Graefes Arch Clin Exp Ophthalmol (2014) 252:1607–1610 DOI 10.1007/s00417-014-2720-5
GLAUCOMA
Can we trust intraocular pressure measurements in eyes with intracameral air? Gauti Jóhannesson & Christina Lindén & Anders Eklund & Anders Behndig & Per Hallberg
Received: 21 February 2014 / Revised: 24 April 2014 / Accepted: 30 June 2014 / Published online: 17 July 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To evaluate the effect of intracameral air on intraocular pressure (IOP) measurements using Goldmann applanation tonometry (GAT) and applanation resonance tonometry (ART) in an in-vitro porcine eye model. Methods IOP was measured on thirteen freshly enucleated eyes at three reference pressures: 20, 30, and 40 mmHg. Six measurements/method were performed in a standardized order with GAT and ART respectively. Air was injected intracamerally in the same manner as during Descemet’s stripping endothelial keratoplasty (DSEK) and Descemet’s membrane endothelial keratoplasty (DMEK), and the measurements were repeated. Results Measured IOP increased significantly for both tonometry methods after air injection: 0.7±2.1 mmHg for GAT and 10.6±4.9 mmHg for ART. This difference was significant at each reference pressure for ART but not for GAT. Conclusions Although slightly affected, this study suggests that we can trust GAT IOP-measurements in eyes with intracameral air, such as after DSEK/DMEK operations. Ultrasound-based methods such as ART should not be used.
Keywords Intraocular pressure . Intracameral air . Goldmann applanation tonometry . Applanation resonance tonometry . DSEK . DMEK
G. Jóhannesson (*) : C. Lindén : A. Behndig Department of Clinical Science, Ophthalmology, Umeå University, Umeå SE-901 85, Sweden e-mail: [email protected] A. Eklund : P. Hallberg Department of Radiation Sciences, Biomedical Engineering, Umeå University, Umeå SE-901 85, Sweden
Introduction Descemet’s stripping endothelial keratoplasty (DSEK) and Descemet’s membrane endothelial keratoplasty (DMEK) have become the major surgical treatments for corneal endothelial dysfunction in recent years. In both techniques, Descemet’s membrane is stripped off; in DSEK it is replaced with a lamellar transplant [1], in DMEK only the endothelium and Descemet’s membrane are implanted [2]. In order to keep the transplant in place, air is injected into the anterior chamber at the end of surgery. This can cause acute increases in intraocular pressure (IOP) [3–6], necessitating various interventions such as tapping air from the anterior chamber or administering IOP-lowering medications. In order to handle the situation correctly, accurate IOP measurement is crucial. However, the accuracy of the IOP measurement under these circumstances is unknown. To date, the potential effect of intracameral air on IOP measurement has not been investigated. Since IOP measurement is an indirect estimation of the true IOP, the properties of the eye contribute to the great number of sources of error connected to different tonometry methods [7–10]. Even though various new tonometry methods have been introduced, each with advantages and disadvantages, Goldmann applanation tonometry (GAT) is still considered the gold standard. It is widely used in clinical ophthalmology, including for IOP measurements in patients with intracameral air after DSEK/DMEK. GAT is assumed to be reliable in these eyes even though the effect of intracameral air on tonometry has yet to be investigated. Thus, the aim of the present study was to investigate, in an in-vitro porcine eye model, whether GAT and applanation resonance tonometry (ART) are affected by the presence of intracameral air.
1608
Graefes Arch Clin Exp Ophthalmol (2014) 252:1607–1610
Materials and methods This in-vitro study was performed at the Department of Ophthalmology, Umeå University Hospital in Sweden. Porcine eyes were enucleated immediately after the animals were put to death at the abattoir (Nylén-Hugosons, Luleå, Sweden). Care was taken to protect the corneal epithelium of the eyes during transport. Measurements were carried out on 13 eyes the same day as the eyes were enucleated. Permission to use animal by-products for research purposes was obtained from the Swedish Board of Agriculture. In-vitro porcine eye model A petri dish fixture was mounted on a biomicroscope (Haag– Streit slit lamp, Bern, Switzerland) to hold the eye in a position comparable to the eye of a patient sitting at the slit lamp (Fig. 1). Each porcine eye was fixed in the centre of a petri dish using agar gel (15 g/dm3). To maintain reference pressure inside the eye, an established method was used where a thinwalled, winged cannula connected to a saline column was inserted through the sclera and into the vitreous chamber [11, 12]. Cyanoacrylate adhesive glue (Loctite® 242, Henkel, Düsseldorf, Germany) was used to seal the interface between the sclera and the cannula to prevent leakage. The IOP was regulated by the height of the saline column [13] and monitored with a standard monitoring kit (BD Infusion Therapy Systems Inc., Sandy, UT, USA), sample rate: 1,000 Hz. Just before measurement, a stopcock between the eye and the saline column was closed to create a closed system that approximated the normal state of the eye [14]. The reference IOP was set to three levels: 20, 30, and 40 mmHg. At each pressure level, IOP was measured 6 times with each tonometry method in a standardized order with GAT first, followed by ART. The same experienced operator (GJ) performed all measurements. ART measurement values are processed electronically and displayed digitally, whereas GAT measurements were blinded to the operator. To keep the eyes moisturized and to imitate eyelid blinking, the eyes were stroked with fluorescein-dyed saline between each measurement using a goat-hair brush. When IOP had been measured at all pressure levels, air was injected into the anterior chamber using a 16-gauge cannula in the same manner as during DSEK/DMEK in clinical practice. IOP was then measured again in the same order as before air injection. Before and after the IOP measurements, the corneal epithelium was inspected in order to eliminate eyes with corneal erosions.
