Nidek NT-4000

Tina Romanay looks at a new non-contact tonometer that claims to compensate for the ocular pulse variation, allowing for accuracy of measurement with just one puffFor many years it has been standard practice to measure intraocular pressure (IOP) through a non-contact means by averaging a series of readings.
This does not imply any loss of accuracy with this technique, merely that it only allows a snapshot of the constantly changing IOP. So any single reading might be at one of several points on an ever changing pressure curve.
One of the main variables influencing transient IOP is the cardiac cycle responsible for the so-called ocular pulse which reflects shifts in IOP of around 2mmHg to 4mmHg in phase with the systemic pulse. A new non-contact tonometer has been introduced by Nidek claiming to monitor ocular pulse allowing a single reading to accurately gauge IOP by compensating for the variation. Before discussing the NT-4000 in practice, a short revision of tonometry may be in order.

How is IOP measured?
The 'tonometer' is the name given to an instrument designed to measure the IOP of the eye in units of millimetres of mercury (mmHg). IOP is clinically estimated either by measuring the degree of corneal deformation produced by a known weight (indentation), or by measuring the weight needed to produce a given degree of deformation (applanation).
Indentation tonometry (as done using, for example, the Schištz tonometer), is based on the degree of indentation made on the cornea, created by a plunger of a known weight, being proportional to the IOP.
The degree of indentation produced by the plunger is displayed on the scale of the instrument. This scale measurement can be converted into mmHg by use of the Friedenwald tables.
Due to the large amount of fluid displaced by the tonometer when placed on the eye, readings with two known weights should be taken to obtain an average IOP with use of the Friedenwald tables. Due to the excessive corneal damage that may be caused by indentation tonometry, it is rarely used in practice.
Contact applanation tonometry (such as Goldmann and Perkins tonometers), as the term implies, applanates or flattens a small area of the cornea. It exploits the Imbert-Fick law which, when applied to the eye, states that the IOP is equal to the tonometer weight divided by the applanated area.
Although the cornea does not strictly satisfy the structural requirements applicable to the Imbert-Fick law, applanating an area with exact diameter of 3.06mm and careful mechanical design of the tonometer allows IOP to be measured over an extensive range.
Prolonged contact of the probe against the cornea means that transient fluctuations in IOP caused by the cardiac cycle, lid and eye muscle movements are averaged out, and therefore only one reading of the IOP need be taken. Recent research has shown that for a truly accurate measurement, when perhaps a decision regarding medical treatment depends on the IOP, corneal thickness needs also to be measured.
Unlike contact applanation and indentation tonometers that employ mechanical contact to flatten the cornea and therefore achieve a reading of the IOP, non-contact tonometers (NCTs) produce a pulse of air to flatten a small region at the apex of the cornea (approximately 3mm).
Grolman introduced the first NCT in 1972 (AO NCT). A piston, which is contained within a cylinder and rapidly moved by a solenoid, produces a puff of air that increases linearly with time and is released through a nozzle until there is sufficient force to momentarily applanate the cornea. The area applanated is detected by an optical system. The instrument then calculates the time required for the air to applanate the eye. This time is related to the IOP of the eye. Many types of NCTs have since been marketed based on this principle.
Calibrations of NCTs have been achieved from clinical studies on both normal and glaucomatous subjects, in which the results of the NCT were compared with those of the Goldmann.
Single measurements of IOP with NCTs are not very repeatable and variations of approximately 3-4mmHg are common due to, among other things, the cardiac cycle. However, this problem has been overcome by taking three readings which, when averaged, give a more reliable representation.

Why use NCT?
IOP as measured with the Goldmann contact tonometer is the clinical standard against which all other tonometers are compared.
Because of its mechanical design, this instrument has been demonstrated to have less variation in repeated measurements of IOP than NCTs. However, for some patients it is not the instrument of choice.
The advantages of non-contact tonometers versus contact tonometers have no doubt been discussed in depth in the past. The most obvious advantage is that it eliminates the need for anaesthesia, as no contact is made between the tonometer and the eye, therefore avoiding problems of corneal damage, infection and allergic reaction to topical drugs.
New NCTs do not need special training of the observer to obtain reproducible and rapid measurements. For these reasons, non-invasive methods for the measurement of IOP have gained popularity with the optometric community.

The Instrument
The NT-4000 is an NCT used to measure the IOP of patients' eyes. It consists of a combined main unit and measuring unit, installed on a base. The main unit houses the monitor, control panel and joystick. The measuring unit houses the air nozzle and photo sensor. The base houses the chin rest and printer.

How does it work?
The NT-4000 increases the air pressure puffed onto the cornea in proportion to time. The shape of the cornea changes from convex to concave as it is flattened by the puff of air.
This change in shape is optically detected by monitoring the maximum amount of reflected light (that is transmitted from a photo transmitter on to the cornea) captured by a photo detector.
The time required to flatten the surface after puffing air out is then calculated by the instrument, and the air pressure used to make the cornea flat is calculated from the time. Finally, the IOP is obtained.

General Functions
Selectable modes of operation
Three modes of operation can be selected. Full auto, where both alignment and focusing are carried out automatically; semi auto where alignment is automatic, focusing is manual; and manual where both focusing and alignment are manual.

Auto alignment mode
As the measuring unit is placed close to the pupil, the instrument automatically focuses the pupil, and executes vertical and horizontal alignment such that a measurement is taken automatically.

Focusing indicator
This shows the distance between the patient's eye and the air nozzle.

Confidence mode
Despite a measurement being taken, any data that is indicated with an asterisk shows a level of unreliability and should be repeated. This kind of measured data is called 'low confidence data'.

What's new?
The NT-4000 has features similar to that of current available models, as mentioned. However, new features incorporated are 'pulse synchronised IOP', 'eyelid checking function', 'expanded auto-focusing detection range' and 'advanced APC (automatic puff control) function'. These claim to help achieve smoother and more stable measurements of IOP.

