|Year : 1977 | Volume
| Issue : 3 | Page : 29-35
Clinical comparative pressure studies with Goldmann; Mackay-Marg and Pneumatic Applanation tonometers
MR Jain, VJ Marmion
Consultant Ophthalmic Surgeon Bristol Eye Hospital, BRISTOL-U.K
M R Jain
Professor of Ophthalmology, S.M.S. Medical College Hospital, Jaipur, India
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Jain M R, Marmion V J. Clinical comparative pressure studies with Goldmann; Mackay-Marg and Pneumatic Applanation tonometers. Indian J Ophthalmol 1977;25:29-35
|How to cite this URL:|
Jain M R, Marmion V J. Clinical comparative pressure studies with Goldmann; Mackay-Marg and Pneumatic Applanation tonometers. Indian J Ophthalmol [serial online] 1977 [cited 2020 Aug 4];25:29-35. Available from: http://www.ijo.in/text.asp?1977/25/3/29/31266
Schiotz tonometer, being cheap and easily portable, gained immense popularity in the recent past, but the results of Schiotz tonometry are known to be affected, to certain extent, by factors like resistance of the scleral sheath to stretching, compressibility of the vascular content of the eye and the ease with which the fluid is expressed through the drainage channels.
Goldmann in 1955 developed an applanation tonometer on the principle of Imbert-Fick's law which states that flattening of a very small area of a fluid filled sphere can be done by a counter pressure exactly equal to the hydraulic pressure within the sphere without raising this hydraulic pressure and hence Goldmann applanation readings approach very close to the levels of pressure in the undisturbed eye. Somehow, there are a few drawbacks which limit its utility. Firstly, the instrument, as a routine, can be used only in sitting posture; secondly, its portability is extremely restricted since it has to be used along with slit lamp; thirdly, it is incapable of recording pressure in ectatic and oedematous corneas and in physically handicapped people with kyphosis or lordosis; lastly, fluoresceine is essential to be used to measure pressure, the use of which is contra-indicated in soft contact lens wearers.
In 1959, Mackay-Marg described a new electronic applanation tonometer. It functions by applanating the cornea with a probe in the centre of which is a small 1.5 mm diameter plunger that senses the ocular tension. The plunger is a fused quartz rod that moves through minute excursions. As the probe is gently pressed against the cornea, the plunger is depressed and the recording stylus traces a pressure curve. A typical curve [Figure - 1] shows an initial rise and a crest followed by a temporary trough and then a rise; when the probe is removed from the cornea, the curve is retraced in a mirror image fashion. About less than half a second's applanation is sufficient to provide a good curve and usually 4 to 5 applanations are done in each eye to get best results.
In 1968, a team of workers headed by Langham and McCarthy developed another rapid acting applanation tonometer, called 'Applanation Pneumatonograph (PTG) [Figure - 2]. This instrument is a combination of pneumatic and electronic system. The pneumatic portion of the instrument measures actual intraocular pressure with the help of a delicate sensor that applanates the cornea and floats almost frictionless on the air bearing. The electronic portion of the instrument translates these measurements into electronic signals which can be shown on the digital display and simultaneorsly recorded on a continuous chart.
The most essential element of PTG is a sensor probe which consists of an air bearing, pressure chamber and a hollow piston (tubing) which is coupled to the silicon membranecovered-tip assembly [Figure - 3]. The gas (dichloro-difloro methane) diffusion into the air bearing provides a cushion effect and causes the piston to float virtually without friction. The gas also flows to the tip and provides an air cushion under the silicon membrane. When the tip of the sensor is applanated to the cornea, the gas flow is restricted and a pressure increase is affected, which continues to increase until it is equal to intraocular pressure. The pressure increase is transmitted to the electronic system which displays the pressure level of the eye. The pressure level and the corresponding amplified (X4) amplitude of ocular pulse can also be recorded precisely with this instrument [Figure - 4]. The PTG has also provision to be used for measuring facility of aqueous outflow by' applying 10 gm weight to the sensor disc.
In our earlier study, we evaluated PTG with Goldmann later-on, while conducting postural studies, Mackay-Marg was evaluated with PTG.
The present study is a continuation of the previous work with a modification that this study is conducted by all the three instruments in the same eyes to further evaluate their clinical correlations.
| Material And Methods|| |
The study was conducted in Bristol Eye Hospital and the patients included were either those attending out-patient clinic or admitted in the wards.
