How is intraocular pressure measured by ophthalmologists

Tonometry: measurement of intraocular pressure

Doctors abbreviate intraocular pressure with IOP for intraocular pressure. It plays an important role in ophthalmology. In healthy people, it is usually between 10 and 21 mmHg. Age, body position, pulse and time of day cause the values ​​to fluctuate.

Why do doctors measure intraocular pressure?

Increased intraocular pressure is the most important risk factor for glaucoma, which doctors call glaucoma. In this disease, the high pressure damages the optic nerve, which can lead to blindness. Many people have had elevated pressure levels for years before they notice the loss of vision. Therefore, the measurement of intraocular pressure using tonometry should detect glaucoma at an early stage. In the case of known glaucoma, it is also used to monitor therapy. In addition to tonometry, the ophthalmologists examine the optic nerve using fundoscopy and determine the visual field using perimetry.

Which procedures are available?

The Goldmann applanation tonometry has been established for many years. The doctor performs it on the seated patient with the help of a so-called slit lamp. Before the examination, he drips a local anesthetic drug and fluorescein drops into the eye. The latter is a dye that glows in blue light. This dye will help the doctor adjust the measuring instrument. Then the doctor presses the approximately three millimeter wide pressure sensor of the slit lamp onto the cornea of ​​the eye. The cornea, which is actually curved outwards, flattens out due to the pressure and is therefore "applanated". The more force it takes to push in, the higher the pressure inside the eye.

A similar principle is used by the "MacKay-Marg tonometer": It is portable and therefore more practicable, but it measures less precisely. It only appears to be superior in the case of corneal changes.

A fairly new development is that Dynamic contour tonometry (DCT), also called "Pascal tonometer". The measuring process is similar to the Goldmann method. However, the pressure sensor does not press the cornea in here, but rather brings it into a relaxed state. In this relaxed state, the pressure on both sides of the cornea is the same. The measuring head on the outside of the cornea of ​​the eye thus measures the same pressure that also prevails inside the eye. In contrast to the Goldmann method, the different properties of the cornea have hardly any influence on the measurement result. Dynamic contour tonometry therefore seems to be superior, especially for patients with particularly thin or thick corneas. In addition, it not only determines a pressure value, but up to a hundred values ​​per second. This method can graphically represent the intraocular pressure, which fluctuates with the pulse, in a so-called "pulse pressure curve".

It comes without touching the cornea Non-contact tonometry out. Therefore, the doctor can do without anesthetic eye drops. With this procedure, a short pulse of air presses in the cornea. A small infrared lamp on the measuring instrument checks whether the cornea is sufficiently flattened. The air pulse then breaks off immediately. From the time required for this, the device calculates the intraocular pressure: the higher this is, the longer the puff of air takes to flatten the cornea. Non-contact tonometry has the disadvantage of being imprecise when measuring relatively low and relatively high intraocular pressures. The same applies if the cornea is abnormally changed.

The portable one Rebound or induction tonometer does not require local anesthesia. This is why this variant is particularly popular when examining bedridden patients and children. Unfortunately, however, it is also prone to incorrect measurements and large spread of the measured values. In rebound tonometry, the instrument lets a measuring head hit the cornea. Depending on the intraocular pressure, it is decelerated to different degrees on the cornea and thrown back. With the help of special magnetic coils, the device converts these values ​​into intraocular pressure.

The Transpalpebral tonometry measures through the eyelid. She uses a pencil-sized device with a freely movable rod as a sensor. The measuring device is placed on top of the eyelid. Then the bar falls on the eye. Depending on the elastic tension of the eye, it springs back and thus measures the pressure conditions.

In addition to these, there are other measuring methods and instruments. Many are in the testing stage or have not yet been able to assert themselves. Doctors and patients hope for progress from "Pressure sensitive contact lenses": These contact lenses could possibly measure intraocular pressure and its fluctuations over many hours.

How high are the risks?

All tonometry procedures are considered low-risk examinations. If the measuring instrument touches the cornea, transmission of germs from patient to patient is conceivable. However, correctly performed disinfection should prevent this. The risk is lower for instruments with a blast of air and those with exchangeable contact bodies. Injuries to the cornea remain the exception if carried out correctly.

When does the health insurance company pay?

The statutory health insurances pay the examination of the intraocular pressure under certain conditions: These conditions are met if there is a reasonable suspicion or an increased risk of glaucoma. The same applies to check-ups in the case of known glaucoma and medical interventions in which a glaucoma must be ruled out in advance.

As a pure early detection measure, i.e. for the so-called screening of the population, the measurement of intraocular pressure is one of the "Individual Health Services" (IGeL). The patient has to pay for this himself. The costs are usually between 10 and 22 euros. For the early detection of glaucoma as IGeL, however, the optic nerve is always examined. Only in this combination is there a chance of recognizing a glaucoma at an early stage.

The health insurance companies refuse to cover the costs with reference to various studies. According to their information, an early detection examination for glaucoma cannot reduce the risk of blindness in a demonstrable way. In addition, in the opinion of the statutory health insurance companies, it seems unclear how reliably doctors detect a glaucoma during a screening.

Consulting expert: Professor Dr. med. Wolfgang Heider is a specialist in ophthalmology and has been a partner of Professor Dr. med. Klaus G. Riedel in the operative group practice at the Herzog Carl Theodor Eye Clinic in Munich. In 1995, Professor Heider was appointed adjunct professor for ophthalmology at the Johann Wolfgang Goethe University in Frankfurt. Professor Heider is a member of numerous German and international specialist societies.

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Important NOTE: This article contains general information only and should not be used for self-diagnosis or self-treatment. He can not substitute a visit at the doctor. Unfortunately, our experts cannot answer individual questions.