Pulse-synchronised IOP measurement
This distinguishing feature allows for a more accurate IOP reading to be taken with consideration for the change in IOP caused by the ocular pulse (cardiac cycle).
A pulse detector on the forehead rest on the measuring unit provides a pulse signal when the patient's head rests against it. When the pulse signal and the alignment of the pupil match, an IOP reading is taken automatically. Pulse detection can be taken at three different levels of the ocular pulse Ð peak, middle and bottom. It can be done either in 'pre-time' Ð pulse sync is controlled based on the prediction of the patient's pulse Ð or in 'real-time' when pulse sync is based on the patient's detected pulse. This feature can be easily deactivated and the NT-4000 can be used as a normal screening system.

Eyelid checking function
To obtain a stable and accurate measurement of the IOP, an automatic reading is taken only when the eye is fully open after alignment and focusing.
Therefore, before taking a reading, the instrument checks the position of the eyelid to avoid an error. This is done by reflecting four detection lights onto the cornea (at 1 o'clock, 4 o'clock, 7 o'clock and 10 o'clock). If the eyelid obstructs any of these reflections, the instrument informs the operator that a reading cannot be taken by wording 'open the eye', and the operator is then required to make appropriate adjustments. This function can be switched off at any time.

Expanded auto-focusing detection range
In the automatic focus/alignment mode, compared to the conventional model, the auto-focusing detection range has been expanded more than 50 per cent in the focusing direction. This allows for quicker and more comfortable measurements to be obtained.

Advanced APC function
This allows for a softer puff of air on to the cornea according to the patient's IOP. In the automatic puff control mode (APC40 or APC60), after the first puff of air is delivered and a measurement is taken, the automatic shut-off function activates to eliminate excess air puffing on to the cornea. The next measurement taken is then performed with a softer puff, based on the former measured data. This means that the eye is protected from surplus pressure which may be uncomfortable for the patient. Compared to the conventional model, the amount of air pressure applied has been reduced by more than 10 per cent, resulting in increased patient comfort.
How was it for me?
On first appearances, the NT-4000 looked very similar to its fellow NCTs. Yet with its compact colour screen, it was striking and noticeably appealing to both the operator and the patient. Surprisingly, I found this simple feature to be helpful and effective in putting the patient at ease with an otherwise intimidating instrument. As we have all no doubt heard at some point before measuring patients IOPs: 'Oh no. Not the puffer test?'
Setting up the patient on the instrument was no different to any other NCT with the aid of the eye level marker, chin rest adjuster and height-adjustable base unit.
The three different modes of operation (auto, semi and manual) proved useful on patients that did not respond to the automatic function, either because of poor alignment, stability on the instrument or corneal deformation. In addition, the auto alignment mode allowed beginners and non-trained staff to use the instrument easily and therefore reduced variability in results obtained due to lack of operator experience.
If a patient's eye was not open enough, or the eyelids/lashes obstructed the light detection system during measurement, a measurement error occurred. Like its fellow NCTs, the Nidek NT-4000 informed the operator if the measurement was inaccurate by either showing APL Ð applanation error; ALM Ð alignment error; BLK Ð blinking and slight movement of the eye made measurement impossible; or PRS Ð pressure error.
To improve measurements relating to eyelid position, I found the eyelid detection mode to be very useful. When using NCTs we cannot always be sure if interference from the eyelids is affecting the automatic measurement mode. One can find oneself adjusting the patient in many ways before realising it is in fact an error due to the eyelids. Therefore, if any of the four reflective lights from the cornea were being obstructed it was immediately apparent to the operator, and therefore appropriate instructions could be given to the patient to open their eyes further, or make necessary modifications before taking a reading.
I did, however, find that subtle lid variations in older patients made the instrument more sensitive to measurements being taken, and the instruction of 'Open eye' came up more than I would have liked.
The manufacturer claims that if the eyelid detection mode is switched off (from the sub switch compartment), increased errors in measurements and fluctuations may occur. Therefore this should only be cancelled when it is very difficult to perform a measurement. When this mode was cancelled, a marker appeared at the bottom right hand corner of the display screen (indicated by a picture of a lower lid with a cross over it).
Measurements were taken on a few patients with and without the eyelid detection mode, and a mean variation of 1mmHg was found respectively. However, it was difficult to determine if this fluctuation was due to the detection mode being switched off or a normal variation in IOP. This function does not operate in the manual mode.
As we know the IOP is not constant within an individual and varies according to the ocular pulse whether the patient is erect or supine, and the time of day at which a reading is being taken. The ocular pulse causes a fluctuation in the IOP of about 2-4mmHg. Therefore, use of the pulse synchronised mode claims to reduce the variability that may arise due to ocular pulse (cardiac cycle), and help the level of accuracy for measurements of IOP obtained.
I found that the patient's forehead needed to be in constant contact with the pulse detector on the forehead rest, for a pulse signal to be detected. The patient also needed to be very still for a reading to be obtained. This meant that the measuring time took longer than the normal screening test by approximately one and a half minutes on patients that were examined. If three readings were taken while the pulse synchronisation feature was still operational, it was interesting to note there was some variation between readings still present. Furthermore, subtle patient movements made taking measurements more difficult and occasionally the pulse detection mode had to be switched off in order to obtain a reading of the IOP.
Use of the pulse detection mode did help to discipline the operator to set the patient up more accurately, as it is quite easy for the patient to veer from the forehead rest while the operator attempts to take readings when the patient is not within the correct working range.
When the pulse synchronised mode is operated, a symbol appears on the print out that indicates at which point a measurement has been taken. This may be at the peak, middle or bottom of the ocular pulse. The factory setting is set at peak, but this can be altered in the parameter change mode.
The manufacturer states that even though the pulse synchronised mode is utilised, three readings are still advised in any of the pulse settings, in order to average the results obtained.
Ten patients that were examined with the NT-4000, showed variations in IOP of approximately 1-2mmHg between the peak, middle and bottom range of the ocular pulse. These patients were also examined on both the NT-3000 and NT-4000. IOP was found to be approximately 1-2mmHg higher with the NT-3000.
Additionally a comparison was made on two patients between the IOP reading obtained at the peak ocular pulse (average of 3mmHg) of the NT-4000, and the Goldmann tonometer. Pressures were found to be approximately 1-2mmHg higher with the NT-4000. These results, although very limited, showed the high reliability and accuracy of the NT-4000.
Those few patients that were tested on both the NT-3000 and NT-4000 noted that the 'air-puff' created while measuring IOP was less uncomfortable with the latter instrument. This effect could be due to the refined automatic puff control mechanism, or simply due to the conditioning effects of applanation.

