Ocular tonometry

With tonometry (from Tonos, τόνος, Greek for " pressure, tension " and metron, Greek for " dimension" ) is the measurement of intraocular pressure refers to the increase in general one of the most important but not the only risk factor for represents glaucoma ( glaucoma). It is measured in mmHg and is in a healthy adult 10-21 mmHg. Processes in which it comes into direct contact between the eye and the instrument, the tonometer, previously require local anesthesia of the cornea by means of drops, so that the measurement runs pain free. Since the intraocular pressure at different times of the day can be of different heights, this is when indicated repeatedly measured over the day and distributed the night and combined to form a so-called daylight pressure profile.

Measurement methods

Applanation tonometry

A common and accurate method is the applanation tonometry ( planus, Latin for "flat, flat, flat" ), which was developed by the Austrian-Swiss ophthalmologist Hans Goldmann. At a special ophthalmologic examination device, the slit lamp, in this case a small measuring body is attached. In the investigation the force is measured, which is necessary with a diameter of 3.06 mm ( area = 7.35 mm ²) to bring his plane front surface in contact with the cornea and flatten them. For visual inspection of the contact between the cornea and measuring body by the examiner, an aqueous solution of the dye fluorescein is instilled into the conjunctival sac before. The applied force is generated by a spring scale which is coupled to a measuring drum. From this one can read off the pressure immediately. There are also applanation tonometer, which are held in the hand in order to carry out an inspection on for example situated people.

Various studies point to the applanation tonometry on a correlation of corneal thickness and measured pressure values. In people with very thick cornea then could the measurement result erroneously be too high, too low in people with very thin corneas. Especially after refractive surgery in which the cornea " diluted " was, should be able to such measurement errors. It is therefore recommended, in particular in respective groups of patients to determine the corneal thickness prior to the measurement of the intraocular pressure. This happens most often with a so-called pachymeter. The correct value can then be determined using a conversion factor based on a correction table. However, other studies have come to the conclusion that applanatorisch measured pressure values ​​in patients with a thick cornea were not an artifact, and recommend in doubt, a pressure value determination by means of intraocular pressure probe.

However, also other corneal properties such as stiffness and viscoelasticity affect the measurement accuracy of the intraocular pressure. If need be, therefore, recommend other cornea independent measurements.

Impressionstonometrie

One older method of intraocular pressure measurement, the Impressionstonometrie ( impressio, Latin for " intrusion impression " ) Represents the Schiötz used for this purpose usually tonometer, named after its developer, the Norwegian ophthalmologist Prof. Hjalmar Schiøtz ( 1850-1927 ) is, only for measurements on persons subject or for late zurückgebeugtem head is held in the hand and placed perpendicular to the cornea. In this investigation will determine how deep a metal pin dents through a well-defined weight the cornea. Scale values ​​, used weights and calculated results are shown in a calibration table. A problem of Impressionstonometrie is that the instruments used are calibrated only on eyes with an average strain capability of the sclera ( rigidity ). When abnormal rigidity ( for example, myopia), the calibration table displays incorrect values ​​for intraocular pressure. For this reason, the applanation tonometry is the method of choice.

Non -contact tonometry (NCT )

A procedure in which there is no contact between the eye and the measuring instrument, the non-contact tonometry (NCT ) dar. The intraocular pressure is measured by a pulse with short-term elevated air pressure. This method is the examination process for on-the subjects, although pleasant, but because of the terms of apparatus, not systemically induced drop in accuracy is not fully meaningful.

While impressions and applanationstonometrische studies represent only an ophthalmological work, find inspections and screening of intraocular pressure by NCT, increasingly, opticians application. The medical and health economic benefits will this remains controversial.

Transpalpebral scleral tonometry

The Transpalpebral scleral tonometry refers to the method for the measurement of intraocular pressure through the eyelid. It avoids the direct contact with the cornea and works on the principle of repulsion, but by the eyelid pass ( transpalpebral ). The user resets the device to the upper eyelid and triggers the measurement. The function of the sensor meets a freely movable rod which cooperates with her sinks with the elastic eye surface in the region of the sclera. The patient can sit or lie down, but needs to look at a 45 ° angle. In applying this method, no local anesthesia is required. With this method, results of intraocular pressure measurement are independent of the biomechanical condition of the cornea and should not be corrected with pachymeter.

Dynamic Contour Tonometry

A new method, the so-called Dynamic Contour Tonometry (DCT ) Represents the dynamic measuring principle is fundamentally different from the static applanation tonometry because it does not flattens the cornea, but the cornea modeled measuring head brings the cornea in its natural, unstressed condition. The curvature of the cornea under the measuring head is only slightly reduced ( flatter). The pressure between the measuring head and the cornea then corresponds to the intraocular pressure. A built- in tonometer pressure sensor can thus absorb the pressure in the eye directly and largely independent of the cornea influences. The achieved precision allows pulse curves of the intraocular pressure, which are triggered by heart, similar to an ECG display.

The accuracy was confirmed in vivo in comparison intracameral measurements. It was shown that the DCT measurements correlate better with an error of 1 mm Hg by the actual pressure in the eye and the corneal thickness has only a minimal effect on the measurement. The repeatability and reproducibility of the dynamic DCT measurements also proved to be better than with the usual static Applanationstonometern or with the non-contact tonometers at single pulse.

After eye surgery such as LASIK or crosslinking, which modify the properties of the cornea, results of DCT measurements are apparently less distorted than in Applanationsmethoden.

Blickrichtungstonometrie

A viewing direction dependent intraocular pressure measurement is performed for diagnostic assessment when symptoms of Graves' ophthalmopathy. Background of this study is that a conditional of this disease, inflammatory increase in volume of the eye muscles lead when changing the viewing direction to a compression of the eyeball and thus the intraocular pressure may increase. The same symptoms can occur during a orbital floor.

Tonography

Unlike the pressure curve a day over a period of a few minutes a constant measurement carried out of the outflow of aqueous humor and the corresponding record of a path curve of the resulting pressure drop is called tonography. The method is a reliable method for early detection of glaucoma simplex.