OT 2016 The Changing Face of Tonometry
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Tonometry, or the measurement of intraocular pressure (IOP), has evolved significantly over time. The Goldmann applanation tonometer, introduced in the 1950s, was long considered the gold standard but does not account for factors like corneal thickness and stiffness. Newer devices use air puffs, indentation, or advanced cameras to record the cornea's response and provide a more accurate IOP reading. Specifically, the Corvis ST uses a high-speed camera to measure hysteresis and account for biomechanical properties, allowing for a truly personalized IOP assessment. As technology advances, tonometry continues to improve our understanding and treatment of glaucoma.
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OT 2016 The Changing Face of Tonometry
- 1. Optometry Tomorrow 2016 The Changing Face of Tonometry Jason Higginbotham BSc (Hons) MCOptom FBDO – Ophthalmic Clinical Advisor Birmingham Optical
- 2. The History of Tonometry • Visual loss related to a ‘hard or marble like’ eye was first reported as far back as the 10th Century in Arabia (Al-Tabari). • In 1622, Bannister first suggested ‘Digital Palpation’ as a technique for checking the ‘pressure of the eyeball’. • In 1862, Von Graefe invented the first indentation Tonometer.
- 3. The History of Tonometry • Ernst Pflüger theorised the link between Intra Ocular Pressure (IOP) and Glaucoma and associated changes to the Optic Disc. • Schiøtz developed the impression tonometer in 1897 for the Sclera before updating this to a Corneal plunger device – still in the Keeler catalogue in the early 1980’s. • Weber (1867) and Maklakov (1885) produced first Applanation Tonometers.
- 4. The History of Tonometry • In 1954, Professor Goldmann invented an applanation tonometer with a plexiglass probe that made contact with the eye. Using a coiled spring and lever system, it was very reliable and accurate. As the area applanated was small, ocular scleral rebound and globe rigidity did not affect the IOP results. • The Goldmann tonometer is still considered the GOLD STANDARD in tonometry to this day. Should this really be the case?
- 6. Draeger Hand Applanation Tonometer Goldmann and Perkins Tonometers
- 7. The History of Tonometry • In 1972, AO introduced the Non Contact Tonometer (NCT). It used a pneumatic ‘air puff’ to applanate the tonometer. Much easier to use, it allowed for quicker IOP screening on all patients. The accuracy and repeatability were not as good as the Goldmann. • In 1986, Keeler introduced the Pulsair. This revolutionised quick IOP screening as it was less noisy or uncomfortable than earlier NCT’s.
- 8. Keeler Pulsair / Intellipuff NCT American Optical NCT Diaton tonometer
- 9. Reichert PT 100 Reichert Tonopen and Tonopen XL Reichert 870
- 10. The History of Tonometry • Current devices most commonly found in everyday practice tend to be combined pneumo tonometers and auto refractors / auto keratometers. • Most recent devices also analyse corneal pachymetry and in some cases, also evaluate corneal hysteresis or bio-mechanical properties. There is also a contact tonometer which evaluates ocular pulse amplitude for short term IOP fluctuations (Pascal).
- 11. Nidek Tonoref III Oculus Corvis ST Reichert Corneal Response Analyser Zeimer Pascal DCT
- 12. Tonometry – relevance? • Most studies show IOP (intraocular pressure) is the biggest risk factor in developing glaucoma • More precise knowledge of how IOP causes mechanical damage or vascular damage or a combination of both now exists • Apoptosis of Ganglion cells and the reduction in axonal transport (RNFL) is the proven cause of actual loss of Ganglion cells and their axons in the RNFL; ONH blood perfusion?
- 13. Tonometry – relevance? • There are numerous papers which show that reducing IOP in normal tension glaucoma is still effective for many patients. • The exact link between IOP and Glaucoma is still not fully understood. Ocular blood flow and even the physical architecture of the Lamina Cribrosa have been shown to have some influence on Glaucoma prevalence.
- 14. Tonometry – Gold Standard • The current recognised Gold Standard tonometer is the Goldmann Applanation Tonometer (GAT). • The GAT is a mechanical device introduced in 1956 and relies on the Imbert Fick law, a 3.06mm diameter probe making contact with the cornea and the assumption that the cornea is 520 microns thick. • There are many ways the measured IOP can be incorrect with GAT.
