|Year : 1966 | Volume
| Issue : 1 | Page : 6-12
Experience with the posner-inglima tonometer
BD Telang, HG Badlani
K. B. Haji Bachooali Eye Hospital, Bombay, India
|Date of Web Publication||12-Jan-2008|
B D Telang
K. B. Haji Bachooali Eye Hospital, Bombay
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Telang B D, Badlani H G. Experience with the posner-inglima tonometer. Indian J Ophthalmol 1966;14:6-12
There have been indeed many instruments and techniques to record the pressure of the contents of the eye ball which for clinical purposes cannot be measured directly as such but indirectly through the state of tension of the cornea and sclera. There are in fact, so many factors with wide variations involved in this indirect deduction of the intra-ocular tension that Sir Stewart Duke-Elder has written that the results of tonometry will always remain as approximations and will always involve uncertainties. The pursuit of an ideal instrument therefore appears to be unending and the scales of tonometer still lure the research worker.
Indentation tonometer appeals to ophthalmologists because the measurement is accomplished quickly. However, there is no scientific foundation for interpreting its results. Applanation tonometer on the other hand, requires more time, but is more accurate and is founded on scientific principles expressed through mathematical formulations. The Maklakov tonometer is the simplest type of applanation tonometer and has been in use for many years in U.S.S.R. and Eastern Europe. It uses a known force to produce an area of corneal flattening "Applanation", as against the Goldman type where the force is varied and the area of applanation is kept constant. The Posner - Inglima Applanometer is a modified Maklakov applanation tonometer. The applanation method of tonometry is indeed the more reliable for the detection of early cases of glaucoma, associated with high myopia and also of glaucoma in young individuals. Because it produces a minimal volume displacement, the applanation measurement is not significantly affected by variations in ocular rigidity and thus permits better differentiation between normal and glaucomatous eyes.
Posner-Inglima applanometer is a set of 4 tonometers and a magnetic handle. Two tonometers are identical and weigh 5 gms. each, one weighs 7.5 gms. and one weighs 10 gins. All the tonometers are similar in size and shape; two truncated cones with their smaller ends facing each other and connected by a cylindrical shaft. The body of the tonometer is made of high grade nonmagnetic stainless steel and contains a freely sliding ballast which gives the tonometer greater stability when it is placed on the cornea, and ensures a low centre of gravity whichever end is being used. The handle is made of stainless steel and is hollowed out to accommodate a permanent magnet with a minimum lifting force of one pound. The handle may be attached to the tonometer instantly by bringing it into contact with the steel tab into which is fitted the stainless steel ring which loosely encircles the shaft of the tonometer. Two 5 gm. weights are provided in each set so that duplicate readings may he taken. (The 5 gm; tonometer is the one most frequently used, since it is to be preferred for normal and border line tensions.) With the tonometer in place and the instrument suspended above the eye, the vertical handle acts as a guide to perpendicularity. The end plates which are 9 mm. in diameter each, and which are polished to optical flatness are made of Pyroceram, an extremely hard, heat-resistant ceramic material. They are fully exposed in order to facilitate cleansing and sterilization. They are highly resistant to scratching by abrasive materials. The end plates are fitted and cemented to the body of the tonometer by a heat resistant adhesive. Since, Pyroceram is a highly polarized material the end plates accept a uniform coating of the brown stain (mild silver protein) and the imprint made by the cornea retains its sharply outlined edge which permits accurate measurement.
The applanometer, thus, is a constant force applanation tonometer and the area of corneal applantion which it produces varies inversely with the height of the intra-ocular pressure. The absence of moving parts eliminates any instrumental error due to friction. Rigid specifications regarding weight of instrument and degree of flatness of the end plates assure absolute uniformity and obviate the need for future re-calibration.
Application of the Tonometer
The patient is placed either in a supine or in a reclining position and any inadvertent pressure on the vessels in the neck e.g. by a tight collar must be removed. He is made to fix his gaze vertically upwards by use of a fixation target. An anesthetic is instilled into the eyes. The end plates should always be wiped clean with a pledge of cotton moistened with alcohol or water, and no finger print should be allowed to remain on the applanating surface. To avert danger of infection each end plate is heated in the flame of a spirit lamp for 4 to 5 seconds and later cooled before use. The mild silver protein should be applied to the end plates in a uniformly thin layer to produce a light tan colour. A cotton-tipped applicator may be used for this purpose. [Figure - 1]
The magnetic handle is applied to the surface of the metal tab. The eye lids are gently retracted, being careful not to exert pressure on the eye ball. The tonometer is suspended above the eye and lowered until the full weight rests on the centre of the cornea. [Figure - 2] The ring of the metal tab should not move more than half way clown the tonometer shaft. The tonometer is then lifted off the eye in a vertical direction. The tonometer should not rest on the cornea for more than one-half of a second.
