A METHOD AND A DEVICE FOR MEASURING INTRAOCULAR PRESSURE

Abstract: A method and a device for measuring intraocular pressure. The device consists of a probe (3) which is made to impact with an eye at a certain velocity and which contains means to register the duration of the impact. It is also possible to measure the deceleration. These values are used to deduce the intraocular pressure.

Description: A METHOD AND A DEVICE FOR MEASURING INTRAOCULAR PRESSURE The invention concerns a method and a device for measuring intraocular pressure. A probe is propelled against the eye at a certain velocity and the impact duration and/or deceleration is recorded. The probe is subject to different impact durations and rates of deceleration, depending on the intraocular pressure. Such methods or devices have not been previously known. This is partly due to the fact that the method is based on an understanding of the laws concerning the impact of an object with the eye. The achievement of this understanding has needed a sufficiently effective and light deceleration sensor, which has only been available for less than ten years. Intraocular pressure is usually measured by tonometers which are placed on the cornea and which measure its elasticity by various methods (Goldmann's tonometer, Schiotz's tonometer, etc.). The two most commonly used principles are the measurement of the force needed to flatten out a certain area of the eye's surface, or the measurement of the diameter of the area flattened out by a certain force. But these methods demand the cooperation of the patient, and are therefore not suitable without general anesthesia for small children, dementia patients or animals. Other methods have been developed in which the surface of the cornea is not touched. In these methods, e.g. in US patent publications 5148807, 5279300 and 5299573, intraocular pressure is measured by means of water or air jets or various waves. These methods are not widely used, however, and the related meters are complex and expensive. It is also known, in US patent publication 5176139, a method wherein a freely falling ball is allowed to drop onto the eyelid, and the ball's rebound height is recorded. The aim of the current invention is to design a meter which is both simple and effective, and which can be used to measure intraocular pressure in patients who are incapable of cooperating and who cannot be held in position for more than a moment. In addition, the meter is suitable for examinations of large numbers of patients at the same time, because the measurement is done quickly, without local anesthesia and without a need for specially trained staff. The above mentioned and other benefits and advantages of the current invention are achieved by a method and a device according to the invention, the characteristic features of which are detailed in the attached patent claims. In short, the method according to the invention is as follows. The probe is propelled at a certain velocity. When the probe impacts with the eye or the closed eyelid, its velocity is suddenly reduced, ceased and rebound back. By using electronics, it is possible to record the decelerations and impact durations, which depend on the elasticity of the surface of the eye (= intraocular pressure). The probe with its sensor is light and the velocities used small, so that there is no danger of damage to the eye. If the intraocular pressure is high, the deceleration of the probe will be greater and the impact durations shorter, whereas if the pressure is low, the decelerations are smaller and the durations longer. The weight of the probe also influences the results, in that the lighter the probe the shorter the impact durations and the greater the decelerations. However, it has been observed that the impact velocity of the probe with the surface of the eye has very little influence on the impact duration. This naturally improves the accuracy of the measurements. The method can also be used when the eyelid is closed. Due to the low velocity of the probe and the small mass involved, the eye does not require a local anaesthetic under any measurement conditions. The meter can be calibrated according to different standards by comparing the results with those obtained by other methods. The following is a detailed description of the invention, with reference to the figures in which Figure 1 shows a schematic diagram of the construction of the measurement section of the meter, in accordance with one embodiment of the invention; Figure 2 shows the deceleration curves obtained in a practical experiment using a meter operating in the manner described in Figure 1 and various pigs' eyes, which have been pressurized to different pressures. Figures 3 A - 3 D show schematically various ways of measuring the impact duration, and Figure 4 shows the results of the interdependence of intraocular pressure and impact duration obtained with a test arrangement according to Figure 3 A. Figure 1 thus shows a simplified picture of an intraocular pressure meter in accordance with the first embodiment of the invention. The