- 17. • Poor alignment of the Mires is the most common cause of incorrect readings and taking too long. Too much Fluorescein Too little Fluorescein Main Errors with Goldmann
- 18. Dial pressure too low, increase pressure on dial so mires just overlap. Dial pressure too high, reduce pressure on dial so mires just overlap. Probe too high, move the Probe. Vice Versa is lower mire is biggest. Main Errors with Goldmann
- 19. Every GAT should be supplied with a calibration rod. Essentially, this is the same as the weights that are used with the Perkins. There are three settings to calibrate against, 0 mmHg, 20 mmHg and 60 mmHg. Goldmann Calibration
- 20. Insert the calibration bar, with the zero notch in the centre first, as shown here. With the dial set below zero, slowly turn the dial until the probe arm just tilts forwards. The scale should read zero if the GAT is properly calibrated for 0 mmHg. If not, it will need sending for recalibration. Repeat this for 20 and 60 mmHg.
- 21. Rebound Tonometry The iCare Tonometer This is particularly useful in domiciliary optometry or where patients refuse anaesthetic for Goldmann.
- 22. Rebound Tonometry The home use version has been shown to be very effective for self checking of IOP and assessment of long term diurnal variation
- 23. Electronic Indentation Tonometry The Tonopen range of tonometers use a combination of applanation and indentation to measure the IOP.
- 24. Tonometry and Pachymetry • Corneal Centre Thickness (CCT) is assumed to be 520 microns with the Goldmann tonometer. Most other tonometers assume either 545 or 550 microns for the CCT. • If the CCT is higher than that assumed by the tonometer, then the device will OVER ESTIMATE the patients IOP. • If the CCT is lower than that assumed by the tonometer, then the device will UNDER ESTIMATE the patients IOP.
- 25. Is Pachymetry enough? • CCT certainly helps us to calculate a more accurate IOP reading. • Remember to consider the patients pulse; take averages. NICE suggests 4 readings per eye with pneumo tonometers. • Also, diurnal variation and even longer term variations have been shown to exist. Phasing of IOP readings might be worthwhile.
- 26. What is Corneal Hysteresis? • The corneal hysteresis is the amount of energy that is absorbed by the system. • Therefore there is a difference in the loading and unloading process. • The area between the loading and the unloading curve is the exact measure of corneal hysteresis. • A perfect elastic material would have no hysteresis
- 27. The Cornea is a Viscoelastic material! • The cornea has an elastic and a viscosic component. • The collagen fibres of the stroma provide the highest contribution for the stiffness of the cornea. • The collagen fibres are embedded in a gel-like substance (proteoglykanes) that cause friction.
- 28. Providing REAL IOP readings Reichert Ocular Response Analyser Oculus Corvis ST Biomechanical Tonometer
- 29. Oculus Corvis 4300 frames per second Scheimpflug camera image records the corneal reactions from a 60mmHg pneumo applanation
- 30. Oculus Corvis Contact Lens Experiment TAN 40 at 0mm Hg adjusted chamber pressure Contact Lens Experiment TAN 40 at 20mmHg adjusted chamber pressure Contact Lens Experiment TAN 40 at 60mmHg adjusted chamber pressure Deformation amplitude significantaly decreases when pressure increases
- 31. Deformation amplitude at 20 mmHg significantely decreases with higher stiffness of material Oculus Corvis
- 32. Beyond Tonometry Conclusion: Corvis ST with best inter- and intra- observer reproducibility compared to NCT (Topcon) and GAT Author: Jiaxu Hong1, Jianjiang Xu1, Anji Wei1, Sophie X. Deng2, Xinhan Cui1, Xiaobo Yu1,
- 33. 0.536 How big is the influence of corneal thickness on IOP-measurements? • A: up to 2 mmHg • B: up to 5 mmHg • C: up to 7 mmHg • D: >10 mmHg Answer: Up to 2 mmHg Pot. error moderate Beyond Tonometry Liu and Roberts, JCRS, January 2005 • The thicker the cornea, the higher the measured pressure • The thinner the cornea, the lower the measured pressure IOP-Error
- 34. How big is the influence of stiffness on IOP-measure- ments? • A: up to 2 mmHg • B: up to 5 mmHg • C: up to 7 mmHg • D: > 10 mmHg Antwort: > 10 mmHg Pot. Error Huge Beyond TonometryIOP-Error • The “stiffer” the cornea, the greater the measured pressure • The “softer” the cornea, the lower the measured pressure
- 35. Corneal Elasticity and Corneal Thickness The effect of CCT depends on Young’s modulus of elasticity Difference = 8.68mmHg Difference = 2.84mmHg Liu and Roberts, JCRS, January 2005
- 36. Difference = 8.68mmHg Older Patient = Stiffer cornea Higher influence of conneal thickness on IOP Difference = 2.84mmHg Young Patient = Weaker cornea Smaller influence of corneal thickness on IOP Takes into account age + corneal thickness IOPcorr.= IOPmeas + constant*age*(540-CCT) Sporl Correction Table
- 37. Data provided by Renato Ambrosio, MD, PhD Normal eye versus Keratonic Eye