The study comprised two groups of patients. The first was 50 non-glaucomatous subjects (100 eyes) presenting with varying Ophthalmic problems or for routine check up. The second group consisted of 35 patients (50 eyes) with Goldmann pressure of 22m Hg. or more.
In both the groups, patients' age ranged between 30 and 80 years with a high proportion of cases (70 percent) between 50 and 70 years. The male female ratio was 5:7.
All needful adjustments, and if necessary, caliberation was done, with all the three instruments before use. Patient was seated erect on the chair and the eyes were anaesthetised with 0.4 percent benoxinate hydrochloride drops. Mackay-Marg tonometer was the first instrument to be used to record the pressure. The Mackay-Marg probe was covered with an autoclaved tonocover and a drop of mineral oil was applied to its tip. While the patient looked straight, the centre of the tip of the probe was applanated to the apical zone of the cornea for about half a second and withdrawn abruptly. Such 4-5 applanations were done in each eye and the recordings obtained [Figure - 5].
After 5 minutes interval, local anaesthesia was repeated. PTG machine was switched to "Tonometry" and while the patient maintained a straight gaze, the tip of the PTG sensor probe was applanated to the apical zone of the cornea for about 3 to 5 seconds. Correct applanation was confirmed by a continuous whistle emmitted from the machine. During applanation, the level of IOP was flashed on the "digital display" and simultaneously the pressure recordings were obtained on the graph paper.
After another 5 minutes, the pressure was recorded with Goldmann's tonometer in the classical way.
An interval of 5 minutes during each recording was allowed to avoid any effect of previous applanation on consecutive recording.
| Results|| |
The results of our mean pressure recordings in group-I, II and I and II combined are compiled in [Table - 1]. It shows that mean pressures in group-1 by Goldmann, PTG and MackayMarg are 16.97, 17.83 and 18.93 respectively, suggesting that PTG and Mackay-Marg recordings give readings of approximately 1 mm and 2 mm higher than Goldmann. In group-II, PTG and Mackay-Marg recordings are slightly higher than in Group-I, suggesting that in glaucomatous subjects PTG and Mackay-Marg give slightly higher readings than in nonglaucomatous subjects.
[Figure - 6],[Figure - 7] are complete scattergrams in all the 150 eyes. Here the PTG and MackayMarg pressure recordings have been plotted against the Goldmann readings. Coorelation coefficient of 0.89 in [Figure - 6] and 0.84 in [Figure - 7] suggest excellent correlation of PTG and Mackay-Marg recordings with Goldmann readings. By intercept points 'a' in both above figures, it is interpreted that PTG and Mackay-Marg recordings are higher by 0.95 mm Hg and 2.09 mm Hg as compared to Goldmann recordings.
Frequency difference between Goldmann and PTG readings and Goldmann and MackayMarg is shown in [Figure - 8]. It is observed that between 36 and 50 per cent of eyes by PTG and 20 and 30 per cent by Mackay-Marg show f same recordings as Goldmann in group I and II respectively, and 8 and 20 per cent by PTG and 2 and 14 per cent by Mackay-Marg show even lower pressure than Goldmann. Pressure difference of 4 mm Hg or more is observed in 4 and 16 per cent by PTG and 12 and 36 per cent by Mackay-Marg, suggesting that the difference of frequency is higher in group-II, specially with Mackay-Marg recordings.
| Discussion|| |
A good clinical correlation between Goldmann and pneumatonograph recordings in 40 normal and equal number of glaucomatous eyes was reported in our earlier study. It was found that pneumatonograph readings were generally higher by 0.75 mm. Hg. in normal and 1.25 mm. Hg. in glaucomatous eyes. In the present study a larger group of normal (hundred) and glaucomatous (fifty) eyes were included and the study was conducted with three applanation instruments with a definite order of Mackay Marg first, pneumatonograph next and the Goldmann last, with an interval of five minutes between each reading. Such an order with an adequate interval was strictly adhered to since an applanation effect usually causes variable degree of hypotension due to neurovascular reflex pressure-lowering phenomenon , and this lowering effect was demonstrated to be comparatively more with Goldmann applanation than pneumatonograph.
