Parasympathetic nervous control of intraocular pressure

Diurnal Variation in Accommodation, Binocular Vergence, and Pupil Size

SIGNIFICANCE

Our results show significant diurnal variations in accommodative function and the magnitude of the phoria. Therefore, when comparing visual measures in clinical or laboratory settings, performing the visual examination at the same time of day (±1 hour) is encouraged.

PURPOSE

The aim of this study was to evaluate the accommodation, binocular vergence, and pupil behavior on three different times during the day.

METHODS

Twenty collegiate students (22.8 ± 2.1 years) participated in this study. Participants visited the laboratory on three different days at 2-hourly intervals (morning, 9:00 to 11:00 am ; afternoon, 2:00 to 4:00 pm ; evening, 7:00 to 9:00 pm ). The binocular vergence and accommodative function were measured using clinical optometric procedures, and the accommodative response and pupil function were evaluated in binocular conditions using the WAM-5500 autorefractometer.

RESULTS

The accommodative amplitude for the right and left eyes showed statistically significant differences for the time interval ( P = .001 and P = .02, respectively), revealing higher accommodative amplitude in the morning and afternoon in comparison with the evening. Participants were more esophoric when assessed in the morning in comparison with the evening at far and near ( P = .02 and P = .01, respectively) and when assessed in the afternoon in comparison with the evening at far distance ( P = .02). The magnitude of accommodative response was higher in the morning, and it decreased throughout the day at 500 ( P < .001), 40 ( P = .05), and 20 cm ( P < .001). No statistically significant differences were obtained for any other variable.

CONCLUSIONS

This study shows small diurnal variations in some accommodative and binocular vergence parameters, but no effects were observed for the pupil response. These outcomes are of special relevance for eye care specialists when performing repeated accommodative or binocular vergence measures. However, the diurnal variations were modest and may not influence a routine orthoptic examination.

Effects of head posture on intraocular pressure and heart rate of human beings

BACKGROUND

The study analyzed the association of head posture on intraocular pressure (IOP). The study aimed to evaluate and measure the changes in IOP and heart rate (HR) of human beings on head-down posture. The study included 105 patients at the department of ophthalmology of a tertiary care center in India.

SUBJECTS AND METHODS

Patients underwent applanation tonometry and HR variability (HRV) analysis before and after 20 min of head-down posture (approximately 20°). The IOP and HRV were measured.

STATISTICAL ANALYSIS USED

The statistical methods of Paired t-test and linear regression analysis were applied. P < 0.05 was defined as statistically significant.

RESULTS

After 20 min of the 20° head-down position, an increase in IOP was significant from 15.0 ± 2.0 mmHg to 18.0 ± 2.3 mmHg (P < 0.001). A decrease in HR was also significant from 78 ± 10.48 bpm to 72 ± 10.52 bpm after the head-down position for 20 min (P < 0.05).

CONCLUSIONS

These outcomes presented the first evidence of the activation of the parasympathetic nervous system in the head-down position which might cause decreased HR and the collapse of Schlemm's canal lumen, which in turn leads to the increased IOP.

Ocular Autonomic Nervous System: An Update from Anatomy to Physiological Functions

The autonomic nervous system (ANS) confers neural control of the entire body, mainly through the sympathetic and parasympathetic nerves. Several studies have observed that the physiological functions of the eye (pupil size, lens accommodation, ocular circulation, and intraocular pressure regulation) are precisely regulated by the ANS. Almost all parts of the eye have autonomic innervation for the regulation of local homeostasis through synergy and antagonism. With the advent of new research methods, novel anatomical characteristics and numerous physiological processes have been elucidated. Herein, we summarize the anatomical and physiological functions of the ANS in the eye within the context of its intrinsic connections. This review provides novel insights into ocular studies.

