Comparison of Intraocular Pressure Measurements With Goldmann Applanation Tonometry, Tonopen XL, and Pascal Dynamic Contour Tonometry in Patients With Descemet Membrane Endothelial Keratoplasty : Journal of Glaucoma

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New Understandings of Glaucoma: Original Studies

Comparison of Intraocular Pressure Measurements With Goldmann Applanation Tonometry, Tonopen XL, and Pascal Dynamic Contour Tonometry in Patients With Descemet Membrane Endothelial Keratoplasty

Yildiz, Izlem MD; Altan, Cigdem MD; Çakmak, Semih MD; Genc, Selim MD; Yildirim, Yusuf MD, FEBO, FICO; Agca, Alper MD

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doi: 10.1097/IJG.0000000000002089
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Descemet membrane endothelial keratoplasty (DMEK), first introduced by Melles, has been applied in Fuchs endothelial dystrophy, bullous keratopathy, and other corneal endothelial disorders.1 DMEK surgery involves selective removal of the patient’s dysfunctional endothelial cell layer and Descemet’s membrane followed by transplantation of donor corneal endothelium and Descemet’s membrane without additional stromal tissue from the donor.2 Compared with penetrating keratoplasty, this surgical technique has resulted in a good visual outcome and fewer postoperative complications, such as glaucoma.3–5

Following DMEK, an increase in IOP and glaucoma can develop. Maier et al6 found that only 12.1% developed a postoperative IOP elevation, excluding cases with postoperative pupillary block. Naveiras et al7 also found a lower 12-month incidence of IOP elevation after DMEK (6.5%). In addition, the 12-month incidence of post-DMEK glaucoma (2.7%) was lower than described for postpenetrating keratoplasty.8–12

Uncontrolled postkeratoplasty IOP can cause endothelial cell loss followed by graft failure and poor visual outcome; in addition, irreversible visual field loss owing to glaucoma can also occur. Regular IOP measurement is essential in eyes undergoing DMEK surgery. But accurate IOP measurement can be difficult because of biomechanical corneal alterations.13

The Goldmann applanation tonometer ([GAT] Haag Streit, Koeniz, Switzerland) is accepted as the gold standard technique for obtaining IOP measurements.14 GAT applies the Imbert–Fick law, which postulates that for an ideal thin-walled dry sphere, the pressure within the sphere is equal to the force needed to flatten its surface. IOP is proportional to the pressure applied to the ocular globe (in practice to the cornea) and the thickness of its walls (such as corneal thickness).15 Corneal biomechanical and geometrical properties, such as corneal curvature, central corneal thickness (CCT), corneal rigidity, and corneal hysteresis (CH) as measured with an ocular response analyzer (Reichert Ophthalmic Instruments, Buffalo, New York, USA) can affect the IOP measurements using GAT. The accuracy of GAT, which was calibrated for a mean CCT of 520 µm, is affected by corneal thickness.16,17 GAT measurements underestimate IOP in eyes with thin corneas and overestimate IOP in eyes with thick corneas.18,19 Postkeratoplasty corneas typically have clinical or subclinical edema, so their thickness is >520 µm; thus, GAT measurements may be inaccurate for postkeratoplasty corneas.20

Dynamic contour tonometry (DCT-Pascal, SMT Swiss Microtechnology, Port, Switzerland), independent of corneal characteristics, is a digital non-applanation contact tonometer.21 This instrument has a 7-mm diameter concave-surface probe that adapts to corneal contour and does not change its shape or curvature. Therefore, the resulting distortion is minimal because the cornea does not need to be flattened for IOP measurement as required for an applanation tonometer. An electronic pressure sensor embedded in the tonometer’s concave probe surface enables direct measurements of transcorneal pressure without being affected by the biomechanical properties of the cornea.22

Tono-Pen XL (Reichert, Depew, New York, USA) is a portable electronic tonometer based on the applanation principle (as is GAT), but its applanation area (2.36 mm2) is smaller than that of GAT (7.35 mm2). Reducing the applanation area leads to a decrease in the difference between applanation pressure and IOP because of the reduced corneal resistance of a smaller contact area.23

This study aimed to compare IOP measurements taken with Pascal DCT, Tono-Pen XL, and GAT in a group of patients who underwent DMEK surgery and to evaluate the effect of CCT on IOP measurements after DMEK.


