Glaucoma - Pathogenesis

Intraocular pressure depends on a number of factors:

1. There is a rich network of blood vessels inside the eye. The value of intraocular pressure is determined by the tone of the vessels, their blood filling, and the condition of the vascular wall;
2. inside the eye there is a continuous circulation of intraocular fluid (the processes of its production and outflow), which fills the posterior and anterior chambers of the eye. The speed and continuity of fluid exchange, intraocular exchange also determine the height of intraocular pressure;
3. An important role in the regulation of intraocular pressure is also played by metabolic processes occurring inside the eye. They are characterized by persistent changes in the eye tissues, in particular swelling of the vitreous colloids;
4. The elasticity of the eye capsule - the sclera - also plays a role in regulating intraocular pressure, but much less than the above factors. Glaucoma is caused by the death of nerve cells and fibers, which disrupts the connection between the eye and the brain. Each eye is connected to the brain by a large number of nerve fibers. These fibers gather together in the optic disc and exit the back of the eye in bundles that form the optic nerve. During the natural aging process, even a healthy person loses some nerve fibers throughout their life. In patients with glaucoma, nerve fibers die much faster.

In addition to the death of nerve fibers, glaucoma also causes tissue death. Atrophy (lack of nutrition) of the optic disc is a partial or complete death of the nerve fibers that form the optic nerve.

In glaucomatous atrophy of the optic nerve head, the following changes are observed: depressions, called excavations, develop on the disc, and glial cells and blood vessels die. The process of these changes is very slow, and can sometimes last for years or even decades. In the area of excavation of the optic nerve head, small hemorrhages, narrowing of blood vessels, and areas of choroidal or vascular atrophy are possible along the edge of the disc. This is a sign of tissue death around the disc.

With the death of nerve fibers, visual functions also decrease. In the early stages of glaucoma, only a disturbance in color perception and dark adaptation is observed (the patient himself may not notice these changes). Later, patients begin to complain of glare from bright light.

The most common visual impairments are defects in the visual fields, and visual field loss. This is due to the appearance of scotomas. There are absolute scotomas (complete loss of vision in some part of the visual field) and relative scotomas (reduced visibility only in a certain part of the vision). Since these changes appear very slowly with glaucoma, the patient often does not notice them, since visual acuity is usually preserved even in cases of severe narrowing of the visual fields. Sometimes a patient with glaucoma can have visual acuity of 1.0 and read even small text, although he already has serious visual field impairments.

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The meaning of intraocular pressure

The physiological role of intraocular pressure is that it maintains a stable spherical shape of the eye and the relationship of its internal structures, facilitates metabolic processes in these structures and the removal of metabolic products from the eye.

Stable intraocular pressure is the main factor protecting the eye from deformation during eyeball movement and blinking. Intraocular pressure protects eye tissues from swelling in case of blood circulation disorders in intraocular vessels, increased venous pressure and decreased blood pressure. Circulating aqueous humor constantly washes various parts of the eye (the lens and the inner surface of the cornea), due to which the visual function is preserved.

Drainage system of the eye

The aqueous humor is formed in the ciliary body (1.5-4 mm/min) with the participation of non-pigment epithelium and in the process of ultrasecretion from the capillaries. Then the aqueous humor enters the posterior chamber and passes through the pupil into the anterior chamber. The peripheral part of the anterior chamber is called the angle of the anterior chamber. The anterior wall of the angle is formed by the corneoscleral junction, the posterior wall is formed by the root of the iris, and the apex is formed by the ciliary body.

The main parts of the eye drainage system are the anterior chamber and the anterior chamber angle. Normally, the anterior chamber volume is 0.15-0.25 cm ³. Since moisture is constantly produced and drained, the eye maintains its shape and tone. The width of the anterior chamber is 2.5-3 mm. The anterior chamber moisture differs from blood plasma: its specific gravity is 1.005 (plasma - 1.024); per 100 ml - 1.08 g of dry matter; pH is more acidic than plasma; 15 times more vitamin C than plasma; less proteins than plasma - 0.02%. The anterior chamber moisture is produced by the epithelium of the ciliary body processes. Three mechanisms of production are noted:

1. active secretion (75%);
2. diffusion;
3. ultrafiltration from capillaries.

The fluid in the posterior chamber bathes the vitreous body and the back surface of the lens; the fluid in the anterior chamber bathes the anterior chamber, the surface of the lens, and the back surface of the cornea. The drainage system of the eye is located in the angle of the anterior chamber.

On the anterior wall of the angle of the anterior chamber is the scleral groove, across which a crossbar is thrown - trabecula, which has the shape of a ring. The trabecula consists of connective tissue and has a layered structure. Each of the 10-15 layers (or plates) is covered with epithelium on both sides and is separated from adjacent layers by slits filled with aqueous humor. The slits are connected to each other by openings. The openings in different layers of the trabeculae do not coincide with each other and become narrower as they approach Schlemm's canal. The trabecular diaphragm consists of three main parts: the uveal trabecula, which is closer to the ciliary body and iris; the corneoscleral trabecula and juxtacanalicular tissue, which consists of fibrocytes and loose fibrous tissue and provides the greatest resistance to the outflow of aqueous humor from the eye. The aqueous humor seeps through the trabecula of the Schlemm's canal and flows out from there through 20-30 thin collecting canals or graduates of the Schlemm's canal into the venous plexuses, which are the final point of outflow of the aqueous humor.