- 38. greater stability in post-op “blue” image than in pre-op “red” image Data provided by Renato Ambrosio, MD, PhD Red: Keratoconus cornea Blue: same cornea after CXL Assessing Collagen Cross Linking (CXL)
- 39. Hong J, Xu J, Wei A, Deng SX, Cui X, Yu X, Sun X.: A new tonometer--the Corvis ST tonometer: clinical comparison with noncontact and Goldmann applanation tonometers. Invest Ophthalmol Vis Sci. 2013 54(1):659-665 Reznicek L, Muth D, Kampik A, Neubauer AS, Hirneiss C.: Evaluation of a novel Scheimpflug-based non-contact tonometer in healthy subjects and patients with ocular hypertension and glaucoma. Br J Ophthalmol. 2013 Leung CK, Ye C, Weinreb RN. An ultra-high-speed Scheimpflug camera for evaluation of corneal deformation response and its impact on IOP measurement. Invest Ophthalmol Vis Sci. 2013 54(4): 2885-2892 Kling S, Marcos S. Contributing factors to corneal deformation in air puff measurements. Invest Ophthalmol Vis Sci. 2013 54(7):5078-5085 Faria-Correia F, Ramos I, Valbon B, Luz A, Roberts CJ, Ambrósio R Jr.: Scheimpflug-based tomography and biomechanical assessment in pressure induced stromal keratopathy. J Refract Surg. 2013 29(5):356-3588 Ambrósio R Jr, Valbon BF, Faria-Correia F, Ramos I, Luz A.: Scheimpflug imaging for laser refractive surgery. Curr Opin Ophthalmol. 2013 Jul;24(4):310-20 References
- 40. College of Optometrists Tonometers Peter C. Kronfeld.: The History of Glaucoma Glaucoma – Volume 3, Chapter 41 Michael Tsatsos, David Broadway Controversies in the history of glaucoma: is it all a load of Greek? Br J Ophthalmol. 2007 Nov; 91(11): 1561 - 1562 Ivan Marjanovic The history of detecting glaucomatous changes in the optic disc http://dx.doi.org/10.5772/52470 Kniestedt C, Nee M, Stamper RL Dynamic contour tonometry: a comparative study on human cadaver eyes. Arch Ophthalmol. 2004 Sep: 122(9): 1287-93 Aghaian E, Choe JE, Lin S, Stamper RL Central corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African Americans and Japanese in a Glaucoma clinic. Ophthalmology. 2004 Dec; 111(12):2211-2219 References
Editor's Notes
- At present, under GOC and GOS requirements, Optometrists have to carry out assessment of the Intra Ocular Pressure (IOP) of patients during an eye examination. IOP is still currently used as a diagnostic marker for Glaucoma and of course Ocular Hypertension. There have been numerous papers and studies on the relevance of IOP in Glaucoma diagnosis and monitoring as well as its treatment. Instead, how IOP is measured and how accurate and reliable those measurements can be is the main topic of this presentation.
- Over the centuries, people have been aware of blindness that seems to have been caused by hard / glassy eyes. The term Glaucosis was originally used to describe a range of conditions causing blindness, without any real understanding of mechanisms. As far back as 800BC, people have referred to the ‘juices of the eye’ causing the pupils to turn ‘grey’. This is most likely a reference to cataracts, but several physicians indepentently also noted that the use of deadly nightshade (Atropine) would cause the grey pupils to turn black. In 1854, Eduard Jaeger first described the optic disc appearance in Glaucoma in relation to some form of raised pressure in the eye. Von Graefe also mentioned noticing the venous pulsation of the glaucomatous eye.
- Indentation and early applanation devices were invented in the late 19th century. This, in particular, was because of the development of topical local anaesthesia. This allowed contact with the patients eye to be made for prolonged periods, though initial problems with sterility did occur.
- The Goldmann tonometer is a very reliable device which is also fairly easy to use. The recent issues with new variant CJD have led to the increased use of disposable tonometer prisms. There appears to be no evidence of a drop in accuracy with such disposable prisms. Glaucoma shared care requires that Goldmann tonometry is the method used to monitor patients.
- Pneumotonometers first appeared in the mid 1960s. Generally, they used a continuous flow of gas which applanated the Cornea continuously and the applied pressure altered until the live IOP reading could be taken. This took too long and caused discomfort and drying. Later devices used more rapid measuring systems, primarily with the development of faster and smaller transistors and micro-processors.
- Since the late 1980’s, auto tonometers have developed into highly reliable and versatile products, many versions adding auto refraction and auto keratometry functionality as well. Also, ‘through the lid’ tonometry was introduced in the late 1990’s; the Diaton promised excellent results without discomfort, but the added range of biomechanical properties introduced by the eye lid and tarsal plate simply makes results less reliable and adds more variables to take into account.