Thirty-four eyes (from 34 patients) who had a successful DMEK surgery for pseudophakic bullous keratopathy and who applied for a control examination between May and June 2020 were included in this cross-sectional, comparative, and single-center study. This study was conducted in accordance with the principles of the Declaration of Helsinki. All patients gave informed consent to participate in the study after being informed about the study. Approval for the study was obtained from the Ethics Committee of Istanbul Training and Research Hospital in May 2020, with the decision numbered 2333.

We included patients over 18 years of age who underwent DMEK surgery at least 1 month before the start of the study, which had an attached endothelium with Descemet membrane and clear cornea, and from whom reliable measurements could be obtained with three tonometers. We excluded patients who had corneal edema, Descemet membrane folds, blepharospasm, previous ocular surgery other than cataract surgery, and a history of ocular trauma. We noted the time between our examination and DMEK surgery.

Surgical Technique

The corneal graft (7.5–8 mm) was prepared before the patient was taken to the operating room. Four limbal side cuts were created, and through an anterior chamber maintainer connected with a continuous irrigation mode of vitrectomy, air was introduced into the anterior chamber. A reverse Sinsky hook was used to perform descemetorhexis (8 mm), and a 23G vitrectomy was used to perform an inferior iridectomy. A 2.8 mm wide clear corneal incision was then made, the maintainer was removed, and the anterior chamber was formed with a balanced salt solution. After the prepared graft was stained with trypan blue, the graft was injected into the anterior chamber through the corneal incision using an intraocular lens cartridge. The incision was closed with a 10/0 Nylon suture. A no-touch technique was chosen to unfold the endothelium by gently touching the cornea from the external epithelial surface. After checking the graft’s position, SF6 20% gas was injected under the graft to fill 80% of the anterior chamber. The patient was instructed to lie down in the supine position for at least 30 min postoperatively and then move to his/her bed.

Postoperatively, all eyes received topical prednisolone acetate (1.5%) four times daily for four weeks. At the 1-month follow-up visit, medication was switched to fluorometholone 0.1% four times daily and then gradually tapered to once per day until the 3-month postoperative visit. Moxifloxacin 0.5% drops four times daily for two weeks. Post-operatively, artificial tears without preservatives were prescribed, to be used for at least 1 year.

After routine ophthalmologic examinations of all eyes (including refractive examination and slit-lamp biomicroscopy) but before the fundoscopic examination, IOP was measured using all methods at 10 min intervals by 3 blinded physicians with properly calibrated tonometers. To avoid changes in corneal thickness early in the morning owing to nocturnal edema, all measurements were performed between 10 AM and 12 AM. Alcaine® 0.5% eye drops (Alcon Laboratories Inc., Fort Worth, TX, USA) were performed to anesthetize the eyes before the first physician took measurements with Tono-Pen XL. A fluorescein strip was then applied by the second physician to the inferior conjunctival fornix, and GAT measurements were taken using a biomicroscope’s cobalt blue filter. Finally, topical anesthesia was applied again, and the third physician took measurements via Pascal DCT. Only quality scores 1, 2, and 3 were accepted, and the average of the two IOP measurements averages were noted. Patients’ CCTs were measured using the DGH-550 ultrasonographic pachymetry (DGH Technology Inc., PA, USA).

GAT readings were considered the gold standard. Each device’s IOP measurements were compared with the GAT readings. Statistical analysis was performed using SPSS (Statistical Package for the Social Sciences) for Windows (IBM, Version 22.0 Armonk, NY, USA). All data were not normally distributed according to the Shapiro-Wilk test for all outcome measures. Nonparametric Wilcoxon signed-rank tests were used for analysis. The deviation of each device from GAT readings was calculated and correlated with the CCT variable using Spearman rank correlation. To assess the agreement between the GAT, Tonopen XL, and DCT, the Bland-Altman method was used. Statistical analyses were two-sided. A P-value of<0.05 was considered statistically significant.


The mean age of 34 patients was 63.8±12.4 years (range 36–83 y; median 64.0 y). Among the enrolled patients, 21 were female (61.8%), and 13 patients were male (38.2%). The mean time between our examination and DMEK surgery was 9.9±13.5 months (range, 1–60 mo). Glaucoma medication was started in 10 of the patients after DMEK. The mean number of glaucoma medications was 0.7±1.1 at our visit. The baseline demographic data during the examination are shown in Table 1.