Intraocular Pressure Reactivity to Social Stressors

Abstract. A field study was carried out in an optometry clinic, aimed at assessing the role of perceived control and aversiveness of non-contact tonometry in intraocular pressure (IOP) reactivity to psychosocial stressors, and analyzing the covariation with cardiovascular and affective reactivity. Forty-four customers volunteered to participate in the study. Perceived control (self-efficacy and threat) was assessed at the onset. IOP, systolic and diastolic blood pressure, heart rate, affect, and aversiveness of the IOP measurement procedure were assessed throughout five phases with a mean duration for each phase of 9 min: arrival, optometry, baseline, stressor task (speech in public task), and recovery. The results suggest that IOP decreases over time and the stressor task induced a remarkable reactivity in all the physiological variables assessed. The interaction between self-efficacy and threat partially explains individual variability in IOP: a high threat combined with a high self-efficacy yielded higher reactivity in IOP or increased tonic values throughout the phases. The aversiveness of the measurement procedure did not affect IOP. Intraocular Pressure (IOP) is reactive to social stressors and perceived control partially explains individual variability. Cardiovascular and IOP reactivity are parallel phenomena but do not share a common regulatory mechanism.

Dual effect of the Valsalva maneuver on autonomic nervous system activity, intraocular pressure, Schlemm's canal, and iridocorneal angle morphology

Background

The Valsalva maneuver (VM) is widely used in daily life, and has been reported to cause high intraocular pressure (IOP). This study aimed to assess changes in IOP, the Schlemm’s canal (SC), autonomic nervous system activity, and iridocorneal angle morphology in healthy individuals during different phases of the VM.

Methods

The high frequency (HF) of heart rate (HR) variability, the ratio of low frequency power (LF) and HF (LF/HF), heart rate (HR), IOP, systolic (SBP) and diastolic blood pressure (DBP), the area of SC (SCAR), pupil diameter (PD), and some iridocorneal angle parameters (AOD500, ARA750, TIA500 and TISA500) were measured in 29 young healthy individuals at baseline, phase 2, and phase 4 of the VM. SBP and DBP were measured to calculate mean arterial pressure (MAP) and mean ocular perfusion pressure (MOPP). HF and the LF/HF ratio were recorded using Kubios HR variability premium software to evaluate autonomic nervous system activity. The profiles of the anterior chamber were captured by a Spectralis optical coherence tomography device (anterior segment module).

Results

Compared with baseline values, in phase 2 of the VM, HR, LF/HF, IOP (15.1 ± 2.7 vs. 18.8 ± 3.5 mmHg, P < 0.001), SCAR (mean) (7712.112 ± 2992.14 vs. 8921.12 ± 4482.79 μm², P = 0.039), and PD increased significantly, whereas MOPP, AOD500, TIA500, and TISA500 decreased significantly. In phase 4, DBP, MAP, AOD500, ARA750, TIA500and TISA500 were significantly lower than baseline value, while PD and HF were remarkably larger than baseline. The comparison between phase 2 and phase 4 showed that HR, IOP (18.8 ± 3.5 vs. 14.7 ± 2.9 mmHg, P < 0.001) and PD decreased significantly from phase 2 to phase 4, but there were no significant differences in other parameters.

Conclusions

The expansion and collapse of the SC in different phases of the VM may arise from changes in autonomic nervous system activity. Further, the effects of the VM on IOP may be attributed to changes in blood flow and ocular anatomy.

Trial registration

This observational study was approved by the ethics committee of Tongji Hospital (Registration Number: ChiCTR-OON-16007850, Date: 01.28.2016).

Mental stress as consequence and cause of vision loss: the dawn of psychosomatic ophthalmology for preventive and personalized medicine

