Shedding More Light on Small-Gauge Vitrectomy
A review of new light sources and visualization techniques in minimally invasive vitrectomy.
DAVID S. DYER, MD
Minimally invasive surgery with 25-gauge and 23-gauge instrumentation continues to improve with technological advances. One of the difficulties with small-gauge surgery, however, has been adequate illumination. In fact, many retina specialists who tried 25-gauge vitrectomy using the standard light sources returned to 20-gauge techniques because of poor illumination. This is understandable given that the standard light sources — halogen or metal halide bulbs — on early small-gauge systems produce about 3 lumens, compared to 10 lumens with 20-gauge surgery. Furthermore, 25-gauge fibers are only half the diameter of 20-gauge fibers, and approximately 60% of the light is lost when using 25-gauge fibers with halogen lighting.
Fortunately, instrumentation and ophthalmic agents have recently been introduced that provide dramatically better visualization during small-gauge surgery. This article will review several of the most promising and their application in vitrectomy.
IMPROVED LIGHT SOURCES
The problem of poor illumination has been overcome with the availability of high-output xenon lights. These can be added to existing vitrectomy systems as a stand-alone external light or, in the case of the CONSTELLATION® Vision System (Alcon, Fort Worth, TX), the xenon light source is incorporated directly into the system's console. With xenon lights, more light is available through a 23- or 25-gauge light pipe than was previously available using standard light sources with 20-gauge vitrectomy systems.
All xenon light sources use filters to reduce UV light, and the color produced by the xenon light depends on the filter that is used. Surgeons may perceive advantages to the various colors, as different hues may help identify different structures inside the eye.
Retina specialists can choose from several types of light pipes that employ these new light sources. Two of the most useful for bimanual surgery are the illuminated infusion cannula, or infusion light pipe, and the chandelier probe.
|David S. Dyer, MD, maintains a private retina practice located in Kansas City, Mo. He is a member of the Retinal Advisory Council of Alcon Laboratories, Inc.|
LIGHTED INFUSION CANNULA
The illuminated infusion cannula has a small light fiber inside, eliminating the need for a fourth cannula, which is placed when using a chandelier light. The user can focus light posteriorly by pushing the fiber into the eye, or can illuminate a wider field by pulling the fiber back toward the tip of the cannula.
A limitation of the infusion light pipe is that the light fiber is smaller than a standard 25-gauge light pipe because both the infusion and the light fiber must fit inside the 25-gauge cannula. Fortunately, the newer, powerful xenon light sources can compensate for much of the downside of a small fiber.
Because it does not have a very wide field of illumination, the infusion light pipe is best used for bimanual membrane dissection in a localized area of the retina. It works best in cases such as proliferative vitreoretinopathy (PVR) inferiorly or traction retinal detachment (RD) from proliferative diabetic retinopathy (PDR) in one quadrant, where the need for illumination is limited to a specific location in the eye.
CHANDELIER AND TORPEDO LIGHT PROBES
Chandelier and torpedo light probes also are available for use in small-gauge surgery. With the powerful xenon light sources, a 25-gauge chandelier can illuminate the entire retina from ora to ora, facilitating bimanual membrane dissection with 25-gauge, 23-gauge or 20-gauge instrumentation. Even intricate and difficult membrane dissection for anterior PVR and vitreous base dissection for severe traction RDs from PDR — procedures that previously could be performed only with 20-gauge surgery — can now be performed with 25-gauge instrumentation.
The new xenon light sources are powerful and allow the user to operate inside the eye with greater light intensity. However, this higher light intensity is associated with greater potential for phototoxicity or thermal toxicity if the light is focused on a specific point on the retina for too long or at too close a distance. Therefore, it is advisable to use the minimum amount of light to achieve the necessary illumination. Furthermore, the higher the power output from the xenon illuminator, the farther away from the retina the light should be.
Pharmaceutical agents have been used for visualization in vitreoretinal surgery for many years. The two most common are indocyanine green (ICG) and triamcinolone.
ICG has been popular for visualizing epiretinal membranes (ERMs) and internal limiting membrane (ILM) during vitrectomy surgery to remove ERMs or to treat macular holes. There have been reports of possible toxicity of ICG to the retina and optic nerve. This risk increases with higher concentrations and prolonged exposure. Triamcinolone has been used for many years, in a fashion similar to ICG, to improve visualization of the clear vitreous body, posterior vitreous base, ILM and ERMs. However, its use carries a risk of retinal toxicity and inflammation from the preservative benzyl alcohol. This risk has been mitigated with the introduction of TRIESENCE® suspension (Alcon), a new, preservative-free formulation of triamcinolone.
CLINICAL EXPERIENCE WITH TRIESENCE®
TRIESENCE® suspension is a synthetic corticosteroid approved by the FDA for visualization during vitrectomy surgery and for the treatment of sympathetic ophthalmia, temporal arteritis, uveitis and ocular inflammatory conditions that are unresponsive to topical corticosteroids. The particle size of TRIESENCE® suspension is optimized to provide the best possible visualization of the vitreous, ERMs and ILMs without obscuring the retina.
A dilute solution of this new formulation of preservative-free triamcinolone appears to be safe and effective for membrane removal and is useful for visualization of the vitreous and vitreous base in selected cases.
This author typically uses TRIESENCE® suspension at a concentration of 5% in BSS® sterile irrigating solution. A volume of 1 cc (0.05 cc of TRIESENCE® suspension plus 0.95 cc of BSS®) is drawn into a TB syringe. Ensure rigorous mixing just prior to injection to prevent settling of the particles in suspension.
With xenon light, more light is available through a 23- or 25-gauge light pipe than was previously available with 20-gauge sources of light. GRIESHABER® DSP end-grasping forceps are used to peel the membrane, which is visualized with TRIESENCE® suspension.
This dilution has the effect of placing a light dusting of TRIESENCE® suspension on the ERM or ILM. The underlying membrane/retina can be visualized easily, and there is no need to aspirate excess particles. Another advantage is that the dilution does not clog the tips of the forceps, eliminating the need to remove the instrument periodically during membrane dissection to clean the ends.
In the author's experience, use of this dilute solution has been associated with no postoperative complications of elevated intraocular pressure or cataract formation, or complaints from patients about floaters from residual particles. By the first postoperative day, no remaining TRIESENCE® can be visualized.
Small-gauge surgery offers the advantages of less invasive techniques but its use has been limited by inadequate visualization. Recent innovations in light sources and pharmaceutical agents have overcome this problem, offering good visualization in most cases. Surgeons who are currently using small-gauge surgery will find these advances very helpful. Those who have previously rejected the approach should consider reevaluating if or how these techniques can fit into their surgical repertoire. RP
1. Sato Y, et al. Evaluation of indocyanine green toxicity to rat retinas. Ophthalmologica. 2006;220:153-158.