TABLE 1 - Baseline Characteristics of Patients (n=34)
Parameter Mean±SD/n-% Range
Age (y) 63.8±12.4 36–83
Female/male ratio 61.8%/38.2%
Eye (right/left) 15/19 (44.1%/55.9%)
Central corneal thickness 559±83 404–821
Best corrected visual acuity (LogMAR) 0.6±0.4 0.1–1.8
Post-DMEK glaucoma 29.4%
Number of glaucoma drugs 0.7±1.1 0.0–4.0
Time (mo) 9.9±13.5 1.0–60.0
DMEK indicates descement membrane endothelial keratoplasty; LogMAR, logarithm of the minimum angle of resolution.

Mean IOP readings measured by the three devices are presented in Table 2 and Figure 1. These values indicate an overestimation of pressure by the DCT compared with the other 2 tonometers.

TABLE 2 - Mean Intraocular Pressure Measurements
Mean±SD (mmHg) Range (mmHg)
Goldmann applanation tonometer 14.9±5.8 6–40
Tonopen XL 16.2±5.5 7–38
Pascal dynamic contour tonometry 19.2±5.0 10.4–38.2
SD indicates standard deviation.

Mean IOP Measurements (GAT, Tonopen XL, DCT) DCT indicates Dynamic Contour Tonometer; GAT, Goldmann Applanation Tonometer; IOP, intraocular pressure.

A statistically significant difference between GAT and Tonopen XL (r=0.942 [0.885–0.971]; P=0.0001) based on the Wilcoxon signed test was found. Figure 2 shows the difference between measurements according to the average IOP value measured with GAT and Tonopen XL. GAT and Tonopen XL showed good agreement, and the mean Tonopen XL value was 1.3 mmHg higher than the GAT value (P<0.01).

Differences between GAT and Tonopen XL. GAT indicates Goldmann Applanation Tonometer.

Also, a statistically significant difference between GAT and DCT measurements with the Wilcoxon signed test was found. Figure 3 shows the difference between measurements according to the average IOP value measured by GAT and DCT (r=0.942 [0.885–0.971]; P=0.0001) based on the Wilcoxon signed test. DCT showed reasonable agreement with GAT but overestimated IOP values than that found with GAT (mean 4.1 mmHg).

Differences between GAT and Pascal DCT. DCT indicates Dynamic Contour Tonometer; GAT, Goldmann Applanation Tonometer.

Correlations between 3 different (GAT, Tonopen XL, and Pascal DCT) IOP measurements from the 3 tonometers and CCT are shown in Figure 4. When analyzed by Spearman correlation analysis, no significant correlations between IOP and CCT in all 3 devices (P and r values were 0.211 and 0.220 for GAT, 0.371 and 0.158 for Tonopen XL, and 0.688 and 0.071 for Pascal DCT, respectively) were noted.

A, Correlation of CCT with GAT. B, Correlation of CCT withTonopen XL. C, Correlation of CCT with Pascal (DCT). CCT indicates Central Corneal Thickness; DCT, Dynamic Contour Tonometer; GAT, Goldmann Applanation Tonometer.


Increased IOP may lead to endothelial cell destruction and thus anatomic and functional failures after successful keratoplasty surgeries.24 Risk factors for increased IOP after DMEK surgery have been described. The study by Maier et al6 showed that steroid-induced glaucoma was the major reason for IOP elevation (8.0%). A preoperative increase in IOP was a risk factor for IOP elevation as demonstrated by the results from the study by Maier et al as described in many studies, the need for an air bubble to fixate the posterior lamella in DMEK led to an increase in the risk of developing postoperative mechanical angle closure glaucoma.7,25,26 Mechanical angle closure glaucoma developed temporarily in 15.4% of eyes within the first postoperative 24 h.7 Corneal biomechanical properties change after DMEK surgery. In this altered corneal structure, an accurate and ideal IOP measurement method is still under investigation.24 To our knowledge, this study is the first one that compares GAT, Tonopen XL, and Pascal DCT with IOP measurements after DMEK surgery. We believe that our study will contribute to the literature in this respect.

