Pharmacological effects on accommodative adaptation

Improvement of Presbyopia Using a Mixture of Traditional Chinese Herbal Medicines, Including Cassiae Semen, Wolfberry, and Dendrobium huoshanense

Background

Presbyopia is a primary cause of a decline in near vision. In this study, we developed a new mixed herbal medicine to retard presbyopic progression and increase the amplitude of accommodation (AA), which is beneficial for near vision.

Methods

A total of 400 participants between the ages of 45 and 70 years were recruited. We designed the mixed herbal drug to include Cassiae Semen (200 mg), wolfberry (200 mg), and Dendrobium huoshanense (DD) (40 mg) in one capsule. In experiment 1, the recruited subjects were directed to perform a push-up test to measure their AA; this was then converted to the additional diopters of reading glasses. In experiment 2, 240 subjects took three capsules daily for six months and then stopped medical therapy for a six-month follow-up. In experiment 3, 160 subjects were randomly categorized into four groups: a placebo group, low-dose group (LDG) (1 capsule daily), middle-dose group (MDG) (two capsules daily), and high-dose group (HDG) (three capsules daily). The 160 volunteers took different doses for six months and then stopped treatment, accompanied by another six-month follow-up. In experiments 2 and 3, the change in AA, uncorrected far visual acuity (UFVA), and uncorrected near visual acuity (UNVA) were recorded each month for one year.

Results

In experiment 1, AA was found to decrease with age and a great deal of additional power was needed in older individuals. In experiment 2, the mean AA reached a maximum value of 2.1D (P < 0.05) after six months, while the UNVA improved by about two to three lines of a Jaeger chart in most of the subjects. At nine months, all the means decreased slightly to 2.0 D (P < 0.05). This meant that the mixed herbal medicine could still maintain AA for another three months because the herbal therapy was stopped at the seventh month. In experiment 3, the maximal AA was 2.8D, 2.9D, and 3.2D (P < 0.05) in the LDG, MDG, and HDG after six-month treatments, respectively. Experiment 3 showed that AA gain occurred in a dose-dependent manner; the higher the dose, the greater the AA value.

Conclusion

Only two studies on the use of herbal drugs for presbyopia have been reported in PubMed. In our study, we found that taking a mixed herbal drug caused an excellent gain in AA. This is the first study to report that the characteristics of the new herbal regimen could retard and even ameliorate presbyopia.

AC/A ratios in myopic and emmetropic Hong Kong children and the effect of timolol§

PURPOSE

Caucasian children with myopia have elevated response accommodative vergence to accommodation (AC/A) ratios. The purpose of this study was twofold: to determine if response AC/A ratios vary with refractive error and with myopic progression rate in Hong Kong Chinese children, and to determine the effect of beta-adrenergic antagonism with topical timolol application on AC/A ratios.

METHODS

Thirty children aged eight to 12 years participated in the study. All refractive errors were corrected with spectacle lenses. Accommodative responses were measured using a Shin-Nippon autorefractor and concurrent changes in vergence were assessed using a vertical prism and a Howell-Dwyer card at three metres and 0.33 metre. Accommodative demand was altered using plus or minus two dioptre lenses and lens- and distance-induced response AC/A ratios were calculated. Measurements were repeated 30 minutes after the instillation of topical timolol maleate (0.5 per cent).

RESULTS

AC/A ratios appeared higher in progressing myopic children but the difference was not statistically significant. Timolol application reduced accommodative convergence (AC) in the stable myopes (reduction = -3 +/- 1.14 prism dioptres) but not in the emmetropes (0.69 +/- 0.96 prism dioptres) or progressing myopes (0.16 +/- 0.43 prism dioptres) and this difference between refractive groups was statistically significant (F(2, 27) = 3.766; P = 0.036). However, timolol did not produce a significant change in the accommodative response to positive or negative lenses or response AC/A ratios.

