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Tonometry
Last reviewed: 20.11.2021
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Tonometry - measurement of intraocular pressure (pressure inside the eye). The instruments used in tonometry, with a small force, deform the surface of the cornea, which is used to calculate intraocular pressure.
Tonometers are applanation and impressional. The work of each type of tonometer suggests that all eyes have the same rigidity, thickness of the cornea and blood flow.
Applanation tonometer
The applanation tonometry, introduced in 1954, is based on the Imbert-Fick law, according to which the intraocular pressure is equal to the force required to flatten the spherical surface of the contact surface of the tonometer. Goldmann's applanation tonometry is the “gold standard”, the most commonly used method of tonometry. The method is used only in the position of the patient at the slit lamp. The cornea is inspected through a double prismatic lens located at the top of the conical tip of the device, highlighted obliquely with cobalt blue light. When the patient's head is stationary, the flattened tip is carefully placed against the anesthetized cornea painted with fluorescein. The doctor sees in the slit lamp a meniscus of a tear film around the tip of the tonometer. These fluorescent rings are combined when the tip pressure becomes equal to the intraocular pressure. The graded scale on the instrument measures the force in grams and translates them into millimeters of mercury by multiplying by ten.
With a diameter of flattening of 3.06 mm, the surface tension of the tear film balances the force necessary to overcome its rigidity. Thus, the applied force corresponds to intraocular pressure. The tip flattens the surface of the cornea by less than 0.2 mm, shifts 0.5 μl of moisture, increases the intraocular pressure by 3% and provides a reliable measurement result of ± 0.5 mm Hg. With high astigmatism (more than 3 diopters), the flattest corneal meridian should be displaced by 45 ° relative to the cone axis. This can be easily achieved by aligning the red line at the top of the tonometer with the same axis of the negative cylinder of the eye.
Shiottsa tonometer
The Schiotz tonometer (Schiotz), used since 1905, is a classic impression tonometer. When tonometry, the patient should lie on his back. In contrast to the applanation tonometer, the degree of cornea indentation with a Schiottz tonometer is proportional to intraocular pressure. Such deformation creates unpredictable and relatively large displacements of intraocular volume. The Shiots tonometer weighs 16.5 g, its main weight is attached to the plunger and weighs 5.5 g. With high intraocular pressure numbers, this weight can be increased to 7.5; 10 or 15 g. The calibrated base of the tonometer is carefully placed on the cornea after its preliminary anesthesia, and the free movement of the attached plunger vertically down determines the readings on the scale. Conversion tables based on empirical data from cadaver eyes and in vivo studies are used to assess intraocular pressure . These tables suggest a standard eye rigidity, so if the sclera is rigid (for example, after surgery for retinal detachment), the Schiötz tonometer may show distorted results.
[6], [7], [8], [9], [10], [11], [12], [13]
Perkins Tonometer
This manual applanation tonometer of the Goldmann type is especially often used in examinations of infants and children. The light source in it works on batteries, the instrument can be used both in the vertical position of the patient, and in the position on the back. The force of applanation is changed by rotating a calibrated dial with the same measuring device as in the Goldmann tonometer.
With tono
Manual tonometer ton-pins (Mentor Oftalmiks, Santa Barbara) can measure the intraocular pressure of both the patient and the patient who is lying on his back. The method is particularly suitable for children and patients with injured or edematous cornea, when it is not possible to use a slit lamp. In ton-type make-ups, which include ton-foams, the effects of corneal rigidity are transmitted to the surrounding sleeve, so the central plate measures only intraocular pressure. The microprocessor in ton-foams, connected to a strain gauge, measures the force with which the central plate with a diameter of 1.02 mm flattens the surface of the cornea. With 4-10 measurements of the same eye, the end result is obtained with a variability between the lowest and the highest permissible results of less than 5.10, 20% or more.
Pneumatic pressure monitor
The pneumotonometer is a hand-held device that can be used in the absence of a slit lamp. During the examination, the patient may sit or lie on his back, the surface of the cornea of the examined eye may be uneven. Like a ton-foam, this Makei-Marg type tonometer has a sensitive surface in the center, and the cushion surrounding it transmits the force necessary to overcome the rigidity of the cornea.
The central sensitive area is the silastic diaphragm covering the air plunger. When this elastic membrane is placed on the cornea, there is an obstacle to the escape of gas from the plunger, which causes an increase in air pressure until it becomes equal to intraocular pressure. An electronic sensor measures the air pressure in the chamber.