Ophthalmology lecture

2021-02-02 12:00 AM

When there is circulatory stasis in the venous area, in the head, face, and neck area such as the patient lying on the slope, the carotid vein is squeezed, ocular thrombophlebitis.

Define

Eye pressure is the pressure of elements in the eyeball acting on the sclera wall and cornea.

Normal eye pressure

Vietnamese eye pressure is considered normal with the following characteristics.

Average value of intraocular pressure when measured with a Malakhov manometer with a 10g weight with values ​​from 16mm to 22 mmHg.

The difference in eye pressure between 2 eyes at the same time is less than 5mmHg.

The difference in eye pressure in one eye in 24 hours is less than 5mmHg.

The role of eye pressure

Keep the eyeballs of a spherical shape.

Ensure the transparency of optical media.

Ensuring the nutrition of the ingredients in the eyeball.

Production of aqueous humor

Aqueous humor is secreted by non-pigmented epithelial cells in the ciliary folds with a daily rate of change of 2-3mm3 per minute. The production of aqueous humor decreases with age. The rate of aqueous production is influenced by many factors: the integrity of the aqueous blood barrier, the flow of blood into the lumen, and the regulation of the nerve-fluids of the blood vessels and the epithelium of the ciliary tract. Aqueous discharge decreases when the eye becomes inflamed or after taking certain medications.

The aqueous circulation

The aqueous humor escapes the eyeball in two ways

Save through the raft area

Most aqueous humor (about 80%) from the back room passes through the gap between the front of the vitreous body and the back of the iris through the pupil into the anterior chamber. The aqueous humor is then released out of the eyeball by the corneal spinal system into the Schlemm tube, then through the water veins into the mesothelioma and general circulation system. The raft area operates like a check valve, allowing a large amount of aqueous humor out of the eye and limiting the flow in the opposite direction.

Circulation through the sclera uvea

A portion of the aqueous humor (about 20%) that is not circulated through the raft will escape along the sclera uvea. Research by markers in humans and animals has shown that aqueous humor can pass through the base of the iris and the interstitial tissue of the ciliary body to reach the parietal cavity. From there, the aqueous humor enters the epithelial cavity either through the sclera around the ciliary artery or drains directly through the sclera collagen. The flow rate of aqueous drainage in this path will increase with the use of the eyelid paralysis agents, the ophthalmic agents of the Prostaglandin group.

Factors affecting eye pressure

The weakness in the eyeballs

Sclerosis:

The sclera is a fibrous tissue that has a certain degree of elasticity. The elasticity of the sclera decreases with age, while the solidity of the sclera will increase. Index of intraocular pressure depends on sclera. In people with severe nearsightedness, thin sclera, the elasticity will be greater, the solidity of the sclera will decrease so the eye pressure is usually lower than that of normal people. Children sclera have many elastic fibers, thin easily, high eye pressure increases eyeball volume.

Circulation of the macula with eye pressure:

The eye pressure is always maintained stable because in each systole there is a certain amount of blood circulating in the dark retina, if this circulation stops, the eye pressure will drop to only 10mmHg. This occurs when the eyeball is removed or after death.

When there is circulatory stasis in the area of ​​the head, face and neck such as the patient lying on the head slope, squeezing the carotid veins, ocular thrombophlebitis, cavernous sinus thrombophlebitis ...

Glass translation:

The vitreous fluid accounts for about 4/5 of the volume of the eyeball. In the vitreous there are two substances, vitrain and hyaluronic acid, which have a very strong absorbent capacity. Normally the glass solution has a pH of 7.7. When the vitreous is alkaline it will retain water and cause glaucoma. 

Glass form:

The vitreous body is a biconvex transparent lens with no blood vessels located just behind the plane of the iris and pupil. Due to the anatomical position of the vitreous body, abnormalities in its anatomical position as well as its structure can affect the drainage of aqueous humor and affect the eye pressure.

Hydrological resistance:

Fluid is the most important factor directly related to eye pressure. The aqueous humor is continuously excreted from the ciliary ligament, circulating from the anterior chamber through the pupil, and then out of the eyeball along the cornea and the uvea sclera.

