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Eye strain and headache are common ocular symptoms, and the main cause in persons aged 15 to 40 is convergence insufficiency. It is more common in females than in males. Many patients are supplied with glasses for eye strain without the causative convergence insufficiency being treated. The treatment in most cases is orthoptic exercises.
Refraction and its components were measured on 96 subjects consisting of 48 Young Adults (19 to 31 years old) and 48 Mature Adults (49 to 61 years old) having refractive errors in the range from +2.50 to -2.50 D. Subjects for the two groups were matched on the basis of sex and refractive error. A quota sampling technique was used to obtain subjects for myopic, emmetropic, and hyperopic refractive-error categories. An autorefractor was used to measure refractive error, an autokeratometer was used to measure corneal radius of curvature, and an A-scan ultrasonometer was used to determine the axial dimensions. Crystalline lens radii were determined by submitting Purkinje image data to a ray-tracing program, with the assumption that the index of refraction of the ocular media is constant throughout life. By comparing the results for the two groups of subjects, it was found that there were no significant age-related differences in mean corneal radius of curvature or in mean axial length. However, the following significant age-related differences were found: the Mature Adult eyes had shallower anterior chambers, thicker lenses, shorter vitreous chambers, steeper anterior and posterior lens surfaces (shorter radii of curvature), more highly powered lenses, and higher ocular refracting power than the Young Adult eyes. It was concluded that a decrease in the gradient-index of the lens occurs with increasing age, acting as an emmetropizing mechanism by compensating for the steepening of both the front and back surfaces of the lens.(ABSTRACT TRUNCATED AT 250 WORDS)
Dyslexia and the effect of visual problems on reading ability have long been subjects of controversy. This paper critically reviews the literature on the relationship between reading disability and visual acuity, refractive error, binocular vision anomalies, eye movements, ocular pathology and the transient and sustained visual systems. The limitations of much of the work in this field are described, with particular reference to the problems of subject selection and group matching. Tentative conclusions, with deference to the complex nature of dyslexia, are drawn and suggestions are made for future research.
Conflicting views have been expressed in the literature regarding the sign and magnitude of the difference between the values of ocular correction deduced by retinoscopy and by subjective refraction. Possible causes for such a discrepancy are briefly reviewed. Calculations based upon experimental data show that neither the reddish colour of the retinoscopic reflex nor the position of the layer in the retina at which the reflex originates are likely to introduce any important discrepancy. The probable significance of other factors is assessed.
Topical application of Prostaglandin F2 alpha (PGF2 alpha) to the eye reduces intraocular pressure (IOP) in all mammalian species studied thus far, including humans. The L-isopropylester derivative is currently the one most commonly used in experimental and clinical studies. Dose-response relationships were determined between PGF2 alpha-IE and IOP, pupillary diameter, and refraction in ketamine-anesthetized ocular normotensive cynomolgus monkeys. Single doses of 10 and 30 micrograms had smaller and less consistent but longer lasting IOP-lowering effects than repeated doses (twice daily for 3 days) of 1-5 micrograms. For repeated dosing in this manner, the just-maximal dose is probably between 2-5 micrograms, producing a approximately 70% reduction in IOP to a final IOP of approximately 5 mm Hg. Continuing treatment for up to 18 days did not further enhance the efficacy of twice daily treatment with a submaximal 1-microgram dose. Partial reversal of anesthesia-induced tonic accommodation occurred with single 10- and 30-micrograms doses and with repeated 1-microgram doses, but additional myopia of 0.5-1.5 diopters was induced with repeated higher doses. These physiologic findings and previous morphologic data are consistent with a proposed dual PG action on the ciliary muscle, one involving a short-onset long-lasting direct effect on the muscle fibers (causing relaxation and narrowing of the muscle bundles) and the second involving a slowly developing but shorter duration dissolution of the intermuscular connective tissues.
The study of ocular mechanisms is often concerned with the possible interaction of two systems. Spectral analysis has been used previously by many investigators as a tool to investigate the interrelationships in the behaviour of two or more systems. This note demonstrates the value of computing coherence functions following spectral analysis to obtain a more rigorous tool for the investigation of synergy between two systems.
