Tonometry English: intraocular pressure measurement
Definition intraocular pressure measurement
By intraocular pressure measurement we understand different mechanisms to measure and determine the pressure present in the anterior segment of the eye.
Need for tonometry
The measurement of intraocular pressure, also known as tonometry, is a standard procedure for the examination and detection of a possibly too high intraocular pressure, the green star (glaucoma). You can find general information on this topic here: Intraocular pressure A light pressure on the eyeball allows a first rough estimate of the pressure prevailing in the eyeball. Very strong deviations from the normal pressure inside the eye can thus be easily detected.
In the case of slight deviations or only moderately increased pressure in the eye, this procedure alone does not provide any information about the extent or severity of the disease. Thus, for an exact estimation of the intraocular pressure, the measurement by means of a tonometer becomes more important. The estimation of the intraocular pressure alone is not decisive and decisive for the development of later consequential damages, or the glaucoma, which has to be given priority in this context.
However, too high pressure in the eye increases the risk of later disease, with loss of optic nerves and fibres, which, depending on their characteristics, can result in more or less severe visual impairment for the person concerned. The most important application of this examination is therefore the diagnosis of glaucoma. Furthermore, it is also used for further monitoring in the case of elevated values.
This means that the intraocular pressure should then be measured at regular intervals of half a year. If there is a family history of glaucoma, regular check-ups should be carried out at intervals of one year as a precautionary measure. The examination can be carried out by an ophthalmologist. Irrespective of any illness or complaints, it is recommended that an intraocular pressure measurement be carried out from the age of 40 onwards, in patients who are to be fitted with glasses.
Mechanism of intraocular pressure measurement
Palpation: Before there were appropriate instruments and apparatus for measuring intraocular pressure, the intraocular pressure was determined by this method. Today, the measurement of intraocular pressure can also be carried out by any non-eye doctor in order to obtain an overview of the pressure conditions inside the eye. With this method, the practitioner faces his patient.
The patient is asked to close his eyes and the examiner applies a careful and light pressure with his two index fingers on one eyeball while the remaining fingers rest on the patient’s forehead. Depending on how far the eyeball surface can be pressed in, a rough estimate of the pressure conditions can be made. The measurement of intraocular pressure must be carried out extremely carefully, but it is not possible to make an accurate pressure measurement with this method.
This examination method is particularly useful for diagnosing a glaucoma attack in which the eyeball cannot be pushed in and is hard as a board. It is also important to compare the sides of the eye. A pressure difference between the left and right eye can give an indication of glaucoma.
Applanation tonometry:Applanation tonometry is performed on a measuring device called a tonometer. The patient rests his chin on a pad while sitting, and his forehead is pressed against a band. The ophthalmologist sitting opposite moves a small cylinder close to the eye and carefully places this cylinder on the patient’s wide open eye.
During applanation tonometry of intraocular pressure measurement, the force required to press an area of 3mm diameter with this cylinder in such a way that it is flattened out is measured. Once this is done, the applied pressure corresponds to the intraocular pressure. The ophthalmologist sees two circles on his side of the device, which have to be moved towards each other by turning a knob (on the side of the tonometer) until they lie on top of each other.
Then the intraocular pressure is read on a scale. Since the eye is sensitive to pain and irritation, it is necessary to anaesthetize the surface of the eye. In addition, a fluorescent liquid is injected into the eye.
The intraocular pressure varies in healthy people and also depends on various factors, such as corneal thickness. The thicker the patient’s cornea is, the more pressure must be applied to dent the surface, which corresponds to a formal increase in intraocular pressure that is not present. For this reason, it is always necessary to determine the patient’s corneal thickness when high values are in question.
Patients who are lying down can be examined by so-called hand applanation tonometer. Such mobile devices are also used for so-called day-night measurements, where intraocular pressure must also be measured at night. Non-contact tonometry:In this method of measuring intraocular pressure, the device does not touch the cornea during measurement.
Instead of the cylinder, the cornea is flattened by a short, strong blast of air. This produces a visible reflex which can be evaluated by the device and shows a corresponding intraocular pressure. Since there is no direct contact with the cornea, there is no need for surface anaesthesia of the cornea.
The possible risks of corneal injuries or infections are also minimized. The results of this measurement of intraocular pressure are not as accurate as those of applanation tonometry. For the patient, non-contact tonometry is also the more unpleasant examination.
Furthermore, the air blast measurement only works if the corneal surface is intact. Incorrect values are displayed if the cornea is scarred or injured (astigmatism and corneal ulcer). Impression tonometryThis is an older method of measuring intraocular pressure, in which a pencil is placed on the cornea and then it is measured how far this pencil penetrates the corneal surface with its weight.
From this, the corresponding intraocular pressure is then determined. In this procedure too, the cornea must be treated with anaesthetic eye drops before the examination. Today, applanation tonometry and non-contact tonometry have largely replaced this procedure. This form of measuring intraocular pressure is still used in patients who have a scarred cornea and the first two measuring methods do not allow reliable values to be obtained. Overall, it must be said that impression tonometry does not give exact values of the intraocular pressure.