The loss of vision after damage to the retina, optic nerve, or brain has often grave consequences in everyday life such as problems with recognizing faces, reading, or mobility. Because vision loss is considered to be irreversible and often progressive, patients experience continuous mental stress due to worries, anxiety, or fear with secondary consequences such as depression and social isolation. While prolonged mental stress is clearly a consequence of vision loss, it may also aggravate the situation. In fact, continuous stress and elevated cortisol levels negatively impact the eye and brain due to autonomous nervous system (sympathetic) imbalance and vascular dysregulation; hence stress may also be one of the major causes of visual system diseases such as glaucoma and optic neuropathy. Although stress is a known risk factor, its causal role in the development or progression of certain visual system disorders is not widely appreciated. This review of the literature discusses the relationship of stress and ophthalmological diseases. We conclude that stress is both consequence and cause of vision loss. This creates a vicious cycle of a downward spiral, in which initial vision loss creates stress which further accelerates vision loss, creating even more stress and so forth. This new psychosomatic perspective has several implications for clinical practice. Firstly, stress reduction and relaxation techniques (e.g., meditation, autogenic training, stress management training, and psychotherapy to learn to cope) should be recommended not only as complementary to traditional treatments of vision loss but possibly as preventive means to reduce progression of vision loss. Secondly, doctors should try their best to inculcate positivity and optimism in their patients while giving them the information the patients are entitled to, especially regarding the important value of stress reduction. In this way, the vicious cycle could be interrupted. More clinical studies are now needed to confirm the causal role of stress in different low vision diseases to evaluate the efficacy of different anti-stress therapies for preventing progression and improving vision recovery and restoration in randomized trials as a foundation of psychosomatic ophthalmology.

Neural control of choroidal blood flow

The choroid is richly innervated by parasympathetic, sympathetic and trigeminal sensory nerve fibers that regulate choroidal blood flow in birds and mammals, and presumably other vertebrate classes as well. The parasympathetic innervation has been shown to vasodilate and increase choroidal blood flow, the sympathetic input has been shown to vasoconstrict and decrease choroidal blood flow, and the sensory input has been shown to both convey pain and thermal information centrally and act locally to vasodilate and increase choroidal blood flow. As the choroid lies behind the retina and cannot respond readily to retinal metabolic signals, its innervation is important for adjustments in flow required by either retinal activity, by fluctuations in the systemic blood pressure driving choroidal perfusion, and possibly by retinal temperature. The former two appear to be mediated by the sympathetic and parasympathetic nervous systems, via central circuits responsive to retinal activity and systemic blood pressure, but adjustments for ocular perfusion pressure also appear to be influenced by local autoregulatory myogenic mechanisms. Adaptive choroidal responses to temperature may be mediated by trigeminal sensory fibers. Impairments in the neural control of choroidal blood flow occur with aging, and various ocular or systemic diseases such as glaucoma, age-related macular degeneration (AMD), hypertension, and diabetes, and may contribute to retinal pathology and dysfunction in these conditions, or in the case of AMD be a precondition. The present manuscript reviews findings in birds and mammals that contribute to the above-summarized understanding of the roles of the autonomic and sensory innervation of the choroid in controlling choroidal blood flow, and in the importance of such regulation for maintaining retinal health.

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

The multifunctional choroid

The choroid of the eye is primarily a vascular structure supplying the outer retina. It has several unusual features: It contains large membrane-lined lacunae, which, at least in birds, function as part of the lymphatic drainage of the eye and which can change their volume dramatically, thereby changing the thickness of the choroid as much as four-fold over a few days (much less in primates). It contains non-vascular smooth muscle cells, especially behind the fovea, the contraction of which may thin the choroid, thereby opposing the thickening caused by expansion of the lacunae. It has intrinsic choroidal neurons, also mostly behind the central retina, which may control these muscles and may modulate choroidal blood flow as well. These neurons receive sympathetic, parasympathetic and nitrergic innervation. The choroid has several functions: Its vasculature is the major supply for the outer retina; impairment of the flow of oxygen from choroid to retina may cause Age-Related Macular Degeneration. The choroidal blood flow, which is as great as in any other organ, may also cool and warm the retina. In addition to its vascular functions, the choroid contains secretory cells, probably involved in modulation of vascularization and in growth of the sclera. Finally, the dramatic changes in choroidal thickness move the retina forward and back, bringing the photoreceptors into the plane of focus, a function demonstrated by the thinning of the choroid that occurs when the focal plane is moved back by the wearing of negative lenses, and, conversely, by the thickening that occurs when positive lenses are worn. In addition to focusing the eye, more slowly than accommodation and more quickly than emmetropization, we argue that the choroidal thickness changes also are correlated with changes in the growth of the sclera, and hence of the eye. Because transient increases in choroidal thickness are followed by a prolonged decrease in synthesis of extracellular matrix molecules and a slowing of ocular elongation, and attempts to decouple the choroidal and scleral changes have largely failed, it seems that the thickening of the choroid may be mechanistically linked to the scleral synthesis of macromolecules, and thus may play an important role in the homeostatic control of eye growth, and, consequently, in the etiology of myopia and hyperopia.

