The blood-ocular barrier consists of the blood-retinal barrier as well as the blood-aqueous barrier and corresponds to a physiological barrier for defense against pathogens and maintenance of biochemically different milieus. Disorders of the blood-retinal barrier cause fluid accumulation in the retinal area, which can lead to retinal detachment. Most commonly, diabetes mellitus causes blood-ocular barrier disorders.
What is the blood-ocular barrier?
The blood-ocular barrier consists of the blood-retinal barrier as well as the blood-aqueous barrier and corresponds to a physiological barrier that serves, for example, to defend against pathogens. The human body has different biochemical compositions at different points. Physiological barriers maintain these biochemical milieu differences to ensure optimal functioning of individual body sections. One of the best known barriers separating different environments is the blood-brain barrier. A similar function to the blood-brain barrier is performed by the blood-eye barrier. This is a physiological barrier within the eye that separates the uveal blood supply area from the retina. Furthermore, the blood-ocular barrier is responsible for separating the anterior chamber space from the vitreous cavity. These two functions are known as the blood-retinal barrier and the blood-aqueous barrier. The blood-retinal barrier corresponds to the barrier between the retina and the vessels supplying the retina. This barrier is divided into an inner and an outer barrier and has selective permeability. In some literature, the blood-retinal barrier is also considered to be the blood-vitreous barrier because of permeabilities toward the vitreous.
Function and Purpose
The blood-ocular barrier performs protective functions. In addition, it maintains the biochemically different composition of the individual eye segments. For example, the blood-chamber water barrier ensures that aqueous humor, vitreous fluid, and blood plasma do not mix. The blood-retinal barrier, in turn, separates the retina from the plasma. In all mammals, the blood supply to the retina is handled by two different vascular systems. The retinal blood vessels feed from the central artery and take over the blood supply of the inner retinal layers together with their granular layer. The outer retinal layers and their granular layer as well as sensory cells feed instead by diffusion from the choroid. The rod and cone outer members of this retinal layer maintain close contact with the pigment epithelium, whose cells are connected by tight junctions. Tight junctions are belt-like fringes around the entire cell circumference that form a diffusion barrier in the form of an epithelial cell association. This type of barrier is also called a paracellular barrier and closes the intercellular space to protect the interior. In summary, the blood supply to the retina (retina) is provided by the ocular branches of the ophthalmic artery, the central retinal artery, and the posterior ciliary arteries. Branches of the centralis retinae artery run into the strati neurofibrarum et ganglionicum and from there supply the inner portions of the neuronal retina. In contrast, blood supply to the outer photoreceptor layer and the retinal pigment epithelium comes from the choroidal vessels of the posterior ciliary arteriae breves. In the blood-retinal barrier, therefore, the anatomist distinguishes in principle an inner from an outer barrier. The expressions inner and outer in this distinction refer to the capillaries. The inner blood-retinal barrier is formed by tight junctions of endothelial cells. The outer blood-retinal barrier forms with tightly connected epithelial cells in the retinal pigment epithelium. Both the blood-retinal barrier of the retinal vasculature and that of the choroidal vasculature keep pathogens and toxins from the bloodstream from entering the intercellular spaces of retinal layers. The barriers are selectively permeable. Thus, they keep molecules from entering depending on specific properties, such as size. Thus, the barriers do not deny passage to all molecules.
Diseases and ailments
Disturbances or pathologic changes in the blood-retinal barrier always indicate pathologic processes within the eye.In addition to macular edema, these pathological processes can be triggered, for example, by retinal hemorrhages and other retinal diseases (retinopathies). Macular edemas are accumulations of extracellular fluid in the region of the yellow spot. This change is reversible and manifests as blurring of the visual field, especially in the zone of sharpest vision. Retinal hemorrhages, on the other hand, are caused by broken blood vessels. This phenomenon can cause severe visual disturbances. If edema as well as retinal hemorrhages are present, the suspicion of a causative blood-retinal barrier disorder is particularly obvious. Isolated disturbances of the blood-ocular barrier occur in the context of different retinal diseases. A so-called integrity disorder of the inner blood-retinal barrier can occur, for example, in diabetic retinopathy. This is a possible complication of diabetes mellitus. Diabetics often suffer from hypertension, which in the long run can pathologically alter the retinal vessels. The changes in the retinal vessels are further favored by elevated or poorly controlled blood glucose. In the course of the process, glycated and chemically otherwise modified macromolecules are deposited in the vessel walls, where they can cause microhemorrhages of the retina. In Germany, blood-retinal barrier disorders as a result of diabetes affect about one-third of all diabetics. The risk of diabetic retinopathy is about 90 percent for patients with diabetes mellitus. Diabetic retinopathies can cause blindness as they progress. Barrier disorders of the blood-ocular barrier, in turn, do not occur exclusively in the context of diabetes. In the pathomechanism of retinopathia centralis serosa, external blood-retinal barrier disorders are discussed, which correspond to a complete loss of function of the physiological barrier. Retinopathia centralis serosa is an acquired retinal disease with partial retinal detachment, as it appears to result from functional blood-retinal barrier disorders. Psychological and physical stress seem to play a major role in the onset of the disease. The pathogenesis of the disease results from an integrity disorder of the pigment epithelium. This outer blood-retinal barrier disruption evokes subretinal fluid accumulation. The fluid accumulation leads to the formation of edema, which appears to cause retinal detachment.