What's the verdict?
I found the NT-4000 easy to use and non-qualified operators could achieve reliable and stable results with this instrument.
The help of various alignment and focusing mechanisms helped to produce repeatable and accurate results. The eyelid checking function facilitated in ensuring that reliable readings of the IOP could be taken in the automatic mode of operation. This mechanism also made it useful for the practitioner to decide if in extreme cases the manual mode of operation should be employed.
The appearance of the tonometer (colour screen) proved to be an interesting discussion point for patients, seemingly putting them at ease with an otherwise intimidating piece of equipment.
The manufacturer claims that the use of the APC increases patient comfort due to the decrease in air puff pressure after the first initial reading, thus improving the experience. Although this was noted by a few patients, the remainder did not notice a considerable difference.
The use of the pulse-synchronised IOP measurement claims to reduce the fluctuation in IOP that may occur due to the ocular pulse. Using this device increased measurement time, occasionally becoming uncomfortable for the patient. Moreover, three readings were still needed to be taken to achieve an average result which seemed to defeat the object.
In theory, the concept of compensating for the ocular pulse is an interesting one. If the pulse was the only variable influencing IOP, then one would expect the three readings taken when the synchronisation was active to be identical; in the default case, representing the IOP at the peak measurement of the cycle. However, the variation of 2-3mmHg still found must reflect either some inaccuracy in the synchronisation or, much more likely, the influence of other variables, such as fluctuations in accommodation, lid tension, pupil diameter, breathing rate and extraocular muscle tension. The introduction of the synchronisation should be applauded but I would argue that one should always remember that ocular pulse is one of several variables and hence the manufacturer's advice to still take three readings is sound.
Although nominal comparison of this instrument was made with its fellow NCT and the Goldmann tonometer, the NT-4000 revealed a good level of accuracy.
The development of NCTs that take the ocular pulse into consideration may increase the accuracy of measurements taken without the need to applanate where it is not possible, thereby aiding practitioners to detect and appropriately refer patients with increased IOP.
It is stressed that further investigation and research is needed on a larger population-based sample to determine the accuracy and reliability of IOP readings obtained with the NT-4000 as compared to the Goldmann tonometer the current 'gold standard'.

Further reading
1 Kini M et al. Prevalence of senile macular degeneration and open angle glaucoma in the Framingham eye study. Am J Ophthal, 1978;85:28-34.
2 Cockburn D. Diagnosis and management of open angle glaucoma: suggested guidelines for optometrists. Clin Exp Optom, 2000;83:3:119-127.
3 Klein B et al. Heritability of risk factors for primary open angle glaucoma. The Beaver Dam Eye Study. Investigative Ophthalmol and Vis Sci, 2004;45:59-62.
4 Le A Mukesh et al. Risk factors associated with the incidence of open angle glaucoma. The Visual impairment project. Investigative Ophthalmol and Vis Sci, 2003 Sep;44 (9):3783-9.
5 M Kroese and H Burton. Primary OAG. The need for consensus case definition. Journal of Epidemiology and Community Health, 2003; 57:752-754.
6 Hollows FC, Graham PA. Intraocular pressure, glaucoma and glaucoma suspects in a defined population. Br J Ophthalmol, 1966; 50:570-586.
7 Klein B et al. Prevalence of glaucoma. The Beaver Dam Study. Ophthalmology, 1992;99:1499-1504.
8 Mitchell P et al. Prevalence of open angle glaucoma in Australia. The Blue Montains Eye Study. Ophthalmology, 1996; 103:1661-1669.
9 Holland G. A review of the Keeler Pulsair EasyEye Tonometer. optician, May 24, 2002; No 5852:Vol 223.
10 Cockburn D. Referrals from Optometrists to Ophthalmologists. Aust J Optom, 1975; 58: 161-164.
11 Henson D. Optometric Instrumentation. Butterworths, London 1983.
12 Nidek Co Ltd. Nidek non-contact tonometer NT-4000, Operators Manual 2002.

optician would like to thank Birmingham Optical Group (0121 442 2222) for the loan of this equipment.

Tina Romanay is clinical manager at the Fight for Sight Optometry Clinic, City University
For many years it has been standard practice to measure intraocular pressure (IOP) through a non-contact means by averaging a series of readings.
This does not imply any loss of accuracy with this technique, merely that it only allows a snapshot of the constantly changing IOP. So any single reading might be at one of several points on an ever changing pressure curve.
One of the main variables influencing transient IOP is the cardiac cycle responsible for the so-called ocular pulse which reflects shifts in IOP of around 2mmHg to 4mmHg in phase with the systemic pulse. A new non-contact tonometer has been introduced by Nidek claiming to monitor ocular pulse allowing a single reading to accurately gauge IOP by compensating for the variation. Before discussing the NT-4000 in practice, a short revision of tonometry may be in order.

