Mechanism Of Aqueous Outflow Research Articles

Evaluation of the Position and Function of Aqueous Drainage Implants With Magnetic Resonance Imaging

To evaluate position and function of antiglaucomatous Ahmed and Molteno shunts using magnetic resonance imaging with head and surface coils. Eight patients (5 males) with shunt implants were included, 4 with Ahmed (FP-7) and 4 with Molteno (s1, single plated). All patients were operated at least 6 months before imaging. In 3 cases (2 with Molteno and 1 with Ahmed shunt), the intraocular pressure (IOP) was above 21 mm Hg, despite maximal medical treatment. The shunt endplate, tube and filtering blebs were identified in T1-weighted and T2-weighted images with both head and surface coils. Volumetric measurements of the orbits, eyeball, and filtering bleb and calculation of the endplate position along sagittal, transverse, and vertical axes were performed in T1-weighted and T2-weighted images using head coils. The shunt endplate was identified in T1-weighted and T2-weighted images (head coils) as a low intensity (dark) circumlinear band at the superotemporal aspect of the eyeball, surrounded by a pocket of water density, corresponding to the filtering bleb. The anterior position of the endplate, and smaller volume of the orbital cavity (less available orbital space) were associated with higher IOP. Filtering bleb volume was inversely correlated with IOP. In the unsuccessful cases, filtering bleb was absent. Magnetic resonance imaging provides insights into the mechanism of aqueous outflow and causes of failure of shunts. A lower orbital volume is associated with anterior position of the shunt endplate and poor shunt performance.

The mechanism of aqueous outflow following trabeculectomy

The morphological effects of trabeculectomy, performed as therapy for a case of chronic simple glaucoma, are described in the two eyes of a patient ten and thirteen months after treatment. Ten months after surgery, a transscleral channel, extending from the position formerly occupied by Schlemm's canal to the conjunctiva, is found. This channel is formed from a network of extended fibroblasts and loosely-packed collagenous tissue, and a large number of vessels, including both dilated blood capillaries and lymphatic vessels, develop within it. The sclera and subconjunctiva react differently to the presence of the nylon sutures. The findings support the hypothesis that the formation of an intrascleral outflow route stimulated by surgical intervention plays an important role in lowering IOP after trabeculectomy.

The mechanism of aqueous outflow in primates, lower mammals and birds: a comparative study

The optical properties of the crystalline lens are determined by its shape and refractive index distribution. However, to date, those properties have not been measured together in the same lens, and therefore their relative contributions to optical aberrations are not fully understood. The shape, the optical path difference, and the focal length of ten porcine lenses (age around 6 months) were measured in vitro using Optical Coherence Tomography and laser ray tracing. The 3D Gradient Refractive Index distribution (GRIN) was reconstructed by means of an optimization method based on genetic algorithms. The optimization method searched for the parameters of a 4-variable GRIN model that best fits the distorted posterior surface of the lens in 18 different meridians. Spherical aberration and astigmatism of the lenses were estimated using computational ray tracing, with the reconstructed GRIN lens and an equivalent homogeneous refractive index. For all lenses the posterior radius of curvature was systematically steeper than the anterior one, and the conic constant of both the anterior and posterior positive surfaces was positive. In average, the measured focal length increased with increasing pupil diameter, consistent with a crystalline lens negative spherical aberration. The refractive index of nucleus and surface was reconstructed to an average value of 1.427 and 1.364, respectively, for 633 nm. The results of the GRIN reconstruction showed a wide distribution of the index in all lens samples. The GRIN shifted spherical aberration towards negative values when compared to a homogeneous index. A negative spherical aberration with GRIN was found in 8 of the 10 lenses. The presence of GRIN also produced a decrease in the total amount of lens astigmatism in most lenses, while the axis of astigmatism was only little influenced by the presence of GRIN. To our knowledge, this study is the first systematic experimental study of the relative contribution of geometry and GRIN to the aberrations in a mammal lens.