CONCLUSIONS

We did not find that AC/A ratios in myopic Chinese children were elevated and therefore, it is unlikely that elevated AC/A ratios are responsible for the high levels of myopia that occur in Hong Kong. The finding that timolol reduced AC in the stable myopes suggests that the autonomic control of accommodative convergence in these children may be different from that in emmetropic children and those with progressing myopia.

Is it possible to relate accommodative visual dysfunctions to neck pain?

The aim of this study was to establish whether there is a relationship between conditions of accommodative visual dysfunctions and cervical complaints. Fifty-two participants were included. Variables were accommodative amplitude, positive and negative relative accommodation (NRA), accommodative response, and accommodative facility. Subjects were classified as accommodative insufficiency, accommodative excess, or normal. Neck complaints were measured with the Neck Disability Index, the Visual Analogue Scale, and by cervical range of motion, deep flexor muscle activation score, and performance index. We found the following significant relationships: between NRA and both performance index and left-side bending; accommodative amplitude right-eye with right-side bending and with left-side bending; accommodative amplitude left-eye with right-side bending; and accommodative facility left-eye with both performance index and left-side bending. In accommodative amplitude right-eye, aIl participants showed significant values and greater than those with accommodative excess. In both groups, performance index values were decreased. Greater pain and lower right-rotation were found in participants with accommodative excess than in those with accommodative insufficiency. We conclude that accommodative dysfunctions are related to low performance index, decreased range of motion, as well as greater neck pain.

Children's accommodation during reading of Chinese and English texts

PURPOSE

Inaccurate accommodation during nearwork and subsequent accommodative hysteresis may influence myopia development. Myopia is highly prevalent in Singapore; an untested theory is that Chinese children are prone to these accommodation characteristics. We measured the accuracy of accommodation responses during and nearwork-induced transient myopia (NITM) after periods spent reading Chinese and English texts.

METHODS

Refractions of 40 emmetropic and 43 myopic children were measured with a free-space autorefractor for four reading tasks of 10-minute durations: Chinese (SimSun, 10.5 points) and English (Times New Roman, 12 points) texts at 25 cm and 33 cm. Accuracy was obtained by subtracting accommodation response from accommodation demand. Nearwork-induced transient myopia was obtained by subtracting pretask distance refraction from posttask refraction, and regression was determined as the time for the posttask refraction to return to pretask levels.

RESULTS

There were significant, but small, effects of text type (Chinese, 0.97 ± 0.32 diopters [D] vs. English, 1.00 ± 0.37 D; F1,1230 = 7.24, p = 0.007) and reading distance (33 cm, 1.01 ± 0.30 D vs. 25 cm, 0.97 ± 0.39 D; F1,1230 = 7.74, p = 0.005) on accommodation accuracy across all participants. Accuracy was similar for emmetropic and myopic children across all reading tasks. Neither text type nor reading distance had significant effects on NITM or its regression. Myopes had greater NITM (by 0.07 D) (F1,81 = 5.05, p = 0.03) that took longer (by 50s) (F1,81 = 31.08, p < 0.01) to dissipate.

CONCLUSIONS

Reading Chinese text caused smaller accommodative lags than reading English text, but the small differences were not clinically significant. Myopic children had significantly greater NITM and longer regression than emmetropic children for both texts. Whether differences in NITM are a cause or consequence of myopia cannot be answered from this study.