meter therefore includes a case 2, which the person carrying out the measurement holds in his/her hand. The meter may also include an additional component, which is not shown in the figure, but which is intended to support the meter at the desired distance from the eye of the patient being measured. The intraocular pressure meter includes, in this embodiment, a probe 3 containing a deceleration sensor 4, in which there is, for reasons of hygiene, an interchangeable head 5, and a system 6 for propelling the probe. A meter according to the invention may include the desired amount of electronics, even all the electronic components required, but the arrangement shown schematically in Figure 1 is also possible. Thus the meter includes a connection card 7 located in the case 2, which contains, for example, an analogue capacitance meter and from which a connection 8 is arranged to an external device, usually a microcomputer, for example via an AD converter, in order to deal with and print out the results. The operation of a device in accordance with the invention is as follows. The system 6, which propels the probe 3, for example spring 9, is tensioned so that probe 3 is in its retracted position. A suitable locking device, which is not shown here, retains probe 3, until it is released, when the probe 3 and its interchangeable head move at a certain velocity in Figure 1 from right to left, and the head 5 strikes the surface of the eye 10 and in relation to the intraocular pressure rebounds and rises from the surface of the eye. In this embodiment, the deceleration sensor 4 measures the deceleration and the impact duration and sends on the data. Figure 2 shows curves A, B and C which show deceleration curves related to different eye pressures and obtained by a meter formed in the way described above. As mentioned, the eyes of the example were pigs' eyes having different pressures: curve A = 10 mmHg, curve B = 30 mmHg and curve C = 60 mmHg. The time scale is divided into milliseconds and the deceleration range seen in the figure is about 0-20 G. As can be seen in the figure, a calibrated meter can give extremely precise measuring results from the obtained curves from which curves it is technically easy to calculate the intraocular pressure. The meter shown in Figure 1 can be connected in various ways to a device, which analyzes the measurement, or the measurement section 1 may contain all the necessary components itself and be equipped with a suitable display to show the intraocular pressure. For example, it is possible to use a capacitive deceleration sensor 4, which is usually connected by means of connector card 7 to an analogue capacitance meter and then through lead 8 via an AD converter to a computer. This type of connection is known to one versed in the art and it is always made in an arrangement, according to the circumstances, and with the aid of the necessary components. It is obvious that the deceleration sensor may be of some other type than the capacitive. What, however, is important, is that the signal from the sensor, no matter what form it is in, can be converted in a suitable manner into a readable quantity, which, when displayed on the desired display device, gives the person carrying out the measurement, information on the intraocular pressure. Figure 3 shows various schematic embodiments of the device, which are simplified in comparison with Figure 1, in which different methods are exploited to measure the impact duration and record the duration measured. One very useful method is to measure the impact duration using a principle that is based on the measurement of conductivity or a mechanical connection, in which the impact causes a change in the position of a switch and at the termination of the impact resets the position. Thus Figures 3 A and 3 B show ways in which the duration is measured by exploiting the conductivity of the object being measured. In these figures reference number 3 represents an entirety formed by the probe 3 and a head 5, which it possibly contains, reference number 11 represents the measurement leads, and reference number 12 represents the measuring/recording device in general. In Figure 3 A, the electrical conductivity of the object being measured is employed in the measurement. One terminal of the electrical connection is connected to the probe by means of lead 11 and the other terminal to a part of the patient close to the eye, for example the eyelid. Meter 12 records the time that probe 3 is in contact with the surface of the eye, the so-called impact duration. The measurement is easy and simple. After all, it is only a matter of a basic measurement as to whether the circuit is open or closed. Figure 3 B shows a variation of the previous version, in which the measurement points are both located in the end of the probe. In this case too, the impact with the wet surfac of the eye closes the circuit and the rebound opens it again. Figure 3 C shows yet another variation of a meter in accordance with the invention, in which a switch 13 attache to the probe 3 is used, and which may be