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ORIGINAL ARTICLE
Year : 2006  |  Volume : 54  |  Issue : 2  |  Page : 95-98

ProTon tonometer determination of intraocular pressure in patients with scarred corneas


Department of Ophthalmology, Post Graduate Institute of Medical Research and Education, Chandigarh, India

Correspondence Address:
Arun K Jain
Advanced Eye Centre, Post Graduate Institute of Medical Research and Education, Chandigarh - 160 012
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0301-4738.25829

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  Abstract 

Purpose: To evaluate the measure of intraocular pressure (IOP) in patients with scarred corneas obtained from the affected and non-affected areas.
Materials and Methods: Eighteen patients with small maculo-leucomatous corneal opacity following microbial keratitis were prospectively included in the study. Goldmann applanation tonometry was done first on the non-affected corneal surface. ProTon tonometry (PT) was then carried out on the same eye, to obtain IOP measurements from the non-affected (PT1) and the affected area (PT2) of the cornea. The IOP measurements were compared using a two-tail paired t test.
Results: The mean of IOP measurements of PT 2 and PT1 was 22 mm of Hg (SD 5.2) and 14.8 mm of Hg (SD 5.4), respectively. The higher reading of PT2 was statistically significant ( P <0.001). The mean Goldmann applanation tonometry was 14.6 mm of Hg (SD 5.4) and it did not differ significantly ( P = 0.86) from the PT1 readings of 14.8 mm of Hg (SD 5.4).
Conclusion: In patients with maculo-leucomatous corneal opacity, determination of IOP by ProTon tonometer varies from the affected to the non-affected area. The ProTon tonometer overestimates the level of IOP when it is applied to a leucomatous corneal opacity.

Keywords: Goldmann applanation tonometer, proton tonometer, tonopen, intraocular pressure, corneal opacity


How to cite this article:
Jain AK, Sukhija J, Gupta A. ProTon tonometer determination of intraocular pressure in patients with scarred corneas. Indian J Ophthalmol 2006;54:95-8

How to cite this URL:
Jain AK, Sukhija J, Gupta A. ProTon tonometer determination of intraocular pressure in patients with scarred corneas. Indian J Ophthalmol [serial online] 2006 [cited 2019 Oct 15];54:95-8. Available from: http://www.ijo.in/text.asp?2006/54/2/95/25829



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The determination of the Intra ocular pressure (IOP) in patients with corneal pathology is problematic. The recordings with the Goldmann applanation tonometer (GAT) are not reproducible, because the irregular corneal surface causes pooling of the fluorescein dye and irregular images of the circles.[1],[2] Other commonly used techniques, such as Schiotz tonometry, non-contact tonometry and palpation method, are not accurate.[3],[4]

Rootman et al reported that the Tono-Pen (TP) provided IOP measurements similar to the MacKay-Marg after keratoplasty, epikeratophakia and in scarred corneas, suggesting that the TP was as accurate as the Mackay-Marg tonometer.[5] The two electronic tonometers, ProTon (PT) and TP share similar measurement principles and are developed on the basis of MacKay-Marg tonometer. In normal corneas, the PT tonometer appears to have a higher level of accuracy than the TP tonometer, using the GAT as a standard method.[6] The PT tonometer has a strain gauge that converts IOP into an electrical signal, transmitted to a microprocessor where it is analyzed for acceptability. The measurements are displayed on a liquid crystal panel. The instrument records multiple measurements and displays the average with a confidence interval. This instrument has certain advantages over the GAT. It is portable and compact, easy to calibrate, has a disposable tip that eliminates contamination risks, can be used regardless of the patient's position and the digital display minimizes user bias. Because of its smaller contact area, the TP is considered more accurate for the measurement of the IOP in eyes with irregular corneas.[5] In eyes with scarred, irregular cornea, measurement of the IOP is difficult and critical, as the corneal opacity or associated media opacity precludes an accurate assessment of optic nerve head and visual field examination.

The reliability of PT tonometry for recording IOP upto 21 mmHg in normal and post kertoplasty eyes have already been proved.[7] The purpose of this study was to analyse the IOP measurements obtained with the PT tonometer and GAT in patients with scarred cornea and also to compare the values obtained from PT tonometry on affected and non-affected areas on the same eye.