Autonomic drugs and the accommodative system in rhesus monkeys

Accommodation and pupil constriction result from parasympathetic stimulation from the Edinger-Westphal (EW) nucleus of the midbrain resulting in release of acetylcholine at the neuromuscular junctions of the ciliary muscle and iris. Cholinergic and adrenergic drugs can be applied topically to evaluate the effects on the pupil and accommodative system without input from the EW nucleus. This study is directed at characterizing how topical low dose echothiophate, an anti-cholinesterase inhibitor (i.e., an indirect cholinergic agonist), epinephrine, an adrenergic agonist, and timolol maleate, a beta adrenergic antagonist, affect pupil diameter, resting refraction and accommodative amplitude and dynamics in rhesus monkeys. The effects of 0.015% echothiophate, 2% epinephrine, 0.5% timolol maleate and saline on pupil diameter and resting refraction were measured in one eye each of four normal rhesus monkeys for 60-90 min following topical instillation. Pupil diameter was measured with infrared videography and refraction was measured with a Hartinger coincidence refractometer. Effects on static and dynamic EW stimulated accommodation were studied in three iridectomized monkeys (ages 5, 6 and 12 years) with permanent indwelling stimulating electrodes in the EW nucleus. Dynamic accommodative responses were measured with infrared photorefraction for increasing current amplitudes before and during the course of action of the pharmacological agents. Echothiophate caused a significant decrease in pupil diameter of 3.07 +/- 0.65 mm (mean +/- SEM, p < 0.01), and a myopic shift in resting refraction of 1.30 +/- 0.39 D (p < 0.05) 90 min after instillation. Epinephrine caused a 2.76 +/- 0.38 mm (p < 0.01) increase in pupil diameter with no change in resting refraction 60 min after instillation. Timolol maleate resulted in no significant change in either pupil diameter or resting refraction 60 min after instillation. There was no significant change in maximum EW stimulated accommodative amplitude after any agent tested. The amplitude vs. peak velocity relationship for accommodation was significantly different after echothiophate and timolol maleate, and for disaccommodation after echothiophate, epinephrine and timolol maleate. In conclusion, when tested objectively in anesthetized monkeys, epinephrine and timolol maleate did not alter resting refraction or accommodative amplitude, but did have small, significant affects on accommodative dynamics. This suggests that there is an adrenergic component to the accommodative system. Low dose echothiophate had significant effects on pupil diameter and resting refraction, with only small effects on the dynamics of the accommodative response.

Incremental retinal-defocus theory of myopia development--schematic analysis and computer simulation

Previous theories of myopia development involved subtle and complex processes such as the sensing and analyzing of chromatic aberration, spherical aberration, spatial gradient of blur, or spatial frequency content of the retinal image, but they have not been able to explain satisfactorily the diverse experimental results reported in the literature. On the other hand, our newly proposed incremental retinal-defocus theory (IRDT) has been able to explain all of these results. This theory is based on a relatively simple and direct mechanism for the regulation of ocular growth. It states that a time-averaged decrease in retinal-image defocus area decreases the rate of release of retinal neuromodulators, which decreases the rate of retinal proteoglycan synthesis with an associated decrease in scleral structural integrity. This increases the rate of scleral growth, and in turn the eye's axial length, which leads to myopia. Our schematic analysis has provided a clear explanation for the eye's ability to grow in the appropriate direction under a wide range of experimental conditions. In addition, the theory has been able to explain how repeated cycles of nearwork-induced transient myopia leads to repeated periods of decreased retinal-image defocus, whose cumulative effect over an extended period of time results in an increase in axial growth that leads to permanent myopia. Thus, this unifying theory forms the basis for understanding the underlying retinal and scleral mechanisms of myopia development.

The effect of a beta-adrenoceptor antagonist on accommodative adaptation in Hong Kong children

PURPOSE

Increased susceptibility to nearwork-induced accommodative adaptation has been suggested as a risk factor for myopia development. We investigated whether accommodative adaptation may explain in part the high prevalence of myopia in Hong Kong children and examined the effect of beta-antagonism with topical timolol maleate on accommodative adaptation.

METHODS

Thirty children (10 emmetropes and 20 myopes) aged between 8 and 12 years were recruited. Tonic accommodation was measured before and after 5 min of video game-playing using an open-field Shin-Nippon autorefractor. Measurements were repeated 30 min after timolol instillation.

RESULTS

Children with progressing myopia demonstrated accommodative adaptation following the near task, whereas stable myopes showed counter-adaptive, hyperopic accommodative changes. Timolol increased the magnitude of accommodative adaptation in the stable myopes but had little effect on responses of the progressing myopes or emmetropes.

CONCLUSIONS

Neuropharmacological modulation of the accommodative system may have a possible etiological role in the progression of myopia.

