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Glaucoma is one of the world's leading causes of blindness. Intraocular pressure (IOP) is an important criterion for decision-making in both the screening and management of this disease.

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In practice, measurement of the IOP is by tonometry. The force required to applanate or flatten the outer coat of the eye is the basis of applanation tonometry.

A true measure of IOP can only be obtained using manometric methods which require a probe to be inserted directly into the anterior chamber.

Tonometry has evolved a long way from the days of digital palpation and the indentation tonometers, developed in the late 1800s1 (see Figure 1). Most instruments now use the basic principle of applanation tonometry, a method derived from the Maklakov-Fick law, alternatively known as the Imbert-Fick law.

Imbert-Fick Law
The Imbert-Fick law states that 'pressure is equal to the force per unit area of applanation for a spherical container', assumed to be infinitely thin, dry and perfectly elastic in form.

The cornea is, however, a rigid structure which has an average thickness of 540<03BC>m, and a tear-moistened outer surface.
Nevertheless, by selecting a circular zone 3.06mm in diameter the smallest area of applanation (7.35mm2) is attained while still giving accurate results, introducing a relative error in IOP of only 2.5 per cent.

This is due to the balance between the four forces acting on the cornea on applanation:

Force of probe + Tear capillary action
= IOP + Ocular rigidity

Furthermore, once the density of mercury (13.6g/cm3) is considered in the final calculation, an easy conversion is produced in which 1g of weight equates to 10mmHg of pressure.

Contact Applanation tonometry
The two main contact applanation tonometers used in the UK are the 'Goldmann' and the 'Perkins', which use the basic principle of varying the force applied to applanate a fixed area. The basic instrumentation consists of a flat applanating 'cone' comprised of two prisms with apices joined together. Its main advantage over the first mechanical indentation designs is the reduction in induced aqueous outflow known as 'ocular massage'. Research has shown there to be a maximum volume displacement of 0.50mm3 which has minimal effects on the IOP.

Goldmann tonometry
This technique should only be performed on completion of all other clinical tests including refraction, as softening of the cornea by anaesthesia in addition to potential corneal abrasions can cause blurred vision. Only dilation is recommended post-Goldmann as the softening action permits increased absorption of the mydriatic across the corneal surface.

1)The instrument should be calibrated regularly depending on usage. The Goldmann is mounted onto the slit lamp with the cone in position. A calibration key comprising a metal rod with three grooves is inserted into the main body of the instrument (Figure 2). The central groove of the key is initially aligned with the marking of the attaching device; in this arrangement the tonometer cone should rock forward at a weight of 0g. The key is then moved towards the practitioner aligning the second (2g) and third (6g) groove, repeating the same procedure in turn. The instrument must be sent for re-calibration if any non-linear error or a linear error outside +/- 2.5mmHg is recorded.4

2)Ascertain any contraindications to contact tonometry such as previous adverse reactions to anaesthetic drops. The practitioner must wash their hands thoroughly using antibacterial soap. Further preliminary checks include the assessment of visual acuity and corneal integrity, using the slit lamp biomicroscope. Initially, a wide parallelopiped with moderate to high magnification is used, followed by a more detailed examination using fluorescein under cobalt blue light. Contact tonometry is inadvisable if any signs of infections or abrasions are found, as direct contact with the eye inevitably risks further damage even when conducted by an experienced practitioner.

3)Non-disposable tonometer cones must be disinfected according to College of Optometrists or ABDO guidelines to prevent any risk of transmitting HIV, vCJD or Hepatitis B. All have been isolated in the tear film. It is advisable to rinse the disinfected cone with saline, before leaving it to air dry to prevent any residual disinfectant causing a toxic reaction. Alternatively, disposable 'Tonosafe' cones are available to use with both the Goldmann and Perkins tonometers, although at increased expense (Figure 3).

4)The tonometer probe is inserted into the main instrument body with prism aligned horizontally, taking care not to touch the sterile cone surface. This can either be approximated visually, or checked by aligning the white line of the probe carrier with the 0/180 marking of the body when using non-disposable cones. However, if the corneal astigmatism is 3DC or above, an elliptical applanation area larger than 7.35mm2 is produced, leading to an IOP error of at least 1mmHg.5 This is avoided by rotating the prism 43 to the flattest corneal meridian. Since the separation between white and red markings of the probe carrier is 43, the red marking simply needs to be aligned with the prism for 'with the rule' or 'against the rule' astigmatism.

5)One drop of anaesthetic (such as benoxinate 0.4 per cent, or a combination such as proxymetacaine 0.5 per cent with fluorescein) is instilled into the lower conjunctival sac. Allow approximately 30 seconds for the drops to take effect, taking this time to record the drug name, expiry date, dose, batch number and time of instillation on the record card. The corneal sensitivity can be checked using either a sterile wisp of tissue or preferably a wetted fluoret strip at the limbal edge.

