The eye is a closed cavity bounded by the outer capsule (sclera and cornea). There is an exchange of fluids in the eye - their inflow and outflow. The main place in their products is occupied by the ciliary body. The fluid it produces enters the posterior chamber of the eye, then passes through the pupil into the anterior chamber, from where, through the angle of the anterior chamber and Schlemm’s canal, it enters the venous network (see Fig. 4). Apparently, the iris also takes part in this. In a normal eye, there is a strict correspondence between the inflow and outflow of ocular fluids, and the eye has a certain density, which is called intraocular pressure. It is designated by the letter T (the initial letter of the Latin word tensio - pressure). Intraocular pressure is measured in millimeters of mercury and depends on many factors. The main factors are the amount of intraocular fluid and blood in the internal vessels of the eye. The technique for studying intraocular pressure is described in Chapter IV.
Sometimes, for various reasons, there is a disproportion between the inflow and outflow of intraocular fluids and intraocular pressure increases, and glaucoma develops. Among the causes of blindness, glaucoma is in first place worldwide - it accounts for up to 23% of blind people.
Glaucoma is a Greek word meaning “green.” Indeed, during an acute attack, the pupil becomes slightly greenish, the eye seems to be filled with greenish water. Hence its name in folk medicine “green water”. There are two types of glaucoma - primary and secondary. Primary glaucoma is those cases of the disease where the cause of the increase in intraocular pressure is unknown. In secondary glaucoma, the reasons for the increase in intraocular pressure are clear (blood in the anterior chamber, circular synechia, corneal scar fused to the iris, etc.). We will consider only primary glaucoma, since the causes and treatment of secondary glaucoma are clear.
The following 3 signs are characteristic of glaucoma: increased intraocular pressure (the main symptom), decreased visual function and excavation of the optic nerve head.
Intraocular pressure is normally 18-27 mmHg. Art. It can change for many reasons. Pressure equal to 27 mm Hg. Art., already makes you wary, but if it is higher, then you need to talk about glaucoma.
With increased intraocular pressure, the light-receiving elements of the retina are damaged, central and peripheral vision decreases. This drop can be short-term, since the increased pressure causes swelling of the cornea (it becomes somewhat dull, its surface looks like foggy glass); Retinal edema usually occurs. The swelling goes away and vision is restored. When the nerve elements of the retina are damaged due to high intraocular pressure, vision loss is permanent. It can no longer be restored, even if the pressure returns to normal. This moment determines the treatment tactics for a patient with glaucoma. With glaucoma, peripheral vision is also impaired (narrowing of the visual field). Glaucoma is characterized by a narrowing of the visual field on the nasal side; this pathology is called “nasal jump”. The field of view can be narrowed and concentric on all sides.
The thinnest part of the sclera is the cribriform plate. Due to increased intraocular pressure, the nerve tissue on the optic disc atrophies, and the cribriform plate itself bends back. Normally, this is a flat place, but with glaucoma, a depression is formed, shaped like a rinse cup. At its bottom, an atrophic optic disc is visible, and on the sides there are bent vessels - excavation of the optic disc.
The organ of vision contains structures without vascular elements. Intraocular fluid provides trophism for these structures, since the absence of capillaries makes typical metabolism impossible. Violation of the synthesis, transport or outflow of this fluid leads to significant disturbances in intraocular pressure and is manifested by such dangerous pathologies as glaucoma, ocular hypertension, and hypotony of the eyeball.
What it is?
Aqueous humor is a clear liquid that is found in the anterior and posterior chambers of the eye. It is produced by the capillaries of the ciliary processes and drains into Schlemm's canal, located between the cornea and sclera. Intraocular moisture constantly circulates. The process is controlled by the hypothalamus. It is located in the perineural and perivasal fissures, retrolental and perichoroidal spaces.
Composition and quantity
The eye fluid is 99% water. 1% includes the following substances:
- Albumin and glucose.
- B vitamins.
- Protease and oxygen.
- Ions:
- chlorine;
- zinc;
- sodium;
- copper;
- calcium;
- magnesium;
- potassium;
- phosphorus.
- Hyaluronic acid.
The production of fluid inside the organs is necessary for hydration so that the visual apparatus functions normally.
Adults produce up to 0.45 cubic centimeters, children - 0.2. Such a high concentration of water explains the need to constantly moisturize the structures of the eye, and there are enough nutrients for the visual analyzer to function fully. The refractive power of moisture is 1.33. The same indicator is observed in the cornea. This means that the fluid inside the eye does not affect the refraction of light rays and is therefore not reflected in the refractive process.
