Tonometry – Principles, Types, and Clinical Significance

This presentation provides a comprehensive overview of Tonometry, an essential diagnostic procedure in optometry and ophthalmology used to measure intraocular pressure (IOP). It explains the principle, classification, and techniques of various types of tonometers including Indentation, Applanation, and Non-contact methods. The slides cover the Goldmann Applanation Tonometry in detail, highlighting its procedure, calibration, advantages, errors, and clinical interpretation.
Special emphasis is given to the clinical importance of IOP measurement in diagnosing and managing glaucoma and other ocular pathologies. Designed for optometry students, eye care practitioners, and educators, this presentation serves as a valuable academic and clinical resource.

1. tonometry Ms. Reshu Yadav Assistant professor Shs csjm university

2. introduction The intraocular pressure (IOP) is measured with the help of an instrument called tonometer. Two basic types of tonometer's available are: Indentation tonometer Applanation tonometer

3. Indentation tonometry Indentation tonometry is based on the fundamental fact that a plunger will indent a soft eye more than a hard eye. The indentation tonometer in current use is that of Schiotz. Because of its simplicity, reliability, low price and relative accuracy, it is the most widely used tonometer

4. Schiotz tonometer It consists of- Handle for holding the instrument in vertical position on the cornea. Footplate which rests on the cornea. Plunger which moves freely within a shaft in the footplate. Bent lever whose short arm rests on the upper end of the plunger and a long arm which acts as a pointer needle. Weights- a 5.5 gm weight is permanently fixed to the plunger, which can be increased to 7.5 and 10 gm.

5. Technique of Schiotz tonometry Before tonometry, the footplate and lower end of plunger should be sterilized. For repeated use in multiple patients, it can be sterilized by dipping the footplate in ether, absolute alcohol, acetone or by heating the footplate in the flame of spirit. After anaesthetizing the cornea with paracaine or 2-4 per cent topical xylocaine, patient is made to lie supine on a couch and instructed to fix at a target on the ceiling. Then the examiner separates the lids with left hand and gently rests the footplate of the tonometer vertically on the center of cornea. The reading on scale is recorded as soon as the needle becomes steady

6. It is customary to start with 5.5 gm weight. However, if the scale reading is less than 3, additional weight should be added to the plunger to make it 7.5 gm or 10 gm, as indicated since with Schiotz tonometer the greatest accuracy is attained if the deflection of lever is between 3 and 4. A conversion table is then used to derive the intraocular pressure in mm of mercury (mm Hg) from the scale reading and the plunger weight. The main advantages of Schiotz tonometer are that it is cheap, handy and easy to use. Its main disadvantage is that it gives false reading when used in eyes with abnormal scleral rigidity.

7. Applanation tonometry The concept of applanation tonometry was introduced by Goldmann in 1954. It is based on Imbert-Fick law which states that the pressure inside a sphere (P) is equal to the force (W) required to flatten its surface divided by the area of flattening (A) i.e., P = W/A. The commonly used applanation tonometer's are: Goldmann tonometer Air puff tonometer (non-contact tonometer) Perkin's applanation tonometer Pneumatic tonometer Pulse air tonometer Tono-Pen

8. Goldmann tonometer It is the most popular and accurate tonometer. It consists of a double prism mounted on a standard slit-lamp. The prism applanates the cornea in an area of 3.06 mm diameter.

9. Technique After anaesthetizing the cornea with a drop of 2% xylocaine and staining the tear film with fluorescein patient is made to sit in front of slit-lamp. The cornea and biprisms are illuminated with cobalt blue light from the slit-lamp. Biprism is then advanced until it just touches the apex of cornea. At this point two fluorescent semicircles are viewed through the prism. Then, the applanation force against cornea is adjusted until the inner edges of the two semicircles just touch. This is the end point.  The intraocular pressure is determined by multiplying the dial reading with ten.

10. Air puff tonometer It is a non contact tonometer based on principle of Goldmann tonometer. In this central part of cornea is flattened by yet of air. This tonometer is very good for mass screening as there is no danger of cross infection & local anaesthetic is not required.

11. Perkin's applanation tonometer This is a hand-held tonometer utilizing the same biprism as in the Goldmann applanation tonometer. It is small, easy to carry and does not require slit-lamp. However, it requires considerable practice before, reliable readings can be obtained.