Factors affecting tonometry measurement.
1 CET POINT
Course code: C-35982 | Deadline: May 9, 2014
To be able to explain to the patient about the implications of tonometry results (Group 1.2.4)
To understand the importance of recording full detail with respect to intraocular pressure measurement (Group 2.2.4)
To be able to interpret the results when undertaking tonometry (Group 3.1.6)
To be able to recognise anomalies in intraocular pressure readings (Group 6.1.5)
To be able to explain to the patient about the implications of intraocular pressure readings (Group 1.2.4)
To be able to understand the implications of intraocular pressure readings (Group 3.1.6)
To be able to assess intraocular pressure using appropriate techniques (Group 2.1.2)
Several different types of tonometer are used in clinical practices throughout the UK, with the most common falling into two groups--contact (Goldmann applanation tonometry and Perkins tonometry) and non-contact tonometry (NCT). (1) The results from tonometry are influenced by the characteristics of the patient's eye, the type of instrument used, and is operator-dependent. This article explores these variables in turn.
The Imbert-Fick principle
A true measure of intraocular pressure (IOP) can be determined by manometry, a procedure where a needle is inserted into the anterior chamber of the eye; this is understandably inappropriate for routine measurement. As an alternative, the IOP can be established indirectly by measuring a force that is applied to the walls of the eyeball, usually the cornea. Goldmann applanation tonometry (GAT), the gold-standard method for the measurement of IOP, and related techniques such as NCT and TonoPen, use the Imbert-Fick principle to determine this value.
The Imbert-Fick principle states that the IOP can be estimated indirectly via measuring the force required to applanate or 'flatten' a known area of the cornea. (2) In its original form, the Imbert-Fick principle requires the applanated surface to be dry, perfectly elastic, infinitely thin and spherical, whereas the cornea meets none of these conditions. In order to compensate for the mismatch between the theoretical assumptions made by the Imbert-Fick principle and reality, GAT was calibrated by its inventors against directly measured manometric readings of the true IOP. (3) During calibration, a range of different applanation diameters were evaluated in order to determine where the GAT IOP most closely matched the true manometric IOP. When the GAT and true IOP readings were comparable, it was assumed that the force resisting applanation from the cornea was equal and opposite to the attractive capillary forces of the tear film (see Figure 1) and thus cancelled each other out. GAT was found to be most accurate between applanation values of 3 and 4mm, with the final value of 3.06mm chosen because it required only a simple multiplier of x10 to convert the scale reading (measured in g) to the IOP (measured in mmHg).
Due to the fact that the GAT was calibrated against a relatively small group of real eyes, it will only produce correct estimates of IOP in patients who share similar corneal and tear film properties with the original calibration group. Therefore, incorrect estimates of IOP will occur in patients whose tear film or cornea differs significantly from the original calibration; this effect is also present in all other tonometers that rely on the Imbert-Fick principle to estimate the IOP, including NCT and the TonoPen. (4)
Corneal effects on tonometry
Central corneal thickness (CCT)
The introduction of accurate corneal pachymetry (corneal thickness measurement), some years after GAT, revealed a much greater variability in central corneal thickness (CCT) than had been anticipated. The average CCT is approximately 530-540um, but it can vary by 60um or 70um either side of this value in healthy eyes. (5) The impact of inter-patient variability in CCT means that readings are underestimated in eyes with thin corneas and overestimated in eyes with thick corneas, with the effect amounting to approximately [+ or -] 5mmHg over the range of CCTs that are likely to be encountered in routine clinical practice.
Once it was known that CCT influenced the accuracy of IOP measurements, attempts were made to develop a method to correct for it. Although CCT compensation methods take various forms (correction tables, formulas, computer programs), the premise is the same; the CCT is measured and the IOP reading is reduced if the CCT is thicker than average or increased if the CCT is thinner than average. (6,7) Anecdotal evidence suggests that this procedure is commonplace in many clinical settings in the UK. However, any method of IOP correction that considers only CCT should be treated with a degree of caution for the following reasons:
* There is conflicting evidence that any correction factor is capable of producing an IOP that is closer to the true IOP than the original GAT measurement (8)
* Many other factors influence the accuracy of GAT; attempting to correct for CCT alone could produce a corrected IOP that is further from the true IOP.
Corneal hydration is used to describe the amount of fluid that is present within the cornea. The level of corneal hydration is linearly related to the corneal thickness; more water means a thicker cornea. (9) Where there is an extreme amount of fluid within the cornea, this is clinically evident as corneal striae, folds and haze of corneal oedema. GAT, or any other form of applanation tonometry, produces a gross underestimation of IOP when the cornea is highly oedematous due to the soft cornea being 'moulded' or 'squashed' giving the appearance of applanation, even though the underlying IOP plays no part in the measurement. (10) Therefore, GAT is an unsuitable technique for IOP measurement in highly oedematous corneas.
