Significant changes in optical axial length occurred in human subjects after 60 minutes of monocular defocus. The bidirectional optical axial length changes observed in response to defocus implied the human visual system is capable of detecting the presence and sign of defocus and altering optical axial length to move the retina toward the image plane.

The human choroid is capable of rapidly changing its thickness in response to a variety of stimuli. However little is known about the role of the autonomic nervous system in the regulation of the thickness of the choroid. Therefore, we investigated the effect of topical parasympatholytic and sympathomimetic agents upon the choroidal thickness and ocular biometrics of young healthy adult subjects. Fourteen subjects (mean age 27.9 ± 4 years) participated in this randomized, single-masked, placebo-controlled study. Each subject had measurements of choroidal thickness (ChT) and ocular biometrics of their right eye taken before, and then 30 and 60 min following the administration of topical pharmacological agents. Three different drugs: 2% homatropine hydrobromide, 2.5% phenylephrine hydrochloride and a placebo (0.3% hydroxypropyl methylcellulose) were tested in all subjects; each on different days (at the same time of the day) in randomized order. Participants were masked to the pharmacological agent being used at each testing session. The instillation of 2% homatropine resulted in a small but significant increase in subfoveal ChT at 30 and 60 min after drug instillation (mean change 7 ± 3 μm and 14 ± 2 μm respectively; both p < 0.0001). The parafoveal choroid also exhibited a similar magnitude, significant increase in thickness with time after 2% homatropine (p < 0.001), with a mean change of 7 ± 0.3 μm and 13 ± 1 μm (in the region located 0.5 mm from the fovea center), 6 ± 1 μm and 12.5 ± 1 μm (1 mm from the fovea center) and 6 ± 2 μm and 12 ± 2 μm (1.5 mm from the fovea center) after 30 and 60 min respectively. Axial length decreased significantly 60 min after homatropine (p < 0.01). There were also significant changes in lens thickness (LT) and anterior chamber depth (ACD) (p < 0.05) associated with homatropine instillation. No significant changes in choroidal thickness, or ocular biometrics were found after 2.5% phenylephrine or placebo at any examination points (p > 0.05). In human subjects, significant increases in subfoveal and parafoveal choroidal thickness occurred after administration of 2% homatropine and this implies an involvement of the parasympathetic system in the control of choroidal thickness in humans.

Homatropine appears to block the thinning effect of hyperopic defocus on choroidal thickness but did not enhance the thickening effect of myopic defocus. The changes in the choroid may relate to the different pathways in the eye's response to myopic and hyperopic blur or reflect an upper limit on the capacity of the choroid to thicken in the short-term.

Purpose. To examine the interaction between a short period of hyperopic defocus and low-dose atropine upon the choroidal thickness and ocular biometrics of healthy myopic subjects. Methods. Twenty young adult myopic subjects had subfoveal choroidal thickness (ChT) and ocular biometry measurements taken before and 30 and 60 min following the introduction of optical blur (0.00 D and −3.00 D) combined with administration of 0.01% atropine or placebo. Each combination of optical blur and drug was tested on different days in a fixed order. Results. The choroid exhibited significant thinning after imposing hyperopic defocus combined with placebo (mean change of −11 ± 2 μm, p<0.001). The combination of hyperopic blur and 0.01% atropine led to a significantly smaller magnitude of subfoveal choroidal thinning (−4 ± 8 μm), compared to placebo and hyperopic defocus (p<0.01). Eyes treated with 0.01% atropine with no defocus exhibited a significant increase in ChT (+6 ± 2 μm, p<0.01). Axial length also underwent small but significant changes after treatment with hyperopic blur and placebo and 0.01% atropine alone (both p<0.01), but of opposite direction to the changes in choroidal thickness. However, the 0.01% atropine/hyperopic blur condition did not lead to a significant change in axial length compared to baseline (p>0.05). Conclusion. Low-dose atropine does inhibit the short-term effect of hyperopic blur on choroidal thickness and, when used alone, does cause a slight thickening of the choroid in young healthy myopic adults.

