ages. Results. Contrast sensitivity at the peak improved almost two log units from infancy to adulthood. Much of the shift in the peak to higher spatial frequencies occurred in infancy. Sensitivity decreased by almost one octave at 0.38 c/deg between 2 and 4 months of age. Sensitivity was not yet at adult levels at 8 years of age. Conclusions. The reduction in contrast sensitivity at the lowest frequency between 2 and 4 months of age suggests an increase in lateral inhibition during early infancy. Contrast sensitivity at the peak increased by two log units from then until adulthood.