Tonometer

details about tonometer

1. TONOMETER BY: JESLIN JOSE

2. What is IOP? The intraocular pressure refers to the pressure exerted by intraocular contents on the coats of eyeball. It is distributed evenly throughout the eye, so that the pressure is always the same in the posterior vitreous as it is in the aqueous humor. It is important in maintaining the shape of eyeball. Normal IOP varies between 10.5 and 20.5 mmHg.

3. Measurement of IOP Manometry Tonometry

4. Manometry Direct method of measuring IOP. In this method, a needle is introduced either into the anterior chamber or into vitreous, which is then connected with a suitable mercury or water manometer to measure the IOP. Disadvantage: Not a practical method for routine in human beings. N eeds general anaesthesia which has its other effects on IOP Uses: Greatest help in continuous measurements over time and recording the changes in IOP induced by physiological and pharmacological manipulations in the experiment research work on animal eyes.

5. Indentation ( Schiotz ) Applanation Air puff tonometer Pulse air tonometer It is an instrument used to measure IOP. The method of measuring IOP with the help of this instrument is known as tonometry .

6. Digital tonometer It provides a rough estimate of IOP. Procedure: Measurement of IOP can be roughly estimated by palpating the eyeball with index finger of both hands after asking patient to look down over the upper eyelid above tarsal plate. Index finger of one hand indents the eyeball , while the index finger of other hand feels the pressure changes produced by the indenting finger. It is a subjective method and needs experience . In cases of raised of IOP, the eyeball is felt hard i.e. stony hard. In cases of decreased IOP the eyeball is felt soft, i.e. water bag.

7. Instrumental tonometer 1.Contact tonometer (a) Indentation tonometer ( Schiotz ) History: It was invented by Schiotz in 1905 and later was modified by Schiotz in 1927. Principle: Degree of indentation is inversely proportional to Intraocular pressure. Indentation α 1 Intraocular pressure A plunger will indent a soft eye more than a hard eye.

8. Errors: Difference in weight of different parts of tonometer . Difference in size, shape and curvature of footplate. Friction arising in working of plunger. Differences in smoothness of gliding movement of pointer on scale. Error due to contraction of Extraocular muscles. Errors due to accommodation. Errors due to ocular rigidity. Error due to variation in corneal curvature. Error in scale reading. Blood volume alteration.

9. (b) Applanation Tonometer Concept introduced by Goldmann in 1954. Principle: It states that the pressure inside a sphere(P) is equal to External force(F) needed to flatten a portion of a sphere divided by the area of sphere. P = F A where, P= Pressure F= Force A= Area Based on Imbert Fick Law. Commonly used applanation tonometers are: ( i ) Goldmann applanation tonometer (ii) Perkin’s applanation tonometer (iii) Pneumatic tonometer

10. ( i ) Goldmann applanation tonometer Most popular and accurate tonometer . Consists of a double prism mounted on a standard slit lamp. This method of tonometry is more accurate than indentation tonometry since because of the very tiny area of cornea applanated the ocular rigidity doesnot interfere with readings. The prism applanates the cornea in an area of 3.06 mm diameter. This is an accessory of slit lamp.

11. Errors: Prism should be stable if rotated then wrong reading. Flourescein stain not much or less it should be in proper amount. Illumination amount should be proper.

12. (ii) Perkin’s applanation tonometer Handheld Tonometer utilizing the same biprism as in goldmann tonometer . Small, easy to carry and doesnot require slit lamp. However, it requires considerable practice before, reliable readings can be obtained.

13. (iii) Pneumatic tonometer In this tonometer the cornea is applanated by touching its apex by a silastic diaphragm covering the sensing nozzle( which is connected to a central chamber containing pressurised air). In this there is a pneumatic-to-electronic transducer, which converts the air pressure to a recording on a paperstrip , from where IOP is used.

14. 2. Non Contact Tonometer (a) Air Puff Tonometer Non contact tonometer based on principle of goldmann tonometer . In this central part of cornea is flattened by a jet of air. This tonometer is very good for mass screening as there is no danger of cross-infection and local anaesthetic is not required.

15. (b) Pulse Air Tonometer Handheld, non contact tonometer that can be used with patient in any position.

16. (c) Tono Pen Computerised pocket tonometer . It employs a microscopic transducer which applanates the cornea and converts IOP into electric waves.