Corneal curvature and astigmatism
When the cornea is applanated, the volume of the eye decreases slightly, thereby causing a small increase in IOP. This increase in IOP was taken into account during the calibration process. Nevertheless, atypical corneal curvatures may produce an error in IOP measurement. As more fluid needs to be displaced from under steep corneas than flat, in comparison to the calibration cornea, steep corneas can overestimate the IOP and flat corneas can lead to an underestimation (see Figure 2). Despite this, the impact of corneal curvature on IOP measurements is usually less than 1mmHg, is constant for each patient, and can, therefore, be safely ignored. (17)
The material properties and the overall structural integrity of the cornea provide resistance that can exceed those produced by physiological variations in corneal thickness. In short, two corneas can be exactly the same thickness, but one will be more pliable than the other depending on whether they have a soft or rigid biomechanical nature. This might be akin to comparing a squash ball to a golf ball; the difference in the material properties of their outer skin makes it easier to squeeze a squash ball into a different shape.
In the average healthy population, the natural variation in corneal biomechanical behaviour is sufficient to cause an error in IOP measurement that is at least as large as the CCT-related errors, (18) if not more so. (19) The clinical dilemma is that soft and rigid corneas will appear to be identical without the use of specialised equipment to assess the biomechanical nature of the cornea. This again highlights the danger of using CCT-based correction factors that ignore the effect of corneal biomechanics; if the cornea were thick but soft, a CCT-based correction factor would result in a corrected IOP that is further from the true IOP than the GAT IOP was. There has been good progress towards the development of a multi-parameter correction factor that accounts for ocular biomechanics as well as CCT, (20) although at present it assumes that corneal rigidity increases with age in a uniform manner. Further work will be needed to establish whether this is indeed the case for all patients.
IOP measurements are usually, but not always, significantly reduced by several mmHg in eyes that have had corneal refractive procedures. (21-23) A reduction in measured IOP also occurs in keratoconus. (24) This is thought to be due, at least in part, to the decrease in CCT and the change of corneal curvature. Additionally, weakening occurs when the collagen fibrils, that would normally traverse across the cornea, are severed during corneal refractive surgery. (21) Care must be taken when interpreting an IOP measurement in eyes under these circumstances, particularly as the measurements change as the cornea heals following refractive surgery, (22) or after corneal cross-linking for keratoconus. (25)
Tear film effects on tonometry
The surface tension of the tear film is responsible for the attractive force between the tears and the tonometer probe tip. Given that GAT was calibrated for eyes with 'average' surface tension, variations from this value can impact upon the validity of IOP readings; increased surface tension causes an underestimation of the true IOP, whereas decreased surface tension results in an overestimation. (3,26) Nevertheless, the error related to changes in the consistency of the tear film is likely to be less than 1mmHg. (27)
Amount of fluid
GAT was calibrated assuming that the width of the fluorescent mires was 0.2mm. (3) The appearance of the mires changes with the amount of fluid in the eye. Thick mires can cause the IOP to be overestimated by as much as 5mmHg due to the decrease in the attractive force of the tear film. (3) Very narrow mires are thought to cause a small underestimation of 0.4mmHg. (3)
Visibility of the mires
GAT mires should be aligned from inner edge to inner edge. The inner edge of the mires, which corresponds to the region immediately adjacent to the tonometer probe, is where the tear film is the thinnest and, therefore, most difficult to see. Under conditions where there is insufficient sodium fluorescein (NaFl), poor illumination, or NaFl quenching, the inner edge will appear to be at a different location resulting in IOP underestimation. Research shows IOP to be 4.6mmHg higher when using a 0.25% NaFl solution compared with the NaFl strips, because the solution produces a higher concentration of NaFl in the tears. (26) It is, therefore, essential that there is a good concentration of NaFl in the tear film and a bright cobalt blue illumination is used.
Even if the utmost care is taken to stabilise the IOP prior to tonometry, it will still fluctuate from day to day. Recent research shows that the IOP can vary by 4-5mmHg per day in patients with ocular hypertension or glaucoma, even when measurements are taken at the same time of day and by the same practitioner. (31) Similar figures have been reported in healthy eyes. (32) Knowing that IOP is unstable can help with interpretation when viewed in a statistical light, where an IOP must change by more than twice the standard deviation of the tonometer before we can be sure that it has changed. For example, an IOP of 19mmHg and 23mmHg for the same patient taken on different days are probably not statistically different even though one of them is technically elevated (that is above 21 mmHg), because the standard deviation of GAT is around [+ or -] 2.5mmHg. (3) Sampling errors can be accounted for by ensuring that the IOP is consistently elevated prior to referral by assessing the IOP on at least two different days.