The interaction between the stain and the pre-corneal film displaces the stain from that portion of the end plate which comes in contact with the cornea. The area of contact appears as a white circular imprint on the end plate, which is sharply demarcated from the surrounding stained area which had not made contact with the cornea. The area of this imprint bears an inverse relationship to the height of the intraocular pressure. The ideal imprint is a white area circular in outline and contains a central spot of stain. This spot of stain is an artefact and is without significance for the measurement. Any deviation from the circular shape indicates that the tonometer has slipped while in contact with the cornea. Such slippage may be caused either by movement of the tonometer or by movement of the patient's eye. If the imprint is oval in shape, it is accepted provided that the shortest diameter is measured. If the imprint is grossly distorted or has indistinct margins, the measurements are repeated. Inspection of the imprint thus permits the operator to verify the correctness of his technique and to control the accuracy of his measurements. This feature is unique to this form of tonometry.
Since the corneal applanation is measured not on the cornea but on the tonometer end plate it is necessary to establish a relationship between the applanated area of the cornea and the area of the imprint. The process of corneal applanation gives rise to the formation of 3 zones; a central zone, an intermediate zone and the outer zone. [Figure - 3] When the tonometer flattens the cornea, the layer of stain on the corresponding portion of the end plate becomes dissipated and mixes with the precorneal film. The dissipation of the stain in this early stage of applanation produces a homogenous coloured circular area which may be referred as the central zone. As the area of applanation increases in size, the outward displacement of a portion of the pre-corneal film results in the gradual formation of a dark coloured liquid ring, which constitutes the intermediate zone. The outer margin of this ring appears sharply defined. The inner margin, which is less clearly defined, represents the outer border of the flattened area and thus is the true measure of the corneal applanation.
Surrounding the liquid ring, there is formed an outer clear zone which is relatively free of stain. While the tonometer is being removed from the cornea, the area of contact becomes smaller and the liquid ring moves towards the centre. The outer margin of the clear zone, however, remains fixed in position and forms a circle the diameter of which is regarded as a measure of the "corneal applanation.". [Figure - 3]
The diameter of the imprint may be measured with a transparent scale supplied with the applanometer or by using a 8 X measuring magnifier with the scale divisions of 0.1 mm. The measuring scale contains two pairs of covergent guide lines and four columns of numbers. The column of number at the left of the scale gives the values for the intraocular pressure in mmHg. when the 5 gm. weight is used; the next two columns of numbers give the values when the 7.5 Gm. and 10 Gm. tonometers are used. The column of number at the right of the scale gives the diameter of the imprint in mm. This is useful for recording the tension by the size of the diameter of the imprint rather than by the conventional method in mmHg. If a measuring magnifier is used instead of the transparent scale, the corresponding intra-ocular pressure in mmHg, is obtained by referring to a conversion table. The transparent measuring scale used for measuring the diameter of the imprint introduces an element of inaccuracy and is not recommended if maximum degree of precision is to be achieved. The sources of error derive partly from parallex produced by the overlying protective plastic film, partly from the fact that the naked eye does not discern the innermost edge of the imprint. The use of the measuring scale with the naked eye generally leads to an overestimation of the diameter and consequently, to an underestimation of the intra-ocular pressure. It is important to bring the imprint into direct contact with the scale so as to avoid parallex. The imprint on the end plate should be measured promptly. It can however, be transferred to a fine grain absorbent paper supplied with the instrument. The edges of the imprint are generally sharper and more distinct on the end plate than they are on the transfer paper. The advantage of the transfer paper, however, is the fact that it enables to keep a permanent record of the measurement. If the ocular tension is expected to be within normal limits the 5 gm. tonometer should be used. If the resulting imprint has a diameter smaller than 4 mm., the measurement should be repeated with the 7.5 gm. tonometer. In cases of high myopia, congenital glaucoma, or any other condition in which the possibility of abnormal ocular rigidity exists then, the 10 gm. tonometer should be used in conjunction with the 5 gm. tonometer. If the difference between the two measurements is greater than 3 mmHg., the eye does not have average ocular rigidity.
Ocular Rigidity - With the applanometer it is possible to estimate ocular rigidity by means of paired readings. This is a valuable diagnostic aid in cases of borderline tension. If the tension as measured with the 7.5 gm. or the 10 gm. tonometer is the same as that obtained with the 5 gm, tonometer, the eye is said to have a "normal ocular rigidity". If the tension readings are higher with the heavier weights, the ocular rigidity is high. If the tension readings are lower with the heavier weight, the ocular rigidity is low. If the ocular rigidity is abnormal in either direction, the readings obtained with the 5 gm. tonometer give the closest approximation to the true intra-ocular pressure. There is no tonometer which measures the true P o state of the eye.