The laboratory and clinical results of tests of different bifocal intraocular lenses (BIOLs) are compared. Included are two-zone refractive BIOLs with a central near zone and an annular distance zone (group 1); three-zone refractive BIOLs with a central distance zone, an annular near zone, and a peripheral distance zone (group 2); diffractive BIOLs (group 3); multizone refractive BIOLs (group 4); and other BIOLs (group 5). All BIOLs cause a significant loss of image contrast and acuity at low contrast. The incidence of visual side effects (e.g., halos, glare) varies but is significantly higher than that obtained with monofocal IOLs. On the other hand, BIOLs provide a depth of focus of 3-4 D as compared with only 1.5 D for monofocal IOLs. In group 1, distance vision is reduced in bright light, which limits the clinical use of these BIOLs. In group 2, distance vision is better than near vision. Thus, a temporary near add is required in some cases. In group 3, distance and near vision are similar, but the incidence of visual side effects is higher than that in group 2. Only BIOLs of groups 2 and 3 can be recommended for clinical use in selected patients. Not enough data are currently available on the other groups. Patient selection is the most critical factor for BIOLs. Ideal patients are relatively young and active, are highly motivated, show no ocular pathology, and have a normal pupil, an axial length of 22-24.5 mm, and a corneal astigmatism of less than 1 D. Reduced image contrast, mainly at near focus, and visual side effects must be acceptable. Bilateral implantation yields the best results. A monofocal fellow eye is a definite contraindication. In summary, BIOLs cannot replace monofocal IOLs. However, they provide a valuable alternative in selected patients.
PURPOSE: The fitting of chick eyes with positive or negative lenses causes eye growth to decelerate or accelerate, respectively, thereby minimizing the imposed blur. This study was conducted to determine whether the eye can initially assess the correct direction of growth or whether it relies on trial and error, reversing its direction if the magnitude of blur increases. The rapid changes in choroidal thickness in response to brief periods of defocus were measured. METHODS: After their eyes were measured by ultrasound biometry, chicks wore either a +10-D lens over one eye for 10 minutes while restrained in the center of a 60-cm drum (to ensure myopic blur), or a negative lens (-7 or -8.6 D) over one eye for 10 minutes or 1 hour in a normal cage environment. They were then kept in darkness until they were remeasured 2 hours, 1 day, or 2 days after the first measurement. Other chicks wore +10 or -8.6-D lenses briefly and were measured several times over the next 7 hours in darkness. RESULTS: Wearing positive or negative lenses for only 10 minutes produced significantly different effects on choroidal thickness measured 2 hours later. Wearing positive lenses for 10 minutes caused an increase in choroidal thickness (in 28 of 32 eyes) and a concomitant decrease in vitreous chamber depth, relative to the amount of change in the untreated fellow eye over the same period. Wearing negative lenses for 1 hour caused significant changes in the opposite direction. Wearing lenses for 2 hours resulted in choroidal changes that persisted in darkness for up to 6 hours after positive lens wear, but returned to normal after negative lens wear. Finally, 1 hour of positive lens wear caused significant inhibition of ocular elongation over the next 2 days. CONCLUSIONS: The eyes of chicks require only a brief period of lens wear to initiate compensation in the appropriate direction. Because the refractive status changes little during the period of lens wear, the authors conclude that eyes can rapidly determine the sign of the imposed blur without resorting to a trial-and-error method.
This study investigated the ocular response of young chicks to astigmatic errors imposed by spectacle lenses and as a related issue, we examined the nature and prevalence of astigmatism in young chicks, and its relation to corneal development and natural emmetropization. Normal hatchling chicks exhibited significant against-the-rule refractive astigmatism (approx. 8 D) of which 60-90% was corneal. Both types of astigmatism decreased in magnitude with normal corneal development as part of emmetropization. The apparent association with corneal growth is consistent with two further observations: (1) that smaller corneas, induced by constant light rearing, had higher than normal astigmatism (1.5 D greater at 15 days), (ii) that enlarged corneas, due to form deprivation, had reduced astigmatism (2.4 D less). When astigmatism was artificially imposed with (+/-10 DC spectacle lenses), altered ocular growth patterns were observed, although the changes were not consistent with the chicks having emmetropized to the imposed astigmatism. Irrespective of the axis setting used in positioning the lenses (45 degrees, 90 degrees, 180 degrees), eyes became hyperopic with +10 DC lenses (+8.8 +/- 1.3 D), and became slightly myopic with 10 DC lenses (-1.8 +/- 1.9 D). These refractive changes are consistent with the chicks having emmetropized to the more myopic meridian rather than the equivalent mean spherical error imposed (responses of control groups to +5 D and -5 D spherical lenses were +5.2 +/- 1.0 D and -5.1 +/- 0.8 D resp.). The same was true for chicks first prevented from accommodating by prior ciliary nerve section, except for one group wearing the 10 DC lens at 45 degrees axis where astigmatic changes consistent with partial compensation were seen, although this may represent an artefact of the surgery. These results argue against 'astigmatic emmetropization' as a normal phenomenon. Also consistent with this finding was the lack of significant astigmatic changes with accommodation-stimulating and inhibiting drugs (nicotine and vercuronium resp.), for normal chicks. These results imply that accommodation, while the most likely mechanism for astigmatic emmetropization, has little capacity to compensate for imposed astigmatic focussing errors.