What functions?
Aqueous humor plays an important role in the functioning of the organ of vision and provides the following processes:
- Plays a major role in the formation of intraocular pressure.
- Performs a trophic function, which is important for the lens, vitreous body, cornea and trabecular meshwork, since they do not contain vascular elements. The presence of amino acids, glucose and ions in the intraocular fluid nourishes these eye structures.
- Protection of the visual organ from pathogens. This is done thanks to immunoglobulins that are part of the aqueous humor.
- Ensuring normal passage of rays to photosensitive cells.
Causes and Symptoms of Outflow Problems
In case of outflow disturbances, intraocular pressure increases, which can cause glaucoma.
During the day, the norm is considered to be the production of 4 ml of aqueous humor with outflow in the same amount. The volume per unit time should not exceed 0.2-0.5 ml. If the cyclicity of this process is disrupted, moisture accumulates, resulting in increased intraocular pressure. Decreased outflow is the basis of open-angle glaucoma. The pathogenetic basis of this disease is the blockade of the scleral sinus, through which the normal outflow of fluid occurs.
The blockade develops due to the following factors:
- congenital developmental anomalies;
- age-related changes in the angle of inclination of Schlemm's canal;
- long-term use of glucocorticosteroids;
- myopia;
- autoimmune diseases;
- diabetes.
For a long period, disturbances in the circulation of intraocular fluid may not appear. Symptoms of this disease include pain around the eyes and in the area of the brow ridges, headache, and dizziness. Patients note deterioration in vision, the appearance of rainbow circles when focusing on light rays, fog or “spots” in front of the eyes, clouding, flickering.
In the first stages, patients do not pay attention to signs of impaired fluid outflow, but as the pathology progresses, it becomes much worse and leads to loss of vision.
- Glaucoma. It is characterized by increased pressure inside the eye, followed by progressive atrophy of the optic nerve and visual impairment. It can be open- or closed-angle, which depends on the causes of its occurrence. This disease is chronic and has a slow development.
- Ophthalmic hypertension. A disease that is an increase in intraocular pressure without damage to the optic nerve head. The causes are infections of the organ of vision, systemic diseases, congenital disorders, and drug intoxication. In this case, the patient feels fullness in the eye, but visual acuity does not change.
- Hypotony of the eyeball. Develops due to a decrease in the amount of aqueous humor. Etiological factors include mechanical damage, inflammatory diseases, and severe dehydration. Clinically, this is manifested by clouding of the cornea, vitreous body and papilledema.
Intraocular fluid or aqueous humor is a kind of internal environment of the eye. Its main depots are the anterior and posterior chambers of the eye. It is also present in the peripheral and perineural clefts, suprachoroidal and retrolental spaces.
In its chemical composition, aqueous humor is analogous to cerebrospinal fluid. Its amount in the eye of an adult is 0.35-0.45, and in early childhood - 1.5-0.2 cm 3. The specific gravity of moisture is 1.0036, the refractive index is 1.33. Consequently, it practically does not refract rays. Moisture is 99% water.
Most of the dense residue consists of anorganic substances: anions (chlorine, carbonate, sulfate, phosphate) and cations (sodium, potassium, calcium, magnesium). Most of the moisture contains chlorine and sodium. A small proportion is accounted for by protein, which consists of albumins and globulins in a quantitative ratio similar to blood serum. Aqueous humor contains glucose - 0.098%, ascorbic acid, which is 10-15 times more than in the blood, and lactic acid, because the latter is formed during the process of lens exchange. The composition of aqueous humor includes various amino acids - 0.03% (lysine, histidine, tryptophan), enzymes (protease), oxygen and hyaluronic acid. There are almost no antibodies in it and they appear only in secondary moisture - a new portion of liquid formed after suction or expiration of the primary aqueous humor. The function of aqueous humor is to provide nutrition to the avascular tissues of the eye - the lens, vitreous body, and partially the cornea. In this regard, constant renewal of moisture is necessary, i.e. outflow of waste liquid and influx of freshly formed liquid.
