Intraocular pressure and eye enlargement in chicks

Relationship between intraocular pressure and eye growth in chick

In infants, abnormally high intraocular pressure (IOP) results in excessive eye enlargement. In the study reported here, we investigated whether IOP might be a determinant of ocular elongation using the chick as an animal model. Specifically, we examined IOP changes in (I). normally developing eyes, and (II). eyes undergoing altered growth. In the first case (I). developmental changes in IOP, axial length and refractive error were assessed at approximately daily intervals during early development (days 2-11 post-hatch, n = 8), and at weekly intervals from weeks 1 to 6 (n = 8). In the second case (II). opposite ocular growth responses were elicited using -15 D and +15 D defocusing spectacle lenses fitted monocularly to 8-day-old chicks (n = 8 and 7, respectively). Treated eyes were measured 3 and 7 h after lens application (between 9 and 10 am), as well as 1, 2 and 4 days later, around the same time as the initial lens application. In normal development (I). IOP increased over the first post-hatch week, peaking at 18.0 +/- 5.1 mmHg, and declined slowly thereafter to be back to near 'hatching' values at 6 weeks (12.7 +/- 5.3 mmHg at day 2 cf. 13.4 +/- 1.9 mmHg). Eyes elongated at an approximately linear rate over the entire monitoring period. The -15 D and +15 D lenses (II). produced opposing effects on eye growth, as indicated by axial length changes of + 0.67 +/- 0.25 mm cf. -0.33 +/- 0.17 mm over the 4-day treatment period. Both groups showed decreases in IOP over the first 20 h of lens wear. The faster growing eyes of the -15 D group exhibited a later relative increase in IOP. The described changes in IOP are compatible with, but do not prove, a modulatory influence of IOP on early eye growth.

Intrinsic neurons in the duck choroid are contacted by CGRP-immunoreactive nerve fibres: evidence for a local pre-central reflex arc in the eye

Intrinsic choroidal neurons represent peripherally displaced autonomic nerve cells supposed to work as a local integrative network similar to the enteric nervous system, to control choroidal vasculature and stromal smooth muscle. A typical feature of such intramural neuronal networks is the innervation by primary afferent collaterals expressing peptides, e.g. CGRP. The present study was aimed at determining primary afferent contacts on nitrergic intrinsic choroidal neurons (ICN) in the duck eye. In addition, a sympathetic innervation of ICN was assessed. Choroids were immunohistochemically processed for the following markers: neuronal nitric oxide synthase (nNOS), galanin (GAL), calcitonin gene-related peptide (CGRP), and tyrosine hydroxylase (TH). For evaluation, fluorescence as well as confocal laser scanning microscopy were used. For electron microscopy, immunoperoxidase staining for CGRP in combination with NADPH-diaphorase histochemistry was applied. ICN immunoreactive for nNOS or GAL spread over the entire choroid, although they were concentrated in an equatorial zone passing obliquely from naso-cranial to temporo-caudal. About 40% of ICN showed close relationships with CGRP-immunoreactive nerve fibres, originating most likely in the trigeminal ganglion, as seen in the fluorescence and confocal laserscanning microscope. These appositions could be ultrastructurally defined as both synapses and close contacts without synaptic specialization. Some ICN endowed with CGRP-positive fibres also received TH-immunoreactive boutons. CGRP-immunoreactive profiles were also detected in close relationship to choroidal non-vascular smooth muscle cells and collagen fibres connected to them. In many instances, they were intercalated between smooth muscle cells and processes of ICN forming triads. These results suggest that ICN, similar to other intramural autonomic systems integrate signals from trigeminal primary afferent collaterals. The 'sensory' terminals of these primary afferents may be located in the anterior eye segment but also within the smooth muscle stroma of the choroid itself. Thus, ocular homeostasis may be regulated via intraocular pre-central reflexes which are probably subject to sympathetic modulation.

Axial length increase in lid-sutured rabbits

Using eyelid sutures, we induced axial elongation in 20 15-day-old rabbits. When the eyelid sutures were opened, the eyes revealed a slight increase in their axial length. This evident axial elongation continued to evolve despite restitution of functional integrity to the previously sutured eyes. We suspect that because the lid sutures altered visual input and inhibited the retina from maintaining control of ocular growth, the retina instigated excessive eye growth, resulting in the initial consequent axial elongation that was evident on removal of the sutures. Additionally, poor quality of the retinal image, probably resulting from the noncorrected myopic ametropia caused by the lid sutures, could be the cause of the continued axial length increase observed after the lid sutures were removed.

