Glaucoma represents a group of eye disorders partly related to raised intraocular pressure (IOP) leading to progressive optic nerve damage resulting in impaired vision and possibly blindness. To assess the suitability of new IOP lowering therapeutic strategies, such as the implantation of glaucoma drainage devices, appropriate animal models have to be used. Currently, a number of rodent glaucoma models are available , however, especially for surgical interventions rodent eyes are too small. Rabbits are much more suitable with respect to dimension. Unfortunately, rabbit glaucoma model systems described in literature are difficult to reproduce or fail totally, associated with a high level of discomfort and pain for treated animals. Therefore, development of an in vivo rabbit glaucoma model is one of the most important goals in glaucoma research. Here, we describe the adaptation of the oculopressure tonometry, an existing method to quantify the outflow of aqueous humor in humans, to generate a transient glaucoma model in rabbits. The existing suction-cup oculopressor (SCOP) is extended with newly designed suction-cups, which are adjusted to the anatomy of the rabbit eye. The modification of the oculopressure tonometry method facilitates an increase in IOP over a time frame of 9 minutes by vacuum induced deformation of the rabbit eye. This method can be used to test functionality of fistulating glaucoma surgeries or implanted drainage devices in a long term follow-up without any side effects and suffering of the animals.
The second leading cause of blindness and the leading cause of irreversible blindness worldwide is glaucoma. Despite many efforts made in the research of glaucoma formation and therapeutic options glaucoma prevalence is still increasing to date. Approximately 80 million people will be affected by glaucoma in 2020 .
For admission of new therapy strategies it is essential to conduct experiments in rabbit model of glaucoma. It is therefore important to have methods at ones disposal to sufficiently increase IOP for the simulation of glaucoma. Different rabbit models for glaucoma induction have been published. Allemann and colleagues chose four of them with respect to ethical, surgical and logistic factors to reproduce in a large scale investigation. Unfortunately, none of these models was reproducible in a satisfactory way .
When looking for another approach to increase IOP in a rabbit glaucoma model the oculopressure tonometry (OPT) of Ulrich and Ulrich came to mind . This method is based on creating an elevated IOP in the eye to determine aqueous humor outflow rate and has been used in general ophthalmology since 1987.
The intention of this study is to adapt this diagnostic tool to the rabbit eye and to establish an alternative to conventional models of glaucoma induction without any surgical interventions of animals involved.
2.1 Estimation of suction-cup diameter for rabbits
For the adaptation of OPT to the rabbit eye New Zealand White rabbits were used (Charles River, Sulzfeld, Germany). As a fundamental requirement for adapting OPT to rabbit eyes, the suction-cup diameter had to be adjusted to the average size of rabbit eyeballs. The diameter ds,r for the suction-cups to be used was designed based on the diameter of suction-cups used for humans (ds,h = 13 mm) and the human or rabbit eyeball diameter (de,h = 22.50 mm and de,r = 18.25 mm, respectively). The ratio ds,r/de,r was calculated for suction-cup diameters of ds,r = 10, 11, 12 and 13 mm and compared to the corresponding ratio ds,h/de,h for human use.
2.2 Design and manufacturing of suction-cups
Suction-cups for rabbit eyes were designed according to original components using Creo Elements/Pro 5.0 (PTC Inc., Needham, USA) (Fig. 2).
Manufacturing of suction-cups was based on Polyoxymethylene (POM, SUSTARIN C, ThyssenKrupp Plastics GmbH, Essen, Germany) machined by a metalworking lathe (EMCOMAT-14D, EMCO Maier Ges.m.b.H, Hallein, Austria). Manufactured suction-cups were finally cleaned for 10 minutes in an ultrasonic bath (SONOREX RK 103 H, BANDELIN electronic GmbH & Co. KG, Berlin, Germany).
2.3 Application of OPT to rabbits
Application of OPT was tested for rabbits under deep anesthesia. The animals were sedated with a subcutaneously injection of 35 mg kg1 Ketamin 10% (Bela-pharm GmbH & Co. KG, Vechta, Germany), 5 mg kg1 Rompun®2% (Bayer HealthCare AG, Leverkusen, Germany). Additionally, local anaesthesia using Proparakain-POS®0,5% drops was administered to the eyes (URSAPHARM Arzneimittel GmbH, Saarbrücken, Germany). During OPT rabbits were retained in a regular sitting position.
SCOP (Fa. B. Boucke, Medizin-Elektronik, Tübingen, Germany) is used to increase IOP about 40 mmHg during a maximum examination period of 9 minutes. The SCOP-device, which is licensed for application in human medicine, consists of a vacuum pump that is connected to the eyeball by a flexible tube. The suction-cup at the end of the tube is positioned on the conjunctiva-covered sclera in the temporal canthus near the corneal limbus. Applying a vacuum to the cone-shaped SC results in an eyeball deformation and subsequently in an IOP increase.