The fact that intraocular fluid is constantly exchanged in the eye was already shown in the time of T. Leber. It was found that the fluid is formed in the ciliary body. It is called primary chamber moisture. It mostly enters the posterior chamber. The posterior chamber is bounded by the posterior surface of the iris, the ciliary body, the zonules of Zinn, and the extrapupillary portion of the anterior lens capsule. Its depth in different sections varies from 0.01 to 1 mm. From the posterior chamber, through the pupil, the fluid enters the anterior chamber - the space limited in front by the posterior surface of the iris and lens. Due to the valve action of the pupillary edge of the iris, moisture cannot return from the anterior chamber back to the posterior chamber. Next, the waste aqueous humor with tissue metabolic products, pigment particles, and cell fragments is removed from the eye through the anterior and posterior outflow tracts. The anterior outflow tract is the Schlemm's canal system. Fluid enters Schlemm's canal through the anterior chamber angle (ACA), an area limited anteriorly by trabeculae and Schlemm's canal, and posteriorly by the root of the iris and the anterior surface of the ciliary body (Fig. 5).
The first obstacle to aqueous humor leaving the eye is trabecular apparatus.
In section, the trabecula has a triangular shape. The trabecula has three layers: uveal, corneoscleral, and porous tissue (or the inner wall of Schlemm's canal).
Uveal layer consists of one or two plates consisting of a network of crossbars, which represent a bundle of collagen fibers covered with endothelium. Between the crossbars there are slots with a diameter of 25 to 75 mu. The uveal plates are attached to Descemet's membrane on one side and to the fibers of the ciliary muscle or the iris on the other.
Corneoscleral layer consists of 8-11 plates. Between the crossbars in this layer there are ellipsoidal holes located perpendicular to the fibers of the ciliary muscle. When the ciliary muscle is tense, the trabecular openings expand. The plates of the corneoscleral layer are attached to the Schwalbe ring, and on the other hand to the scleral spur or directly to the ciliary muscle.
The inner wall of Schlemm's canal consists of a system of argyrophilic fibers enclosed in a homogeneous substance rich in mucopolysaccharides. This fabric has fairly wide Sondermann channels ranging from 8 to 25 mu in width.
Trabecular slits are abundantly filled with mucopolysaccharides, which disappear when treated with hyaluronidase. The origin of hyaluronic acid in the chamber corner and its role are not fully understood. Apparently, it is a chemical regulator of intraocular pressure levels. Trabecular tissue also contains ganglion cells and nerve endings.
Schlemm's canal is an oval-shaped vessel located in the sclera. The average channel lumen is 0.28 mm. 17-35 thin tubules extend from Schlemm's canal in the radial direction, ranging in size from thin capillary filaments of 5 mu to trunks up to 16 mu in size. Immediately at the exit, the tubules anastomose, forming a deep venous plexus, representing clefts in the sclera lined with endothelium.
Some tubules go directly through the sclera to the episcleral veins. From the deep scleral plexus, moisture also goes to the episcleral veins. Those tubules that go from Schlemm's canal directly into the episclera, bypassing the deep veins, are called aqueous veins. In them, for some distance, you can see two layers of liquid - colorless (moisture) and red (blood).
Posterior outflow tract These are the perineural spaces of the optic nerve and the perivascular spaces of the retinal vascular system. The angle of the anterior chamber and the Schlemm's canal system begin to form already in a two-month-old fetus. In a three-month-old child, the corner is filled with mesoderm cells, and in the peripheral parts of the corneal stroma the cavity of Schlemm’s canal is distinguished. After the formation of Schlemm's canal, a scleral spur grows in the corner. In a four-month fetus, corneoscleral and uveal trabecular tissue differentiates from mesoderm cells in the corner.
The anterior chamber, although morphologically formed, however, its shape and size are different from those in adults, which is explained by the short sagittal axis of the eye, the unique shape of the iris and the convexity of the anterior surface of the lens. The depth of the anterior chamber in the center of a newborn is 1.5 mm, and only by the age of 10 does it become like that of adults (3.0-3.5 mm). With old age, the anterior chamber becomes smaller due to the growth of the lens and sclerosis of the fibrous capsule of the eye.
What is the mechanism of formation of aqueous humor? It has not yet been finally resolved. It is regarded both as a result of ultrafiltration and dialysate from the blood vessels of the ciliary body, and as an actively produced secretion of the blood vessels of the ciliary body. And whatever the mechanism of formation of aqueous humor, we know that it is constantly produced in the eye and flows out of the eye all the time. Moreover, the outflow is proportional to the inflow: an increase in inflow increases the outflow, and vice versa, a decrease in inflow reduces the outflow to the same extent.
The driving force that determines the continuity of outflow is the difference - higher intraocular pressure and lower pressure in Schlemm's canal.
Aqueous humor circulates along the episcleral and intrascleral venous network of the anterior segmented area of the eyeball. It supports metabolic processes and the trabecular apparatus. Under normal circumstances, the human eye contains 300 mm of the component or 4% of the total volume.
