Pneumotonometer

Measurement of intraocular pressure is one of the diagnostic methods used to study the condition of the organs of vision. In this way, the physician evaluates the degree of pressure of the vitreous body and eye fluid on the capsule of the eye, which they exert on it from the inside to maintain the overall tone of the eye.

The normal state of pressure in the eye, firstly, maintains the spherical shape of the eyeball, and secondly, it contributes to sufficient nutrition of the eye itself. However, in some pathological conditions and diseases, this category may deviate from the norm, causing inconvenience to a person – from a decrease in visual acuity to a general deterioration in well-being and complete blindness.

Violations of the level of intraocular pressure: why they appear, why they are dangerous

If the degree of pressure in the capsule of the eye deviates from the norm, this, first of all, indicates the presence of disturbances in the functioning of the visual apparatus. An increase in this indicator can be triggered by an increase in the production of intraocular fluid, congenital abnormalities in the development of the eye, or the development of cardiovascular diseases.

Pressure drops are less common. The problem is formed against the background of eye injuries, after surgical interventions, and also due to the underdevelopment of the eyeball. Prolonged low pressure causes malnutrition, as a result of which irreversible changes, such as necrosis, may begin in the tissues of the organ.

Constantly elevated pressure forms glaucoma in a person. If the balance of production and outflow of intraocular fluid is disturbed in the patient’s body, it begins to accumulate in the limited shell of the eye, thereby increasing pressure in it. The patient feels a decrease in visual acuity, gradually his peripheral vision is disturbed, the field of view and the visibility zone narrow. Over time, complete blindness sets in. Changes of this nature are irreversible, so timely diagnosis and properly prescribed treatment is the only real way to save vision.

Glaucoma can also form with normal intraocular pressure – in this case, the patient has a pronounced deterioration in the blood circulation of the optic nerve, a violation of its functioning.

In the risk group for the formation of pathology are people over the age of 40 with atherosclerosis or diabetes mellitus. The presence of hereditary predisposition also has some influence.

It is almost impossible to determine glaucoma based on external symptoms. A person notes a gradual decrease in vision, and attributes this to normal age-related changes. Detection of the disease is possible only during an ophthalmological examination.

Rarely enough, those affected have pain in the temples and superciliary arches, blurred vision, the appearance of a halo around light sources. In such cases, the disease proceeds with periodic bouts of increased intraocular pressure. A person at this time feels nausea, general weakness and malaise.

The only way to prevent the development of the disease and the onset of complete blindness is to periodically visit an ophthalmologist, undergo preventive examinations, including measurements of intraocular pressure.

Ophthalmic tonometry: concept and types

The procedure performed by ophthalmologists to calculate and measure the level of intraocular pressure is called ophthalmic tonometry.

Depending on the method and the devices used in the process, tonometry can be of several types:

• applanation tonometry: intraocular pressure (IOP) is measured based on the effort applied to form a flat area of ​​the cornea of ​​a given area;
• goldman’s: performed using a disinfected prism and a tonometer with a cobalt blue filter, it is considered one of the most effective and objective non-invasive methods;
• dynamic contouring: instead of aplanat, the contour of the cornea is used, and it avoids deformation of the cornea in the process, therefore it does not depend on its thickness;
• non-contact, or air jet tonometry: it uses a fast air pulse for applanation, the strength of which is used to calculate IOP;
• with a response analyzer: a non-contact type of measurement that uses the ability of a pulsed air flow to deform the cornea, forming a slight concavity in it;
• electronic identification: the device uses a freely located sensor to detect the transmitted pressure value in the eyeball;
• rebound: such tonometers determine the level of intraocular pressure by striking the cornea with a small probe with a plastic tip;
• impressive: occurs with the measurement of the depth of indentation of the cornea, which is carried out by a plunger of small weight and size;
• transpalpebral: carried out through the eyelid in the sclera, excludes contact of the sensor with the cornea;
• pneumotonometry.

Pneumotonometry is a measurement process that uses a pneumatic sensor, namely a piston with an air bearing. Filtered air, bypassing the piston, is conducted through a hole in the membrane with a diameter of up to 5 millimeters, located at the end of the sensor. Calculation of the resistance of the cornea to air flow is the basis for calculating the intraocular pressure in this way.

There is also a method of palpation, or digital tonometry, but it is extremely unreliable. In the process, the physician makes a light pressure with a finger against the cornea with the eye closed.

Non-contact pneumotonometers: how and why they are used

Measurement of IOP is usually included in the complex of computer vision diagnostics. The pressure formed in the eyeball appears due to the aqueous humor formed by filtering the liquid part of the blood. This fluid penetrates from the posterior chamber of the eye through the pupil into the anterior part, while it washes the cornea and lens. Further, moisture passes through the angle of the anterior chamber of the eye into the vessels.

