ORIGINAL ARTICLE Year : 2020  |  Volume : 22  |  Issue : 2  |  Page : 207-211

Comparative study of measurement of intraocular pressure in eyes with corneal opacity and normal cornea by Tonopen Avia, rebound tonometer (I-care), and noncontact tonometer

Sridhara Reddy1, Vijay K Sharma2, Partha Sarathy Moulick3, Atul Kumar Singh4
1 Assistant Professor, Department of Ophthalmology, Air Force Central Medical Establishment, New Delhi, India
2 Professor, Department of Ophthalmology, Army Hospital (Research and Referral), New Delhi, India
3 Consultant, Apollo Gleneagles Hospital, Kolkata, India
4 Associate Professor, Department of Ophthalmology, Command Hospital (Air Force) Bangalore, India

Date of Submission11-Feb-2020Date of Decision06-Mar-2020Date of Acceptance30-May-2020Date of Web Publication02-Sep-2020

Correspondence Address:
Wg Cdr Sridhara Reddy
Department of Ophthalmology, Air Force Central Medical Establishment (AFCME), New Delhi - 110010
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jmms.jmms_10_20

Purpose: The most common indications for keratoplasty are patients with corneal opacity. The presence of glaucoma prior to keratoplasty is an important risk factor, eventually may lead to failure of the corneal graft. Currently, we have insufficient evidence of comparison of intraocular pressure (IOP) measurement in patients with corneal opacity. The purpose of this study is to analyze the IOP measurements obtained with Tonopen (TP) Avia, I-Care rebound tonometer, and noncontact tonometer (NCT) in patients with corneal opacity and normal cornea. Materials and Methods: It is a prospective, comparative study using a convenience cohort of eyes with corneal opacity. Patients having corneal opacity and normal cornea had IOP measured using TP (Avia-Reichert, Depew, Newyork, USA), rebound tonometer (I-care), and NCT (Reichert AT 555). IOP measurements were compared and agreement assessed. Bland–Altman plots, box–whisker plot, and error–bar diagram were used for assessing agreement. Results: Thirty eyes with corneal opacity and thirty eyes with normal cornea of the same patients were included in the study. The mean IOP readings in eyes with normal cornea taken by NCT, rebound tonometer, and TP Avia were 13.71 mm Hg with standard deviation (SD) of 2.73, 14.85 mm Hg with SD of 2.13, and 14.73 mm Hg with SD of 2.27, respectively. The mean IOP readings in eyes with corneal opacity taken by NCT, rebound tonometer, and TP Avia were 17.35 mm Hg with SD of 5.82, 15.83 mm Hg with SD of 3.2, and 15.98 mm Hg with SD of 3.53, respectively. Bland–Altman analysis plots show good agreement between all three devices. Conclusion: IOP measurements in normal cornea showed underestimation by NCT and overestimation by rebound tonometer with TP as standard, whereas in eyes with corneal opacity, IOP measurements showed overestimation by NCT and underestimation by rebound tonometer. TP Avia was proved to be a useful instrument because of its smaller contact area and repeatability.

Keywords: Corneal opacity, I-care pro, intraocular pressure, noncontact tonometer, Tonopen

How to cite this article:
Reddy S, Sharma VK, Moulick PS, Singh AK. Comparative study of measurement of intraocular pressure in eyes with corneal opacity and normal cornea by Tonopen Avia, rebound tonometer (I-care), and noncontact tonometer. J Mar Med Soc 2020;22:207-11
How to cite this URL:
Reddy S, Sharma VK, Moulick PS, Singh AK. Comparative study of measurement of intraocular pressure in eyes with corneal opacity and normal cornea by Tonopen Avia, rebound tonometer (I-care), and noncontact tonometer. J Mar Med Soc [serial online] 2020 [cited 2021 Dec 5];22:207-11. Available from: https://www.marinemedicalsociety.in/text.asp?2020/22/2/207/294190   Introduction 

Corneal blindness is the major cause of avoidable blindness worldwide after cataract and glaucoma. Ocular trauma and corneal ulceration are significant causes of corneal blindness responsible for 1.5–2.0 million new cases of monocular blindness every year.[1],[2] The most common indications for keratoplasty are patients with corneal opacity.