Fig. 1 Measurement set-up. The porcine eye is moulded in an agar plate. The reference pressure is set by a saline column through a thin-walled cannula placed in the vitreous behind the lens
corneal area (Imbert–Fick’s law). The method is described in detail elsewhere [15]. ART is a recently developed tonometry method [11, 16, 17]. It is slit-lamp-mounted, and based on resonance technique. A piezoelectric element oscillates in its resonance frequency. When in contact with the cornea, it produces a shift in resonance frequency that is proportional to the applanated contact area. IOP is estimated through the relationship between area (frequency shift) and the applied force by nonstop sampling of both parameters. The tonometer rests on the cornea for 2 s, and collects data from which IOP is calculated.
Tonometry methods Statistical analysis GAT is considered the reference standard for tonometry. It is slit-lamp-mounted. GAT estimates IOP by using the relationship between the force needed to applanate a pre-defined
IBM SPSS version 19.0 was used for data analysis. For statistical analysis, the mean of each set of six measurements
Graefes Arch Clin Exp Ophthalmol (2014) 252:1607–1610
1609
was calculated. To analyze change in IOP from before to after air injection into the anterior chamber, paired t-test was used. Power calculation for a 1.5 mmHg difference showed that≥10 eyes were needed to acquire a power of 90 %. Statistical significance was considered when p
GLAUCOMA
Can we trust intraocular pressure measurements in eyes with intracameral air? Gauti Jóhannesson & Christina Lindén & Anders Eklund & Anders Behndig & Per Hallberg
Received: 21 February 2014 / Revised: 24 April 2014 / Accepted: 30 June 2014 / Published online: 17 July 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To evaluate the effect of intracameral air on intraocular pressure (IOP) measurements using Goldmann applanation tonometry (GAT) and applanation resonance tonometry (ART) in an in-vitro porcine eye model. Methods IOP was measured on thirteen freshly enucleated eyes at three reference pressures: 20, 30, and 40 mmHg. Six measurements/method were performed in a standardized order with GAT and ART respectively. Air was injected intracamerally in the same manner as during Descemet’s stripping endothelial keratoplasty (DSEK) and Descemet’s membrane endothelial keratoplasty (DMEK), and the measurements were repeated. Results Measured IOP increased significantly for both tonometry methods after air injection: 0.7±2.1 mmHg for GAT and 10.6±4.9 mmHg for ART. This difference was significant at each reference pressure for ART but not for GAT. Conclusions Although slightly affected, this study suggests that we can trust GAT IOP-measurements in eyes with intracameral air, such as after DSEK/DMEK operations. Ultrasound-based methods such as ART should not be used.