Thus, IOP is created due to the ratio between the formation and outflow of fluid, when a stable level of tone is maintained in the eye, normal nutrition and hydration of all elements of the visual apparatus, a sufficient level of metabolism and microcirculation.

All methods for measuring pressure inside the eye are divided into two groups:

• contact;
• contactless.

In the second case, a pneumotonometer can be used. In the process, the patient puts and fixes his head on a special stand, looks forward without squinting. The nozzle of the device directs a strictly dosed air flow to the cornea, having a given speed.

The intraocular pressure is calculated based on the reaction rate of the cornea and airflow. The procedure is painless, does not cause trauma to the cornea, and does not require anesthesia.

Modern pneumotonometers automatically record erroneous data if the patient blinked or closed his eyes in the process of taking measurements, and indicate them in brackets. At the same time, the devices are distinguished by the high accuracy of the information received, the stability of the calculated information, since they reduce the dependence of data on external factors to a minimum. The procedure does not cause any practically any sensations in the subject, except for the feeling of blowing the cornea with air.

The devices are equipped with a digital screen, which displays the test results just a few seconds after the completion of the procedure, while the measurement error remains minimal.

Pneumotonometers usually take a series of measurements, after which they display the average value for each eye, taking into account the range from 0 to 60 millimeters of mercury. More modern models are equipped with a self-control function, as well as protection against incorrect measurement.

Today, not only stationary, large and heavy devices have been developed, but also portable, ergonomic devices that can be carried. They are equipped with rechargeable batteries, so they can be used on field trips to examine the elderly and those who cannot move independently.

One of the most popular models used today in ophthalmology offices is the HNT-7000 HUVITZ, made in South Korea. The device is equipped with a motorized chin rest, automatic eye search along three coordinate axes, a touch screen, due to which the number of function buttons is reduced. The test measurement mode allows you to demonstrate to the patient the degree of exposure of the air to his eye. The thermal printer makes it possible to immediately print the results obtained.

The Reichert 7 pneumotonometer is made in the USA. The model is calibrated by the Goldman tonometer, and has a liquid crystal touch display, automatic sensor guidance in accelerated mode. It works in two modes – a single shot of air, or a series of three shots. This type of pneumotonometer makes all settings automatically before starting the operation – the physician only needs to start the measurement process.

The non-contact tonometer model Pulsair intelliPuff, according to the registration certificate, has additional accessories. It is produced by the British company Keeler. The device has a manual non-contact modification. For the study, the patient does not need to place the chin on the stand. The device is equipped with high-precision optics, sensitive sensors for determining the position of the eye, a pointing and focusing system, and built-in LEDs.

The measurement range is from 5 to 50 millimeters of mercury. The permissible error is 0,1 mm Hg. The output of information is carried out through the built-in thermal printer.

Canon has released a new model of pneumotonometers Canon TX-20p. The device combines the functions of an automatic non-contact tonometer and a pachymeter. The device calculates not only intraocular pressure, but also the thickness of the cornea, as well as compensated IOP.

This line of blood pressure monitors is equipped with an intelligent pupil detection system, an alert system when a low or high IOP is detected. The tonometer has a rotary liquid crystal display, a built-in printer, and an automatic three-dimensional alignment system.

The search and detection of the left and right pupil of the subject occurs automatically. Immediately after detection, without the participation of a doctor, intraocular pressure and corneal thickness are measured. The built-in printer makes it possible to immediately print the received calculations.

The weight of the device is 15 kilograms, so it is not suitable for portable use, but is installed in ophthalmology rooms.

Through the USB port, the device can be connected to a desktop computer or laptop, save or record the results of the examination to any digital media.

The attending ophthalmologist, having on hand the results of measurements of intraocular pressure, can determine the presence of an examined pathology or disease of the visual organs. The norm for a healthy eye is in the range of 10 to 25 millimeters of mercury. Any deviations in these figures are dangerous for a person by disruption of the optic nerve, the death of its nerve fibers, the appearance of visual field defects, the gradual and irreversible onset of blindness. In addition, changes in IOP in itself are a symptom of certain diseases and disorders, for example, retinal or choroidal detachment, trauma, complications after surgery.

Non-contact pneumotonometers are devices designed to take measurements of pressure inside the eye. Their indicators are considered one of the most informative tools for ophthalmic diagnostics. Electronic devices make it possible to quickly and accurately measure IOP values ​​without causing any discomfort to the patient.