Optician Online - CPD Archive

At present, under GOC and GOS requirements, optometrists would be expected, where appropriate, to carry out assessment of the intraocular pressure (IOP) of patients during an eye examination. IOP is still currently used as a primary diagnostic marker for glaucoma and, of course, defines ocular hypertension. There have been numerous papers and studies on the relevance of IOP in glaucoma diagnosis and monitoring as well as its treatment. This article is an introduction to the development of IOP measurement techniques and will be followed by further articles later in the year looking at the interpretation of the results and any management that may be required.

IOP and eye disease

Before looking further at the technique of tonometry, it may be of value to remind readers of the role of IOP with relation to glaucoma and other eye disorders. Glaucoma describes a group of diseases characterised by the degeneration of retinal nerve fibres causing sight loss. The degeneration may begin with the death of retinal nerve fibre axons which result in the release of trigger substances which lead to apoptosis (pre-programmed death) of surrounding cells. Genetic profile is a major predictor of this process, but elevated IOP is likely to make the degenerative process more likely. There is also evidence that elevated IOP may cause damage to microvascular circulation leading to neuronal damage, and yet more that implies the raised IOP may directly cause mechanical damage to tissues. Whatever the inter-relationship of these underlying theoretical explanations for the optic neuropathy, it is quite clear the IOP plays a major role and, importantly, currently is the only influence over which control may be exerted in management strategies. Figure 1 shows the relationship between primary open angle glaucoma and IOP.

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Many patients inherit a form of glaucoma with progressive nerve damage despite having IOP within the expected range. This normal tension glaucoma is challenging to treat, yet management still involves IOP adjustment.

Elevated IOP may be the cause of retinal venous shut down and should always be measured when confronted with a retinal vein occlusion. Furthermore, patients who present with signs or symptoms raising suspicions of retinal detachment or damage need IOP assessment. This is not just as a precautionary pre-dilation measure, but also to see if there is a recorded drop in IOP which can suggest retinal tearing.

Origins and development

As far back as the 10th century, Al-Tabari in Arabia reported ‘hard or marble-like’ eyes in some people who had gone blind. Over subsequent centuries, people became increasingly aware of cases of blindness that seemed to have been caused by hardening of the eyes. The term ‘glaucosis’ was originally used to describe a range of conditions causing blindness, without any real understanding of the underlying pathological processes and including many varied signs and symptoms, such as discolouration of the pupil. This is most likely a reference to cataract.

In 1622, a physician named Richard Bannister considered it possible to measure the pressure within the eye by using touch; digital palpation. He wrote: ‘If one feele the Eye by rubbing upon the Eie-lids, that the Eye be growne more solid and hard then naturally it should be.’

For over two hundred years, this was really the only way doctors could evaluate the intraocular pressure. In 1826, Sir William Bowman emphasised that experienced physicians should be able to accurately assess the level of tension in the eye and from this even suggest possible risks to sight. He suggested that, when a patient presented with visual problems, the first thing to do would be to carry out digital palpation.

Eventually, after examination of the optic disc became possible with the development of ophthalmoscopy and increasing evidence from histological analysis of ‘glaucotic’ optic nerve heads, it became evident that raised IOP was linked to damage of the optic nerve. In 1854, Eduard Jaeger became the first to describe the optic disc appearance in glaucoma in relation to raised pressure in the eye. Ernest Pflüger hypothesised a definite link between IOP and glaucoma and this led to speculation that reducing the pressure might lead to a cure. From this speculation and the subsequent development of therapies, physicians began to look for more accurate, reliable and repeatable methods of assessing IOP. This led to numerous efforts to develop mechanical devices to perform IOP measurement in a more controlled and repeatable way than was possible using simple palpation.

It was von Graefe (in 1862) who first considered a mechanical approach to measuring IOP. He never actually produced a prototype of his theoretical indentation tonometer, but his idea certainly spawned numerous devices constructed by later eminent physicians.

In 1865, Donders took von Graefe’s ideas and developed an indentation tonometer to measure the IOP. It relied on applying force to the sclera which would displace internal ‘ocular fluids’. A similar approach was invented in 1884 by Priestley Smith. In both cases, the radius of curvature of the sclera needed to be measured first. It was evident that measuring the radius of curvature of the cornea was easier due to its transparency and its more regular curvature than the sclera. Also, the cornea is more easily accessible than the sclera and it is far more likely that a similar point of indentation will be used again and again on all subjects. The problem, of course, is that the cornea is extremely sensitive to touch. Beyond the discovery of cocaine in 1884 and subsequently other topical anaesthetics, it was realised that corneal tonometry devices could be used.