How is IOP measured?
The 'tonometer' is the name given to an instrument designed to measure the IOP of the eye in units of millimetres of mercury (mmHg). IOP is clinically estimated either by measuring the degree of corneal deformation produced by a known weight (indentation), or by measuring the weight needed to produce a given degree of deformation (applanation).
Indentation tonometry (as done using, for example, the Schištz tonometer), is based on the degree of indentation made on the cornea, created by a plunger of a known weight, being proportional to the IOP.
The degree of indentation produced by the plunger is displayed on the scale of the instrument. This scale measurement can be converted into mmHg by use of the Friedenwald tables.
Due to the large amount of fluid displaced by the tonometer when placed on the eye, readings with two known weights should be taken to obtain an average IOP with use of the Friedenwald tables. Due to the excessive corneal damage that may be caused by indentation tonometry, it is rarely used in practice.
Contact applanation tonometry (such as Goldmann and Perkins tonometers), as the term implies, applanates or flattens a small area of the cornea. It exploits the Imbert-Fick law which, when applied to the eye, states that the IOP is equal to the tonometer weight divided by the applanated area.
Although the cornea does not strictly satisfy the structural requirements applicable to the Imbert-Fick law, applanating an area with exact diameter of 3.06mm and careful mechanical design of the tonometer allows IOP to be measured over an extensive range.
Prolonged contact of the probe against the cornea means that transient fluctuations in IOP caused by the cardiac cycle, lid and eye muscle movements are averaged out, and therefore only one reading of the IOP need be taken. Recent research has shown that for a truly accurate measurement, when perhaps a decision regarding medical treatment depends on the IOP, corneal thickness needs also to be measured.
Unlike contact applanation and indentation tonometers that employ mechanical contact to flatten the cornea and therefore achieve a reading of the IOP, non-contact tonometers (NCTs) produce a pulse of air to flatten a small region at the apex of the cornea (approximately 3mm).
Grolman introduced the first NCT in 1972 (AO NCT). A piston, which is contained within a cylinder and rapidly moved by a solenoid, produces a puff of air that increases linearly with time and is released through a nozzle until there is sufficient force to momentarily applanate the cornea. The area applanated is detected by an optical system. The instrument then calculates the time required for the air to applanate the eye. This time is related to the IOP of the eye. Many types of NCTs have since been marketed based on this principle.
Calibrations of NCTs have been achieved from clinical studies on both normal and glaucomatous subjects, in which the results of the NCT were compared with those of the Goldmann.
Single measurements of IOP with NCTs are not very repeatable and variations of approximately 3-4mmHg are common due to, among other things, the cardiac cycle. However, this problem has been overcome by taking three readings which, when averaged, give a more reliable representation.

Why use NCT?
IOP as measured with the Goldmann contact tonometer is the clinical standard against which all other tonometers are compared.
Because of its mechanical design, this instrument has been demonstrated to have less variation in repeated measurements of IOP than NCTs. However, for some patients it is not the instrument of choice.
The advantages of non-contact tonometers versus contact tonometers have no doubt been discussed in depth in the past. The most obvious advantage is that it eliminates the need for anaesthesia, as no contact is made between the tonometer and the eye, therefore avoiding problems of corneal damage, infection and allergic reaction to topical drugs.
New NCTs do not need special training of the observer to obtain reproducible and rapid measurements. For these reasons, non-invasive methods for the measurement of IOP have gained popularity with the optometric community.

The Instrument
The NT-4000 is an NCT used to measure the IOP of patients' eyes. It consists of a combined main unit and measuring unit, installed on a base. The main unit houses the monitor, control panel and joystick. The measuring unit houses the air nozzle and photo sensor. The base houses the chin rest and printer.

How does it work?
The NT-4000 increases the air pressure puffed onto the cornea in proportion to time. The shape of the cornea changes from convex to concave as it is flattened by the puff of air.
This change in shape is optically detected by monitoring the maximum amount of reflected light (that is transmitted from a photo transmitter on to the cornea) captured by a photo detector.
The time required to flatten the surface after puffing air out is then calculated by the instrument, and the air pressure used to make the cornea flat is calculated from the time. Finally, the IOP is obtained.

General Functions
Selectable modes of operation
Three modes of operation can be selected. Full auto, where both alignment and focusing are carried out automatically; semi auto where alignment is automatic, focusing is manual; and manual where both focusing and alignment are manual.

Auto alignment mode
As the measuring unit is placed close to the pupil, the instrument automatically focuses the pupil, and executes vertical and horizontal alignment such that a measurement is taken automatically.

Focusing indicator
This shows the distance between the patient's eye and the air nozzle.

Confidence mode
Despite a measurement being taken, any data that is indicated with an asterisk shows a level of unreliability and should be repeated. This kind of measured data is called 'low confidence data'.

What's new?
The NT-4000 has features similar to that of current available models, as mentioned. However, new features incorporated are 'pulse synchronised IOP', 'eyelid checking function', 'expanded auto-focusing detection range' and 'advanced APC (automatic puff control) function'. These claim to help achieve smoother and more stable measurements of IOP.

Pulse-synchronised IOP measurement
This distinguishing feature allows for a more accurate IOP reading to be taken with consideration for the change in IOP caused by the ocular pulse (cardiac cycle).
A pulse detector on the forehead rest on the measuring unit provides a pulse signal when the patient's head rests against it. When the pulse signal and the alignment of the pupil match, an IOP reading is taken automatically. Pulse detection can be taken at three different levels of the ocular pulse Ð peak, middle and bottom. It can be done either in 'pre-time' Ð pulse sync is controlled based on the prediction of the patient's pulse Ð or in 'real-time' when pulse sync is based on the patient's detected pulse. This feature can be easily deactivated and the NT-4000 can be used as a normal screening system.

Eyelid checking function
To obtain a stable and accurate measurement of the IOP, an automatic reading is taken only when the eye is fully open after alignment and focusing.
Therefore, before taking a reading, the instrument checks the position of the eyelid to avoid an error. This is done by reflecting four detection lights onto the cornea (at 1 o'clock, 4 o'clock, 7 o'clock and 10 o'clock). If the eyelid obstructs any of these reflections, the instrument informs the operator that a reading cannot be taken by wording 'open the eye', and the operator is then required to make appropriate adjustments. This function can be switched off at any time.

Expanded auto-focusing detection range
In the automatic focus/alignment mode, compared to the conventional model, the auto-focusing detection range has been expanded more than 50 per cent in the focusing direction. This allows for quicker and more comfortable measurements to be obtained.

