Tonometry.pptx

Introduction to Intraocular pressure and tonometry
Classfication of tonometry
Newer tonometers
Tonometry for Special Clinical Circumstances

1. Tonometry Presenter Anisha Heka 22 nd batch MMC, IOM

2. Presentation Layout

3. Intraocular pressure (IOP) is  the fluid pressure of the eye . So what exactly is a normal IOP? According to the American Academy of Ophthalmology , a normal IOP is between 10-20 mm Hg, though wide variation exists . In a large population based survey conducted on caucasians, it was observed that 95% of individuals had an average IOP of 15.7 + 2 S.D (standard deviation) which if translated in integers, falls between 10 and 21 mm Hg.

4. IOP is a complex trait determined by many genetically and environmentally determined factors including but not limited to aqueous humor flow, uveoscleral outflow, trabecular outflow, episcleral venous pressure, and blood pressure. Studies conducted in south Indian population (Vellore and Andhra Pradesh Eye Disease studies) also reported mean IOP of 15.5 mm Hg (2 SD: 9-22.6 mm Hg). In individuals who are susceptible to glaucoma, “normal” intraocular pressure (IOP) may be defined as that pressure which does not lead to glaucomatous damage of the optic nerve head.

5. Tonometry is the procedure for the measurement of intraocular pressure . It is the most important risk factor for the development and progression of glaucoma. It is the only factor that can be treated and modified in the management of glaucoma. Therefore, the need for accurate and reproducible measurement of IOP is of utmost importance . G old standard for the measurement of IOP is the Goldmann applanation tonometer.

6. Direct Method A catheter is inserted into the anterior chamber of the eye and the other end is connected to a manometric device from which the pressure is calculated. Though this is the most accurate method available, it is not feasible in human being because of its invasive nature

7. Palpation Method : Intraocular pressure (IOP) is estimated by response of eye to pressure applied by finger pulp (indents easily/firm to touch). This method cannot give accurate IOP reading and is not used as an IOP measurement technique. A “rock hard” eyeball only occurs in acute angle closure glaucoma. Slight increases in intraocular pressure such as occur in chronic glau-coma will not be palpable .

8. Indirect method The two basic types of tonometers differ according to the shape of the deformation: Indentation and Applanation (flattening) Contact tonometers

9. Indentation tonometers In indentation tonometry, a known weight is placed on the cornea, and the IOP is estimated by measuring the deformation or indentation of the globe(shape of the deformation with this type of tonometer is a truncated cone). It displaces large intraocular volume so conversion tables based on empirical data is used to estimate IOP. The Schiøtz tonometer is the prototype for this class of instruments.

10. Schiotz tonometer The Schiotz tonometer consists of a metal plunger that slides through a hole in a concave metal footplate. The plunger supports a hammer device connected to a needle that crosses a scale. The plunger, hammer, and needle weigh 5.5g. This can be increased to 7.5, 10, or 15g by the addition of appropriate weights. The more the plunger indents the cornea, the higher the scale reading – that is, the lower the IOP, the higher the scale reading. Each scale unit represents a 0.05mm protrusion of the plunger .

11. Principle The weight of tonometer on the eye increases the actual IOP (Po) to a higher level (Pt ). The change in pressure from Po to Pt is an expression of the resistance of the eye (scleral rigidity) to the displacement of fluid. Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables.

12. Procedure The patient is supine with the cornea anesthetized. The patient is asked to fixate, tonometer footplate is applied to the cornea, the tonometer is kept vertical to allow free movement of the plunger to indent the cornea. The needle will oscillate with the ocular pulse, and the midpoint of the excursion is used as the scale reading. If the value is not greater than 4 units, additional weight is added. Then IOP read from the appropriate table.