The presence of glaucoma prior to keratoplasty is an important risk factor for postoperative rise of intraocular pressure (IOP). An early and accurate detection of increased IOP in eyes with corneal opacity prior to penetrating keratoplasty is important, as increased IOP in these patients may cause progressive endothelial cell loss and eventually may lead to failure of the corneal graft.[3],[4] The diagnosis of glaucoma after penetrating keratoplasty is a major clinical problem because of inconsistency in IOP measurements caused by graft irregularities, thick or astigmatic corneal graft, and irregular corneal surface epithelium.[5],[6]

Currently, we have insufficient evidence comparing different tonometers for the measurement of IOP in patients with corneal opacity. There is a need to compare the IOP measurement using tonometers in patients with corneal opacity.

The purpose of this study is to compare and analyze the IOP measurements taken with Tonopen (TP) (Avia-Reichert, Depew, New York, USA), rebound tonometer (I-care Finland Oy, Vantaa, Finland), and noncontact tonometer (NCT) (Reichert AT 555) in patients with corneal opacity and normal eye of same patient and assess agreement between these devices.

  Materials and Methods 

It was a prospective, comparative study at a tertiary care hospital. Institutional ethical clearance and written informed consent were taken from all the patients. The study was performed in accordance with the tenets of the Declaration of Helsinki. Patients with maculoleucomatous corneal opacity up to 6 mm in one eye and normal cornea of other eye were included in the study. Patients who had keratoconus, prior ocular surgery, nystagmus, and known glaucoma history were excluded from the study.

A total of sixty eyes of thirty patients were studied. All the patients underwent slit-lamp biomicroscopic examination, followed by IOP measurement using NCT, rebound tonometer, and TP Avia by a single examiner (author). The sequence of examination was constant and in comfortable sitting posture.

IOP was measured using NCT, where three readings were taken, followed by rebound tonometer and finally by TP Avia. Central corneal thickness (CCT) was also measured with ultrasonic pachymeter (model – DGH Ultrasound Pachymetry-Pachy 2) in both the eyes. The regular calibration of tonometers was performed by the trained personnel as per the manufacturer's recommendation.

For measuring IOP using NCT, the patient was asked to sit in the chair opposite eyepiece of NCT. Power button was turned on and the instrument was allowed to warm up for 30 s. The eyepiece was adjusted till the reticule was in good focus. The instrument was adjusted so that the light from the instrument objective shines in the center of the patient's pupil. The patient was asked to see a green dot between the two red dots. Once the alignment was proper, the air pulse button was pressed and three IOP readings were taken.

For measuring IOP using rebound tonometer, the patient was asked to sit in the chair. The instrument power button was turned on. The adjustable forehead rest of the instrument was placed on the patient's forehead, so that the probe is 4–8 mm from the cornea. When the measurement button was pressed, a round tipped probe of 0.9 mm radius held in position by an electromagnetic field collides with the patient's cornea. Five consecutive readings were taken and average readings of IOP displayed in the monitor were noted along with the standard deviation (SD) variability.

For measuring IOP using TP Avia, the patient was asked to sit in the chair. The cornea is anesthetized by 0.5% proparacaine. Instrument tip was brought in contact with the cornea, which in turn activates a strain gauge that senses the force generated by a central 1.02 mm diameter movable plunger to applanate the central cornea. When the instrument made an audible click sound, a measurement is taken. A total of five measurements were taken. The final averaged reading along with the percentage variability between the highest and lowest value was recorded.

As CCT is a known modifier of tonometer readings, CCT was also measured with ultrasound pachymetry in the supine position. The cornea was anaesthetized with topical 0.5% proparacaine. After 5 min, the patient was asked to look straight above. The pachymeter probe was placed on the center of the cornea and then the thickness was measured with a SD range to be within 2.5. A drop of topical antibiotic ciprofloxacin 0.3% was instilled in both eyes after the procedure.