Advanced APC function
This allows for a softer puff of air on to the cornea according to the patient's IOP. In the automatic puff control mode (APC40 or APC60), after the first puff of air is delivered and a measurement is taken, the automatic shut-off function activates to eliminate excess air puffing on to the cornea. The next measurement taken is then performed with a softer puff, based on the former measured data. This means that the eye is protected from surplus pressure which may be uncomfortable for the patient. Compared to the conventional model, the amount of air pressure applied has been reduced by more than 10 per cent, resulting in increased patient comfort.
How was it for me?
On first appearances, the NT-4000 looked very similar to its fellow NCTs. Yet with its compact colour screen, it was striking and noticeably appealing to both the operator and the patient. Surprisingly, I found this simple feature to be helpful and effective in putting the patient at ease with an otherwise intimidating instrument. As we have all no doubt heard at some point before measuring patients IOPs: 'Oh no. Not the puffer test?'
Setting up the patient on the instrument was no different to any other NCT with the aid of the eye level marker, chin rest adjuster and height-adjustable base unit.
The three different modes of operation (auto, semi and manual) proved useful on patients that did not respond to the automatic function, either because of poor alignment, stability on the instrument or corneal deformation. In addition, the auto alignment mode allowed beginners and non-trained staff to use the instrument easily and therefore reduced variability in results obtained due to lack of operator experience.
If a patient's eye was not open enough, or the eyelids/lashes obstructed the light detection system during measurement, a measurement error occurred. Like its fellow NCTs, the Nidek NT-4000 informed the operator if the measurement was inaccurate by either showing APL Ð applanation error; ALM Ð alignment error; BLK Ð blinking and slight movement of the eye made measurement impossible; or PRS Ð pressure error.
To improve measurements relating to eyelid position, I found the eyelid detection mode to be very useful. When using NCTs we cannot always be sure if interference from the eyelids is affecting the automatic measurement mode. One can find oneself adjusting the patient in many ways before realising it is in fact an error due to the eyelids. Therefore, if any of the four reflective lights from the cornea were being obstructed it was immediately apparent to the operator, and therefore appropriate instructions could be given to the patient to open their eyes further, or make necessary modifications before taking a reading.
I did, however, find that subtle lid variations in older patients made the instrument more sensitive to measurements being taken, and the instruction of 'Open eye' came up more than I would have liked.
The manufacturer claims that if the eyelid detection mode is switched off (from the sub switch compartment), increased errors in measurements and fluctuations may occur. Therefore this should only be cancelled when it is very difficult to perform a measurement. When this mode was cancelled, a marker appeared at the bottom right hand corner of the display screen (indicated by a picture of a lower lid with a cross over it).
Measurements were taken on a few patients with and without the eyelid detection mode, and a mean variation of 1mmHg was found respectively. However, it was difficult to determine if this fluctuation was due to the detection mode being switched off or a normal variation in IOP. This function does not operate in the manual mode.
As we know the IOP is not constant within an individual and varies according to the ocular pulse whether the patient is erect or supine, and the time of day at which a reading is being taken. The ocular pulse causes a fluctuation in the IOP of about 2-4mmHg. Therefore, use of the pulse synchronised mode claims to reduce the variability that may arise due to ocular pulse (cardiac cycle), and help the level of accuracy for measurements of IOP obtained.
I found that the patient's forehead needed to be in constant contact with the pulse detector on the forehead rest, for a pulse signal to be detected. The patient also needed to be very still for a reading to be obtained. This meant that the measuring time took longer than the normal screening test by approximately one and a half minutes on patients that were examined. If three readings were taken while the pulse synchronisation feature was still operational, it was interesting to note there was some variation between readings still present. Furthermore, subtle patient movements made taking measurements more difficult and occasionally the pulse detection mode had to be switched off in order to obtain a reading of the IOP.
Use of the pulse detection mode did help to discipline the operator to set the patient up more accurately, as it is quite easy for the patient to veer from the forehead rest while the operator attempts to take readings when the patient is not within the correct working range.
When the pulse synchronised mode is operated, a symbol appears on the print out that indicates at which point a measurement has been taken. This may be at the peak, middle or bottom of the ocular pulse. The factory setting is set at peak, but this can be altered in the parameter change mode.
The manufacturer states that even though the pulse synchronised mode is utilised, three readings are still advised in any of the pulse settings, in order to average the results obtained.
Ten patients that were examined with the NT-4000, showed variations in IOP of approximately 1-2mmHg between the peak, middle and bottom range of the ocular pulse. These patients were also examined on both the NT-3000 and NT-4000. IOP was found to be approximately 1-2mmHg higher with the NT-3000.
Additionally a comparison was made on two patients between the IOP reading obtained at the peak ocular pulse (average of 3mmHg) of the NT-4000, and the Goldmann tonometer. Pressures were found to be approximately 1-2mmHg higher with the NT-4000. These results, although very limited, showed the high reliability and accuracy of the NT-4000.
Those few patients that were tested on both the NT-3000 and NT-4000 noted that the 'air-puff' created while measuring IOP was less uncomfortable with the latter instrument. This effect could be due to the refined automatic puff control mechanism, or simply due to the conditioning effects of applanation.

What's the verdict?
I found the NT-4000 easy to use and non-qualified operators could achieve reliable and stable results with this instrument.
The help of various alignment and focusing mechanisms helped to produce repeatable and accurate results. The eyelid checking function facilitated in ensuring that reliable readings of the IOP could be taken in the automatic mode of operation. This mechanism also made it useful for the practitioner to decide if in extreme cases the manual mode of operation should be employed.
The appearance of the tonometer (colour screen) proved to be an interesting discussion point for patients, seemingly putting them at ease with an otherwise intimidating piece of equipment.
The manufacturer claims that the use of the APC increases patient comfort due to the decrease in air puff pressure after the first initial reading, thus improving the experience. Although this was noted by a few patients, the remainder did not notice a considerable difference.
The use of the pulse-synchronised IOP measurement claims to reduce the fluctuation in IOP that may occur due to the ocular pulse. Using this device increased measurement time, occasionally becoming uncomfortable for the patient. Moreover, three readings were still needed to be taken to achieve an average result which seemed to defeat the object.
In theory, the concept of compensating for the ocular pulse is an interesting one. If the pulse was the only variable influencing IOP, then one would expect the three readings taken when the synchronisation was active to be identical; in the default case, representing the IOP at the peak measurement of the cycle. However, the variation of 2-3mmHg still found must reflect either some inaccuracy in the synchronisation or, much more likely, the influence of other variables, such as fluctuations in accommodation, lid tension, pupil diameter, breathing rate and extraocular muscle tension. The introduction of the synchronisation should be applauded but I would argue that one should always remember that ocular pulse is one of several variables and hence the manufacturer's advice to still take three readings is sound.
Although nominal comparison of this instrument was made with its fellow NCT and the Goldmann tonometer, the NT-4000 revealed a good level of accuracy.
The development of NCTs that take the ocular pulse into consideration may increase the accuracy of measurements taken without the need to applanate where it is not possible, thereby aiding practitioners to detect and appropriately refer patients with increased IOP.
It is stressed that further investigation and research is needed on a larger population-based sample to determine the accuracy and reliability of IOP readings obtained with the NT-4000 as compared to the Goldmann tonometer the current 'gold standard'.