Rapid emmetropization is described in pediatrically normal infants from 9 months of age during the following year. The infants, obtained from various categories of the Cambridge population screening program, provided a broad range of refractive errors. The large group of 254 nonanisometropic infants studied allowed the mean rate of change and dependence on the initial refraction value to be determined. Refraction was measured by cycloplegic retinoscopy. Rapid emmetropization changes occurred in the following refractive components: mean spherical equivalent (MSE), astigmatism magnitude, the horizontal astigmatism component, the infant's most positive meridian, and the infant's most negative meridian. The MSE and astigmatism rates of change (diopters/year), were highly dependent on their respective initial powers (r = -0.61 and r = -0.76). The percentage weighted mean proportional rate of change for MSE was -30% (SE 4%) and for astigmatism magnitude it was -59% (SE 14%). There was much individual variation, with some exhibiting fast emmetropization and others not. The MSE and astigmatism changes, however, were almost independent of each other. The refractive errors of the most positive and most negative meridians emmetropize because they are both derived from the MSE and half the astigmatism. With-the-rule astigmatism was more prevalent than against-the-rule astigmatism at 9 months of age, and with-the-rule astigmatism exhibited a significantly greater proportional rate of change. The relationship of emmetropization and refractive screening is considered. A new component "MOMS" is introduced, the maximum ocular meridional separation, when both eyes are considered. Thus incorporating astigmatism and anisometropia may be a good single indicator of conditions associated with later amblyopia. The almost independent emmetropization of the MSE and astigmatism components is an important result to consider in theories of emmetropization, refractive screening, clinical prescribing, and the evaluation of infants in treatment trials.
The lens-rearing paradigm has developed great importance in the field of experimental myopia. Although an apparently simple paradigm, the results of any experiment can be influenced by a variety of factors including habitual viewing distance, ocular refraction, oculomotor performance and the spatial sensitivity of the retinal elements involved in retinal image assessment. Computer modelling has been used to evaluate the expected impact when lenses are placed in front of a primate eye as a function of the above parameters. Spatial band-pass responses of the mechanisms responsible for emmetropization are predicted to lead to a limited range of retinal defocus over which compensation to lenses could occur. Even assuming an equal ability to detect hyperopic and myopic defocus, it is predicted that primate eyes should be able to compensate for a much larger range of minus lenses than plus lenses. This derives from the ability of the accommodation system to keep retinal defocus within this operating range over a wider range of minus lenses than plus lenses. The range over which compensation can occur will depend on the spatial tuning of the elements responsible for detecting retinal defocus and capabilities of the accommodation system. The observed asymmetry in responses of the primate eye to rearing with plus and minus lenses and observed differences between primates and chickens in lens rearing studies may therefore be partly attributable to optical and neurophysiological considerations.
Previous work has suggested that cerebellum may control the size of the pupil and transmission in the pathway mediating the pupillary light reflex. In this work we found that electrical stimulation of the cerebellar nuclei (nucleus fastigius and nucleus interpositus bilaterally) evoked a discharge in the short ciliary nerves which innervate the intraocular muscles. The latency was 5.7 msec, which is short enough for a direct excitatory connexion from cerebellar efferents onto the preganglionic neurones in the Edinger-Westphal nucleus. These neurones are controlling the ciliary muscle of the lens system as well as the constrictor muscle of the pupil and it was therefore important to elucidate whether both of these groups participate in the discharge. Studies on interaction between the response to cerebellar stimulation and 'pupilloconstrictor' responses from optic tract fibres as well as recording from individual cells in the ciliary ganglion (identified with diffuse light stimuli) led to the conclusion that the short latency excitation is more or less confined to units controlling accommodation. Cerebellar inhibition (or disfacilitation) of longer latency affects both pupilloconstrictor and accommodation units.