13. Eyes with high ocular rigidity give falsely high Schiøtz IOP readings, whereas eyes with low ocular rigidity give falsely low Schiøtz IOP readings. High Ocular Rigidity Low Ocular Rigidity High Hyperopia Chronic Glaucoma Vasoconstrictor therapy High Myopia Miotic therapy (especially cholinesterase inhibitors) After retinal detachment surgery Intravitreal injection of gas Vasodilator therapy It is possible to estimate ocular rigidity by comparing applanation and Schiøtz measurements

14. Applanation tonometers The shape of the deformation with these tonometers is a simple flattening . and because the shape is constant, its relationship to the IOP can, in most cases, be derived from mathematical calculations. The applanation tonometers are further differentiated on the basis of the variable that is measured. a. Variable force : Area of cornea on applanation held constant, force varies. Prototype is Goldmann tonometer . b. Variable area : Force applied to cornea held constant, area varies. Prototype is Maklakov tonometer . The volume displacement is sufficiently large to require a conversion table.

15. Goldmann Applanation Tonometry Principle M odification of the Maklakoff–Fick law, also referred to as the Imbert–Fick law . This law states that an external force (W) against a sphere equals the pressure in the sphere (Pt ) multiplied by the area flattened (applanated) by the external force (A).

16. Why modification necessary? The validity of the law requires that the sphere be (1) perfectly spherical, (2) dry, (3) perfectly flexible, and (4) infinitely thin . The cornea fails to satisfy any of these requirements. The moisture creates a surface tension (S), and the lack of flexibility requires a force to bend the cornea (B), which is independent of the internal pressure. In addition, because the cornea has a central thickness of approximately 550 µm, the outer area of flattening (A) is not the same as the inner area (A1 ).

17. When A1 equals 7.35 mm2 , S balances B, and W equals Pt . This internal area of applanation is obtained when the diameter of the external area of corneal applanation is 3.06 mm, which is used in the standard instrument. W + s = PA + b (equation 2) The volume of displacement produced by applanating an area with a diameter of 3.06 mm is approximately 0.50 mm3 , so that Pt is very close to P0 , and ocular rigidity does not significantly influence the measurement

18. Procedure The GAT is available as a slit-lamp mounted or hand-held device. Corneal anesthesia is required . The use of fluorescein sodium facilitates the IOP reading: The tears fluorescein a cobalt blue light, making it easier to determine the area applanated. The slit-lamp illumination arm is positioned at an angle to the observation arm to maximize illumination of the tonometer head.

19. The instrument is brought closer to the eye until corneal contact is made. When viewed through the slit-lamp, a biprism within the tonometer tip splits the image seen into two semicircular rings. The height of the slit-lamp is raised or lowered so that the semicircles are equal in size. A dial on the side of the tonometer is adjusted to vary the force applied to the eye. This causes a movement of the rings towards or away from each other. The correct area is applanated when the inner edges of the semicircles touch.

20. Sources of error 1. Corneal curvature : In patients with, with-the-rule astigmatism regular corneal astigmatism over 4 D will result in an underestimate of IOP and against-the-rule astigmatism, an overestimate of IOP by 1 mm Hg . A practical way to overcome this problem is to take two successive measurements, one with the prism oriented horizontally and the other vertically and then simply take the average of the two.

21. 2.Magnitude of capillary attraction : The radius of contact is greater with wide mires which leads to overestimation and is lesser with thin mires leading to underestimation. To get consistent readings, optimal mire widths should be between 0.4 to 0.6 mm that corresponds approximately to ¼ to 1 /3 of the radius of the applanated area . 3.Adequate tear film fluorescence : It is needed for the clear visualization of the contact point between the GAT tip and the cornea . Hypofluorescence leads underestimation of IOP and hyperfluorescence leads overestimation of IOP

22. 4.Obesity : Measuring the IOP with the Goldmann tonometer in obese patients may give high readings. This is due to thorax compression and breath-holding, which causes transitory rises of IOP. In overweight patients, Perkins hand-held applanation tonometer is recommended to obtain accurate IOP.

