Glaucoma: pathogenesis

Intraocular pressure depends on a number of factors:

1. inside the eye there is a rich network of blood vessels. The magnitude of intraocular pressure determines the tone of the vessels, their blood filling, the state of the vascular wall;
2. inside the eye, the circulation of the intraocular fluid (the processes of its production and outflow) is continuously circulating, 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 intra-ocular pressure is also played by metabolic processes that occur inside the eye. They are characterized by a persistent change in the tissues of the eye, in particular by swelling of the vitreous colloids;
4. the elasticity of the capsule of the eye - sclera - also has importance in the regulation of intraocular pressure, but much less than the above factors. With glaucoma, nerve cells and fibers die, so the connection between the eye and the brain is broken. Each eye is connected to the brain by a large number of nerve fibers. These fibers collect together in the optic disc and come out from the back of the eye in the beams forming the optic nerve. In the process of natural aging, even a healthy person loses some of the nerve fibers throughout his life. In patients with glaucoma, nerve fibers die much faster.

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

With glaucomatous atrophy of the optic nerve disc, the following changes are noted: on the disc, dents develop, called excavation, the death of glial cells and blood vessels. The process of these changes is very slow, sometimes it can last for years or even decades. In the field of excavation of the optic nerve disk along the edge of the disk, small hemorrhages, constriction of the blood vessels and the zone of atrophy of the choroid or vascular membrane are possible. This is a sign of tissue death around the disc.

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

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

[1], [2], [3], [4], [5], [6],

The value of intraocular pressure

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

Stable intraocular pressure is the main factor in protecting the eye from deformation during the movement of the eyeball and when blinking. Intraocular pressure protects the eye tissue from swelling in case of blood circulation disorders in the intraocular vessels, increased venous pressure and blood pressure decrease. Circulating water moisture constantly flushes the various parts of the eye (the lens and the inner surface of the cornea), thereby preserving the vision function.

Drainage system of the eye

Watery moisture 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 watery moisture enters the back chamber and through the pupil passes into the anterior chamber. The peripheral part of the anterior chamber is called the angle of the anterior chamber. The anterior wall of the corner is formed by the corneoscleral joint, the posterior by the root of the iris, and the apex by the ciliary body.

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

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

The moisture in the back chamber, washing the vitreous body and the back surface of the lens; the moisture of the anterior chamber flushes the anterior chamber, the surface of the lens and the posterior surface of the cornea. In the corner of the anterior chamber is the drainage system of the eye.

On the front wall of the angle of the anterior chamber there is a scleral groove through which the crossbar - the trabeculae, which has the form of a ring - is thrown. Trabecula consists of connective tissue and has a layered structure. Each of the 10-15 layers (or plates) on both sides is covered with epithelium and separated from adjacent layers by slits filled with watery moisture. The slits are interconnected by holes. The holes in the different layers of the trabecula do not coincide with each other and become narrower when approaching the helmet canal. Trabecular diaphragm consists of three main parts: the uveal trabeculae, which is closer to the ciliary body and the iris; corneoscleral trabeculae and juxtacanalicular tissue, which consists of fibroblasts and loose fibrous tissue and exerts the greatest resistance to outflow of watery moisture from the eye. Watery water seeps through the trabeculae to the helm-channel and flows from there through 20-30 thin collector channels or graduates of the helmet canal into the venous plexus, which is the final point of outflow of watery moisture.

Thus, trabeculae, helmet dripping and collecting channels are the drainage system of the eye. The resistance to movement of liquid through the drainage system is very significant. It is 100,000 times greater than the resistance to movement of blood throughout the entire vascular system of man. This provides the necessary level of intraocular pressure. Intraocular fluid meets an obstacle in the trabecula and helmet canal. It maintains the tone of the eye.

[7], [8]

Hydrodynamic parameters

Hydrodynamic parameters determine the state of hydrodynamics of the eye. Hydrodynamic parameters, in addition to intraocular pressure, include the pressure of outflow, the minute volume of watery moisture, the rate of its formation, and the ease of outflow from the eye.

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

The minute volume of watery moisture (F) is the rate of outflow of watery moisture, expressed in cubic millimeters per minute.

If the intraocular pressure is stable, then F characterizes not only the rate of outflow, but also the rate of formation of watery moisture. The value indicating how much liquid (in cubic millimeters) flows out of the eye in 1 minute per 1 mm Hg. Art. Pressure of the outflow, is called the coefficient of ease of outflow (C).

The hydrodynamic parameters are related by an equation. The value of P0 is obtained with tonometry, C - with the help of topography, the value of PV varies from 8 to 12 mm Hg. Art. This indicator in clinical conditions does not determine, but is taken equal to 10 mm Hg. Art. The above equation gives the values obtained, calculate the value of F.

In the case of tonography, it is possible to calculate how much of the intraocular fluid is produced and administered in a unit of time, and to record changes in intraocular pressure per unit of time with eye strain.

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

C - coefficient of ease of outflow, i.e. For 1 min from the eye flows 1 mm ³ with pressure on the eye 1 mm od.

F is equal to the minute volume of the liquid (its production per 1 min) and is 4.0-4.5 mm ³ / min.

PB - indicator of Becker, in the norm PB is less than 100.

According to the alastosterium, the coefficient of rigidity of the eye is measured: C less than 0.15 - outflow is difficult, F is more than 4.5 - hyperproduction of intraocular fluid. All this can solve the problem of the genesis of increased intraocular pressure.

Investigation of intraocular pressure

An approximate method is a palpation study. For a more accurate measurement of intraocular pressure (with digital indications) use special tools called tonometers. In our country, the domestic tonometer of the professor of the Moscow Eye Clinic LN Maklakova is used. 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 smeared with a hot 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 from the fact that the upper area of the tonometer at this point is above the handle. The tonometer, naturally, will flatten the cornea the more, the lower the intraocular pressure. At the moment of flattening, a part of the paint remains on the cornea, and on the plate of the tonometer a circle is formed which is devoid of paint, the diameter of which can be used to judge the state of intraocular pressure. To measure this diameter, make an imprint of the disc circle on paper moistened with alcohol. This impression is then superimposed on a transparent graduated scale, the scale readings are converted into millimeters of mercury by a special table of Professor Golovin.

The normal level of the true inside the eye pressure varies from 9 to 21 mm Hg, st., The standards for the 10-g Maklakov tonometer are from 17 to 26 mm Hg. With a mass of 5 g - from 1 to 21 mm Hg. Art. Pressure approaching 26 mm Hg. Is considered suspicious, if the pressure is higher than the specified figure, then it is clearly pathological. Elevated intraocular pressure can not always be determined at any time of the day. Therefore, with any suspicion of increased intraocular pressure, its systematic measurement is required. To this end, resort to the definition of the so-called daily curve: the pressure is measured at 7 am and 6 pm. Pressure in the morning is higher than in the evening. The difference between them is more than 5 mm considered pathological. In doubtful cases, patients are placed in a hospital, where systematic monitoring of intraocular pressure is established.

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

The level of intraocular pressure depends on the circulation of watery moisture 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.