The influence of visual experience on ocular development in higher primates is not well understood. To investigate the possible role of defocus in regulating ocular growth, spectacle lenses were used to optically simulate refractive anomalies in young monkeys (for example, myopia or nearsightedness). Both positive and negative lenses produced compensating ocular growth that reduced the lens-induced refractive errors and, at least for low lens powers, minimized any refractive-error differences between the two eyes. These results indicate that the developing primate visual system can detect the presence of refractive anomalies and alter each eye's growth to eliminate these refractive errors. Moreover, these results support the hypothesis that spectacle lenses can alter eye development in young children.
Ocular dimensions and lens refractive properties were studied in enucleated eyes from 5-day old chicken (Gallus domesticus). Measurements were made in frozen hemisections and in fresh eyes sectioned through a para-equatorial plane. In these specimens, lens properties were examined after systematical removal of the vitreal mass. This procedure caused significant changes in lens diameter and refractive properties. Our data indicate that avian lens shape could be controlled by vitreal pressure and/or scleral support.
In a double blind study, the influence of the tranquilizer Camazepam on different ophthalmological parameters was examined. The 12 test persons were between 20 and 30 years of age and had entirely normal ocular findings. Visual acuity was uninfluenced by a single dose of 40 mg of Camazepam, reflecting the very slightly hypnotic effect of the drug. Roughly half of the patients showed a considerable widening of the visual field, thought to be related to the stimulation of attention caused by Camazepam. Finally the lack of muscle relaxing properties of Camazepam was confirmed as regards the extraocular muscles, since their balance remained uninfluenced as well.
The purpose of this article is to provide a guide to assist the Emergency Physician in examining the eye. The evaluation of a patient with eye problems consists of a history, visual acuity, pupil examination, external examination, extra ocular movements, visual fields, and color vision. The patient is then examined at the slit lamp. After the slit lamp examination, the fundus and optic nerve is examined with a direct ophthalmoscope and intraocular pressure is measured. Special tests such as a plain film study and computed tomography (CT) scan may be obtained when indicated and, finally, referral to an ophthalmologist can be made for a dilated fundus examination, ultrasound studies of the eye and orbit, and surgical treatment.
Contact lens-spectacle telescopic systems provide an alternative to conventional spectacle-mounted telescopes. There is a need to describe the parameters of such systems, the interrelations between them, and the use of such relations in selecting values for prescribing. Graphical representation of ocular (contact) lens power, objective (spectacle) lens power, separation (vertex distance), and magnification are presented. Skillful prescribing involves selection of the best set of parameters to achieve the desired visual goal in spite of the practical limitations imposed by facial features, lens availability, and appearance. Additional factors (not graphically represented) are induced vergence amplification, altered depth perception, and apparent motion, all unique to telescopic systems. One patient is described and selection of appropriate parameters is discussed. General application of the selection process is described.
Small doses of nondepolarizing relaxants are often recommended as prior medication to suxamethonium in order to avoid or attenuate the side effects elicited by the latter drug. After D-tubocurarine (0.05 mg/kg b.w. and 0.075 mg/kg b.w.) in unmedicated volunteers, a dose-dependent recession of the monocular near point of accomodation was recorded. This recession, which parallelled the decrease in grip strength, was 4 cm and 9 cm, respectively. The same doses evoked an exophoria of 14 and 17 prism diopters, respectively. Intraocular pressure was significantly lowered by D-tubocurarine 0.05 mg/kg b.w., from 2.01 kPa (15.1 mmHg) to 1.64 kPa (12.3 mmHg), and by pancuronium 0.015 mg/kg b.w. from 1.90 kPa (14.3 mmHg) to 1.44 kPa (10.8 mmHg). The decreases lasted for 7 min. Gallamine 0.3 mg/kg b.w. also decreased intraocular pressure, but to a lesser degree, and with statistical significance only during the fourth minute after administration. Relaxation of extraocular muscles may change the slightly ovoid shape of the eyeball into a more spherical one, thereby inducing a fall in intraocular pressure and a recession of the near point of accommodation.