23. Non-contact Tonometer U ses a jet of air to applanate the anterior corneal surface . The system consists of a central air plenum flanked either side by a light emitter and detector. When the cornea is in the resting state, light emitted is scattered by the convex corneal surface. As the pressure of the air pulse directed to the cornea increases to deform the cornea, the corneal surface behaves like a plane mirror reflecting light to the detector. At the point of maximal light detection when the cornea is completely applanated, the instrument switches off the air pressure pulse. . The prototype was introduced by Grolman in 1972

24. Other applanation tonometers Perkins Applanation Tonometer Draeger Applanation Tonometer Mackay Marg Tonometer

25. Newer tonometers Non-contact Tonometry Ocular Response Analyser Tonopen Pneumatic tonometer Dynamic Contour Tonometer – Pascal Rebound Tonometer Transpalpebral Tonometer Continuous IOP Monitoring Systems Diaton

26. Ocular response analyser interesting adaptation of the non-contact tonometer. an air puff tonometer that directs the air jet against the cornea and measures not one but two pressures at which applanation occurs – when the air jet flattens the cornea as the cornea is bent inward (force in pressure-P1) and as the air jet lessens in force and the cornea recovers (force out pressure-P2) The first is the resting intraocular pressure. The difference between the first and the second applanation pressure is called corneal hysteresis and is a measure of the viscous dampening and, hence, the biomechanical properties of the cornea .

27. The ORA produces two measurements of corneal biomechanical properties, corneal hysteresis (CH) and the corneal response factor (CRF). While CH represents the absolute difference between the applanation pressures P1 and P2, the CRF is derived from the formula (P1- kP2), where k is a constant. It has been suggested that CH predominantly reflects the viscous properties of the cornea, whilst CRF better reflects the elastic properties

28. Tonopen T he force that is required to keep the flat plate of plunger flush with a surrounding sleeve against the pressure of corneal deformation is measured. The effect of corneal rigidity is transmitted to the sleeve. The IOP is recorded as wavefront.

29. Pneumatic tonometer It applanates cornea with a probe that is supported by a column of gas and hence, the sensing device is air pressure . A transducer converts the gas pressure in the chamber into an e­lectrical signal that is recorded on a paper strip

30. Dynamic Contour Tonometer – Pascal The principle is based on contour matching , which assumes that if the eye were enclosed by a contoured, tight fitting shell, the forces generated by IOP would act on the shell-wall . Replacing part of the shell-wall with a pressure sensor would enable measurement of these forces and therefore the IOP. The DCT has a contoured tonometer tip surface that aims to match the contour of the cornea The OPA value displayed on the LCD screen is a peak-topeak difference of the average systolic IOP and average diastolic IOP, in units of mm Hg

31. Rebound Tonometer A new and updated version of an indentation tonometer. A very light, disposable, sterile probe is propelled forward into the cornea by a solenoid ;. the time taken for the probe to return to its resting position and the characteristics of the rebound motion are indicative of the IOP (and also the biomechanical properties of the cornea ). The time taken for the probe to return to its resting position is longer in eyes with lower IOP and faster in eyes with higher IOP

32. Transpalpebral Tonometer  Phosphene tonometry is a psychophysical test for self-tonometry. The pencil-shaped instrument is pressed with its probe against the upper lid with increasing pressure until visual phenomena are detected. Phosphene appears opposite to pressure applied.

33. Tonometry for Special Clinical Circumstances Tonometry on Irregular Corneas Pneumatic tonometer Tonometry Over Soft Contact Lenses P neumotonometry and the Tono- Pen can measure with reasonable accuracy the IOP through bandage contact lenses Tonometry With Gas- Filled Eyes Intraocular gas significantly affects scleral rigidity, rendering indentation tonometry particularly unsatisfactory Tono- Pen

34. Tonometry in Paediatric Patients Perkins Tonometer: Perkins applanation tonometer is ideal for examining babies, children, uncooperative patients, wheelchair-bound patients, and bedridden patients as well as during an examination under anesthesia ( EUA ) Tonopen Post LASIK Status Dynamic contour tonometer

35. References https://www.eophtha.com/posts/a-postgraduates-guide-to-tonometry