Between 1897 and 1905, Schiøtz developed his tonometer, eventually incorporating a corneal ‘plunger’ into the device. By applying downward force via the weight of the plunger, a degree of indentation would be measurable. The amount of indentation was directly related to the IOP; the higher the IOP, the higher the amount of corneal resistance and the lesser the amount of indentation. A special calibration scale was produced (by Friedenwald) that took into consideration the fact that applying the plunger itself would affect the IOP. Around 1910, it became accepted as the new ‘gold standard’ for tonometry as, up until then, digital palpation was still considered the best technique! The Schiøtz device was the main measuring tool for IOP until well beyond the 1950s and even later in some countries (in fact, it was in the Keeler catalogue until the early 1980s). One issue with the Schiøtz tonometer was the prolonged period of contact with the eye and the risks of cross infection or mechanical damage.

Indentation tonometers were highly inaccurate devices as the results were so variable, primarily due to varying amounts of pressure displacing similar amounts of ocular ‘fluids’.

Weber (1867) and Maklakov (1885) produced the first applanation tonometers. These used a more precise form of indentation, where a specific area of cornea was flattened, by a known amount. For some time, such devices were not considered as accurate. Most tonometers used weights to produce the indentation or applanation. It was later versions of such devices that used Laplace’s pressure measurement unit of mmHg (millimetres mercury) as the accepted unit for IOP.

Goldmann applanation tonometry

In 1954, Professor Hans Goldmann invented an applanation tonometer with a Plexiglas probe that could be pressed directly on to the cornea (Figure 3). Using a coiled spring and lever system, it was possible to apply a controlled and measurable force to flatten the cornea. As the area applanated was small, ocular scleral rebound and globe rigidity and the counteractive influence of the tear surface tension did not significantly affect the IOP results. The underlying principle of the Goldmann applanation tonometer (GAT) took a lot of inspiration from the 1892 Maklakov updated design.

Goldmann used a variation of the Imbert Fick Law. This law is based on an assumed sphere of fluid confined by an infinitely thin membrane (like a ball of water in zero gravity) and relates the force applied to deform the sphere to the pressure of the fluid within as follows;

Po = (F/C) + Pv

This is where;

Po = IOP (mmHg) F = rate of aqueous formation (µl/min)

C = facility of outflow (µl/min/mmHg) Pv = episceral venous pressure (mmHg)

C can be replaced with R, where R is resistance to outflow as an inverse of C

Since the development of the Goldmann device, it has become accepted as the current ‘gold standard’ for tonometry. Many people nowadays consider it may be time to change this view to something more reliable and repeatable less subjective, and that takes into account corneal thickness and its viscoelastic properties.

The Plexiglas probe (Figure 2, left) is still used in many hospital eye departments, where sterilisation, sometimes with an autoclave, is used. Since the issues of bovine encephalopathy and subsequent new human variant Creutzfeldt-Jakob disease (vCJD) became more prevalent in the late 1980s and early 1990s, disposable Goldmann probes (prisms) have been introduced (Figure 2 right) and are considered essential in optometric practice under NICE guidelines. There is no evidence to suggest that use of these disposable prisms reduces the accuracy of the device, though cheap copies are prone to greater variation in weight and quality and may produce poorer repeatability and reliability of results.

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It is estimated that only 83% of GATs sold undergo any calibration and this is often 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 even suggest doing this daily!

Since the introduction of the Goldmann, it has changed little. There is now a digital version of the device from Keeler (Figure 4), which records IOP to an accuracy of 0.1 mmHg. Calibration is needed less often. One major advantage is that, in the dark, the practitioner can still read off the IOP readings from the digital display. Also, as there are fewer mechanical parts so is less prone to damage.

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As with all applanation tonometers, the Goldmann relies on an adaptation of the Imbert Fick law. In applanation tonometry, the calculation uses a derivative of this law, with an assumption of a specific corneal thickness. With the Goldmann, this is 520µ. which is thinner than the average cornea.

Handheld tonometers

The Perkins tonometer (Figure 5) uses the same principle as the Goldmann, but is handheld and portable. It has been shown to correlate quite well with the Goldmann, but does tend to measure slightly higher. It requires greater skill to use, but is ideal for disabled patients and domiciliary practice, especially for assessing supine patients.

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More recent contact tonometers include the Tonopen and Tonopen XL (Figure 6), both of which use 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 (Figure 7). 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.

The Diaton tonometer (Figure 8) was designed as a contact tonometer which avoids the need to touch the cornea at all. It is used by applying force to the closed eyelid and measuring resistance on a gauge linked to tiny internal sensors. The problem with this device is that it is difficult to know if the device is over the cornea through the closed lids and the variability in corneal properties are exacerbated by variations in thickness and rigidity of the tarsal plate.