  Materials and Methods Top


In this prospective study, eighteen consecutive patients with scarred cornea (small maculo-leucomatous corneal opacity of size <3.5 mm) were selected from the cornea clinic of the department of Ophthalmology, over a 3 months period. The GAT and the PT tonometer (Tomey Technology Inc, Cambridge, MA) were calibrated as per the instruction manual before each day of use. The Goldmann applanation tonometry (GATY) was performed first, from the non-affected area of the cornea. Astigmatism was not considered and the biprism was adjusted such that the interface between the two prisms was oriented horizontally. Two measurements were performed on each eye and averaged. However, if the difference between the two measurements exceeded 2 mm Hg, three readings were averaged. If the GATY was not possible from the non-affected area of the cornea, the patient was excluded from the study. PT tonometry was then carried out immediately on the same eye, to obtain a measurement of the highest reliability (SEM <0.5 mmHg) from the non affected (PT1) and the affected area (PT2) of the cornea. However we did not randomize the sequence of measurement. The two-tailed paired t test analysis was used to analyze the data. A P -value of less than 0.05 was considered statistically significant. The reading discrepancies between GATY and PT1, GATY and PT2, PT1 and PT2, were compared using the regression test and paired t test. Ninetyfive percent confidence interval was calculated and level of agreement between each of the three sets was seen by the Altman Bland plots. The study was approved by the institutional research review board.


  Results Top


Eighteen patients with small leucomatous corneal opacity following microbial keratitis were included in the study. The age of the patients varied from 10 years to 61 years [mean 36.7 15.7]. The results of IOP measured with GATY, PT1 and PT2, are shown in [Table - 1]. GAT IOP readings varied from 10 mm of Hg to 26 mm of Hg.PT1 IOP ranged from 9 mmHg to 30 mmHg and PT2 from14 mmHg to 33 mmHg. The mean GAT reading of 14.6mm of Hg (SD 4.5) did not differ significantly from the mean PT1 reading of 14.8 mm of Hg (SD 5.4). The average of PT2 readings of 22.7 mm of Hg (SD 5.2) was significantly higher ( P <0.001) than mean IOP obtained by the GAT (14.6 mm of Hg SD 4.5) and PT1 (14.8 mm of Hg (SD 5.4). Altman Bland graph shows the level of agreement between IOP by GATY and PT1.This was significant. ( P = 0.034). Overall, the mean difference between the two methods of measurement (GATY and PT1) was negligible [Figure - 1]. Mean differences (and 95% confidence intervals) between GATY and PT1, between GAT and PT2 and between PT1 and PT2 were - 0.28 (-4.24 to + 3.69), -8.17 (-13.80 to -2.53), -7.89

(-13.78 to -1.99), respectively. There was a wide variation in the level of agreement between GATY and PT2, PT1 and PT2 for all levels of IOP [Figure - 2] and [Figure - 3]. The correlation coefficient between GATY and PT1, GATY and PT2, PT1 and PT2, were 0.94, 0.83 and 0.84, respectively. A scatter plot and linear regression analysis of IOP measurements can be observed in [Figure - 4] and [Figure - 5].


  Discussion Top


The present study demonstrates that, in eyes with leucomatous corneal opacity, PT tonometry readings varies from the affected to the non-affected area. PT readings are significantly higher from the affected area, when compared to readings from non-affected area. Somewhat similar findings have been reported in a study where TP was used to record IOP from the affected (band shaped keratopathy or glued corneas) and nonaffected part of the same cornea.[8] According to literature, the TP is not as accurate as the GAT in normal corneas, especially when measuring IOP above 30 mm Hg.[9],[10],[11] In normal corneas, the PT tonometer appears to have a higher level of accuracy than the TP tonometer, using GAT as a standard method.[6] Considering its limitations, the IOP estimation by PT tonometer may be clinically useful in several clinical situations, especially in patients with corneal edema or scarring, because of its similarities with the MacKay-Marg tonometer.[12],[13] The TP is reported to be as accurate as the MacKay-Marg tonometer in measuring IOP after keratoplasty and in scarred corneas in which Goldmann is not useful.[5] The precision and reliability of the PT tonometer have not been addressed in this study. Therefore the question whether the obtained IOP recording through non-affected cornea or affected cornea accurately reflects the current IOP cannot be answered with certainity. The issue of precision and reliability of the PT tonometer in normal and post keratoplasty eyes is addressed elsewhere.[7] The readings obtained by GAT in this study were similar to the PT values from the non-affected area of cornea. Khan et al compared TP readings obtained from various corneal and scleral locations and concluded that mean readings from the mid peripheral and clear limbal cornea did not differ significantly from the central corneal readings over a 10-35 mmHg IOP range.[14] In our study, range of IOP was 10-26 mmHg with GAT and 9-30 mmHg with PT tonometer from the non-affected area. Acceptable regular mires were obtained in all patients, because a small corneal opacity causes less irregularity of the cornea outside the area of opacity. It is suggested that an average error of more than 3 mmHg cannot be tolerated in the diagnosis and treatment of vision threatening diseases.[15],[16] In our series, none of the differences between GAT and PT from the non-affected area was more than 3 mmHg, except in one case where it was 4 mmHg. The mean difference between GATY and PT1 was only -0.28. Based on these observations, we hypothesise that the PT readings obtained from non-affected corneas do not differ greatly from IOP measured by GAT. The higher levels of IOP readings obtained with the PT from affected areas are fictitious and probably due to the hardness associated with the leucomatous corneal opacity. In this study, patients with only small maculo-leucomatous corneal opacities were included. Whether patients with other grades of opacity (nebular or maculo-nebular) will show similar changes in measurement of IOP with different tonometers, cannot be commented upon from this study. It may still have an upper edge over GAT, when measuring IOP in more scarred cornea which makes the GATY impossible to be performed. One limitation of the study was that, we did not randomize the sequence of measurements, which could be source of error due to repeated measurement of IOP.