In previous studies, IOP measurements based on DCT have shown higher values than GAT, ranging from 0.7 to 4.4 mmHg in both healthy corneas and eyes after DMEK surgery.21,27–37 In the present study, despite the reasonable agreement between DCT and GAT measurements (P<0.01), DCT appears to show statistically significant higher IOP values than GAT (mean 4.1 mmHg). Our findings are compatible with previous studies in the literature. Also, Maier et al28 found that DCT overvalued the GAT readings between 3.35 and 4.77 mmHg at all follow-up points. Their work prospectively investigated IOP measurements taken at pre-l and postoperative first and third postoperative month follow-ups after DMEK. Our study’s findings are consistent with these cited studies.

In our study, the Tonopen XL yielded reasonable agreement with the other measurements in terms of IOP measurements, but the mean Tonopen XL-derived value was 1.3 mmHg higher than the mean GAT value (P<0.01). Similarly, Ohana et al38 found that Tonopen XL readings agreed with the GAT measurements with a mean difference of 1.2 mmHg after Descemet’s stripping endothelial keratoplasty. However, as Salvetat et al23 demonstrated in eyes with primary open angle glaucoma and eyes with a healthy cornea, Tonopen XL showed poor agreement with GAT; the average measurement difference was −0.50 mmHg. To our knowledge, no study in the literature has compared Tonopen XL and GAT in post-DMEK patients. Our study makes an important contribution to the literature in this respect. Prospective, comparative studies with more patients are likely to provide more enlightening results concerning this subject.

While interpreting our results, it should be considered that the true IOP could not be measured. Although the ultimate gold standard to measure the real IOP is cannulation of the anterior chamber with manometry to detect the direct pressure, we compared GAT and two different techniques for IOP measurement.39 Furthermore, in our study, the mean difference between Tonopen XL and DCT was 3.00 mmHg.

Maier et al compared IOP measurements using noncontact pneumatic tonometry (NCT), iCare, GAT, and DCT at preoperative and postoperative 1-month and 3-month follow-ups after DMEK surgery. These authors reported agreement in IOP measurements at postoperative 1-month follow-up between GAT and NCT, GAT and iCare, and iCare and NCT, and at postoperative 3-month follow-up between iCare and NCT.28 In our study, GAT, TonopenXL, and DCT showed good agreement, but the IOP measurement results were not interchangeable.

In our study, no correlation between CCT and IOP measurements by GAT were found (P=0.21). Similarly, Maier et al28 did not observe any correlation between IOP readings and CCT values either preoperatively or postoperatively. However, Lleo et al showed that IOP readings obtained by GAT were correlated with CCT.40 Correspondingly, Kniestedt et al found a good correlation between GAT measurements and CCT.41 This difference likely occurred because these studies enrolled patients with normal corneas. In the current study, a correlation between CCT values and IOP measurements taken via Tonopen XL was not found (P=0.37). Similarly, Ohana et al38 showed no correlation between CCT values and IOP readings based on Tonopen XL in post-Descemet’s stripping endothelial keratoplasty patients. However, Tonnu et al42 found a high correlation between CCT values and IOP measurements based on Tonopen XL in normal corneas. It is known that DCT is less dependent on CCT.41,43 Similarly, the results of this study did not exhibit any correlation between CCT values and IOP measurements conducted using DCT.

Siggel et al evaluated the changes in corneal biomechanics in terms of CH and corneal resistance factor after DMEK surgery. These authors concluded that although the DMEK graft is much thinner, DMEK surgery significantly affected both measured corneal biomechanical properties. This finding should be considered when interpreting IOP measurements.43

Some limitations in this study should be discussed. Our study was cross-sectional. We aimed to compare measurements between different devices. Thus, patients were examined only once without any follow-up examinations. We had a relatively small cohort. Larger prospective studies can be planned, including large groups with normal eyes and eyes that have undergone DMEK surgery. Also, only the CCT values were considered when investigating influences on IOP measurements, but no other corneal biomechanical factors were considered, such as CH.

In conclusion, our results demonstrate that measurements of IOP are higher with both Tonopen and DCT than GAT in eyes undergoing DMEK surgery. In contrast, GAT, Tonopen XL, and DCT measurements showed a good and reasonable agreement when evaluating post-DMEK IOP. In principle, all 3 measurement techniques can be practical in routine postoperative examinations after DMEK surgery. However, it is recommended to use the same device for patient follow-up.


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descemet membrane endothelial keratoplasty; intraocular pressure; goldmann applanation tonometry; tonopen XL; pascal dynamic contour tonometry; central corneal thickness

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