An excellent correlation of pneumatonograph readings with Goldmann has been demonstrated in our present study. In the normal eyes, the mean pneumatonograph readings compared to Goldmann were noted to be higher by 0.86 mm. Hg. and in glaucomatous eyes by 1.16 mm. Hg. Both the figures are in close agreement with our earlier study and with the studies of Quigley and Langham where they noted a difference of 1.30 mm. Hg. In a series of 100 eyes, with mean Goldmann and pneumatonograph pressures of 20.2 + 1.0 and 21.50 + 0.81 mm. Hg. respectively. In their study, the order of recording was the pneumatonograph first and the Goldmann afterwards but when this order was reversed, the difference was reduced to only 0.45 + 0.24 mm. Hg., suggesting a significant applanation hypotensive effect of Goldmann tonometer. Both instruments gave exactly similar readings in 43 per cent of our total eyes and maximum difference of 4 mm. Hg, was observed only in 10 per cent of eyes. The difference frequency was, however, significantly less in normotensive eyes as compared to glaucomatous eyes.
The Mackay-Marg pressure recordings also showed an excellent correlation with Goldmann readings. It was found that mean MackayMarg readings were generally higher by 2.06 mm. Hg. when plotted in all the 150 eyes but when separately calculated the difference was 1.96 mm. Hg. in normal eyes and 2.12 mm. Hg. in glaucomatous eyes. A high clinical correlation was earlier reported ,, with a basic agreement that mean Mackay-Marg pressures were generally higher than Goldmann and pneumatonograph recordings. Tierney and Rubin studied a large group of 113 normal and 248 mixed group of eyes (normal & Pathological) and noted a mean difference of 1.81 mm. Hg. in the former and 2.13 in the latter group of eyes which appears to be very close to our findings. Earlier, while studying postural pressure changes in glaucomatous and non-glaucomatous eyes, we noted that Mackay-Marg recordings were higher approximately by 1 mm. than pneumatonograph readings which is indirectly in agreement to our present findings. The data obtained from frequency difference histogram shows that the MackayMarg and Goldmann readings are equal in 25 per cent of eyes and pressure difference more than 4 mm. was noted in 24 per cent of eyes, suggesting a higher reliability of pneumatonograph as compared to Mackay-Marg tonometer.
It can be concluded that pneumatonograph and Mackay-Marg tonometres are both versatile instruments to record intraocular pressure and they bear an excellent clinical correlation to Goldmann tonometer. There also appears to be a general agreement that these instruments record pressures higher than Goldmann, specially the Mackay-Marg tonometer.
Both rapid applanation tonometres have the advantage of being used in any posture,,, are easily portable, do not require the aid of a slit lamp or the use of fluorescein and can record pressure even in ectatic ,, or oedematous corneas, in animals and in immediate postoperative period.,
The disadvantages of Mackay-Marg tonometer are its generally high pressure recordings, need for frequent caliberations, repeated applanations to get correct tracings and their interpretations. Whereas applanation pneumatonograph has few more significant advantages. Firstly, when applanated, it's correct applanation is confirmed by a continuous whistle emitted from the machine; secondly, the intraocular pressure is simultaneously flashed on the digital display as well as graphically recorded. The recordings do not have the potential inaccuracy of visual mean estimation as in Goldman tonometer or the possible bias of choosing a correct recording and interpreting it as in Mackay-Marg recordings. Thirdly, the instrument can simultaneously by used to study the facility of aqueous outflow and lastly, it can provide amplified recordings of ocular pulse whose importance is recently realized in the diagnosis of border line cases of glaucoma" and carotid insufficiency.
The only disavantage of pneumatonograph is its high cost and dependence on a scarcely available gas (dichloro-difluro-methane) which under specific federal law has to be imported from U.S.A.
| Summary|| |
Intraocular pressure recorded with three modern applanation tonometers in a definte order of Mackay-Marg first, pneumatonograph next and the Goldmann last, in 100 normal and 50 glaucomatous eyes. Scattergrams drawn to compare the recordings show an excellent clinical correlation between these readings. It was found that the mean pneumatonograph readings were generally higher by 0.95 mm. Hg. and Mackay-Marg by 2.06 mm. Hg. when compared to Goldmann readings. In general, the mean difference was more in glaucomatous eyes than in normal eyes. The merits and demerits of each instrument were dsscussed at length.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8]
[Table - 1]