In conclusion, in eyes with leucomatous corneal opacity, the IOP varies from the affected to the non-affected area, while using the PT tonometer. The PT appears to overestimate the level of IOP when it is applied to areas with leucomatous corneal opacity. One should avoid taking IOP readings with the PT tonometer from the area of opacity. It may also be deduced that, in cases where the total cornea is scarred, one may overestimate IOP with PT tonometer. There is no study that compares PT or TP recorded IOP to manometrically measured IOP readings, in scarred irregular corneas. Further studies are suggested to compare PT and GAT recording of IOP in manometrically measured IOP in scarred irregular corneas.

 
  References Top

1.
Moses RA. The Goldmann applanation tonometer. Am J Ophthalmol 1958;46:865-9.  Back to cited text no. 1
[PUBMED]    
2.
MacMilan F, Forster RK. Comparison of MacKay-Marg, Goldmann and Perkins tonometers in abnormal corneas. Arch Ophthalmol 1975;93:1420-4.  Back to cited text no. 2
    
3.
Kaufman HE. Pressure measurement: which tonometer? Invest Ophthalmol Vis Sci 1972;11:80-5.  Back to cited text no. 3
[PUBMED]    
4.
Baum J, Chatuved N, Netland PA, Dreyer EB. Assessment of intraocular pressure by palpation. Am J Ophthalmol 1995;119:650-1.   Back to cited text no. 4
    
5.
Rootman DS, Insler MS, Thompson HW, Parelman J, Poland D, Unterman SR. Accuracy and precision of the TonoPen in measuring intraocular pressure after keratoplasty and epikeratophakia and in scarred corneas. Arch Ophthalmol 1988;106:1697-700.  Back to cited text no. 5
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6.
Midelfart A, Wigers A. Clinical comparison of the Proton and Tono-Pen tonometers with the Goldmann applanation tonometer. Br J Ophthalmol 1994;78:895-8.  Back to cited text no. 6
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7.
Pandav SS, Sharma A, Gupta A, Sharma SK, Gupta A, Patnaik B. Reliability of ProTon and Goldmann Applanation Tonometers in normal and post keratoplast eyes. Ophthalmology 2002;109:979-84.  Back to cited text no. 7
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8.
Azuara-Blanco A, Bhojani TK, Sarhan AR, Pillai CT, Dua HS. Tono-Pen determination of intraocular pressure in patients with band keratopathy or glued cornea. Br J Ophthalmol 1998;82:634-6.  Back to cited text no. 8
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9.
Kao SF, Lichter PR, Bergstrom TJ, Rowe S, Musch DC. Clinical comparison of Oculab Tono-Pento the Goldmann applanation tonometer. Ophthalmology 1987;94:1541-4.  Back to cited text no. 9
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10.
Frenkel REP, Hong YJ, Shin DJ. Comparison of the Tono-Pen to the Goldman applanation tonometer. Arch Ophthalmol 1988;106:750-3.  Back to cited text no. 10
    
11.
Menage MJ, Kaufman PL, Croft MA, Landay SP. Intraocular pressure measurements after penetrating keratoplasty: minified Goldmann applanation tonometer, pneumatoto-onometer and Tono-Pen versus manometry. Br J Ophthalmol 1994;78:671-6.  Back to cited text no. 11
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12.
Geyer O, Matron Y, Lowenstein A, Neudorfer M, Rothkoff L, Lazar M. Tono-Pen tonometry in normal and in post-keratoplasty eyes. Br J Ophthalmol 1992;76:538-40.  Back to cited text no. 12
    
13.
Minckler DS, Baerveldt G, Heuer DK, Quillen-Thomas B, Walonker AF, Weiner J. Clinical evaluation of Oculab Tono-Pen. Am J Ophthalmol 1987;104:168-73.  Back to cited text no. 13
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14.
Khan JA, Davis M, Graham CE, Trank J, Whitacre MM. Comparison of oculab Tono-Pen readings obtained from various corneal and scleral locations. Arch Ophthalmol 1991;109:1444-6.  Back to cited text no. 14
[PUBMED]    
15.
Armstrong TA. Evaluation of the Tono-Pen and the Pulseair Tonometers. Am J Ophthalmol 1950;109:716-20.  Back to cited text no. 15
    
16.
Higginbotham EJ. Clinical evaluation of the Oculab Tono-Pen. Am J Ophthalmol 1988;105:1.  Back to cited text no. 16
[PUBMED]    


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