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Non-contact tonometry

In 1971, Grolman developed, along with the company American Optical, the idea of non contact tonometry using air to applanate the cornea. This pneumotonometry was a novel approach, disposing of the need to anaesthetise the cornea and the use of fluorescein. Although the first device, the AO Non Contact Tonometer (AO NCT), gave repeatable results, it didn’t compare as closely with the Goldmann as more modern non-contact equivalents, especially at the upper limits of normal IOP. Often, non-contact tonometry (NCT) results were higher than Goldmann results by a fairly predictable and repeatable amount.

The device still relied on producing applanation, but it did so by using a puff of air. The release of air was highly controlled, increasing linearly from 0 to 60mmHg in a matter of milliseconds. At the point where applanation occurs, a beam of red light, incident at 45°, is reflected from the cornea acting as a plane mirror, with an angle of reflection of 45°. A sensor detects the fullest intensity of the reflected beam at exactly the instant (the speed of light for such a tiny distance may as well be considered that instant) the cornea is applanated (Figure 9). The device can then correlate the pressure it was producing on the linear gradient with the point where applanation occurred to calculate the IOP.

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This is essentially how non-contact tonometers have worked ever since. The AO NCT was massively successful as it changed the whole practice of tonometry in ophthalmology and optometry. Patients, even to this day, do find the ‘air puff’ can make them jump. It turns out this wasn’t just the reflex reaction to the air puff hitting the cornea and lashes/lids, but also the loud noise the actuator produced when releasing the air puff.

In the early 1980s, Keeler developed its version of a pneumo tonometer, the Pulsair. Since then it has been redesigned numerous times and renamed the EasyEye and the Intellipuff (Figure 10). This hugely successful device has become smaller and smaller over the years. Since its inception, the device’s key selling point was its hand held tonometer head, which allowed easy tonometry upon wheelchair bound patients or simpler access to patients sat in the test chair. It provided and still provides accurate, fast and repeatable tonometry, but with a less aggressive and much quieter air puff. Keeler is still selling this device in large quantities and regularly provides updates to the design of the device.

Since the Pulsair, throughout the 1990s and into the 21st century, a large number of manufacturers have developed non-contact tonometers. The main development since the Pulsair was the incorporation of pneumotonometers into autorefractors and the development of auto tonometry in such devices, along with greater accuracy and repeatability in comparison with the Goldmann. Multi-functional devices have developed over the last two decades to include auto tonometry, auto refraction, auto keratometry and more recently also auto pachymetry.

As mentioned earlier, for many years, applanation tonometry (contact and non contact) relied on an assumed corneal centre thickness (CCT), ranging roughly from 520µ to 560µ, depending on the device. This fairly notable variation in assumed CCT means that one device might produce an IOP just below ‘normal’ limits and another device could read just above the ‘normal’. Without knowing the CCT, many incidents of false positive referrals have taken place. More worryingly, many patients will have been considered to have normal IOP’s (false negatives) when, in fact, they would have benefited, according to the guidelines at least, from being referred for raised IOP / risk of glaucoma.

Some of the notable multi use devices include the Topcon TRK-2P, the Nidek Tonoref II and Tonoref III, the Visionix VX 120 and others. There have been numerous auto-tonometers developed by the likes of Nidek, Oculus, Topcon, Rodenstock, Huvitz, Canon and many, many more (Figure 11).

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The Topcon TRK-2P (Topcon UK) was the first 4-in-1 device, which also combined auto pachymetry with auto refraction, auto keratometry and auto tonometry. This allowed for calculation of the ‘corrected IOP’, where the knowledge of the actual CCT is used to compensate the measured IOP from the assumed CCT used by the device.

The Nidek Tonoref III (Birmingham Optical Group) is a new version of the Tonoref II, which now includes the auto pachymetry measurement. Again, highly repeatable tonometry results are made more useful with auto correction of IOP based on the patients actual CCT.

The Visionix VX 120 (Grafton Optical) is an intriguing 7-in-1 device, which not only performs auto refraction, auto keratometry, auto tonometry and auto pachymetry, but also carries out topography, anterior chamber assessment and cataract assessment too.

The rapid development of technology and computing has allowed for this rapid improvement of tonometers over the last thirty years. As with other areas of medicine, the last few decades have seen meteoric advances in knowledge and technology. This continues at an increasing pace today.

Apart from contact and non contact applanation tonometers developed over the last 60 years, there are a few devices that hold a novel place in the field of tonometry.

Rebound Tonometers

The iCare rebound tonometer is a handheld, lightweight device (Figure 12) which uses a tiny probe that is propelled gently but quickly towards the patient’s cornea from a roughly fixed distance. The force of projection is fixed and the force and velocity of the rebound are measured by the device, allowing for a calculation of the IOP to be made.