Suitable precautionary measures were taken to disinfect the tonometer tips as per the guidelines. All data were coded and analyzed through SPSS statistics 20.0 (IBM SPSS statistics for Windows, version 20.0, Armonk, NY, USA).

  Results 

We examined thirty eyes with corneal opacity and thirty eyes with normal cornea of the same patients. The age of the patients ranged from 28 to 78 years with a mean of 53 years. Out of which, 20 patients were males (66.6%) and 10 were females (33.3%).

The mean IOP readings in eyes with normal cornea taken by NCT, rebound tonometer, and TP Avia were 13.71 mm Hg with SD of 2.73, 14.85 mm Hg with SD of 2.13, and 14.73 mm Hg with SD of 2.27, respectively [Table 1].

The mean IOP readings in eyes with corneal opacity taken by NCT, rebound tonometer, and TP Avia were 17.35 mm Hg with SD of 5.82, 15.83 mm Hg with SD of 3.2, and 15.98 mm Hg with SD of 3.53, respectively [Table 2].

Bland–Altman plots, showing the average of the IOP measurement by two devices, is plotted on the X-axis and the difference between devices (TP-NCT, TP-rebound tonometer) is plotted on the Y-axis. The analysis of mean difference of IOP measurement in normal cornea between TP and NCT was − 1.22 with SD (2.69), which as statistically significant (P = 0.019) [Figure 1]. Similarly, the mean difference of IOP measurement between TP and rebound tonometer in normal cornea was 1.17 with SD (2.73) and was statistically significant (P = 0.026) [Figure 2].

Figure 1: Bland–Altman analysis of intraocular pressure in normal cornea by Tonopen and noncontact tonometer

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Figure 2: Bland–Altman analysis of intraocular pressure in normal cornea by Tonopen and rebound tonometer

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Bland–Altman analysis plot of IOP measured in corneal opacity eyes with TP and NCT showed mean difference of 0.99 with SD (5.13), which was not statistically significant (P = 0.299) [Figure 3]. Similarly, between TP and rebound tonometer showed mean difference of −0.77 with SD (2.83) which as not statistically significant (P = 0.186) [Figure 4].

Figure 3: Bland–Altman analysis of intraocular pressure in eyes with corneal opacity by Tonopen and noncontact tonometer

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Figure 4: Bland–Altman analysis of intraocular pressure in eyes with corneal opacity by Tonopen and rebound tonometer

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Error bar diagram of comparison of the mean values of the IOP in normal cornea by TP was 14.9 mm Hg, NCT 13.6 mm Hg and rebound tonometer 16 mm Hg. By keeping TP as standard, variation of IOP by NCT underestimated by 1.3 mm Hg and that with rebound tonometer overestimated by 1.1 mm Hg. The range of variation was 1.1–1.3 mm Hg, which was in acceptable range [Figure 5].

Figure 5: Error bar chart comparison of the mean values of the intraocular pressure in normal cornea using the three methods

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Error bar diagram of comparison of the mean values of the IOP in eyes with corneal opacity by TP was 16.3 mm Hg, NCT 17.3 mm Hg and rebound tonometer 15.6 mm Hg. By keeping TP as standard, variation of IOP by NCT overestimated by 1 mm Hg and that with rebound tonometer underestimated by 0.7 mm Hg. The range of variation was 0.7–1 mm Hg, which was in acceptable range [Figure 6].

Figure 6: Error bar chart comparison of the mean values of the intraocular pressure in eyes with corneal opacity using the three methods

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CCT in eyes of corneal opacity showed mean readings of 510.17 with SD of 61.06. CCT in normal cornea showed mean readings of 539.63 with SD of 42.42.

  Discussion 

IOP measurements play an important role in ophthalmology in the diagnosis and follow-up of patients with ocular hypertension, glaucoma, and postpenetrating keratoplasty. Corneal opacity alters the corneal thickness and affects corneal biomechanical properties, making measurement of IOP inaccurate.[7] Patients who have undergone corneal transplant surgery for corneal opacity may lose vision due to preexisting undiagnosed glaucoma. 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. In irregular corneal surface, use of Goldmann applanation tonometer (GAT), which is a gold standard technique, yields inaccurate readings due to pooling of fluorescein dye and irregular images of the mires.[8],[9]

This study included IOP measurements in thirty eyes with normal cornea and thirty eyes with corneal opacity in the same patient. The mean age group of the study was 53 years with a range of 28 and 78 years. There was a male predominance in this study compared to females. This could be because males had better access to medical facilities whereas females did not have any support system to offer them help to seek medical advice.