The mechanism of aqueous outflow in birds: II. An ultrastructural study of perfused eyes

In order to detect the route of flow of the large molecules of the aqueous humour, a colloidally suspended electron-dense tracer (Thorotrast—particle size approx. 10 nm) was injected in vivo into the anterior chambers of chicken eyes at a controlled intraocular pressure. After about 20 min, the eyes were fixed and processed for electron microscopy. It was observed that almost all vacuoles in the endothelial lining of the aqueous sinus were filled with the tracer element from their basal openings but a leakage of this material into the lumen of the aqueous sinus was seen only from a small proportion of vacuoles which have openings on the luminal aspect. No leakage of the tracer element occurred, however, across the endothelial cell junctions and the majority of micropinocytotic vesicles were devoid of tracer particles. Variable concentrations of the tracer particles were also seen in the lumen of the aqueous sinus and the intrascleral collector channels. It is concluded that the porosity of the endothelial lining of the aqueous sinus is dependent upon the dynamic process of endothelial vacuolation and that the bulk transfer of the aqueous humour takes place via the transient transcellular channels created during this vacuolation cycle. Reference has been made to the factors which could possibly influence this process. A close similarity of this mechanism of aqueous outflow in the eyes of birds to that of primates ( Tripathi, 1969a,b, 1971a) and lower mammals ( Tripathi and Tripathi, 1972) indicates that the dynamic process of endothelial vacuolation is a fundamental biological process, probably common to the eyes of many vertebrates.

The mechanism of aqueous outflow in birds: I. An ultrastructural study of normal eyes

The morphological organization of the angle of the anterior chamber and the angular aqueous drainage channels, designated in this report as “angular aqueous sinus or plexus” in the eyes of chickens, pigeons and snow goose is described. Certain anatomical and physiological features of the avian drainage angle, including the ultrastructural constitution of the walls of the aqueous sinus (endothelial lining, basement membrane and supporting tissue zone), are described and discussed. The angular aqueous sinus, a conspicuous and circumferentially located structure, lies in the inner aspect of the limbus in a scleral hollow. In meridional sections, the sinus appears elliptical in shape, often divided into two, and rarely three or four, lumina by intervening or bridging septae, the main septum containing a muscular-walled artery. On the external aspect, the lumen of the sinus is in direct communication with the intrascleral collector channels and plexus of veins. No such communications are, however, seen on the inner aspect, facing the anterior chamber. Ultrastructurally, the endothelial lining of the aqueous sinus is a continuous single-layered membrane, adjacent cells being joined by occluding zonules. Frequently, individual endothelial cells lining the trabecular wall (and rarely in the scleral wall) contain electron-optically empty giant vacuoles which project into the lumen of the sinus. Serial section analysis revealed that at a given time the majority of vacuoles are in fact basal invaginations having open communication with the extracellular space on the anterior chamber aspect; a small proportion however, show openings on both surfaces (i.e. basal and luminal) thus representing transcellular channels or pores. Occasionally, wandering cells were seen in transit via the transcellular channels. That these channels really represent the final exit pathway of the aqueous humour from the anterior chamber is further investigated in part II of our communication ( Tripathi and Tripathi, 1973).

The mechanism of aqueous outflow in lower mammals

Ultrastructural studies of the angular aqueous plexuses in lower mammals revealed that normally they were lined by a continuous endothelium, adjacent cells being joined by occluding zonules. Many endothelial cells contained giant vacuoles. Serial sections showed that the majority of these vacuoles have openings towards their basal aspect and only a small proportion had openings on both surfaces thereby constituting a transcellular channel. Apart from traces of flocculent material in occasional vacuoles, almost all vacuoles were electron optically empty. Following in vivo intracameral perfusion with an electron dense tracer (Thorotrast, particle size approximately 10 nm), the majority of vacuoles in the endothelial cells lining the aqueous plexus were readily filled with the tracer element. No leakage of this material was seen across the endothelial cell junctions. Serial sections further revealed that the transcellular channels formed by the vacuoles provided a pathway for the tracer element and wandering cells across the endothelial barrier of the aqueous plexus. It is concluded that the outflow of aqueous humour across the intact endothelial barrier of the aqueous plexus is achieved by the dynamic process of endothelial vacuolation. In this process, the role of many factors, including the unique property of the cell and the pressure gradient across the endothelial barrier of the aqueous plexus, is briefly discussed. From the present work, it is clear that the mechanism of aqueous outflow in lower mammals is essentially similar to that occurring in primate eyes ( Tripathi, 1971a). This evidence, together with the work in progress, favours our hypothesis advanced earlier that endothelial vacuolation is a fundamental mechanism for the outflow of the aqueous humour in the eyes of many vertebrates.