Tonometry and tonography

tonometry and tonography

1. Tonometry and tonography -Dr samarth mishra

2. Tonometry : Measurement of IOP is known as tonometry . A true measurement of IOP requires a direct fluid connection to the anterior chamber .(= manometry ) Cannulation of the anterior chamber for measurement of IOP is used frequently in the laboratory and occasionally during surgery. This approach entails too many hazards for the routine clinical management of glaucoma.

3. . Therefore, we generally use indirect measurements of IOP— tonometry . It is of 2 types: -indentation tonometry . - applanation tonometry . Schiotz developed first device that quantifies IOP with relative reproducibility.

4. . Used quite often d/t its :- a ) Simplicity b) Easy transportability. - schiotz tonometer is based on the principles of indentation tonometry .

5. Indentation tonometry Schiotz tonometer is an example of indentation tonometry . Consists of 3 parts:- a) footplate (radius of curvature=15mm ) b) plunger c) scale A series of known weights of 5.5gm , 7.5gm , 10.0gm and 12.5gm are applied to the plunger. With the smallest plunger weight (5.5 g), the total weight of the instrument on the eye is 16.5 g. The plunger is kept on the cornea.

6. . The plunger indents the cornea which results in the deformation of the globe. This deformation is measured by the scale attached to the plunger. The reading on the scale is converted to IOP with the help of a nomogram k/a friedenwald nomogram . it was published after modification in 1955 . The formula requires a constant ‘k’ which is the cofficient of ocular rigidity .

7. . It is the measure of the resistance of the eye to the distending force. Clinical technique: Pt lies supine: Procedure is explained to pt. Topical anaesthetic drops are instilled. Pt is asked to look at a fixed target Footplate is kept on cornea and reading taken. It is then correlated with the nomgram .

8. . Limitations: ‘k’ value of ocular rigidity is kept average for all eyes. The tables that are commonly used to estimate IOP based on a Schiøtz scale reading assume an eye with normal scleral rigidity. T he tables give inaccurate estimates of IOP in eyes that do not have normal scleral rigidity. so,in eyes where ocular rigidity is high or low,the value becomes unreliable. HIGH OCULAR RIGIDITY SEEN IN : hypermetropia , chronic glaucoma and chronic vasoconstrictor therapy . So, the recorded iop will be higher in these eyes.

9. . Low ocular rigidity is seen in :- a)high myopia b)mitotic therapy c)after retinal detechment surgery d) intravitreal injection of gas e)vasodilator therapy . false high readings are obtained with very thick and steep corneas. Unreliable readings are found in scarred corneas with significant pathology.

10. Applanation tonometry Based on Imbert fick’s law . States that in an ideal round,dry,elastic,infinitely thin walled sphere P=F/A p =pressure F =force required to flatten the cornea A =area

11. . In applanation tonometry 3mm of cornea is flattened. This minimally displaces fluid of 5 micro litre . The force required to flatten a circle of 3.06 mm is in grams a tenth(1/10) of IOP in mmHg. Therefore, a force of 1.6 g is required to flatten this circular area when IOP is 16 mmHg. The 3.06-mm diameter circle of applanation was chosen because of this simple 10:1 relation between IOP and grams of force .

12. . This area is within the range in which the natural bending force of the cornea is canceled by the capillary attraction created by the tear film between the tonometer head and the cornea. Flattening so small an area of the cornea creates little fluid displacement within the eye. Therefore , scleral rigidity is not a factor in Goldmann applanation tonometry .

13. Clinical technique In goldmann applanation tonometry , when the cone tip is pressed against the anesthetized cornea, a small circular area is flattened. The applanated area appears as a dark circle surrounded by a narrow ring of fluorescent tear film. Opposing prisms in the tip of the cone split the image, so that the viewer sees two dark half circles, each with a narrow fluorescent outer border. The force on the cone is adjusted until the inner corners of the fluorescent half rings just touch. At this optical end point, the applanated area is correct: a circle with diameter = 3.06 mm.

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15. . Applanation tonometry can be divided into two subtypes: -variable area -variable force. Variable area: These measure the area of the corneas flattened by a known amount of force. E.g maklakov tonometer .

16. . Variable force:- These measure the force that is needed to flatten a standard area of the cornea. E.g goldmann applanation tonometer . ( it is the gold standard )

17. Goldmann applanation tonometer It is mounted on a standard slit lamp. It has a plastic biprism,which is used to flatten the cornea after anaesthetizing. The prism is mounted on a rod. Before touching the cornea with biprism , sodium fluorescin dye is instilled & cobalt blue filter is switched on.

18. . When the observer views from the slit lamp uniocularly,two semi-circles are seen. The knob of the tonometer is adjusted so that the inner margins of both the semi-circles meet and start pulsating. This is the end point where reading is taken.

19. Perkin’s tonometer : Hand held tonometer . Based on same principle as goldmann tonometer . Advantage is that no slit lamp is required. IOP can be recorded with the patient in supine position or when under anaesthesia ,uncooperative patients and in children.

20. Perkin’s tonometer

21. pneumatonometer Measures the IOP by flattening the cornea with graded flow of gas against a flexible diaphragm. Principle is similar to that of mackey - marg tonometer ,but the sensor is air pressure. Useful for assessing the IOP in : a) scarred cornea . b ) edematous cornea . c) assessment of IOP over soft contact lens.