Further reading
1 Kini M et al. Prevalence of senile macular degeneration and open angle glaucoma in the Framingham eye study. Am J Ophthal, 1978;85:28-34.
2 Cockburn D. Diagnosis and management of open angle glaucoma: suggested guidelines for optometrists. Clin Exp Optom, 2000;83:3:119-127.
3 Klein B et al. Heritability of risk factors for primary open angle glaucoma. The Beaver Dam Eye Study. Investigative Ophthalmol and Vis Sci, 2004;45:59-62.
4 Le A Mukesh et al. Risk factors associated with the incidence of open angle glaucoma. The Visual impairment project. Investigative Ophthalmol and Vis Sci, 2003 Sep;44 (9):3783-9.
5 M Kroese and H Burton. Primary OAG. The need for consensus case definition. Journal of Epidemiology and Community Health, 2003; 57:752-754.
6 Hollows FC, Graham PA. Intraocular pressure, glaucoma and glaucoma suspects in a defined population. Br J Ophthalmol, 1966; 50:570-586.
7 Klein B et al. Prevalence of glaucoma. The Beaver Dam Study. Ophthalmology, 1992;99:1499-1504.
8 Mitchell P et al. Prevalence of open angle glaucoma in Australia. The Blue Montains Eye Study. Ophthalmology, 1996; 103:1661-1669.
9 Holland G. A review of the Keeler Pulsair EasyEye Tonometer. optician, May 24, 2002; No 5852:Vol 223.
10 Cockburn D. Referrals from Optometrists to Ophthalmologists. Aust J Optom, 1975; 58: 161-164.
11 Henson D. Optometric Instrumentation. Butterworths, London 1983.
12 Nidek Co Ltd. Nidek non-contact tonometer NT-4000, Operators Manual 2002.

optician would like to thank Birmingham Optical Group (0121 442 2222) for the loan of this equipment.

Tina Romanay is clinical manager at the Fight for Sight Optometry Clinic, City University
For many years it has been standard practice to measure intraocular pressure (IOP) through a non-contact means by averaging a series of readings.
This does not imply any loss of accuracy with this technique, merely that it only allows a snapshot of the constantly changing IOP. So any single reading might be at one of several points on an ever changing pressure curve.
One of the main variables influencing transient IOP is the cardiac cycle responsible for the so-called ocular pulse which reflects shifts in IOP of around 2mmHg to 4mmHg in phase with the systemic pulse. A new non-contact tonometer has been introduced by Nidek claiming to monitor ocular pulse allowing a single reading to accurately gauge IOP by compensating for the variation. Before discussing the NT-4000 in practice, a short revision of tonometry may be in order.

How is IOP measured?
The 'tonometer' is the name given to an instrument designed to measure the IOP of the eye in units of millimetres of mercury (mmHg). IOP is clinically estimated either by measuring the degree of corneal deformation produced by a known weight (indentation), or by measuring the weight needed to produce a given degree of deformation (applanation).
Indentation tonometry (as done using, for example, the Schištz tonometer), is based on the degree of indentation made on the cornea, created by a plunger of a known weight, being proportional to the IOP.
The degree of indentation produced by the plunger is displayed on the scale of the instrument. This scale measurement can be converted into mmHg by use of the Friedenwald tables.
Due to the large amount of fluid displaced by the tonometer when placed on the eye, readings with two known weights should be taken to obtain an average IOP with use of the Friedenwald tables. Due to the excessive corneal damage that may be caused by indentation tonometry, it is rarely used in practice.
Contact applanation tonometry (such as Goldmann and Perkins tonometers), as the term implies, applanates or flattens a small area of the cornea. It exploits the Imbert-Fick law which, when applied to the eye, states that the IOP is equal to the tonometer weight divided by the applanated area.
Although the cornea does not strictly satisfy the structural requirements applicable to the Imbert-Fick law, applanating an area with exact diameter of 3.06mm and careful mechanical design of the tonometer allows IOP to be measured over an extensive range.
Prolonged contact of the probe against the cornea means that transient fluctuations in IOP caused by the cardiac cycle, lid and eye muscle movements are averaged out, and therefore only one reading of the IOP need be taken. Recent research has shown that for a truly accurate measurement, when perhaps a decision regarding medical treatment depends on the IOP, corneal thickness needs also to be measured.
Unlike contact applanation and indentation tonometers that employ mechanical contact to flatten the cornea and therefore achieve a reading of the IOP, non-contact tonometers (NCTs) produce a pulse of air to flatten a small region at the apex of the cornea (approximately 3mm).
Grolman introduced the first NCT in 1972 (AO NCT). A piston, which is contained within a cylinder and rapidly moved by a solenoid, produces a puff of air that increases linearly with time and is released through a nozzle until there is sufficient force to momentarily applanate the cornea. The area applanated is detected by an optical system. The instrument then calculates the time required for the air to applanate the eye. This time is related to the IOP of the eye. Many types of NCTs have since been marketed based on this principle.
Calibrations of NCTs have been achieved from clinical studies on both normal and glaucomatous subjects, in which the results of the NCT were compared with those of the Goldmann.
Single measurements of IOP with NCTs are not very repeatable and variations of approximately 3-4mmHg are common due to, among other things, the cardiac cycle. However, this problem has been overcome by taking three readings which, when averaged, give a more reliable representation.

Why use NCT?
IOP as measured with the Goldmann contact tonometer is the clinical standard against which all other tonometers are compared.
Because of its mechanical design, this instrument has been demonstrated to have less variation in repeated measurements of IOP than NCTs. However, for some patients it is not the instrument of choice.
The advantages of non-contact tonometers versus contact tonometers have no doubt been discussed in depth in the past. The most obvious advantage is that it eliminates the need for anaesthesia, as no contact is made between the tonometer and the eye, therefore avoiding problems of corneal damage, infection and allergic reaction to topical drugs.
New NCTs do not need special training of the observer to obtain reproducible and rapid measurements. For these reasons, non-invasive methods for the measurement of IOP have gained popularity with the optometric community.