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Studies have shown a good correlation of reliability and repeatability with the Goldmann within the normal range of IOPs. This device is excellent for patients who cannot sit at a slit lamp or desk mounted auto tonometer device. It is also excellent for domiciliary purposes. From personal experience, when examining a patient with dementia who is nervous, pneumo tonometry is not a good idea! The gentle touch of the probe usually produces no response, even in such challenging patients and there is no need for fluorescein or anaesthesia. The probes are disposable. Measuring supine patients was near impossible with the original iCare as the probe would fall out when the device was tilted downwards. The iCare Pro holds the probe in place and avoids this problem. The new iCare ic200 has now been launched (Figure 13).

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To allow phasing (measurement of IOP regularly over a day or more to reflect diurnal variation), a version of the tonometer has been developed to allow self-monitoring by the patient at home (Figure 14).

Dynamic Contour Tonometry

Another interesting device is the Pascal Dynamic Contour Tonometer (Figure 15), which uses a 10.5mm probe which matches the contour of the cornea. Once contact has been established, no forward force is applied and, instead, a directly applied sensor takes around eight seconds to detect fluctuations in IOP which are independent of CCT and corneal rigidity. The device takes 100 measurements of IOP per second and shows the fluctuation of pressure with the pulse.

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Cornea Compensated Tonometry

The latest developments in the accuracy and value of tonometry have come from the full analysis of the corneal response to an air puff. Although knowing the CCT for the patient is valuable, the Imbert Fick law still doesn’t adequately relate to the reality of a complex multi-layered surface like the cornea. The cornea has numerous qualities that can affect the way it responds to the ‘air puff’ and thus how much it deflects and how quickly it flattens. Such corneal properties include the elasticity and viscosity, which along with other factors, control the overall hysteresis of the cornea.

For example, consider two patients with the same IOP’s. One has oedematous corneas, which are extremely thick, but are also very ‘floppy’. If compared with another patient, whose corneas are the same thickness, but not through oedema, then the corneal response will differ massively. However, just using the first point of applanation and the knowledge of the CCT to calculate the IOP, there will be a wildly different result for IOP for the two patients.

Likewise, two patients with the same IOP may have very thin corneae, but one is much older than the other. The young cornea will have far less natural cross linking of the collagen fibrils than the older cornea. The older patient will have naturally undergone collagen cross linking (CXL) via much more UV exposure and natural corneal morphology changes over the years.

Reichert have produced three versions of their Corneal Response Analyser and or Ocular Response Analyser (ORA). These devices take a detailed measure of the corneal hysteresis and produce extremely accurate, repeatable and reliable IOP results (Figures 16 and 17)

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Scheimpflug Adapted Tonometers

The Oculus Corvis ST (Figure 18) is a novel and interesting device which utilises a Scheimpflug imaging system and a high speed camera (for a video of the cornea deformation with this instrument see the online version of this article).

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The device uses pneumotonometry to assess the total corneal response. Not only are both applanation points measured (ie depressing and recovering) but also the total concavity of the corneal indentation and the speed and ‘wobble’ of the corneal response. All of these factors together produce a highly accurate, reliable, precise and repeatable IOP measure (Figure 19). However, the use of this technology also allows for assessment of keratoconus, risk of ectasia and the effect of CXL on such patients. Comparing pre- and post-CXL corneal responses can lead to real understanding of the effectiveness of such CXL treatments.

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One other useful aspect of this device is that it can e-mail patients and clinicians with analyses of their results and provide information and teaching that can be viewed on tablet devices like an iPad (Figure 20).

Final Thoughts

I think the development of tonometry since the Goldmann of 1954 has been so considerable that now is the time to define a new ‘gold standard’ method and even device for tonometry. Considering the number of variables involved, a device which measures hysteresis surely must be the next ‘gold standard’ and I would propose that the Oculus Corvis ST is such a device.

With the development of OCT, knowledge of the processes taking place in glaucoma has been enhanced massively. Visual field screeners will only detect perimetric glaucoma, which will usually mean around 50% of the neural tissue in or around the optic nerve head (ONH) will already be lost. OCTs have helped show a much larger prevalence of normal and low tension glaucoma than previously thought.

Ultimately, IOP is still a highly important and essential measurement to take, despite the changing understanding of its link to glaucoma and other ocular conditions. Most studies still show that IOP is the most important risk factor in patients developing glaucoma and also that reducing the IOP is the only effective way of trying to treat this condition. Some studies also show that reducing an already ‘normal’ IOP in glaucoma patients is still beneficial in at least slowing the progression of the disease.

Jason Higginbotham is an optometrist and head of clinical affairs at Birmingham Optical group

C52054: The changing face of tonometry