PurposeTo evaluate retrobulbar blood flow characteristics of glaucoma suspects with glaucomatous optic disc appearance (GODA) in comparison to healthy control group (CG) and primary open angle glaucoma patients (POAG) and assess the effect of age.Methods145 patients from a single glaucoma clinic were enrolled and classified into two diagnostic groups (GODA and POAG). Third group of subjects consisted of 67 age matched individuals (CG). Retrobulbar blood velocity measurement in central retinal artery was performed using color Doppler imaging (CDI). CDI images were processed in custom software leading a range of parameter estimates from a continuous waveform signal. The effect of age on the estimated parameters was evaluated with the stepwise forward regression and ANCOVA in which age was used as a continuous factor. One-way ANOVA was used to test for the differences in the CDI parameters between the three considered groups. Correlation between restive index (RI) and pulsatility index (PI) was assessed with a bilinear fitting guaranteeing no discontinuities in RI intercept estimate. Fisher test was used to assess the applicability of a bilinear PI/RI relationship, while the statistics of the RI intercept estimate were evaluated using the bootstrap.ResultsANCOVA showed significant interaction between age and group (p<0.05) for five out of nine considered CDI parameters. The RI intercept for CG and GODA groups was 0.602±0.047, and 0.574±0.044 respectively, while the RI intercept of 0.934±0.066 was found for the POAG.ConclusionsThe observed similarity of CG and GODA group and dissimilarity between GODA and POAG groups in terms of PI/RI relationship is remarkable. Age may play some role in the different mechanisms occurring in blood velocity dynamics in GODA and POAG subjects but it is not a strongly determining factor.

v AbstractThe impact of myopia is greater than just its optical consequences and in severe cases it can lead to a loss of vision. It is a highly prevalent eye disorder, with about one third of the population worldwide affected by this refractive error. Despite extensive research, the pathogenesis of myopia is still only partially understood. An improved knowledge of mechanisms underlying myopia is critical to developing new therapies to prevent myopia or to slow its progression.Although there is no fully effective and satisfactory myopia control treatment available, growing evidence has suggested that the choroid has a passive or active role in the regulation of eye growth and refractive error development. Numerous human and animal studies have shown that the retina can be moved forward or backward towards the image plane through changes in choroidal thickness, and these changes are likely to be associated in the longer term with eye growth and the development of refractive errors. As the choroid receives a rich innervation from the autonomic nervous system, many previous animal studies have utilized pharmacological agents and neurotoxins to elucidate the possible role of muscarinic signaling in the regulation of choroidal thickness, but only a few studies have investigated this issue in humans. An improved understanding of the mechanisms that control choroidal thickness may be important in developing new therapies for combating the development and progression of myopia.The research described in the thesis aimed to investigate the role of the autonomic nervous system in the control of choroidal thickness, axial length and the morphology In a series of experiments, we have confirmed that in humans under normal visual conditions, the parasympathetic nervous system is involved in the short-term regulation of subfoveal and parafoveal choroidal thickness, axial length and the morphology of the anterior sclera. The muscarinic blocker homatropine caused a significant subfoveal and parafoveal choroidal thickening, shortening of axial length, an enlargement of Schlemm's canal area and an increase in thickness of the conjunctival/episcleral complex. On the other hand, no notable changes in ocular parameters, except a decrease in the thickness of the conjunctival/episcleral tissues were found after instillation of the sympathomimetic drug phenylephrine.In an attempt to further improve our understanding of the myopigenic mechanisms influencing the thickness of the choroid in humans, we investigated the short-term influence of mixed interventions (positive and negative lens-imposed defocus / 2% homatropine) on choroidal thickness and axial length in emmetropic and myopic subjects. Homatropine prevented choroidal thinning in response to hyperopic defocus.This finding is consistent with the anti-myopia effects of muscarinic blockers and supports the potential importance of hyperopic defocus in the development of human myopia. By contrast, co-administration of homatropine with myopic defocus did not enhance the thicken...

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