The measurement of IOP using NCT showed initially a learning curve especially in eyes with corneal opacity as patient has to focus on to the focusing light for proper alignment. In patients having adherent leukoma, photophobia, and bilateral corneal opacity, IOP could not be measured. A minimum of three readings of IOP within 3 mm of Hg were taken as it increases the probability that an instantaneous pressure measurement will lie within a given range of mean IOP.

In our study, IOP measurement between TP and NCT in normal cornea showed that NCT tends to overestimate IOP by 1.22 mm Hg on an average over IOP measurement by TP. Similarly, IOP measurement between TP and rebound tonometer in normal cornea showed that rebound tonometer tends to underestimate IOP by 1.17 mm Hg. The results using this method in normal cornea showed good agreement with acceptable readings.

IOP measurement between TP and NCT in eyes with corneal opacity showed that NCT tends to underestimate IOP on an average by 0.99 mm Hg and similarly IOP measurement between TP and rebound tonometer showed that rebound tonometer tends to overestimate IOP by on an average of 0.70 mm Hg.

Fernandes et al.[10] in their study with 46 healthy eyes of students, the rebound tonometer was compared to applanation tonometry. The rebound tonometer overestimated the IOP value by 1.34 mm Hg on average when compared with the GAT. Rebound tonometer was also compared to TP and noncontact tonometry (Pulsair 3000). Measurements performed with the rebound tonometer and the TP were in good agreement with that of the GAT. There were no study found in the literature, where similar studies done comparing IOP in corneal opacity eyes using these tonometers.

Using the box-and-whisker plot analysis and error bar diagram, the median and mean value of IOP measurements in normal cornea showed underestimation by NCT and overestimation by rebound tonometer with TP as standard, whereas in eyes with corneal opacity, the median and mean value of IOP measurements showed overestimation by NCT and underestimation by rebound tonometer with TP as standard.

Rootman et al. and Chou et al.[11],[12] in their study reported that the 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 because of its smaller contact area and repeatability.

CCT is known to alter IOP readings of tonometer and hence several correction formulas incorporating CCT are reported. The use of such correction formulas is debatable and a recent report found that the application of such formulas did not yield better agreement between different tonometers.[13] As our primary goal was agreement between devices, no such correction as utilized in our study. Pachymetry readings in eyes with normal cornea showed mean of 539.63 with SD of 42.42 and 510.17 with SD of 61.06 in eyes with corneal opacity. There was no significant correlation found between pachymetry and IOP measured by three methods.

  Conclusion 

The determination of IOP in patients with corneal pathology is challenging. Accurate diagnosis and follow-up of elevated IOP in eyes with corneal opacity, who are ideal patients for corneal transplant, is very important.

From this study it can be concluded that in eyes with corneal opacity IOP measurement by NCT tends to underestimate and rebound tonometer tends to overestimate in comparison to TP. TP Avia proved to be a useful instrument because of its smaller contact area and repeatability. Both NCT and rebound tonometer were found to be user friendly for the operator and caused less discomfort to the patient, as there is no need for local anesthesia and no risk of corneal abrasion or cross infection.

We also carried out study of correlation between CCT and IOP readings by three methods. No significant correlation was found in our study, which was in consonance with a study done by Vandewalle et al.[14] and Liu and Roberts et al., which also have shown that the relationship between CCT and IOP is nonlinear.[15]

Our study has several limitations, a relatively small cohort and nonhomogenized patient population. Yet, this study is still one of the useful reports of agreement of IOP measuring devices in corneal blindness patients. Further such studies are needed to better examine the correlation between corneal opacities and IOP measurement.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
  [Table 1], [Table 2]