During OPT the IOP was measured using a rebound tonometer (TAO1, Icare Finland Oy, Vantaa, Finland). This procedure is also used for IOP measurements in human ophtalmological daily routine. Tonometric IOP recordings pic with the TAO1 device were corrected according to Löbler et al. considering differences in thickness and viscoelastic properties between rabbit and human corneas. Corrected IOP for rabbits pcorr is calculated according to formula 1 .
Prior to anesthesia initial IOP was measured. To record pressure decay during OPT, the IOP is measured every minute (Fig. 3). Finally, IOP was recorded one minute after termination of OPT.
In order to prevent desiccation during OPT, physiologic salt solution was applied to the eyes every two minutes. Upon completion of a series of measurements a moisturizing gel (Vidisic®; Bausch & Lomb/Dr. Mann Pharma, Berlin, Germany) was applied to the eyes.
For adaptation of OPT to the rabbit eye three demagnified suction-cups were manufactured. Table 1 illustrates that suction-cups with a diameter of d s, r = 11 mm yield the best match with regard to the suction-cup/eyeball diameter ratio used in human measurements.
|Diameter of suction-cup ds,h or ds,r||Diameter ratio ds,h/de,h for human eyeball (de,h = 22.50 mm)||Diameter ratio ds,r/de,r for rabbit eyeball (de,r = 18.25 mm)|
|ds,h = 13 mm||0.58|
|ds,r = 12 mm||0.66|
|ds,r = 11 mm||0.60|
|ds,r =10 mm||0.55|
During OPT the measured IOP values obtained by suction-cups with a diameter of ds,r = 10 mm (SC10) generally were below IOP values obtained by suction-cups with a diameter of ds,r = 11 mm and 12 mm (SC11 and SC12) (Fig. 4). As a maximum IOP 34 mmHg was measured after one minute for SC10. Maximum IOP for SC11 and SC12 was 51 mmHg and 47 mmHg after one minute, respectively. IOP decrease was similar for all tested suction-cup sizes during the first five minutes of OPT. In the second half of OPT the pressure decay varied depending on the suction-cup diameter. While IOP decrease decelerated during measurements with SC10 and SC11, IOP decreased more rapidly using SC12. After 9 minutes IOP was still at 37 mmHg for SC11, whereas it had decreased to 23 mmHg with SC12. IOP measurements during OPT confirmed the suction-cup with ds,r = 11 mm (SC11), which is closest to the ratio of cup diameter to eyeball diameter applied in human diagnostics, is most suitable for rabbit OPT.
In a second experiment series, IOP increase on both rabbit eyes was evaluate by OPT using SC11 (Fig. 5). OPT was performed on the right (OD) and left (OS) eye at various points in time (n = 9).
IOP for OD and OS was 45.8±14.2 mmHg and 50.0±12.3 mmHg (n = 9) after one minute, respectively. After nine minutes IOP decreased to 29.9±13.3 mmHg and 31.0±11.5 mmHg (n = 9) for OD and OS, respectively.
In the presented study OPT was adapted to rabbit eyes in an effort to create a transient rabbit glaucoma model. Suction-cups with different diameters where tested for their suitability in rabbit OPT.
Using SC10 it was not possible to increase IOP in a satisfactory way. This is due to the fact that the ratio between SC10 to the rabbit eyeball diameter is too small. The effect on the eye was not efficient.
In contrast with SC12 it was possible to reach an appropriate IOP elevation but it had shortcomings when compared to SC11. On the one hand the diameter ratio of SC12 to the rabbit eye is larger than the corresponding human ratio (Table 1). On the other hand the pressure decay with SC12 was faster than that of smaller suction-cups. Additionally, the dimensions of SC12 led to an unsatisfactory handling. Positioning of suction-cup reached either the conjunctival fornix or threatened to slide on the cornea, which could also explain the steeper decrease of IOP values.
Measurements with SC11 accomplished highest IOP values compared with the other suction-cup diameters. During the first minutes of OPT almost a linear pressure decay was detected, which slowed down at the end of OPT. This phenomenon could be explained with the decline in pressure difference between eyeball and drained aqueous humor. High IOP values cause a high outflow rate, which results in lower IOP values. Ongoing constant production of aqueous humor compensates the increasing amount of drained liquid under lower IOP conditions, which results in a slower decrease of IOP. The results from OPT measurements confirmed the theoretical estimation of the ideal suction-cup diameter. A suction-cup with ds,r = 11 mm is most suitable for rabbit OPT.
In conclusion, adopting OPT to the rabbit eye successfully elevated IOP to defined pressure values over a short period of time with SCOP. Elevation of IOP with SC11 was reproducible on right and left eyes. It was possible to establish a non-invasive, transient rabbit glaucoma model. This model system allows verification of new glaucoma therapy interventions, like implanted drainage devices with minimal stress of the examined animals. It might also be of help to measure outflow facilities after application of new potentially IOP lowering medications.
This project was partially funded by the Federal Ministry of Education and Research (BMBF) within the research project REMEDIS “Höhere Lebensqualität durch neuartige Mikroimplantate”.
Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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