Neville A. McBrien

THE EFFECT OF REFRACTIVE ERROR ON THE ACCOMMODATIVE RESPONSE GRADIENT

Abstracts— The accommodative responses of early‐onset myopes, late‐onset myopes, emmetropes and hyperopes were measured over a range of 5 dioptres using an objective infra‐red autorefractor. Differences were found between the four refractive groups, with hyperopes accommodating more for near targets than emmetropes, followed by early‐onset myopes then late‐onset myopes. Moreover a strong correlation between the accommodative response gradient and refractive error was found, suggesting that hyperopes accommodate more to a particular target than do emmetropes or myopes. The results are interpreted in the context of a dual innervation to the ciliary muscle.

A biometric investigation of late onset myopic eyes

Abstract There is a paucity of biometric information on late onset myopia (onset after 15‐years‐old). The present investigation compared the ocular component dimensions of 30 late onset myopes and 30 emmetropes, who were both age and sex matched. Measurement techniques included A‐scan ultrasonography and keratometry. The findings revealed that the ultimate cause of late onset myopia is vitreous chamber elongation. Late onset myopes were also found to have significantly deeper anterior chambers and thinner crystalline lenses, no differences being found in corneal curvature measures. Possible mechanisms leading to these observed biometric differences are discussed in the light of recent findings.

Lid-suture myopia in tree shrews with retinal ganglion cell blockade.

To determine whether central communication of retinal signals is necessary for the development of an experimentally induced myopia, tree shrews were exposed to monocular deprivation (MD) while the action potentials of retinal cells in the deprived eye were blocked with intravitreally injected tetrodotoxin (TTX-MD animals). TTX injections (0.6 microgram in 3 microL) and MD began about 15 days after eye opening, at the start of the susceptible period for the development of lid-suture myopia. Six injections were given, one every second day to produce 12 days of MD and TTX-blockade. Control TTX animals (TTX-open) received TTX in one eye, but not MD, on the same injection schedule and were always found to be behaviorally unresponsive to visual stimuli through the injected eye indicating that TTX blocked central communication of action potentials. Other control animals received intravitreally injected saline in either an open eye (saline-open), or an MD eye (saline-MD). A sham-injected group (sham-inj-MD) received MD and all anesthetic and surgical manipulations except for penetration of the sclera. In all groups, one eye in each animal was an untreated control. Two effects were found. All MD groups, including the TTX-MD animals, developed a significant vitreous chamber elongation in the deprived eye, indicating that an experimental myopia developed despite ganglion cell blockade. Thus, retinal mechanisms in tree shrew can detect the presence of a degraded visual image and produce an experimental myopia that does not depend on the receipt of visual messages by central neural structures. In addition, eyes in which the sclera was punctured had smaller vitreous chamber depths than comparable uninjected eyes, indicating that puncturing the sclera reduced the normal elongation. These data suggest that forces within the eye normally contribute to its expansion and may be resisted by the choroid and/or the sclera.

The effects of blockade of retinal cell action potentials on ocular growth, emmetropization and form deprivation myopia in young chicks

To investigate the influence of brain mediated functions on control of ocular growth, young chicks were treated monocularly with intravitreally injected tetrodotoxin (TTX) to block retinal ganglion cell action potentials. TTX injections (0.7 micrograms in 7 microliters) were given on day 6 after hatching in both binocularly open and monocularly deprived chicks. Injections were repeated every 48 hr for a period of 8 days (TTX-open; TTX-MD). Control groups of animals received intravitreally injected phosphate buffered saline (PBS-open; PBS-MD) to one eye on the same schedule. There was a minimum of eight animals in each group. Recovery from form-deprivation myopia during blockade of retinal cell action potentials was also investigated. Results demonstrate that blockade of retinal cell action potentials by TTX produces reduced growth of the anterior segment of the eye and crystalline lens in both binocularly open and MD chicks. Blockade of retinal cell action potentials does not prevent form-deprivation induced vitreous chamber elongation and myopia. Form deprived myopic eyes were found to emmetropize despite blockade of retinal ganglion cell action potentials giving further evidence for local ocular control of emmetropization. Blockade of retinal ganglion cell action potentials did not prevent changes in choroidal thickness in eyes developing axial myopia or eyes recovering from induced myopia.

Pirenzepine prevents form deprivation myopia in a dose dependent manner

Previous studies have demonstrated that muscarinic antagonists, such as atropine and pirenzepine, block form deprivation myopia in avian and mammalian models. The aim of the present investigation was to establish dose-response curves for intravitreal and subconjunctivally injected pirenzepine and to determine receptor specificity. Chicks were monocularly deprived of form vision for five days and received daily injections of either pirenzepine or saline. Keratometry, retinoscopy and A-scan ultrasonography of axial ocular dimensions were then taken. Intravitreally injected pirenzepine was effective at preventing form deprivation myopia in a dose dependent manner with an ED50 of 175 micrograms. A 500 micrograms dose totally prevented induced myopia (+0.9 D versus -13.7 D) and axial enlargement (-0.14 mm versus +0.32 mm). Daily subconjunctival injection of pirenzepine was significantly less effective in preventing form deprivation myopia. Form deprivation myopia could still be induced in animals which had undergone pirenzepine treatment. Pirenzepine was effective in preventing the axial elongation associated with experimental myopia in a dose dependent manner and via a functional not toxic mechanism.

The effect of accommodation on oblique astigmatism and field curvature of the human eye

Oblique astigmatism and field curvature of the human eye were measured at various levels of accommodation up to a maximum of SD and angles of eccentricity up to 60°. The results indicate that accommodation has an effect on the level of these aberrations at the larger angles of eccentricity. Comparisons with paraxial schematic eyes are favourable, with paraxial schematic eyes showing the same trend.

The role of muscarinic antagonists in the control of eye growth and myopia

The success of cycloplegic agents such as atropine in stopping the progression of juvenile myopia in adolescent children has given strong support for a major role for accommodation in the control of ocular growth and myopia. This paper presents data from two recent studies which demonstrate that atropine is effective in preventing axial myopia development via a nonaccommodative mechanism and that the effect is mediated through an M1 muscarinic receptor process.

Accommodative hysteresis under static and dynamic stimulus conditions

The influence of continuous variation in dioptric demand on the accommodative hysteresis induced at near distances was examined in 14 visually‐normal young adults. Tonic accommodation was measured before and after 10 minutes of sustained focus using a constant stimulus at 5 D, 6.5 D, and 8 D, as well as a stimulus which slowly and alternately increased and decreased over the continuous range from 5 D to 8 D. For approximately half the subjects, dioptric demand had to be very high (8 D) under static conditions to produce moderate but significant hysteresis, yet little or no attenuation of the effect occurred under the dynamic condition. For other subjects who consistently showed very large tonic changes (1.4 D or more) under static conditions, the hysteresis effect generated under dynamic conditions was greatly reduced (approximately 50 per cent) in magnitude. These findings suggest that the degree to which continuous variation in dioptric demand will disrupt the adaptive process may depend on individual differences in the rate and/or maximum level of tonic accommodative change. Such a relationship could have bearing on the particular strategy recommended for individuals who tend to experience blur at distance following nearwork.