6)The Goldmann is mounted onto the slit lamp via a foot plate with two grooves. The procedure is conducted through only one eyepiece depending on whether the left or right groove is used. Setting the base into the nasal groove is advised as it compensates for convergence of the eye. Initially, the 'force' is set to an average reading 13mmHg,6 as this reduces contact time with the cornea and prevents vibration of the probe. The slit lamp is set to maximise the light illuminating the cone head:

• Wide angle of illumination (60)
• High illumination to account for light absorbance through cobalt blue filter
• Widest beam width and height
• Low to moderate magnification (10-16X).

7)Applanation is initially attempted without manipulating the lids as the patient stares at a fixation target straight ahead. If this is unsuccessful, the practitioner's free hand is used to carefully part the lids using the forefinger to hold the upper lid against the superior orbital ridge and the thumb to draw the lower lid down, taking care not to press on the globe and cause a change in IOP.

8)The slit lamp is carefully advanced to bring the cone within 2-3mm of the central cornea. Final contact is made using the joystick for fine control. This will cause a slight backward tip of the cone and create a circumferential fluorescein limbal glow. An image of two horizontally separated semicircular rings should now be seen through the eyepiece. The ideal thickness of each ring is 1/10th of the arc diameter; if the rings are too thin or thick, the pressure can be underestimated or overestimated respectively. The cone position may also need to be altered to ensure that the rings are equal in size, always moving the cone towards the larger ring, with the cone slightly off the cornea to avoid trauma for large movements.

The practitioner should ensure they have located the dial on the Goldmann base unit which alters the force applied prior to applanation (Figure 4). This force is varied to alter the ring separation until a final endpoint is reached, where the inner rings just touch (Figure 5). If the rings are separated too far, the force must be increased and vice versa.

The fluorescein rings can commonly be seen to pulsate with the venous pulse. In such cases, the force is either adjusted so pulsation occurs equally either side of the endpoint, or an average reading of the highest and lowest point of the pulse is taken.

9)Retract the cone off the cornea and gently release the lids. Record the final result along with the time, instrument name and eye applanated. Only one accurate reading is required per eye as the increased corneal contact time accounts for any variation of IOP with the cardiac cycle.

10)Re-check the corneal integrity. Assess the severity of any staining and manage accordingly by rinsing the fluorescein from the eye using sterile saline minims, and advising prophylactic antibiotics/ocular lubricants as necessary. Ideally, the patient should not leave the practice until corneal sensitivity has been sufficiently regained. Advise the patient to avoid rubbing the eyes and protect from foreign bodies for at least another half hour. A written sheet of information of potential side effects and subsequent action to take should ideally be given to the patient.

Perkins Tonometry
The Perkins tonometer is essentially a handheld version of the Goldmann. It has an internal illumination system making it perfectly portable and ideal for domiciliary visits, or patients with mobility difficulties.

1)Calibration should be checked regularly depending on usage, employing a similar procedure to Goldmann (see Step 1). The instrument (with battery pack removed) is set horizontally on a metal rest (supplied with the instrument) with cone in position. Initially, calibration is checked with no weight in place (0g), and then free weights of 2g and 5g are placed in turn onto the cone head. Correct calibration results in the cone rocking forwards on reaching the equivalent reading on the dial.

2)Follow Steps 2, 3 and 5 as for Goldmann procedure. Perkins can be conducted with the patient supine or seated upright. Extend the instrument headrest setting it firmly on the patient's forehead using the two middle fingers of your free hand. Use the thumb of this hand to lift the patient's upper lid as required (Figure 6).

3)Advance the cone carefully towards the eye, with the patient fixating a target just above the straight ahead position. Ensure the rings are the correct thickness and equal in size, making any minor adjustments off the cornea, before reaching the final endpoint using the same principles as Goldmann (see Step 8).

4)Follow post-applanation Steps 9 and 10 as with Goldmann.

Non-Contact Tonometry
Although Goldmann tonometry is considered the 'gold standard' tonometer, most general practices in the UK use non-contact tonometry (NCT).
While initial costs of NCTs are high, they are quick, easy to use and less invasive, and can therefore be delegated to non-optometric members of staff who have received adequate training. Multiple readings are, however, required to average out variations of IOP with the cardiac cycle. The vast array of models available commercially is so great that it is impossible to cover them all in this article. Nonetheless a summary of three widely known NCTs can be seen in Table 1.

All three NCTs employ the same basic principle in which a jet of air is used to applanate the central cornea. The point of applanation is assessed using an infra-red, electro-optical system in which a collimated beam of light is directed at the cornea from an oblique angle. Complete applanation is reached when maximum reflected light is received by a photoreceptor diode:

1)Once the patient has been seated comfortably, check instrument calibration while demonstrating the air puff on the patient's hand. This also clears any dust particles from the nozzle.