Autonomic denervations influence ocular dimensions and intraocular pressure in chicks

Choroidal thickness and axial eye length in the chick undergo day/night fluctuations that can also be modulated by visual experience. In the present study, we tested the effect of parasympathetic and sympathetic denervations on both day/night and image dependent changes in ocular dimensions. We also correlated such changes with fluctuations in intraocular pressure. Parasympathectomy influenced choroidal thickness and its day/night fluctuation, but had no effect on vision dependent choroidal thickness modulation. Parasympathectomy also influenced-to a lesser extent-axial length and reduced the axial growth response to form vision deprivation. Sympathectomy had little effect on ocular dimensions, but reduced the day/night differences in intraocular pressure. We conclude that (a) the parasympathetic nervous system influences both choroidal thickness and axial length and participates in the neural control mechanism leading to form deprivation myopia and, (b) the day/night fluctuations of choroidal thickness and axial length are unlikely to be explained by fluctuations in intraocular pressure. For the regulation of choroidal thickness, we hypothesize the existence of two independent mechanisms. One involves the parasympathetic nervous system; it influences the day/night choroidal thickness fluctuation. The other uses a separate pathway and is driven by visual input.

The ciliary ganglion and vitreous cavity shape

PURPOSE

To learn the influence of the ciliary ganglion on the postnatal growth of eyes with unimpaired visual input and of eyes beneath an image diffusing goggle.

METHODS

Newborn chicks received unilateral ciliary ganglionectomy or unilateral sham operation and were reared either with or without a goggle ipsilateral to the surgical procedure. Ocular refractions and ultrasound measurements were made on anesthetized chicks; eyes enucleated postmortem were measured in axial and equatorial dimensions with calipers and studied histologically.

RESULTS

Excessive growth of open eyes in the equatorial dimensions followed ciliary ganglionectomy and became more pronounced as the chicks grew older. There was only a modest increase in axial growth. Ganglionectomy also induced relative hyperopia; lens thinning contributed to this effect and likely was a direct result of disrupted parasympathetic input to the ciliary muscle. Ganglionectomy also slightly increased the thickness of the choroid in the posterior pole but not in more peripheral locations.

CONCLUSION

We conclude that the ciliary ganglion exerts an inhibitory influence on the postnatal growth of open eyes; the main effect is in the equatorial dimension of the vitreous cavity, with a smaller effect on axial length. Ciliary ganglionectomy exerted minimal influence on the development of experimental myopia, known to be induced by the goggle regimen. The amount of equatorial expansion in goggle-induced myopia was greater than after ganglionectomy alone, indicating that other factors besides the ciliary ganglion can influence the equatorial dimension of the vitreous cavity.

Association of intraocular pressure and myopia in children

PURPOSE

While elevated intraocular pressure (IOP) is associated with myopia in adults, its potential influence on the growth of eyes in juveniles without glaucoma is controversial. To address this issue, a possible relation between IOP and refraction in children was sought.

METHODS

A cross-sectional survey of IOP was conducted in children presenting to the Division of Pediatric Ophthalmology at The Children's Hospital of Philadelphia for a complete eye examination. Measurement of IOP was attempted in all children, including those with amblyopia, prematurity, and strabismus. Exclusion criteria were abnormalities of the posterior pole and/or conditions such as cataract that precluded assessment of refractive error. For analysis, myopia was defined as a spherical equivalent of more than 1 diopter (D) of myopia. Logistic regression was used to assess the association between other patient characteristics and presence of myopia.

RESULTS

Intraocular pressure testing was attempted in all age groups, but was more successful in older children. Reliable readings were obtained on 321 subjects. The mean age was 9.8 years, with a mean IOP of 17.3 mmHg in the right eye and 17.2 mmHg in the left and a mean spherical equivalent of +0.2 D in the right eye and +0.1 D in the left. Increasing age, a family history of myopia, and amblyopia were associated myopia. Increasing IOP also was related to myopia. Even when patients with amblyopia, strabismus, and prematurity were exclude, age, family history of myopia, and IOP again were associated with myopia.

CONCLUSIONS

These results indicate that IOP in children may be higher in myopic than nonmyopic eyes. Whether IOP could contribute to the mechanisms causing the abnormal eye growth of childhood myopia requires further study.