Investigation of intraocular pressure

The level of intraocular pressure can be determined in various ways: tentatively (palpation), using tonometers of the applanation or impression type, and also in a non-contact way.

[1], [2], [3], [4], [5], [6], [7], [8], [9]

Approximate (palpatory) examination

It is carried out when the head is stationary and the patient looks down. At the same time, the doctor places the index fingers of both hands on the eyeball through the skin of the upper eyelid and alternately presses against the eye. The resulting tactile sensations (compliance of varying degrees) depend on the level of intraocular pressure: the higher the pressure and the denser the eyeball, the less the mobility of its wall. The intraocular pressure thus determined is designated as follows: Tn is the normal pressure; T + 1 - moderately elevated intraocular pressure (eye slightly dense); T + 2 - significantly increased (the eye is very tight); T + 3 - sharply elevated (the eye is hard, like a stone). With decreasing intraocular pressure, three degrees of its hypotension are also distinguished: T-1 - the eye is slightly softer than normal; T-2 - the eye is soft; T-3 - the eye is very soft.

This method of examining intraocular pressure is used only in cases where it is impossible to conduct its instrumental measurement: with injuries and diseases of the cornea, after surgical interventions with opening the eyeball. In all other cases, tonometry is used .

[10], [11], [12]

Appointment tonometry

In our country, this study is carried out according to the method proposed by AN Maklakov (1884), which consists in installing a standard 10 g weight on the surface of the patient's cornea (after its dripping anesthesia). The weigher is a hollow metal cylinder 4 mm in height, which is expanded and equipped with milk-white porcelain grounds with a diameter of 1 cm. Before measuring the intraocular pressure, these areas are covered with a special paint (mixture of collargol and glycerin), and then using a special holder, the eyes of the patient lying on the couch are placed on the cornea of the doctor's wide fingered doctor.

Under the influence of the weight of the weight, the cornea is flattened and the paint is washed off at the point of its contact with the platform of the weigher. On the platform of the weights there is a circle, devoid of paint, corresponding to the area of contact between the surface of the weights and the cornea. The obtained imprint from the weight area is transferred to pre-alcohol soaked paper. In this case, the smaller the circle, the higher the intraocular pressure and vice versa.

To translate linear quantities in millimeters of mercury, SS Golovin (1895) compiled a table on the basis of a complex formula.

Later, BL Polyak transferred these data to a transparent measuring ruler, with which one can immediately obtain an answer in millimeters of the mercury column at the point near which the imprint of the weight of the tonometer fits.

Intraocular pressure, determined in this way, is called tonometric (P _m ), because under the influence of the load, the eyeball increases in the eye. On average, with an increase in the mass of the blood pressure monitor by 1 g, the intraocular pressure rises by 1 mmHg, that is, the smaller the mass of the tonometer, the tonometric pressure is closer to the true pressure (P ₀ ). Normal intraocular pressure when measured with a weight of 10 g does not exceed 28 mm Hg. With daily fluctuations of not more than 5 mm Hg. The set contains weights of 5; 7.5; 10 and 15 g. A consistent measurement of the intraocular pressure is called elastotonometry.

[13]

Impression tonometry

This method, proposed by Schiotz, is based on the principle of pressing the cornea with a rod of constant cross section under the influence of weights of different weights (5.5, 7.5 and 10 g). The magnitude of the resulting corneal depression is determined in linear terms. It depends on the weight of the weight used and the level of intraocular pressure. To translate the readings in millimeters of mercury, use the nomograms attached to the instrument.

Impression tonometry is less accurate than applanation, but is indispensable in cases where the cornea has an uneven surface.

At present, the shortcomings of contact applanation tonometry have been completely eliminated thanks to the use of modern contactless ophthalmologic tonometers of various designs. They realized the latest achievements in the field of mechanics, optics and electronics. The essence of the study is that, at a certain distance, a portion of the compressed air, dosed out by pressure and volume, is sent to the center of the cornea of the eye being examined. As a result of its influence on the cornea, its deformation arises and the interference pattern changes. By the nature of these changes, the level of intraocular pressure is determined. Such instruments allow to measure intraocular pressure with high accuracy, without touching the eyeball.

Investigation of eye hydrodynamics (tonography)

The method allows to obtain quantitative characteristics of production and outflow from the eye of the intraocular fluid. The most important of these are: the coefficient of ease of outflow (C) of chamber moisture (normally not less than 0.14 (mm ³ -min) / mmHg), the minute volume (F) of watery moisture (about 2 mm ³ / min ) and the true intraocular pressure P ₀ (up to 20 mm Hg).

To perform tonography, devices of various complexity, up to electronic ones, are used. However, it can also be carried out in a simplified version of Calfa-Plushko using applanation tonometers. In this case, the intraocular pressure is initially measured using successively weights of 5; 10 and 15 g. Then install a weight of 15 grams of clean area on the center of the cornea for 4 minutes. After such compression, the intraocular pressure is again measured, but the weights are used in the reverse order. The obtained circles of flattening are measured with a ruler of the Polyak and, according to the established values, two elastomers are constructed. All further calculations are made using a nomogram.

According to the results of tonography, the form of glaucoma can be differentiated from the hypersecretory (increase in fluid production) by differentiating the retentive (reducing the flow of fluid outflow ).