Self-circulating aqueous humor needs two conditions:

The difference in pressure between the inside and outside of the eyeball (i.e., the difference between the pressure in the eyeball and the pressure of the water vein).

The circulation in the pupil and the corneal raft area.

Any because that affects the two factors above affects eye pressure. If the pressure in the eyeball is PO (intraocular pressure), the water venous pressure is PV (Normal from 9mmHg - 10 mmHg), the aqueous flow is D (equal to 1.9mm3 / 1min), and R is the resistance. hydrocephalus (due to all factors that interfere with the circulation of aqueous humor in the eyeballs).

According to the Goldman formula

              PO - PV = D x R Or is PO = D x R + PV

When Pv increases i.e., the pressure of the water veins increases as in ocular occlusion, cavernous sinus thrombophlebitis increases the eye pressure.

When R glaucoma also increased for example

When the iris is attached to the vitreous front.

Confused pupils due to vitreous or vitreous.

When the corner of the front room gets stuck, the base of the iris becomes attached to the corner.

Factors beyond the eyeballs

Nerve

It is thought that there is an intraocular pressure regulation center in the hypothalamus, but that is only hypothetical, but it does explain why glaucoma often occurs on emotional sites.

Cervical sympathetic nerve stimulation causes the eye pressure to drop. The neck sympathetic stimulation will constrict the blood vessels in the eyeballs, causing a decrease in corneal circulation, reducing capillary permeability, constricting the smooth muscles in the eye sockets.

Triangular nerve stimulation causes vasodilation, more blood flow to the eyes and increased eye pressure.

Changes in eye pressure during the day

Eye pressure changes according to the cycle of the day. If the difference in intraocular pressure during the day between top and bottom is more than 10mmHg with Goldman’s manometer and more than 5mmHg with Malakhov manometer, it is considered pathological. The mechanism of intraocular pressure change during the day is not yet clear. There was a relationship between adrenocortical hormone with eye pressure. The change in blood cortisone levels is parallel with fluctuations in eye pressure.

Change of posture

When changing position from sitting to lying eye pressure increases by 0.3-0.6mmHg. Postural effects on eye pressure are more pronounced in glaucoma eyes, in eyes with blocked central retinal artery, or in people with hypertension. The mechanism of this phenomenon is due to increased pressure in the central vein of the retina.

The direct method of measuring eye pressure

This method is only performed in a laboratory that measures animals by inserting a needle into the anterior chamber, which is connected to a mercury manometer.

Indirect methods of measuring intraocular pressure

Manually estimate eye pressure

Use 2 index fingers to feel the eyeball through the lashes, assess the eyeball tension under the finger. If one eye has a higher-than-normal eye pressure when palpable, that side will feel more tense. This method can only distinguish when the eye pressure increased or decreased markedly. In cases of acute high glaucoma, the eyeball can be as stretched as a ball.

Measure with a manometer

Malakhov glaucoma can estimate intraocular pressure by measuring the diameter of the crushed cornea with a fixed force (P = W / A where P is the eye pressure, W is the weight of the weight, A is the area crushed area). Set of eye pressure includes four weights weighing 5g, 7.5g, 10g, 15g. The inner cylindrical weight contains lead pellets, with both ends open. The sides of the weights are constructed of two flat stone surfaces with a diameter of 10mm. When measuring the pressure, take the ink on both ends of the weights, place the patient in a lying position, numb the corneal surface. Lift the weight of the weight over an alcohol flame to disinfect, after the tip has cooled, gently place the weight on the cornea, lower the weight so that the entire weight is on the eyeball. Lift the weight off the surface of the cornea, then bring the printed weight on paper that has been wet with alcohol and read the result with the specified ruler.

In addition, in Vietnam today, some other eye pressure gauges are also used such as: Goldman, Schiotz.

Some cases need contraindications to measuring eye pressure with eye pressure such as: When the eye has contagious conjunctivitis, the eye has damage to the corneal epithelium (ulcer, corneal slide), the first days. after surgery.

Methods of monitoring eye pressure

For early detection of Glaucoma, one can monitor the patient's eye pressure with a measuring pad 2 to 6 times a day. Continuous monitoring for 3 days to detect high time in the day.