The fluid is produced from the blood by special cells that are part of the structure of the ciliary body. The human eye produces 3-9 ml of the component per minute. The outflow of moisture occurs through the episcleral vessels, uveoscleral system and trabecular meshwork. Intraocular pressure is the ratio of the produced component to the withdrawn component.
What is aqueous humor?
Aqueous humor (intraocular fluid)- a colorless, jelly-like liquid that completely fills the two eye chambers. The composition of the element is very similar to blood. Its only difference is its lower protein content. Moisture is produced at a rate of 2-3 µl/min.
Structure
The aqueous humor of the eye is almost 100% water. The dense component includes:
- anorganic components (chlorine, sulfate, etc.);
- cations (calcium, sodium, magnesium, etc.);
- insignificant proportion of protein;
- glucose;
- ascorbic acid;
- lactic acid;
- amino acids (tryptophan, lysine, etc.);
- enzymes;
- hyaluronic acid;
- oxygen;
- a small amount of antibodies (formed only in the secondary fluid).
Functions
The functional purpose of the liquid consists of the following processes:
- nutrition of the avascular elements of the organ of vision due to the amino acids and glucose included in the component;
- removal of potential threatening factors from the internal environment of the eye;
- organization of a light-refracting environment;
- regulation of intraocular pressure.
Symptoms
The amount of fluid inside the eye may change due to the development of eye diseases or when exposed to external factors (trauma, surgery).
If the moisture outflow system is disrupted, a decrease in intraocular pressure (hypotension) or an increase (hypertonicity) is observed. In the first case, it is likely to appear, which is accompanied by deterioration or complete loss of vision. With increased pressure inside the eye, the patient complains of headache, blurred vision, and the urge to vomit.
The progression of pathological conditions leads to the development of disruption of the process of fluid removal from the organ of vision and its tissues.
Diagnostics
Diagnostic measures for suspected development of pathological conditions in which the intraocular fluid for some reason is in excess, in deficiency, or does not go through the entire circulation process inside the eye, are reduced to the following procedures:
- visual inspection and palpation of the eyeball(the method allows you to determine visible deviations and location of pain);
- ophthalmoscopy of the fundus– a procedure to assess the condition of the retina, optic nerve head and vascular network of the eye using an ophthalmoscope or fundus lens;
- tonometry– an examination that allows you to determine the level of change in the eyeball when exposed to the eye cornea. With normal intraocular pressure, deformation of the sphere of the organ of vision is not observed;
- perimetry– a method for determining visual fields using computer technology or special equipment;
- campimetry– identification of central scotomas and size indicators of the blind spot in the visual field.
Treatment
For the above-mentioned disorders, as part of the therapeutic course, the patient is prescribed medications that restore intraocular pressure, as well as medications that stimulate blood supply and metabolism in the tissues of the organ.
Surgical treatment methods are applicable in cases where drugs do not have the desired effect. The type of surgery performed depends on the type of pathological process.
Thus, the intraocular fluid is a kind of internal environment of the organ of vision. The composition of the element is similar to the structure of blood and provides the functional purpose of moisture. Local pathological processes include disturbances in fluid circulation and deviations in its quantitative indicator.
The fluid is continuously produced by the ciliary crown with the active participation of the non-pigmented retinal epithelium and, in smaller quantities, in the process of ultrafiltration of the capillary network. Moisture fills the posterior chamber, then enters the anterior chamber through the pupil (it serves as its main reservoir and has twice the volume than the posterior one) and flows mainly into the episcleral veins through the drainage system of the eye, located on the anterior wall of the anterior chamber angle. About 15% of the fluid leaves the eye, leaking through the stroma of the ciliary body and sclera into the uveal and scleral veins - the uveoscleral outflow tract. A small part of the liquid is absorbed by the iris (like a sponge) and the lymphatic system.
Regulation of intraocular pressure. The formation of aqueous humor is under the control of the hypothalamus. A certain influence on secretory processes is exerted by changes in pressure and the rate of blood outflow in the vessels of the ciliary body. The outflow of intraocular fluid is regulated by the ciliary muscle – scleral spur – trabecula mechanism. The longitudinal and radial fibers of the ciliary muscle are attached to the scleral spur and trabecula with their anterior ends. When it contracts, the spur and trabecula move posteriorly and inwardly. The tension of the trabecular apparatus increases, and the openings in it and the scleral sinus expand.
Loading...