Tonometry and tonography

The presentation is about the tonometer, its principle, its use and also it covers tonography

1. TONOMETRY and TONOGRAPHY Presented by Loknath Goswami B.Sc optom 2 nd year

2. Important terms IOP : The pressure exerted by intraocular fluids on the coats of the eyeball Unit : mm of Hg Range : 10 to 21 mm of Hg Indentation : Depression Applanate : Flatten

3. Tonometer and Tonometry Tonometer Instrument that exploits the physical properties of the eye to permit measurement of pressure without the need to cannulate the eye TONOMETRY The procedure eye care professionals perform to determine the intraocular pressure, the fluid pressure inside the eye It is a non invasive measurement of measuring the IOP

4. History

5. History

6. Grolman tonometer Ballistic tonometer Maklakov Goldmann Schiotz Classification

7. Maklakov tonometer The first practical tonometer was the Maklakov tonometer It has a fixed force and a flat bottom that was smeared with ink When the tonometer first touched and then flattened the cornea, the ink was transferred to the cornea The tonometer was then printed on a piece of paper

8. Maklakov tonometer The area (as determined by the diameter) in the center of the inkblot that was devoid of ink was proportional to the IOP If the eye moved during the time tonometer was on the eye, more ink was transferred to the cornea than was necessary due to applanation alone and the IOP was underestimated

9. Applanation tonometry The theory of applanation tonometry comes from the Imbert -Fick law which states for an ideal sphere the internal pressure of a very thin-walled sphere can be obtained by knowing the force required to flatten a known sphere The formula is : P = F/A or F = PA P = Pressure inside the sphere F = Force required to applanate its surface A = Area of flattening

10. Ideal sphere

11. The force of capillary attraction (T) between the tonometer head and the tear film is additive in the external force A force (C), independent of IOP, is required to flatten the relatively inflexible cornea F = PA becomes F + T = PA + C => P = F + T – C/ A

12. The Goldmann applanation tonometer is designed such that A is equal to 7.35 mm 2 With this value for A, the opposing forces of capillary attraction and corneal flexibility cancel out P = F / 7.35 mm 2

13. Goldmann tonometer After anaesthetising the cornea with a drop of 2% xylocaine and staining the tear film with fluorescein patient is made to sit in front of the 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 area of cornea

14. Goldmann tonometer 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 IOP is determined by multiplying the dial reading with 10

15. Conditions

16. SCHIOTZ TONOMETER Measures degree of corneal decompensation by a known weight placed on cornea. Weights – 5.5g, 7.5g, 10g, 15g. Concave foot assembly rests on cornea Forms a reference level from which plunger further indents Degree of indentation is displayed on scale.

17. SCHIOTZ TONOMETER scale Needle plunger holder Foot plate Lever Additional weights

18. Procedure After anaesthetizing the cornea with paracaine or 2-4 % xylocaine , patient is made to lie supine on a couch and instructed to fix at a target on the ceiling We should separate the lids and lower the tonometer plate on the cornea so that the plunger is free to move vertically

19. Procedure The reading on scale is recorded as soon as the needle becomes steady It is customary to start with 5.5 gm weight 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 S chiotz tonometer the greatest accuracy is attained if the deflection of lever is between 3 and 4

20. Procedure In end, tonometer is lifted and a drop of antibiotic is instilled A conversion table is then used to derive the IOP in mmHg from the scale reading and the plunger weight

21. Friedenwald conversion table

22. Dynamic contour tonometry Dynamic contour tonometry (DCT) uses the principle of contour matching instead of applanation The tip contains a hollow the same shape as the cornea with a miniature pressure sensor in its centre

23. Dynamic contour tonometry In contrast to applanation tonometry it is designed to avoid deforming the cornea during measurement and is therefore thought to be less influenced by corneal thickness The probe is placed on the pre-corneal tear film on the central cornea and the integrated   piezoresistive  pressure sensor automatically begins to acquire data, measuring IOP 100 times per second The tonometer tip rests on the cornea with a constant appositional force of one gm

24. Dynamic contour tonometry When the sensor is subjected to a change in pressure, the electrical resistance is altered and the tonometer's computer calculates a change in pressure according to the change in resistance A complete measurement cycle requires about eight seconds of contact time The device also measures the variation in pressure that occurs with the cardiac cycle

25. TONOGRAPHY

26. Tonography Tonography is a clinical test of aqueous humor dynamics that was introduced by W. Morton Grant in 1950 Grant showed that analysis of a continuous recording from an electronic Schiotz tonometer yielded estimates the rate of aqueous flow

27. Grant recorded the output of an electronic tonometer on a strip-chart recorder and showed that this data combined with the tonometer calibration of Friedenwald could be used to provide a quantitative expression relating the outflow of aqueous humor to the driving pressure. Grant called this value “the coefficient of aqueous outflow facility”( C ) The C-value is expressed as aqueous outflow in microlitres per minute per millimeter of mercury

28. For a graphic record the electronic Schiotz tonometer is used by placing it on the eye for 4 minutes C-value is calculated from special tonographic tables taking into consideration the initial IOP (P o ) and the change in scale reading over the 4 minutes Although in general, C-values more than 0.20 are considered normal, between 0.2 and 0.11 border line, and those below 0.11 abnormal

29. A tonogram from a patient with glaucoma The initial scale reading of 4.5 with a 7.5-g weight means that the P was 28 The P t was initially 44.4 and fell gradually to 39.5 over 4 minutes, yielding an average pressure during tonograghy ( P tav ) of 41.9 This reflects a low aqueous outflow

30. References Pearls of glaucoma management, 2 nd edition, JoAnn A. Giaconi , pg no. 92 https:// www.ncbi.nlm.nih.gov/pubmed/21150677 https:// en.wikipedia.org/wiki/Ocular_tonometry http:// eyewiki.aao.org/IOP_and_Tonometry http:// www.oculist.net/downaton502/prof/ebook/duanes/pages/v3/v3c046.html

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