Change in shape of the aging human crystalline lens with accommodation

The objective was to measure the change in shape of the aging human crystalline eye lens in vivo during accommodation. Scheimpflug images were made of 65 subjects between 16 and 51 years of age, who were able to accommodate at least 1D. The Scheimpflug images were corrected for distortion due to the geometry of the camera and the refraction of the cornea and anterior lens surface, which is necessary to determine the real shape of the lens. To ensure accurate correction for the refraction of the anterior lens surface, the refractive index of the crystalline lens must be determined. Therefore, axial length was also measured, which made it possible to calculate the equivalent refractive index of the lens and possible changes in this index during accommodation. The results show that during accommodation there is a decrease in both the anterior and the posterior radius of the lens, although the change in mm per diopter of the latter is much smaller. The increase in lens thickness with accommodation is higher than the decrease in the anterior chamber depth, indicating that the posterior lens surface moves backwards with accommodation. During accommodation the anterior lens surface becomes more hyperbolic. Furthermore, an increase in the equivalent refractive index during accommodation was determined.

Clinical Research. Effect of whiplash injury on accommodation

BACKGROUND

Accommodative disturbance has been cited as one of the causes of visual disturbance following whiplash injury but to date, with one exception, none of the studies have incorporated a group of control subjects; none of the results have been statistically analysed. The aim of the present study was to examine the amplitude of accommodation in a group of whiplash subjects, the majority of whom had specifically complained of visual/ocular symptoms.

METHODS

The whiplash group consisted of 19 subjects and the control group consisted of 43 subjects. The amplitude of accommodation of the right and left eyes of the whiplash and control group subjects was measured and the results of the two groups compared.

RESULTS

There was reduced amplitude of accommodation of the right (P = 0.02) and left (P = 0.02) eyes in the whiplash group subjects compared to the control group subjects.

CONCLUSION

These results indicate that whiplash was associated with defective accommodation in the present select group of whiplash subjects.

The autonomic control of accommodation and implications for human myopia development: a review

Prolonged nearwork has long been associated with myopia development, however, there is no well described linking mechanism. One theory suggests that if accommodation accuracy during nearwork is not maintained, the defocused retinal image leads to myopia development. Here we review the findings of research aimed at determining whether the autonomic inputs to the ciliary smooth muscle are involved in this type of environmental myopia. We examine whether an autonomic imbalance could be a precursor to axial elongation and the resulting myopia. Accommodation responses, such as tonic accommodation and nearwork-induced accommodative adaptation, as a function of refractive error, are described in relation to an autonomic imbalance model. The collective results of this research point to anomalous accommodation responses, possibly as a result of underlying anomalous autonomic input to the ciliary muscle, being involved in myopia development and progression.

Effect of beta-adrenoceptor antagonists on autonomic control of ciliary smooth muscle

PURPOSE

Pharmacological intervention with peripheral sympathetic transmission at ciliary smooth muscle neuro-receptor junctions has been used against a background of controlled parasympathetic activity to investigate the characteristics of autonomic control of ocular accommodation.

METHODS

A continuously recording infra-red optometer was used to measure accommodation on a group of five visually normal emmetropic subjects under open- and closed-loop conditions. A double-blind protocol between saline, timolol and betaxolol was used to differentiate between the localised action on ciliary smooth muscle and effects induced by changes in stimulus conditions. Data were collected before and 45 min following the instillation of saline, timolol or betaxolol. Open-loop post-task decay was investigated following 3 min sustained near fixation of a stimulus placed 3 D above the subject's pre-task tonic accommodation level. Closed-loop dynamic responses were recorded for each treatment condition while subjects viewed sinusoidally (0.05-0.6 Hz) or stepwise vergence-modulated targets over a 2 D range (2-4 D).