Role of muscarinic cholinergic transmission in Edinger-Westphal nucleus-induced choroidal vasodilation in pigeon

Activation of the parasympathetic ciliary ganglion input to the choroid causes increases in choroidal blood flow. We examined the role and the type of muscarinic receptors within the choroid that are involved in these increases in choroidal blood flow, using electrical stimulation of the nucleus of Edinger-Westphal (EW) to activate the ciliary ganglion input to choroid in ketamine anesthetized pigeons. Baseline choroidal blood flow and its EW-evoked increases measured as peak and total (area under the curve) responses were determined using laser Doppler flowmetry. The EW-evoked responses were reduced dose-dependently after administration of 4-diphenyl-acetoxy-N-methylpiperedine (4-DAMP), a relatively selective antagonist of M3 type muscarinic receptors, with a maximal mean decrease of 86% (peak response) and 93% (total response) at a dose of 10 microg kg(-1)i.v. without a significant effect on baseline choroidal blood flow, heart rate or systemic arterial blood pressure. Atropine, a non-selective antagonist of muscarinic receptors, decreased the EW-evoked responses to a lesser extent than 4-DAMP after intravenous administration of 1 mg kg(-1)(by 67% for peak response and by 53% for total response) or topical administration of a 5% solution (by 41% for peak response and by 62% for total response), both of which increased heart rate and systemic arterial blood pressure without a consistent effect on baseline choroidal blood flow. In contrast, himbacine (i.p. 10 microg kg(-1)), a relatively selective antagonist of M2 type muscarinic receptors, increased the EW-evoked parasympathetic cholinergic vasodilation (by 93% for the peak response and by 142% for the total response) without a significant effect on heart rate, systemic arterial blood pressure or baseline choroidal blood flow. The results of our study suggest a major role of M3 type muscarinic receptors in the EW-evoked increases in choroidal blood flow. Based on findings that the ciliary ganglion input to choroid does not synthesize nitric oxide but inhibitors of NO production do block EW-evoked choroidal vasodilation, it seems likely that the M3 receptors acted on by 4-DAMP are present on choroidal endothelial cells and mediate choroidal vasodilation via stimulation of endothelial release of nitric oxide. In contrast, M2 muscarinic receptors may play a presynaptic role in downregulating EW-evoked parasympathetic cholinergic vasodilation in avian choroid.

Influence of ophthalmic nerve fibers on choroidal blood flow and myopic eye growth in chicks