- Here we have examples of three varieties of device. The Tonopen uses a slightly different method of measuring the intra ocular pressure (see later).
- These more modern devices add more than just standard tonometry (see later).
- Intra ocular pressure was considered a primary diagnostic factor in glaucoma. However, the definition of glaucoma is an optic neuropathy presenting progressive loss of neural tissue leaving the eye at the optic nerve head. There is no mention in this definition of intra ocular pressure. More and more cases of ocular hypertension without associated field loss OR low / normal tension glaucoma without raised intra ocular pressure have now been documented which cast doubt on IOP as the primary cause. Where IOP is a cause, there are two main likely causes of cell death; Direct mechanical damage and indirect ischaemia to neural tissue. This can lead to vascular damage, closing off of axonal transport in the RNFL and the release of glutamate in the vitreous thought to cause excitotoxicity to the ganglion cells. Programmed cell death results. The cause of this relies on two theories; Selective Damage Theory and Reduced Redundancy Theory.
- The Bi Prism of the Goldmann allows the user to know when the probe is lined up correctly and when applanation has been achieved.
- Adding too much Fluorescein can make the mires too thick. If this occurs, ask the patient to blink rapidly for a half a minute. You could use the edge of an absorbent tissue in the lower fornix whilst the patient looks up to remove excess Fluorescein, but avoid abrasion or adding lint to the tear film. If the mires are too thin, simply add a little more Fluorescein, either with a strip or adding more of the drop.
- Other errors occur when the patient has not been settled and their IOP is raised due to them being nervous and apprehensive. Also, remember to take averages due to the pulse and remember diurnal variation too.
- It is estimated only 83% of GAT’s sold undergo calibration and often this is sporadic and not routine. Regular calibration is essential and a log of each check (the time, date and who did it) should be kept. Every three months is a fair period to repeat this, but some suggest even doing this daily!
- Some papers do suggest over estimation of the IOP with the iCare compared to other portable tonometers where as other studies show good correlation with the Goldmann and the iCare. Rebound technology is based on a rebound measuring principle, in which a very light-weight probe is used to make momentary contact with the cornea. In rebound technology, the motion parameters of the probe are recorded during the measurement. Induction based coil system is used for measuring the motion of the probe. Advanced algorithm combined with state of the art software analyses the probe deceleration, contact time and other parameters of the probe while it touches the cornea. The deceleration and other rebound parameters of the probe change as a function of IOP. In simple terms, the higher the IOP, the faster the probe decelerates and the shorter the contact time.
- The Tono-Pen involves both applanation and indentation processes. It is a small, handheld, battery-powered device. The tonometer has an applanating surface with a tiny plunger protruding microscopically from the centre. As the tonometer -makes contact with the eye, the plunger gets resistance from the cornea and IOP producing a rising record of force by a strain gauge. At the moment of applanation, the force is shared by the foot plate and the plunger resulting in a momentary small decrease from the steadily increasing force. This is the point of applanation which is read electronically. Multiple readings are averaged. Because the area of applanation is known, the IOP can be calculated. The readings correlate well with Goldmann tonometry within normal IOP ranges
- Measuring pachymetry is clearly important in correctly assessing a patients IOP. In Scotland, this is now a requirement and recently a directive to supply every practice with a pachymeter was put into place. Soon, every practice in Scotland will receive an Accutome pachymeter from Keeler. REMEMBER post LASIK / LASEK patients.
- Now that we have examined briefly the potential pitfalls of simple applanation tonometry, the value of measuring pachymetry and the added value of understanding corneal hysteresis, we can consider devices which take all of these things into consideration and put them together. The results from such devices are shown to be far more reliable, repeatable and accurate. However, such devices can even go beyond the realms of just providing real IOP measurements.
- This shows the affect of IOP on the same thickness contact lens in a pressure chamber. Notice the deformation decreases as the IOP increases, but not linearly.
- Here, we can see the affect where a different thickness of material changes the corneal response with the same IOP. Again, this is not a linear relationship.
- The Corvis shows the best reproducibility of IOP results when compared to the ‘Gold Standard’ Goldmann Applanation Tonometer (GAT) or to one of the more accurate Non Contact Tonometers.
- There is a non linear relationship between the difference in CCT (Corneal Centre Thickness) and how much the IOP is affected.
- More worryingly, corneal stiffness (hysteresis) produces huge potential errors in the IOP measurements of standard tonometers, including GAT with pachymetry. This is why I propose the Corvis become the new Gold Standard in Optometry.
- We may therefore use this device to identify patients at risk of or suffering from keratoconus or other ectasia (Pellucid marginal degeneration for example).
- Here we can see the affect of collagen cross linking on the stiffness of the patients cornea. This shows the added value of modern biomechanical tonometers.