The Instrument
The NT-4000 is an NCT used to measure the IOP of patients' eyes. It consists of a combined main unit and measuring unit, installed on a base. The main unit houses the monitor, control panel and joystick. The measuring unit houses the air nozzle and photo sensor. The base houses the chin rest and printer.

How does it work?
The NT-4000 increases the air pressure puffed onto the cornea in proportion to time. The shape of the cornea changes from convex to concave as it is flattened by the puff of air.
This change in shape is optically detected by monitoring the maximum amount of reflected light (that is transmitted from a photo transmitter on to the cornea) captured by a photo detector.
The time required to flatten the surface after puffing air out is then calculated by the instrument, and the air pressure used to make the cornea flat is calculated from the time. Finally, the IOP is obtained.

General Functions
Selectable modes of operation
Three modes of operation can be selected. Full auto, where both alignment and focusing are carried out automatically; semi auto where alignment is automatic, focusing is manual; and manual where both focusing and alignment are manual.

Auto alignment mode
As the measuring unit is placed close to the pupil, the instrument automatically focuses the pupil, and executes vertical and horizontal alignment such that a measurement is taken automatically.

Focusing indicator
This shows the distance between the patient's eye and the air nozzle.

Confidence mode
Despite a measurement being taken, any data that is indicated with an asterisk shows a level of unreliability and should be repeated. This kind of measured data is called 'low confidence data'.

What's new?
The NT-4000 has features similar to that of current available models, as mentioned. However, new features incorporated are 'pulse synchronised IOP', 'eyelid checking function', 'expanded auto-focusing detection range' and 'advanced APC (automatic puff control) function'. These claim to help achieve smoother and more stable measurements of IOP.

Pulse-synchronised IOP measurement
This distinguishing feature allows for a more accurate IOP reading to be taken with consideration for the change in IOP caused by the ocular pulse (cardiac cycle).
A pulse detector on the forehead rest on the measuring unit provides a pulse signal when the patient's head rests against it. When the pulse signal and the alignment of the pupil match, an IOP reading is taken automatically. Pulse detection can be taken at three different levels of the ocular pulse Ð peak, middle and bottom. It can be done either in 'pre-time' Ð pulse sync is controlled based on the prediction of the patient's pulse Ð or in 'real-time' when pulse sync is based on the patient's detected pulse. This feature can be easily deactivated and the NT-4000 can be used as a normal screening system.

Eyelid checking function
To obtain a stable and accurate measurement of the IOP, an automatic reading is taken only when the eye is fully open after alignment and focusing.
Therefore, before taking a reading, the instrument checks the position of the eyelid to avoid an error. This is done by reflecting four detection lights onto the cornea (at 1 o'clock, 4 o'clock, 7 o'clock and 10 o'clock). If the eyelid obstructs any of these reflections, the instrument informs the operator that a reading cannot be taken by wording 'open the eye', and the operator is then required to make appropriate adjustments. This function can be switched off at any time.

Expanded auto-focusing detection range
In the automatic focus/alignment mode, compared to the conventional model, the auto-focusing detection range has been expanded more than 50 per cent in the focusing direction. This allows for quicker and more comfortable measurements to be obtained.

Advanced APC function
This allows for a softer puff of air on to the cornea according to the patient's IOP. In the automatic puff control mode (APC40 or APC60), after the first puff of air is delivered and a measurement is taken, the automatic shut-off function activates to eliminate excess air puffing on to the cornea. The next measurement taken is then performed with a softer puff, based on the former measured data. This means that the eye is protected from surplus pressure which may be uncomfortable for the patient. Compared to the conventional model, the amount of air pressure applied has been reduced by more than 10 per cent, resulting in increased patient comfort.
How was it for me?
On first appearances, the NT-4000 looked very similar to its fellow NCTs. Yet with its compact colour screen, it was striking and noticeably appealing to both the operator and the patient. Surprisingly, I found this simple feature to be helpful and effective in putting the patient at ease with an otherwise intimidating instrument. As we have all no doubt heard at some point before measuring patients IOPs: 'Oh no. Not the puffer test?'
Setting up the patient on the instrument was no different to any other NCT with the aid of the eye level marker, chin rest adjuster and height-adjustable base unit.
The three different modes of operation (auto, semi and manual) proved useful on patients that did not respond to the automatic function, either because of poor alignment, stability on the instrument or corneal deformation. In addition, the auto alignment mode allowed beginners and non-trained staff to use the instrument easily and therefore reduced variability in results obtained due to lack of operator experience.
If a patient's eye was not open enough, or the eyelids/lashes obstructed the light detection system during measurement, a measurement error occurred. Like its fellow NCTs, the Nidek NT-4000 informed the operator if the measurement was inaccurate by either showing APL Ð applanation error; ALM Ð alignment error; BLK Ð blinking and slight movement of the eye made measurement impossible; or PRS Ð pressure error.
To improve measurements relating to eyelid position, I found the eyelid detection mode to be very useful. When using NCTs we cannot always be sure if interference from the eyelids is affecting the automatic measurement mode. One can find oneself adjusting the patient in many ways before realising it is in fact an error due to the eyelids. Therefore, if any of the four reflective lights from the cornea were being obstructed it was immediately apparent to the operator, and therefore appropriate instructions could be given to the patient to open their eyes further, or make necessary modifications before taking a reading.
I did, however, find that subtle lid variations in older patients made the instrument more sensitive to measurements being taken, and the instruction of 'Open eye' came up more than I would have liked.
The manufacturer claims that if the eyelid detection mode is switched off (from the sub switch compartment), increased errors in measurements and fluctuations may occur. Therefore this should only be cancelled when it is very difficult to perform a measurement. When this mode was cancelled, a marker appeared at the bottom right hand corner of the display screen (indicated by a picture of a lower lid with a cross over it).
Measurements were taken on a few patients with and without the eyelid detection mode, and a mean variation of 1mmHg was found respectively. However, it was difficult to determine if this fluctuation was due to the detection mode being switched off or a normal variation in IOP. This function does not operate in the manual mode.
As we know the IOP is not constant within an individual and varies according to the ocular pulse whether the patient is erect or supine, and the time of day at which a reading is being taken. The ocular pulse causes a fluctuation in the IOP of about 2-4mmHg. Therefore, use of the pulse synchronised mode claims to reduce the variability that may arise due to ocular pulse (cardiac cycle), and help the level of accuracy for measurements of IOP obtained.
I found that the patient's forehead needed to be in constant contact with the pulse detector on the forehead rest, for a pulse signal to be detected. The patient also needed to be very still for a reading to be obtained. This meant that the measuring time took longer than the normal screening test by approximately one and a half minutes on patients that were examined. If three readings were taken while the pulse synchronisation feature was still operational, it was interesting to note there was some variation between readings still present. Furthermore, subtle patient movements made taking measurements more difficult and occasionally the pulse detection mode had to be switched off in order to obtain a reading of the IOP.
Use of the pulse detection mode did help to discipline the operator to set the patient up more accurately, as it is quite easy for the patient to veer from the forehead rest while the operator attempts to take readings when the patient is not within the correct working range.
When the pulse synchronised mode is operated, a symbol appears on the print out that indicates at which point a measurement has been taken. This may be at the peak, middle or bottom of the ocular pulse. The factory setting is set at peak, but this can be altered in the parameter change mode.
The manufacturer states that even though the pulse synchronised mode is utilised, three readings are still advised in any of the pulse settings, in order to average the results obtained.
Ten patients that were examined with the NT-4000, showed variations in IOP of approximately 1-2mmHg between the peak, middle and bottom range of the ocular pulse. These patients were also examined on both the NT-3000 and NT-4000. IOP was found to be approximately 1-2mmHg higher with the NT-3000.
Additionally a comparison was made on two patients between the IOP reading obtained at the peak ocular pulse (average of 3mmHg) of the NT-4000, and the Goldmann tonometer. Pressures were found to be approximately 1-2mmHg higher with the NT-4000. These results, although very limited, showed the high reliability and accuracy of the NT-4000.
Those few patients that were tested on both the NT-3000 and NT-4000 noted that the 'air-puff' created while measuring IOP was less uncomfortable with the latter instrument. This effect could be due to the refined automatic puff control mechanism, or simply due to the conditioning effects of applanation.