Ophthalmic sensory nerve fibers containing substance P and calcitonin gene-related peptide' innervate the choroid in mammals and are known to vasodilate choroidal blood vessels. The avian choroid is also innervated by ophthalmic nerve fibers containing substance P and calcitonin gene-related peptide. The present studies were carried out to determine the influence of these sensory fibers on choroidal blood flow in birds and characterize their interaction with manipulations affecting eye growth. In these studies, ChBF was measured using laser Doppler flowmetry in both eyes in the following groups of birds: (1) normal chicks; (2) chicks with right optic nerve transected for 2 weeks; (3) chicks with right optic nerve transected and a goggle over the right eye for 2 weeks; and (4) chicks with right optic and ophthalmic nerves transected and a goggle over the right eye for 2 weeks. The eyes were refracted and various ocular dimensions measured after the blood-flow measurements. It was found that optic nerve transection reduced ChBF to 30% of normal. Placing a goggle (which increases ocular temperature by 4 degrees C) over an optic nerve transected eye nearly doubled choroidal blood flow over that in an optic nerve transected eye without a goggle. Additional transection of the ophthalmic nerve in a goggled optic nerve-transected eye, yielded choroidal blood flow that was indistinguishable from that in a nongoggled optic nerve-transected eye. Optic nerve transection had a slight stunting effect on axial growth of the eye. While myopic axial elongation was observed in goggled eyes with the optic nerve cut, the extent of myopia was less than in normal goggled eyes. Ophthalmic nerve transection further reduced the myopia induced by goggling in an optic nerve cut eye. These results suggest that ophthalmic nerve input to the choroid exerts a vasodilatory influence, which is activated in a goggled eye. This increased choroidal blood flow may be in response to elevated ocular temperatures caused by the goggling and this increase appears to be masked in goggled eyes with an intact optic nerve by the reduction in choroidal blood flow normally accompanying myopic eye growth. Our results thus show that the induction of myopic eye growth (as in our optic nerve cut eyes with a goggle) need not be accompanied by a decrease in choroidal blood flow from the baseline no-goggle condition (in this case, with the optic nerve cut).

Distribution within the choroid of cholinergic nerve fibers from the ciliary ganglion in pigeons

The distribution of the ciliary ganglion (CG) innervation to the pigeon choroid was determined immunohistochemically, using antisera against choline acetyltransferase (CHAT) and a neurofilament-related protein (the 3A10 antigen). Single-labeling revealed that the nerve fibers containing these two antigens were similarly distributed in the pigeon choroid, with the superior and temporal quadrants of the eye containing the most fibers. Both types of fibers surrounded and ramified on choroidal blood vessels. Additionally, CHAT+ varicosities were evident among vessels in the choroid and choriocapillaris. Double-label immunofluorescence revealed that CHAT and the 3A10 antigen were almost completely colocalized in choroidal nerve fibers, but absent from CHAT+ varicosities. Substance P-containing and calcitonin gene-related peptide-containing choroidal nerve fibers were poor in 3A10+ labeling. Transection of the postganglionic fibers of the CG reduced CHAT+ and 3A10+ nerve fibers in the choroid to 3-5% of normal abundance, with most of the residual fibers being located in the nasal and inferior quadrants. The present results suggest that the CG in pigeon preferentially influences choroidal blood flow in the superior and temporal parts of the eye, which are involved in high acuity and binocular vision.

Embryonic development of the cornea in the eye of the clearnose skate, Raja eglanteria: I. Stromal development in the absence of an endothelium

Embryos of the clearnose skate, Raja eglanteria, develop in sea water at 20-22 degrees C, hatching after 82 +/- 4 days (Luer and Gilbert, Environ. Biol. Fishes, 13:161-171, 1985). Eyes develop as steadily enlarging spheres whose corneas have the same radius of curvature as the sclera. The cornea begins development as a 2-cell thick epithelium beneath which by Day 12 there is only a basal lamina and a wispy matrix separating it from the underlying lens. This matrix, modified by Day 16, is displaced on Day 22 by a few orthogonal plies of fibrillar primary stroma. Ply number increases to at least 13 by Day 30, reaching the final number of 20 +/- 2 by Day 42. Stromal fibroblasts (keratocytes) appear at the corneal periphery by Day 22, and in increased numbers by Day 30, a time at which no keratocytes are seen in the central stroma. However, by Day 40, many fibroblasts are present at the corneal periphery, invading the primary stroma between plies, occasionally reaching even the central cornea. By Day 53, keratocytes are present between all plies, from corneal periphery to center. Thickness of each ply in this secondary stroma increases, but the number of plies remains the same as in the primary stroma. Bowman's layer, non-invaded matrix beneath the epithelial basal lamina, is not evident until Day 53. Sutural fibers, first seen on Day 22, originate in the corneal epithelial basal lamina, traversing perpendicularly the plies of the primary stroma. Sutural fibers persist throughout development of the secondary stroma and into adulthood. In contrast to chicks, skate corneas remain transparent throughout development, and never form an endothelium.