based on deceleration or else is mechanical and is connected in such a way that the force caused by the impact of the probe causes it to move from off to an on position or vice versa. In any event, the impact duration can also be recorded by this embodiment of the device. Figure 3 D in turn shows an embodiment, in which a mechanical switch 14 is located in the end of the probe 3. It is obvious that there are other ways of measuring the impact period than those described above, but also that the all belong to the field of the basic concept according to the invention, to the extent determined by the accompanying Claims. Figure 4 shows the measurement result of the test arrangement, which is as follows. Separate pigs' eyes are pressurized to various pressures using water. The pressure is measured using the height of a column of water as a meter. For purposes of comparison it can be stated that a column of water 80 cm high corresponds to a pressure of 60 mmHg. A measurement arrangement in accordance with Figure 3 A was mainly used, in which a measurement potential of 6 V DC was connected between the probe 3 and the other electrode, which was also attached to the surface of the eye. The horizontal axis of Figure 4 represents the height of the water column and the vertical axis the impact duration in milliseconds. It can be seen from the figure that the measurement results are very evenly distributed and that the curve in question is a very accurate method of measurement when it is calibrated. The invention makes possible many variations, which are not depicted above. Thus, even though spring 9 is a very useful method of creating movement in probe 3, it is certainly possible to use any other method and device for this purpose. Another method is to use a propelling force based on magnetism, such as a solenoid-type solution. An electrical solution might be even more accurate than a mechanical one. It is possible to use any other applicable device whatever. If the device in accordance with the invention is made so that it can be easily held in the hand and contain all the components necessary for carrying out measurements, dealing with the results, and displaying them, the device would be independent and very easy to use. The invention is not limited to the form presented in the descriptions and figures, but can be adapted within the framework of the attached patent claims.

Claims: Claims 1. A method for measuring intraocular pressure by propelling a moving probe (3) into contact with an eye or eyelid, characterized in that the duration of the impact is measured and the intraocular pressure is derived from the results obtained. 2. A method in accordance with Claim 1, characterized in that in addition to the duration of the impact, its deceleration is also measured. 3. A method in accordance with Claim 1, characterized in that the measurement of the impact period is achieved by measuring the duration of the contact with the surface of the eye, either as the duration of an electrical circuit being closed or, alternatively, open. 4. A method in accordance with Claim 3, characterized in that the impact duration is measured by employing the electrical conductivity of the object being measured to close/open an electrical circuit. 5. A method in accordance with Claim 3, characterized in that the impact duration is measured by using a switch (13, 14), which alters its position on the basis .of contact or deceleration. 6. A method in accordance with one of the above Claims, characterized in that the measurement is carried out through a membrane placed on top of the eyelid or transmitted by an intermediate piece. 7. A method according to claim 1 , characterized in that the certain velocity of the probe (3) is achieved by means of a spring (9) . 8. A method according to claim 2, characterized in that the deceleration is measured with a capacitive acceleration sensor ( 4 ) . 9. A device for measuring the intraocular pressure by means of a device, which includes a probe (3), a device (9) for propelling the probe at a certain velocity, and devices for handling and displaying data, characterized in that it includes an device (4, 13, 14), either in the probe (3) or connected to it, for measuring the impact period and/or deceleration. 10. A device in accordance with Claim 9, characterized in that the device for measuring the impact period and/or deceleration is a decleration sensor (4), a closing/opening electrical circuit, or a switch (13, 14) that alters its position mechanically or on the basis of deceleration. 11. A device according to claim 10, characterized in that the acceleration sensor (4) is a capacitive acceleration sensor. 12. A device according to claim 9, characterized in that the device giving a certain force to the probe (3) is a spring (9). 13. A device according to claim 9, characterized in that the probe (3) has a disposable, exchangeable head (5).

Inventors: KONTIOLA ANTTI ILMARI, KONTIOLA, ANTTI, ILMARI
Applicants: KONTIOLA ANTTI ILMARI, KONTIOLA, ANTTI, ILMARI
Application year: 1995
Main classification: A61B3
IPC codes: A61B3