Viability of Topical Endoscopic Imaging System for Vitreous Surgery in Rabbit Eyes

Introduction

To observe the fundus during vitreous surgery, either contact lenses or a non-contact wide-viewing system and a surgical microscope have been used clinically.^1–3 When using contact lenses, it becomes possible to observe the fundus with high resolution. However, when trying to visualize the peripheral retina, a fairly complex technique is required. Although wide-angle viewing systems have made it possible to simultaneously observe both the posterior and peripheral retina, these systems require a device that converts the image to a normally oriented view of the field.

Guyomard et al. first reported the usefulness of topical endoscopy for imaging of the fundus of small animals.^4,5 This technique can be used to produce wide-angle images without extensive training, in addition to being able to focus on the periphery by simply tilting the endoscope. To investigate the viability of this new methodology, we used the technique to perform vitrectomy in rabbit eyes.

Design And Methods

Equipment Required for Topical Endoscopic Observation

The endoscopic imaging system used in this study was similar to that employed by Guyomard et al.⁴ In our study, we used a veterinary endoscope with an 8.5-cm long otoscope that had a 5-mm outer diameter (67260A; Karl Storz, Tuttlingen, Germany) with an oval illuminating tip. A xenon lamp (Xenon Nova 175; Karl Storz) was used as the light source and linked to the endoscope by a flexible optic fiber. The power of the light was adjusted to that of the standard illumination system used for vitrectomy (Fig. 1). The endoscope, which was fixed on a flexible arm, was connected to a digital camera that used a 400,000-pixel charge-coupled device image sensor unit (Telecam-C and Telecam DXII; Karl Storz). After applying a viscoelastic gel to protect the corneal surface and to create an interface that would improve the quality of the image, the endoscope was slowly moved toward the corneal surface of each of the test animal eyes (Fig. 2).

Figure 1. Comparison of spectral irradiance of the xenon lamp used in this study (Xenon Nova 175; Karl Storz, Tuttlingen, Germany) and that used for a standard vitrectomy (Xenon BrightStar illumination system; DORC, Zuidland, The Netherlands). The strength of the spectral irradiance of the xenon lamp used in this study, between 400 and 600 μm, was adjusted to that of the xenon lamp used in a standard vitrectomy (through a 23-gauge light pipe at 50% of maximal strength equipped with a 420-nm cut-off filter).

Figure 2. Endoscope set-up prior to surgery. (A) Preoperative view of the positioning of the endoscopic system. (B) Schematic diagram of the three-port vitrectomy that was performed when using the topical endoscopic imaging system.

Findings

The endoscope proved easy to handle and was able to obtain good resolution (Figs. 3A, 3B, 3C, and 3D). Due to the inherent depth of field, focusing was not necessary. During the surgery, the endoscope was fixed to observe the posterior fundus. Even prior to inserting endoillumination from the light pipe, the posterior fundus could be viewed well simply by using the transcorneal illumination of the endoscope without any heat production at the corneal surface (Fig. 3A). During the vitrectomy, the location of the vitreous cutter was confirmed by using the endoillumination of the light pipe. As seen in the figures, illumination produced a shadow of the cutter on the retinal surface (Figs. 3B and 3C). Therefore, it was possible to easily induce artificial vitreous detachment without any inadvertent retinal damage (Fig. 3B). Use of the new technique made it possible to make retinal breaks that were accurate in both location and size (Fig. 3C), in addition to being able to employ photocoagulation to precisely outline the retinal break after the intravitreal injection of perfluoro-n-octane liquid (Fig. 3D). Furthermore, by tilting the endoscope toward the ciliary body, it was possible to observe not only the location of the trocar ports but also the distance from the ciliary process (Figs. 3E and 3F). The fundus examination performed after vitrectomy showed there was resolution of the intravitreal air within the following 4 days. Retinal reattachment with a photocoagulation scar around the retinal break was observed in all eyes.

Figure 3. Fundus imaging during the surgery. (A) Preoperative image of the fundus using only the endoscope-equipped illumination. (B) Fundus image during vitrectomy and induction of the artificial vitreous detachment. (C) Fundus image of the retinal tear formation. (D) Fundus image of the laser photocoagulation around the retinal tear after the perfluorocarbon injection. (E) Schematic diagram showing the tilting of the endoscope, which allowed observation of the peripheral retina. (F) Fundus image showing the trocar insertion sites.

Discussion

The current study showed the viability of using the topical endoscopic imaging system for vitreous surgery. Use of this method made it possible to perform vitrectomy without the assistance of a conventional surgical microscope. The topical endoscopic imaging system was first reported by Guyomard et al. in a study that examined the fundus in small animals. Results of their study indicated that the topical endoscopic imaging allowed for high-resolution and wide-field fundus imaging in small animal eyes. Another potential benefit of the topical endoscopic imaging system is that it is both compact and relatively inexpensive.⁴ Additionally, the technique produces a high contrast image without unnecessary corneal reflections and the device can be easily moved to obtain images of the extreme periphery. Even during surgical use, we found the system to be simple to set up, with the image obtained similar to that seen when using a wide-angle viewing system. Although corneal opacity or cataracts often cause serious difficulty of observation during surgery, the image becomes clear if the endoscope is simply moved to the clear area of the cornea in the former case or if endoillumination from a light pipe is introduced through the scleral incision port in the latter case.

The introduction of new minimally invasive vitrectomy systems has led to the need for the development of smaller fiberoptic devices. In addition, the intraocular fiberoptic endoscope systems that are currently being used clinically have a relatively low resolution due to the limited number of glass fibers that are present.^6–8 In contrast, the new topical endoscopic imaging system that was examined in this study employs an endoscope with a diameter that does not restrict the size of either the trocar or the scleral incision because the endoscope is placed directly on the corneal surface.

Although it appears that there are many potential benefits associated with this new technique, the topical endoscopic imaging system is limited by the fact that the image obtained is a monocular image, similar to that of the currently used fiberoptic endoscope. Therefore, manipulation of the macula, such as peeling away the epiretinal membrane or internal limiting membrane of the macula, would be rather difficult compared to such an operation using a binocular observation system. This limitation will undoubtedly be overcome with the development of a three-dimensional endoscopic system in the near future.⁹ We are currently working to establish a fundus imaging system with a next-generation endoscope. In addition, intraoperative optical coherence tomography is useful for evaluating the macular morphology.^10,11 The latest endoscopes used for the gastrointestinal tract or other organs are equipped with ultrasonography or optical coherence tomography. Thus, this endoscopic system seems to have potential to provide new insights and strategies into surgical procedures, including telesurgery.

One other limitation of this new technique is that the resolution of the image obtained can also be restricted by the performance of the charge-coupled device. However, newer high-density video cameras can now produce images that are clearer than those obtained from a standard surgical microscope. In addition, a recently introduced image processing system may also improve performance by being able to produce disease-oriented imaging, such as images of the surface structure in epiretinal membrane, the structure of intraocular masses, or subretinal lesions.

The current study demonstrated that the topical endoscopic imaging system is a convenient method for obtaining not only wide-field viewing images but also for panfocal imaging. Thus, this new methodology appears to be a new observational tool that can be easily used during intraocular surgeries performed in a clinical setting.