What's the verdict?
I found the NT-4000 easy to use and non-qualified operators could achieve reliable and stable results with this instrument.
The help of various alignment and focusing mechanisms helped to produce repeatable and accurate results. The eyelid checking function facilitated in ensuring that reliable readings of the IOP could be taken in the automatic mode of operation. This mechanism also made it useful for the practitioner to decide if in extreme cases the manual mode of operation should be employed.
The appearance of the tonometer (colour screen) proved to be an interesting discussion point for patients, seemingly putting them at ease with an otherwise intimidating piece of equipment.
The manufacturer claims that the use of the APC increases patient comfort due to the decrease in air puff pressure after the first initial reading, thus improving the experience. Although this was noted by a few patients, the remainder did not notice a considerable difference.
The use of the pulse-synchronised IOP measurement claims to reduce the fluctuation in IOP that may occur due to the ocular pulse. Using this device increased measurement time, occasionally becoming uncomfortable for the patient. Moreover, three readings were still needed to be taken to achieve an average result which seemed to defeat the object.
In theory, the concept of compensating for the ocular pulse is an interesting one. If the pulse was the only variable influencing IOP, then one would expect the three readings taken when the synchronisation was active to be identical; in the default case, representing the IOP at the peak measurement of the cycle. However, the variation of 2-3mmHg still found must reflect either some inaccuracy in the synchronisation or, much more likely, the influence of other variables, such as fluctuations in accommodation, lid tension, pupil diameter, breathing rate and extraocular muscle tension. The introduction of the synchronisation should be applauded but I would argue that one should always remember that ocular pulse is one of several variables and hence the manufacturer's advice to still take three readings is sound.
Although nominal comparison of this instrument was made with its fellow NCT and the Goldmann tonometer, the NT-4000 revealed a good level of accuracy.
The development of NCTs that take the ocular pulse into consideration may increase the accuracy of measurements taken without the need to applanate where it is not possible, thereby aiding practitioners to detect and appropriately refer patients with increased IOP.
It is stressed that further investigation and research is needed on a larger population-based sample to determine the accuracy and reliability of IOP readings obtained with the NT-4000 as compared to the Goldmann tonometer the current 'gold standard'.

Further reading
1 Kini M et al. Prevalence of senile macular degeneration and open angle glaucoma in the Framingham eye study. Am J Ophthal, 1978;85:28-34.
2 Cockburn D. Diagnosis and management of open angle glaucoma: suggested guidelines for optometrists. Clin Exp Optom, 2000;83:3:119-127.
3 Klein B et al. Heritability of risk factors for primary open angle glaucoma. The Beaver Dam Eye Study. Investigative Ophthalmol and Vis Sci, 2004;45:59-62.
4 Le A Mukesh et al. Risk factors associated with the incidence of open angle glaucoma. The Visual impairment project. Investigative Ophthalmol and Vis Sci, 2003 Sep;44 (9):3783-9.
5 M Kroese and H Burton. Primary OAG. The need for consensus case definition. Journal of Epidemiology and Community Health, 2003; 57:752-754.
6 Hollows FC, Graham PA. Intraocular pressure, glaucoma and glaucoma suspects in a defined population. Br J Ophthalmol, 1966; 50:570-586.
7 Klein B et al. Prevalence of glaucoma. The Beaver Dam Study. Ophthalmology, 1992;99:1499-1504.
8 Mitchell P et al. Prevalence of open angle glaucoma in Australia. The Blue Montains Eye Study. Ophthalmology, 1996; 103:1661-1669.
9 Holland G. A review of the Keeler Pulsair EasyEye Tonometer. optician, May 24, 2002; No 5852:Vol 223.
10 Cockburn D. Referrals from Optometrists to Ophthalmologists. Aust J Optom, 1975; 58: 161-164.
11 Henson D. Optometric Instrumentation. Butterworths, London 1983.
12 Nidek Co Ltd. Nidek non-contact tonometer NT-4000, Operators Manual 2002.

optician would like to thank Birmingham Optical Group (0121 442 2222) for the loan of this equipment.

Tina Romanay is clinical manager at the Fight for Sight Optometry Clinic, City University