2)Ask the patient to close their eyes while the instrument is locked into position 1cm from the upper lid using the 'stopper safety' device.

3)Ask the patient to blink between readings to re-moisten the anterior surface. Record the individual and final average IOPs, time of day and instrument name. Take care to avoid hitting the patient's nose when moving between eyes by pulling the instrument back sufficiently.

Recent Developments
The accuracy of Goldmann has been called into question over recent years as many studies have shown biomechanical features of the cornea such as thickness, rigidity and hydration to have a significant effect on IOP readings.

Goldmann was originally calibrated for an average corneal thickness of 545<03BC>m,8 but, in reality, thickness can vary greatly in the normal population. A thicker cornea requires a greater force to applanate a given area, thereby producing a higher IOP. Conversely, a thinner cornea will produce an underestimation. The importance of corneal thickness has been highlighted by the recent increase in refractive surgery procedures.
A study conducted by Ehlers in the 1970s revealed an underestimation of 7mmHg with every 100<03BC>m reduction in corneal thickness.9 Thus, greater care needs to be taken in patients following refractive surgery - correction of refractive error of -3.00DS removes 30<03BC>m of tissue, underestimating the IOP by approximately 2mmHg.10

So the question arises, 'Should pachymetry be conducted in everyday practice?' The search for answers has led to the development of a new generation of 'dynamic' contact tonometers which assess and compensate for corneal biomechanics.

Tonopen
The Tonopen is a portable, electronic contact tonometer with a stainless steel probe transducer which measures the force applied for full applanation.

Results correlate well with Goldmann, with minor overestimations of lower IOPs and under-estimations of higher IOPs. Although Tonopen uses traditional 'static' applanation principles, studies suggest that it gives a truer reading of IOP when applied to the peripheral cornea in eyes which have undergone refractive surgery.11

Ocular Response Analyser
The Ocular Response Analyser (ORA) is a dynamic contact tonometer, which uses a collimated air pulse to flatten the cornea to full applanation and then beyond into concavity, creating a second applanation event once the air shuts off.

The force required for repeated applanations is measured and displayed in graphical form. The figures are used to calculate the IOP, and give an indication of corneal elasticity known as 'corneal hysteresis'.12

ICARE
The ICARE is a portable contact tonometer which also uses 'rebound' technology, but unlike the ORA it requires no anaesthetic. The probe momentarily touches the cornea, applying such a light force that it barely elicits a corneal reflex.13

Pascal
The Pascal dynamic contour tonometer is a slit-lamp-mounted device similar in design to the Goldmann. Instead of a flat cone, the Pascal has a curved head to match the corneal contour, thereby reducing distortion on applanation. It compensates for ocular blood flow, measuring IOP along the whole course of the pulse amplitude, gathering 100 readings per second.

Conclusion
While NCT results correlate well with Goldmann in the low to medium IOP range, this relationship breaks down with higher readings, particularly over 30mmHg. It is therefore advisable to repeat any high or asymmetrical measurements with Goldmann before making final clinical decisions.
Although it is still early days for the ORA and Pascal, their potential use as clinical tools in decision making for glaucoma referrals is substantial. Furthermore, the prevalence of glaucoma is expected to increase with life expectancy, placing heavy demands on our health system, making quick and accurate detection is essential.

References
1 www.medcompare.com/spotlight.asp?spotlightid=185&headerid=0
2 Doshi S, Harvey W. Investigative Techniques and Ocular examination, 2003; p64, (London: Butterworth-Heinmann).
3 Doshi S, Harvey W. Investigative Techniques and Ocular examination, 2003; p64, (London: Butterworth-Heinmann).
4 www.evidence-based-eyecare.com/pt/re/ebeye/abstract.00061198-200506000-00009.htm.
5 Elliot DB, Clinical Procedures in Primary
Eyecare, 1997; p259, (London, Butterworth-Heinmann).
6 Elliot DB. Clinical Procedures in Primary Eyecare, 1997, p 261, (London, Butterworth-Heinmann).
7 NCT manuals for Pulsair, NT-3000 and AO NCT.
8 www.opt.indiana.edu/Riley/HomePage/new_Goldmann_tonometry/2Goldmann_Tonometry.html
9 Kotecha A. Central corneal thickness and IOP, OT, 2005; 22.
10 www.opt.indiana.edu/Riley/HomePage/new_Goldmann_tonometry/2Goldmann_Tonometry.html
11 Garzozi, HJ, Chung HS, Lang Y et al. Intraocular pressure and photorefractive keratectomy; a comparison of three different tonometers. Cornea, 2005; 2001; 20, 33-36.
12 Pepose J, Sanderson JP, Qazi MA. How should we measure IOP after LASIK? Cataract and Refractive Surgery Today, 2005; 52-54.
13 www.tiolat.fi/tiolat.php?id=29

• Priya Dabasia is a visiting clinician at the Fight for Sight optometry clinic, City University

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