Thus, the trabeculae, Schlemm's canals and collecting canals are the drainage system of the eye. The resistance to fluid movement through the drainage system is very significant. It is 100,000 times greater than the resistance to blood movement through the entire human vascular system. This ensures the necessary level of intraocular pressure. The intraocular fluid encounters an obstacle in the trabeculae and Schlemm's canal. This maintains the tone of the eye.

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Hydrodynamic parameters

Hydrodynamic parameters determine the state of the eye's hydrodynamics. In addition to intraocular pressure, hydrodynamic parameters include outflow pressure, minute volume of aqueous humor, the rate of its formation, and the ease of outflow from the eye.

Outflow pressure is the difference between the intraocular pressure and the pressure in the episcleral veins (P0 - PV). This pressure pushes fluid through the drainage system of the eye.

The minute volume of aqueous humor (F) is the rate of outflow of aqueous humor, expressed in cubic millimeters per 1 min.

If the intraocular pressure is stable, then F characterizes not only the outflow rate, but also the rate of formation of aqueous humor. The value showing what volume of fluid (in cubic millimeters) flows out of the eye in 1 min per 1 mm Hg of outflow pressure is called the outflow ease coefficient (C).

Hydrodynamic parameters are related to each other by an equation. The value of P0 is obtained by tonometry, C - by topography, the value of PV fluctuates from 8 to 12 mm Hg. This parameter is not determined in clinical conditions, but is taken to be equal to 10 mm Hg. The above equation, the obtained values, calculate the value of F.

With tonography, it is possible to calculate how much intraocular fluid is produced and stored per unit of time, and to record changes in intraocular pressure per unit of time with the load on the eye.

According to the law, the minute volume of liquid P is directly proportional to the value of the filtration pressure (P0 - PV).

C is the coefficient of ease of outflow, i.e. 1 mm3 flows out of the eye in 1 min ^with a pressure on the eye of 1 mm od.

F is equal to the minute volume of fluid (its production in 1 min) and is 4.0-4.5 mm3 ^/ min.

PB is the Becker index, normally PB is less than 100.

The coefficient of eye rigidity is measured by the alastocurve: C is less than 0.15 - the outflow is difficult, F is more than 4.5 - hyperproduction of intraocular fluid. All this can resolve the issue of the genesis of increased intraocular pressure.

Intraocular pressure test

The approximate method is palpation examination. For more accurate measurement of intraocular pressure (with digital readings), special instruments called tonometers are used. In our country, they use the domestic tonometer of Professor L. N. Maklakov of the Moscow Eye Clinic. It was proposed by the author in 1884. The tonometer consists of a metal cylinder 4 cm high and weighing 10 g, on the upper and lower surfaces of this column there are round plates made of milky-white glass, which are lubricated with a thin layer of special paint before measuring the pressure. In this form, the tonometer on the handle is brought to the eye of the lying patient and quickly released to the center of the pre-anesthetized cornea. The tonometer is removed at the moment when the load falls on the cornea with all its weight, which can be judged by the fact that the upper platform of the tonometer at this moment will be above the handle. The tonometer will naturally flatten the cornea the more, the lower the intraocular pressure. At the moment of flattening, some of the paint remains on the cornea, and a circle without paint is formed on the tonometer plate, the diameter of which can be used to judge the state of the intraocular pressure. To measure this diameter, an imprint of the plate circle is made on paper moistened with alcohol. A transparent graduated scale is then placed on this imprint, the scale readings are converted into millimeters of mercury using a special table by Professor Golovin.

The normal level of true intraocular pressure varies from 9 to 21 mm Hg, the standards for a 10 g Maklakov tonometer are from 17 to 26 mm Hg, and for a 5 g tonometer, from 1 to 21 mm Hg. Pressure approaching 26 mm Hg is considered suspicious, but if the pressure is higher than this figure, it is clearly pathological. Increased intraocular pressure cannot always be determined at any time of the day. Therefore, any suspicion of increased intraocular pressure requires its systematic measurement. For this purpose, they resort to determining the so-called daily curve: they measure the pressure at 7 a.m. and 6 p.m. The pressure in the morning hours is higher than in the evening. A difference of more than 5 mm between them is considered pathological. In doubtful cases, patients are placed in a hospital, where they establish systematic monitoring of intraocular pressure.

Intraocular pressure is subject not only to individual fluctuations, it can also change during life and with some general and eye diseases. Age-related changes in intraocular pressure are small and have no clinical manifestations.

The level of intraocular pressure depends on the circulation of aqueous humor in the eye, or the hydrodynamics of the eye. The hemodynamics of the eye (i.e. the circulation of blood in the vessels of the eye) significantly affects the state of all functional mechanisms, including those that regulate the hydrodynamics of the eye.