RESULTS

Open-loop data demonstrate a rapid post-task regression to pre-task tonic accommodation levels for saline and betaxolol control conditions. A slow positive post-task shift was induced by timolol indicating that sympathetic inhibition contributes to accommodative adaptation during sustained near vision. Closed-loop accommodation responses to temporally modulated sinusoidal stimuli showed characteristic features for both saline and betaxolol control conditions. Timolol induced a reduced gain for low- and mid-temporal frequencies (< 0.3 Hz) but did not affect the response at higher temporal frequencies. Response times to stepwise stimuli increased following the instillation of timolol for the near-to-far fixation condition compared with the controls and was related to the period of sustained prior fixation.

CONCLUSIONS

Modulation of accommodation under open- and closed-loop conditions by a non-selective beta-blocker is consistent with the temporal and inhibitory features of sympathetic innervation to ciliary smooth muscle. Although parasympathetic innervation predominates there is evidence to support a role for sympathetic innervation in the control of ocular accommodation.

Effects of viewing distance on accommodative and pupillary responses following a three-dimensional task

We have investigated accommodation and pupil responses after viewing stereoscopic images related to the resting position of accommodation. After a 10-min visual task viewing stereoscopic three-dimensional images, measurements of dynamic accommodation and pupil response were made using an infrared optometer and a pupil analyser. Four conditions were given for the viewing distance: 0.4 m (closer than the subjects' resting position), 1 m (approximating to their resting state), and 1.5 and 3 m (farther than the resting position). For the 0.4- and 1-m groups, a delay in the accommodation response for near-to-far movement was shown after the task, from 817 to 1120 ms and from 830 to 898 ms, respectively, but there was no change in the pupillary response. In the 1.5- and 3-m groups, on the other hand, a significant delay in the accommodation response for far-to-near movement was shown, from 827 to 912 ms and from 857 to 1150 ms, respectively, and greater miosis was found, from 7.2 to 9.2 mm2 and from 8.0 to 10.4 mm2. The data reveal that there are different after-effects on accommodation and pupillary functions when subjects perform the task within their accommodative resting position, than when the task is performed outside the resting position.

M3-type muscarinic receptors predominantly mediate neurogenic quick contraction of bovine ciliary muscle

1. The present experiments were designed to investigate which subtypes of muscarinic receptors are involved in the neurogenic quick contraction of bovine ciliary muscle in connection to quick eye focal accommodation. 2. Transmural electrical stimulation (TES) produced a transient contraction, which was abolished in the presence of 3 x 10(-7) M tetrodotoxin and 10(-6) M atropine, but greatly augmented by 3 x 10(-7) M physostigmine. 3. The exogenously applied acetylcholine (ACh: 10(-9) to 3 x 10(-6) M) produced a concentration-dependent contraction, which was competitively antagonized by 10(-6) M atropine and augmented by 3 x 10(-7) M physostigmine, but unaffected by 3 x 10(-7) M tetrodotoxin. 4. The magnitude and time to peak of the maximal contraction produced by TES were significantly greater (1267.5 +/- 86.0 mg, P < 0.005) and shorter (9.0 +/- 0.2 sec, P < 0.005) than corresponding values (97.0 +/- 9.9 mg and 20.3 +/- 2.1 sec, respectively) of the phasic contraction caused by exogenously applied 10(-5) M ACh, at which concentration the agonist caused the maximal contraction. The velocity (140.6 +/- 7.8 mg/sec) of the transient contraction caused by TES was approximately 28-fold greater than that of the phasic contraction caused by ACh (5.1 +/- 0.9 mg/sec). 5. The contractions produced by TES were greatly attenuated by 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) as an M3 antagonist and slightly by pirenzepine as an M1 antagonist (20.2 +/- 7.9% inhibition at the highest concentration), but not by methoctramine (MET) as an M2 antagonist. The IC50 value (-log M) for 4-DAMP was determined to be 7.17 +/- 0.14. 6. Scatchard plot analysis of [3H]-quinuclidinylbenzilate (QNB) binding revealed that the binding sites constituted a single population with a Kd of 31.2 +/- 0.8 pM and a Bmax of 895.5 +/- 93.2 fmol/mg protein. The activity in inhibiting [3H]-QNB binding was most potent with 4-DAMP (-log Ki = 7.98 +/- 0.02), but less potent with pirenzepine (-log Ki = 6.43 +/- 0.04) and MET (-log Ki = 7.32 +/- 0.16). 4-DAMP was approximately 35- and 5-fold more potent than pirenzepine and MET in terms of -log Ki values, respectively, suggesting the predominant localization of M3 receptor subtypes in the bovine ciliary muscle membrane. 7. These results suggest that TES produces a neurogenic quick contraction of the bovine ciliary muscle, which would be mediated mainly by ACh released from the intramural nerve terminals and subsequent excitation of M3 receptor subtypes localized on the ciliary muscle cells, and that neurogenic quick contraction of the ciliary muscle is possibly involved in part in eye focal accommodation.

