Intraocular Pressure

INTRAOCULAR PRESSURE, the state of tension of the eyeball, which is felt upon touching the eye and which expresses the pressure exerted by intraocular fluids on the dense elastic wall of the eyeball. This state of tension of the eye allows it to maintain constant shape, which is essentially necessary for the proper function of the eye as an optical instrument. No less important is the state of intraocular pressure for the processes of nutrition of the eye. The proper exchange of intraocular fluids is possible only on condition of maintaining a certain intraocular pressure. This, in turn, depends on the correctness of the inflow and outflow of fluids in the eye. Thus, here we have to deal with a self-regulating apparatus, the slightest disturbance of the function of which leads to an increase in the tone of the eye or, conversely, to a fall below normal. The normal intraocular pressure of the human eye can be taken as its height between 18 and 30 mm of mercury. Pressure below or above these limits already indicates a disturbance in the exchange of intraocular fluids. A decrease in intraocular pressure often indicates the beginning of atrophy of the eye. An increase in intraocular pressure is observed in glaucoma (see). A very large number of experimental and clinical studies have been devoted to studying the influence of various factors on the level of intraocular pressure. These studies have also established that sharp fluctuations in blood pressure can definitely affect the state of the tone of the eye. There is, however, no direct dependence between these values. The chemical composition of the fluid filling the chambers of the eye also does not correspond to the composition of the blood. True, the composition of the chamber moisture includes only those elements that are also present in the blood, but the percentage ratio between them is completely different. Some elements present in the composition of the blood are completely absent in intraocular fluids. All this speaks against the assumption of the origin of intraocular fluid by simple filtration of it from the vessels of the uveal tract into the chambers of the eye. At present, there is no unanimity on the question of the origin of intraocular fluids, which has enormous significance for understanding the origin of ocular tonus. Perhaps here there is the presence of ultrafiltration phenomena (Dieter), in which the colloids of the blood do not pass into the chambers of the eye, but between the crystalloids there arises a complex relationship expressed by the Donnan formula for fluids separated by a semi-permeable membrane, in those cases when one of these fluids, in addition to crystalloids, contains protein particles incapable of diffusing through the membrane (Meesmann). The possibility of the origin of intraocular fluid by secretion (secretio) is not excluded. As a secretory apparatus of the eye, some authors accept the epithelium of the ciliary body. The distinct reaction of the cells of the ciliary epithelium to oxidases testifies to the active work of these cells, which, in the opinion of supporters of this view, could only be expressed in the separation of intraocular fluid. The explanation of the origin of intraocular fluid by osmosis from the vessels is contradicted by the relationship between the pressure in them and the intraocular pressure. Taking the normal tone of the eye as 25 mm Hg and the colloid-osmotic pressure as 25 mm Hg, there should be a pressure in the capillaries of the eye of not less than 50 mm Hg for osmosis to be possible. Meanwhile, the research of Serr shows that the capillary pressure in the vessels of the eye does not exceed 30-35 mm Hg. As for the pathways of outflow of intraocular fluids, most authors tend to the view that this outflow occurs by simple filtration of fluid from the angle of the anterior chamber through the so-called Fontana spaces into the canal of Schlemm, which is a special venous sinus located in the angle of the anterior chamber and separated from it by the pectinate ligament (ligamentum pectinatum). Thus, the fluid filtering from the eye enters directly into the venous system, bypassing the lymphatic pathways. The attempts of Lebouck, and more recently Magnus and Stubel, to prove the existence of lymph vessels in the eye serving for the outflow of intraocular fluid were not successful. The only recognized draining apparatus of the eye is therefore the canal of Schlemm. The obstruction of this pathway, encountered in certain forms of iritis or in subluxation of the lens, can lead to an increase in intraocular pressure and the development of so-called secondary glaucoma. In cases of congenital underdevelopment of the canal of Schlemm, a special form of childhood glaucoma develops - buphthalmos, bull's eye (see). As for primary glaucoma, its origin can only be explained by assuming some, as yet not fully clarified, disturbances in the process of separation of intraocular fluid itself. The interpretation of glaucoma proposed by Fischer as a process associated with the swelling of colloids of the vitreous body of the eye under the influence of a change in the concentration of metallic and acid ions in it, meets with a number of serious objections. - Besides the significant fluctuations in the tone of the eye that occur in pathological cases, there are also certain small physiological fluctuations of intraocular pressure in a completely normal eye. The tone of a normal human eye is not even the same during the day. Diurnal fluctuations of intraocular pressure were first described by Maslennikov. He found that the maximum pressure of a normal human eye is found early in the morning. During the day the pressure decreases, so that even on normal eyes by evening it is already 2-2.5 mm lower than in the morning. Even greater diurnal fluctuations occur in glaucomatous eyes. Significant fluctuations in intraocular pressure also depend on nervous influences. According to Samoylov, any painful irritation of the eye leads to a rapid increase in pressure in it to 40-50 mm Hg, which is soon replaced by a fall in intraocular pressure below normal. The connection of these fluctuations in the tone of the eye with painful irritation can be clearly shown in experiments on animals at least by the fact that any anesthesia prevents the occurrence of such a reaction, whereas curare, which does not cause loss of sensitivity, has no effect on the course of the process. This process, called by Samoylov reactive hypertension of the eye, is obviously based on a change in the permeability of the walls of the intraocular capillaries. In reactive hypertension, the chemical composition of the fluid filling the chamber of the eye changes, which in this case approaches in composition to the blood. Its protein and sugar content sharply increases; antitoxins, absent in normal chamber fluid, penetrate into it in reactive hypertension. All this speaks in favor of the fact that the normal process of separation of intraocular fluid is replaced in reactive hypertension of the eye by simple filtration from the blood vessels. The subsequent fall in the tone of the eye is apparently connected with the fact that the process of reactive hypertension caused disturbances in the normal separation of intraocular fluid and this process cannot be immediately resumed after the cessation of the intensified filtration of fluid caused by irritation of the eye. The uncertainty of many aspects of the question of the origin of intraocular pressure and its close connection with the elucidation of the pathogenesis and therapy of such a common and serious eye disease as glaucoma makes the question of ocular tonus one of the most important and most urgent problems of modern ophthalmology.