Another type of sampling error can arise if only one measurement of IOP is taken. Although it is standard practise to undertake four non-contact tonometry readings due to the presence of the ocular pulse, some practitioners advocate taking one measurement with GAT; it is suggested that one reading is not statistically different from using the average of three readings, (33) although others argue that two or three measurements should be taken, (34) and possibly discarding the first because it tends to be higher than subsequent readings due to anxiety. (35)
Intra- and inter-observer variability
Variability in measures is not due to fluctuations in IOP alone, but also differences in judgement from one day to the next for a single practitioner (intra-observer variability) or between practitioners (inter-observer variability). Intra-observer variability is indistinguishable from the day-to-day fluctuations in IOP given that it is impossible to tell whether the IOP has changed or whether the observer's judgement has altered. Inter-observer variability is slightly different, as it can lead to systematic errors, where one practitioner always measures the IOP higher or lower than another. Inter-observer variability can occur for both subjective, such as GAT, and objective, such as Pascal Dynamic Contour Tonometry (PDCT) techniques, (36) perhaps due to technique or patient instructions. Systematic bias is thought to be less than 2mmHg in experienced practitioners. (36)
Probe contact with other structures, such as eyelids or lashes, can cause the IOP to appear to be much higher than it truly is. Applanation of the para-central cornea can occur if the patient's gaze is not aligned with the probe, and although the error is theoretically very small, should be avoided when performing applanation tonometry.
The duration of ocular contact is important because leaving the probe on the eye for too long will cause a decrease in IOP. Schmidt recommended a maximum applanation duration of 30 seconds, (28) but subsequent research shows that even this duration is too long, with IOP falling by 2-4mmHg after this time. (27,37) The number of tonometer applications within a short timeframe is also important, as repeated applanation can cause the IOP to fall by 2-3mmHg if IOP is measured every minute for five minutes. (37) It is, therefore, recommended that GAT is completed as quickly as possible and the contact with the eye is kept short. (34) It is worth noting that repeated use of NCT does not cause the IOP to decrease. (38)
Tonometers need to be checked regularly to ensure that they are correctly calibrated. Surveys show that almost 70% of respondents checked the calibration of their GAT at less than the recommended monthly frequency, (39) and an estimated 40-50% of GATs are thought to be inaccurate by more than [+ or -] 2.5mmHg at any given time. (40) GAT should be checked monthly and returned to the supplier for re-calibration if found to be inaccurate by 2.5mmHg or more, or if the error is non-linear. For non-contact tonometers, practitioners are advised to contact their supplier for the recommended service interval.
While Perkins tonometry is often considered to be interchangeable with GAT due the similarity in operating principles of the two devices, there is surprisingly little evidence to confirm this practice. Compared to GAT, Perkins tonometers have lower magnification and dimmer illumination; both of these factors could considerably reduce the visibility of the inner edge of the mires, thus causing an underestimation of IOP measurements.
Non-contact tonometry (NCT) was introduced by Grolman in 1972 as an alternative to GAT. (42) Like GAT, it measures the force required to applanate the cornea, though instead achieves applanation via the rapid application of a collimated air-pulse. Due to the fact that NCT was calibrated to give readings that were consistent with GAT, and are also obtained via a form of applanation, it too is sensitive to most of the corneal factors that affect GAT. Recent research suggests that NCT is actually more sensitive to variations in corneal properties, particularly in thicker corneas. (43) This may be because NCT encounters more corneal resistance as it applanates a larger area, and is a more rapid process that is likely to increase viscoelastic resistance. When the average of four readings is used, the results of most modern NCT devices are considered to be roughly equivalent to GAT except at high levels of IOP, where NCT tends to overestimate the IOP, (38) hence the importance of verifying borderline or high NCT readings with GAT.