22. Mackey- marg tonometer It is an electronic applanation tonometer . It functions by applanating the cornea with a probe which has a 1.5mm fused quartz plunger that records the IOP. Recording is through an attached stylus that documents the pressure curve. In this method, the applanation force is sensed electronically through a sensor attached to the central cylinder As observation of the mires on the patient’s cornea is not a prerequisite for the assessment of IOP, it can be used in scarred and irrregular cornea. the major advantage is its portability .

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24. Air puff tonometer Puff of compressed air is blown through a nozzle towards the patient cornea. invented by Bernard Grolman of Reichert, Inc (formerly American Optical ). IOP is measured based on the physical relationship of the flattening of cornea of a required measure to the amount of compressed air blown through the nozzle as per a predetermined pressure-time charecteristic curve. The moment of flattening is recorded optoelectronically and converted into an estimate of IOP by a computer in the machine This is the physical basis of non contact tonometer .

25. Dynamic contour tonometer It is a digital tonometer . 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 . 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 and other biomechanical properties of the cornea . B ecause the tip shape is designed for the shape of a normal cornea, it is more influenced by corneal curvature .

26. . The probe is placed on the pre-corneal tear film on the central cornea and the integrated peizoresistive pressure sensor automatically begins to acquire data, measuring IOP 100 times per second . A complete measurement cycle requires about 8 seconds of contact time . The device also measures the variation in pressure that occurs with the cardiac cycle Sensitive enough to detect the ocular pulse amplitude(OPA)due to patient’s heartbeat . Provides direct trans corneal measurement of IOP . Eliminates the systematic errors inherent in all previous tonometers , such as the influence of corneal thickness and rigidity.

27. The PASCAL dynamic contour tonometer

28. Rebound tonometer Rebound tonometers determine intraocular pressure by bouncing a small plastic tipped metal probe against the cornea . The device uses an induction coil to magnetise the probe and fire it against the cornea . As the probe bounces against the cornea and back into the device, it creates an induction current from which the intraocular pressure is calculated. Simple ,portable & cheap device. Of use in children, uncooperative patients.

29. Ocular response analyzer This tonometer utilizes the principle of air-puff tonometery . It uses an air pulse to deform the cornea into a slight concavity. provides additional information on the biomechanical properties of the cornea. It corrects the effect of corneal hysteresis on the IOP measurment .

30. Non-corneal transpalpebral tonometer Measures the IOP through the eyelid , overlying the sclera. The response of free falling rod,rebounding against the tarsal plate, gives the measure of IOP . The patient is positioned so that the tip of the device and lid are overlying sclera. Non-corneal and transpalpebral tonometry does not involve contact with the cornea . D oes not require topical anesthetic during routine use This is of use in children & uncooperative patients.

31. Diaton transpalpebral tonometer

32. palpation Palpation (also known as digital tonometry ) is the method of estimating intraocular pressure by gently pressing the index finger against the cornea of a closed eye . This method is unreliable

33.    Ocular/ Periocular Anomalies Lid, muscle, orbit malformation, infiltration, or congestion. Corneal anomalies : thickness, scarring, edema. Absence of “aqueous free space” behind cornea . Abnormal scleral rigidity (indentation tonometry ) Sources of error in tonometry

34. . Patient-induced Lid squeezing Breathholding , constrictive clothing Eye/head movement Unsuspected/unreported drug effects (usually → lower IOP) (e.g., recent ethanol ingestion, marijuana, systemic beta blockers) Recent exercise .

35. . Instrument Error Poor maintenance, cleaning Out of calibration    Operation Error Failure to consider/observe any of the above Applying pressure to the lids Using inappropriate fluorescein concentration Failure to establish steady state through patient observation, repeat measurement Failure to record time of day.

36. T onography 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 Schiøtz tonometer yielded estimates of aqueous outflow and rate of aqueous flow . The principle of the test may be traced to the massage effect , whereby pressure on the eye leads to a softening of the globe due to an increased outflow of aqueous humor induced by the higher pressure. Grant recorded the output of an electronic tonometer on a strip-chart recorder .

37. 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 coefficient of aqueous outflow facility is calculated from Grant's formula: [ episcleral venous pressure rises an average of 1.25 mmHg during Schiøtz tonometry ; therefore, the formula is usually corrected by adding 1.25 to P . ] C = Δ VT ( P tav - P ) Δ V =change in ocular volume T=time interval P tav = average pressure during tonography . .

38. . The output of the electronic tonometer is traced on a strip chart . A good test shows a gentle downward trend in the scale reading, with fine oscillations of the ocular pulse superimposed on the tracing. Scale readings at 0 and 4 minutes are read from a smooth pencil line, which is drawn through the tracing to make a good visual approximation of the average slope . When the tracing is a good one, this average slope is easy to recognize and draw . If the tracing is of poor quality, the approximation is difficult to draw and should be a signal to the examiner that the record is probably not reliable.

39. sources for error in the calculation of outflow facility The calculation assumes that the pressure change that results from placing the tonometer on the eye does not induce a change in the rate of production of aqueous humor or in the resistance of the outflow channels . There is evidence that increased IOP results in some decrease in aqueous formation . In standard tonography , this effect is indistinguishable from true outflow facility and has therefore been called “ pseudofacility . Also increased IOP causes some increase in the resistance to aqueous outflow.

40. The units for the outflow facility are given as μL /minute/mmHg . A tonogram from a patient with glaucoma C = Δ VT ( P tav - P .

41. Electronic schiotz tonographer

42. THANK YOU .