The effects of endothelin-1 on isolated bovine ciliary muscles

The effects of endothelin-1 on bovine ciliary muscle were investigated in vitro using muscle strips prepared in two different directions (longitudinal and circular). Fifty-two bovine ciliary muscle strips (4 x 6 mm) were prepared. Chemicals were added to both types of strips suspended in an organ chamber, and their changes in isometric tension were recorded. The concentration-response relationship of endothelin-1 (n = 20), the magnitudes of contractions caused by endothelin-1 and carbachol (n = 12), and the effects of an endothelin receptor subtype A antagonist BQ123 (n = 4) and an endothelin receptor subtype B agonist IRL1620 (n = 4) were studied. The cumulative addition of endothelin-1 caused relaxation at low concentrations (10(-11) and 10(-10) M), while it caused contraction of the ciliary muscles at high concentrations (10(-9), 10(-8) and 3 x 10(-8) M). The magnitude of the contraction caused by 10(-8) M endothelin-1 was about 30% in the longitudinal muscle strips and 29% in the circular muscle strips, relative to the contraction caused by 10(-5) M carbachol, a muscarinic agonist. The contractile response caused by 10(-8) M endothelin-1 was abolished and converted to relaxation by pretreatment with BQ123, a selective endothelin receptor subtype A antagonist. Single addition of IRL1620, a selective endothelin receptor subtype B agonist, caused only relaxation. These results suggest that, endothelin-1 causes not only contraction at high concentrations, but also relaxation at low concentrations in bovine ciliary muscle. Also, it suggested that the relaxation is mediated by endothelin receptor subtype B, whereas the contraction is mediated by endothelin receptor subtype A.

The effect of topical beta-adrenoceptor antagonists on accommodation in emmetropia and myopia

Using topical instillation of non-selective and selective beta-adrenoceptor antagonists we examine the proposal that a deficit in inhibitory sympathetic innervation of ciliary smooth muscle may be a specific precursor to the onset and development of late-onset myopia (LOM). Post-task accommodative hysteresis, a possible consequence of such a deficit, was assessed by measuring the time-course of regression of accommodation when open-loop (darkness) conditions were immediately imposed following far and near tasks. For the proposal to be feasible only LOMs should exhibit post-task responses which fail to differentiate the type of beta-antagonist employed. As the overall profile of responses to beta-adrenoceptor antagonism was equivalent for each of the three different refractive groups (emmetropes N = 6; early-onset myopes N = 5; LOMs N = 5) we conclude that a propensity to LOM is not associated with a deficit in sympathetic inhibition.

Tonic accommodation: a review. I. Basic aspects

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Temporal characteristics of proximally-induced accommodation

This study sought to determine whether proximally-induced accommodation (PIA) is capable of sustaining an accommodative response over time. Accordingly, the steady-state PIA response was measured during the course of a continuous 5 min stimulus period for targets located at distances of 0.2 or 6 m. The vergence and accommodation loops were opened by subjects (n = 8) monocularly viewing the targets through a 0.5 mm pinhole, while accommodation was assessed using an objective, open-field, infrared optometer. The mean PIA response did not change significantly during the fixation period. This finding suggests that PIA is indeed capable of maintaining a sustained response. This observation is consistent with the constancy of apparent target distance perception during extended fixation of a stationary object of regard under these conditions.