Keywords Intraocular pressure . Intracameral air . Goldmann applanation tonometry . Applanation resonance tonometry . DSEK . DMEK
G. Jóhannesson (*) : C. Lindén : A. Behndig Department of Clinical Science, Ophthalmology, Umeå University, Umeå SE-901 85, Sweden e-mail: [email protected] A. Eklund : P. Hallberg Department of Radiation Sciences, Biomedical Engineering, Umeå University, Umeå SE-901 85, Sweden
Introduction Descemet’s stripping endothelial keratoplasty (DSEK) and Descemet’s membrane endothelial keratoplasty (DMEK) have become the major surgical treatments for corneal endothelial dysfunction in recent years. In both techniques, Descemet’s membrane is stripped off; in DSEK it is replaced with a lamellar transplant [1], in DMEK only the endothelium and Descemet’s membrane are implanted [2]. In order to keep the transplant in place, air is injected into the anterior chamber at the end of surgery. This can cause acute increases in intraocular pressure (IOP) [3–6], necessitating various interventions such as tapping air from the anterior chamber or administering IOP-lowering medications. In order to handle the situation correctly, accurate IOP measurement is crucial. However, the accuracy of the IOP measurement under these circumstances is unknown. To date, the potential effect of intracameral air on IOP measurement has not been investigated. Since IOP measurement is an indirect estimation of the true IOP, the properties of the eye contribute to the great number of sources of error connected to different tonometry methods [7–10]. Even though various new tonometry methods have been introduced, each with advantages and disadvantages, Goldmann applanation tonometry (GAT) is still considered the gold standard. It is widely used in clinical ophthalmology, including for IOP measurements in patients with intracameral air after DSEK/DMEK. GAT is assumed to be reliable in these eyes even though the effect of intracameral air on tonometry has yet to be investigated. Thus, the aim of the present study was to investigate, in an in-vitro porcine eye model, whether GAT and applanation resonance tonometry (ART) are affected by the presence of intracameral air.
1608
Graefes Arch Clin Exp Ophthalmol (2014) 252:1607–1610
Materials and methods This in-vitro study was performed at the Department of Ophthalmology, Umeå University Hospital in Sweden. Porcine eyes were enucleated immediately after the animals were put to death at the abattoir (Nylén-Hugosons, Luleå, Sweden). Care was taken to protect the corneal epithelium of the eyes during transport. Measurements were carried out on 13 eyes the same day as the eyes were enucleated. Permission to use animal by-products for research purposes was obtained from the Swedish Board of Agriculture. In-vitro porcine eye model A petri dish fixture was mounted on a biomicroscope (Haag– Streit slit lamp, Bern, Switzerland) to hold the eye in a position comparable to the eye of a patient sitting at the slit lamp (Fig. 1). Each porcine eye was fixed in the centre of a petri dish using agar gel (15 g/dm3). To maintain reference pressure inside the eye, an established method was used where a thinwalled, winged cannula connected to a saline column was inserted through the sclera and into the vitreous chamber [11, 12]. Cyanoacrylate adhesive glue (Loctite® 242, Henkel, Düsseldorf, Germany) was used to seal the interface between the sclera and the cannula to prevent leakage. The IOP was regulated by the height of the saline column [13] and monitored with a standard monitoring kit (BD Infusion Therapy Systems Inc., Sandy, UT, USA), sample rate: 1,000 Hz. Just before measurement, a stopcock between the eye and the saline column was closed to create a closed system that approximated the normal state of the eye [14]. The reference IOP was set to three levels: 20, 30, and 40 mmHg. At each pressure level, IOP was measured 6 times with each tonometry method in a standardized order with GAT first, followed by ART. The same experienced operator (GJ) performed all measurements. ART measurement values are processed electronically and displayed digitally, whereas GAT measurements were blinded to the operator. To keep the eyes moisturized and to imitate eyelid blinking, the eyes were stroked with fluorescein-dyed saline between each measurement using a goat-hair brush. When IOP had been measured at all pressure levels, air was injected into the anterior chamber using a 16-gauge cannula in the same manner as during DSEK/DMEK in clinical practice. IOP was then measured again in the same order as before air injection. Before and after the IOP measurements, the corneal epithelium was inspected in order to eliminate eyes with corneal erosions.
Fig. 1 Measurement set-up. The porcine eye is moulded in an agar plate. The reference pressure is set by a saline column through a thin-walled cannula placed in the vitreous behind the lens
corneal area (Imbert–Fick’s law). The method is described in detail elsewhere [15]. ART is a recently developed tonometry method [11, 16, 17]. It is slit-lamp-mounted, and based on resonance technique. A piezoelectric element oscillates in its resonance frequency. When in contact with the cornea, it produces a shift in resonance frequency that is proportional to the applanated contact area. IOP is estimated through the relationship between area (frequency shift) and the applied force by nonstop sampling of both parameters. The tonometer rests on the cornea for 2 s, and collects data from which IOP is calculated.
Tonometry methods Statistical analysis GAT is considered the reference standard for tonometry. It is slit-lamp-mounted. GAT estimates IOP by using the relationship between the force needed to applanate a pre-defined
IBM SPSS version 19.0 was used for data analysis. For statistical analysis, the mean of each set of six measurements
Graefes Arch Clin Exp Ophthalmol (2014) 252:1607–1610
1609
was calculated. To analyze change in IOP from before to after air injection into the anterior chamber, paired t-test was used. Power calculation for a 1.5 mmHg difference showed that≥10 eyes were needed to acquire a power of 90 %. Statistical significance was considered when p