Ocular Response Analyzer
The Reichert Ocular Response Analyzer (ORA) is a non-contact tonometer that uses additional computerised analysis to identify and eliminate the effects of corneal biomechanical behaviour on IOP measurements. (44) It produces two measurements--IOPg (the raw IOP measurements) and [IOP.sub.CC] (IOP measurements corrected for corneal properties). Evidence shows that the IOPg may compare favourably to GAT, (45) as might be expected of a raw NCT measurement. However, by design, [IOP.sub.CC] does not correspond to GAT because the IOP reading is relatively free from the effects of corneal interference. Although research shows that [IOP.sub.CC] may be suitable for the measurement of IOP in atypical corneas, such as following corneal refractive surgery or in keratoconus, this method of IOP measurement is not currently recognised by the International Standards Organisation as a gold-standard, or by NICE CG85; this lack of recognition can make it difficult to make clinical judgements or referrals based on [IOP.sub.CC], even if the measurement may be more representative of the true IOP. In addition, there isn't at present a good understanding of the origins of the corneal biomechanical output of the ORA. (46)
In order to counteract the complicated relationship between the ocular properties and the Imbert-Fick principle, there are several commercially available tonometers that use other methods to obtain the IOP measurement.
The Pascal Dynamic Contour Tonometer (PDCT) is a slit-lamp mounted tonometer that looks very similar to GAT, but measures pressure via a sensor embedded in a concave probe tip. (50) The tip is used to avoid applanation thereby vastly reducing, although not eliminating, the interference of corneal properties. (13,47) In almost all studies, IOP tends to be 1-2mmHg higher with a PDCT than with GAT, (47) though it may be less variable. (36) As with the ORA, the IOP measurements obtained by PDCT cannot be used interchangeably with GAT. PDCT is promising as a replacement for GAT, as evidence shows that it is much better at measuring IOP in eyes with corneas that have been compromised by oedema, (51) keratoconus, (24) corneal refractive surgery, (52) and keratoplasty. It does, however, take considerably more time as up to six readings per eye may be required, and the disposable probe tips are a relatively expensive on-going cost; (47) this may limit its adoption for screening purposes.
Optimising IOP measurements
Although it should be recognised that no form of tonometry is immune from measurement inaccuracies, the advice outlined in Figure 3 may help to optimise IOP measurements.
GAT remains the gold-standard device for the measurement of IOP in the UK and should be the tonometer of choice whenever possible, although non-contact tonometry presents a valid alternative for general screening purposes. All forms of applanation tonometry are sensitive to the properties of the cornea and tear film, and, therefore, can provide inaccurate results in some patients. Newer technologies are continually being developed to address the limitations of applanation tonometry, however, further research is needed to determine whether these emerging techniques are a valid alternative to GAT for clinical decision making and referral purposes.
References Visit www.optometry.co.uk/clinical, click on the article title and then on 'references' to download.
Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams. Please complete online by midnight on May 9, 2014. You will be unable to submit exams after this date. Answers will be published on www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.
Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills-- how will you change the way you practice? How will you use this information to improve your work for patient benefit?
Kirsten Hamilton-Maxwell PhD, BOptom (Hons), FHEA
Dr Kirsten Hamilton-Maxwell is a lecturer and clinical optometric supervisor in the School of Optometry and Vision Sciences at Cardiff University, where she teaches a range of clinical techniques to undergraduate students. Her research interests include tonometry, corneal properties and intraocular pressure.
Figure 3 Factors to consider when making clinical decisions based on IOP measurements. Understand your patient Take a good history so that you are aware of any factors that may affect the IOP Consider whether the tonometer that you are using is suitable for your patient * Consider the pros and cons of non-contact applanation tonometry (NCT) * Consider the pros and cons of contact applanation tonometry (GAT or Perkins) * Remember that all tonometers have their own limitations, and that some of them are not yet officially recognised as an alternative to GAT Prepare your equipment * Check the calibration regularly * For GAT, ensure that your slit lamp has the best illumination possible and use an adequate concentration of sodium fluorescein Prepare your patient * Make sure that your patient is relaxed and comfortable (ie not straining) * Ask them to loosen any tight clothing around the neck * Ask them to look at a distant target and to breathe normally Perform the procedure using a standard protocol * Use the protocol recommended by the manufacturer * Take multiple readings where required Where relevant, perform additional tests to help interpret the IOP * For example, slit lamp to look for corneal pathology or pachymetry to measure central corneal thickness (CCT) to look for a very thick cornea * Remember that you should not be correcting IOP for CCT When making referrals under NICE CG85 on the basis of IOP * Remember that reproducibility of IOP measurement is poor no matter what tonometer you use--this is because the IOP itself is fluctuatin * Repeat raised IOPs prior to referral wherever possible and refer only if the IOP is consistently elevated above 21mmHg (exclusions apply to certain age groups visit College of Optometrists website for guidance)
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|Title Annotation:||CONTINUING EDUCATION & TRAINING|
|Date:||Apr 11, 2014|
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