However, we may assume that the measurements obtained with the Goldman Tonometer come close to the value of P,. The Applanometer at best expresses values similar to those obtained with the Goldman Tonometer.
| Study Report|| |
The series under the present study consist of 152 normal and glacomatous eyes examined with the new applanometer and Schiotz tonometer. Selection of a particular weight of the tonometer was based, as in the case of Schiotz tonometer, on the tension of the eye ball. 5 gm. weight is used for pressures upto 35 mmHg, 7.5 gm. weight for pressure ranging between 35 - 49 mmHg and 10 gm. weight for pressures above 49 mmHg.
Each case was first examined with the Applanometer and then with the Schiotz tonometer. Generally corresponding weights were used in the two instruments. Normal cases were selected from the refraction department who showed no evidence of any eye disease on examination ' or interrogation. Glaucoma cases were chosen from those attending the `glaucoma clinic' at the Haji Bachooalli Eye Hospital, Bombay, and included treated as well as untreated cases.
The report is as follows:[Table - 1]
Average normal intra-ocular pressure in this series according to Applanometer was found to be 15.1 mmHg. The range of intra-ocular pressure varied from 10 to 18 mmHg.
The average normal intra-ocular pressure for the same subjects examined with the Schiotz Tonometer was found to be 16.7 mmHg and the range varied from 10 to 21 mmHg.
All the tonometric findings are charted on a graph for comparison. There appears to be a general consistency of the readings on the two instruments for the lower ranges of tension. However, it appears that for higher tension, there is sometimes a great difference between the readings obtained from the two instruments. [Figure - 4]
An average difference of 2.5 mmHg was noticed. However, in terms of percentage difference, there was a significant increase of 9.5% in the total figure value of the Schiotz readings as compared with the same of the Applanometer, the maximum difference between the readings of the two instruments was noted as high as 26 mmHg.
From these figures it appears that the Applanometer registers slightly lower values than the Schiotz Tonometer for eyes with normal and slightly raised tensions. However, in cases of high tension, this difference becomes at times, very marked. These differences can be due to many factors including the factor of ocular rigidity.
Extensive series of cases have been examined by Posner-Inglima and others to substantiate the accuracy and utility of this instrument. Our own experience with this instrument, although small, has impressed us of the many advantages that it possesses.
Comparison with Schiotz Tonometer
1. - The Applanometer is claimed to be a permanent tonometer which does not need any re-testing. The Schiotz tonometer which is a most common instrument in use has the greatest drawback of becoming defective. From our own experience as well as from the personal communications with other colleagues, we feel that the Schiotz Tonometer often suffers from mechanical defects causing great uncertainty of its readings. In our country specially, where replacing or repairing and retesting of the instrument is not an easy proposition, an introduction of a permanent reliable instrument would be naturally most welcome.
2. - The calibration of Applanometer like that of Goldman Tonometer is based on scientific laws of mathematics unlike that of Schiotz indentation tonometer where the calibration is done on empirical basis which undergoes repeated changes.
3. - The instrument is non-traumatic. The mirror polish of the end plates ensures that no corneal injury takes place. In our work, we did not notice a single case of corneal abrasion.
4. - The method is more sensitive than Schiotz Tonometry.
5. - The inaccuracies caused by frictions in the instrument are entirely eliminated.
6. - Errors caused by abnormal ocular rigidity are reduced. This is particularly important for the diagnosis of glaucoma with the presence of High Myopia etc.
Advantages over the Goldman Tonometer:
1. -It is a simple portable instrument and does not need the use of a Slit Lamp.
2. -It is easy to handle and causes minimum trauma to the eye and hence can be conveniently used even in children and nervous patients.
3. - It can be sterilized and hence may be used in the Operation Theatre.
Other features: 1. A permanent record of the actual measurements may be obtained by transferring the imprint from the end plate to the paper.
2. The reliability of the readings can be checked by examining the regularity of the outline of the imprint.
3. It may be used in conjunction with Tonography for the direct measurements of P o value.
4. The precision readings can be obtained specially for research work by utilising the special magnification system for measuring the diameter of the imprint.
| Summary|| |
A new type of applanometer is described in which intra-ocular pressure is determined by measuring the area of flattening of cornea caused by resting on it a known weight with a perfectly flat surface.
The results in 46 normal and 106 glaucomatous eyes are compared with those obtained by a Schiotz tonometer.
The advantages of this simple portable device over a Schiotz Tonometer or a Goldmann's Applanometer are stated.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
[Table - 1]