Shedding Some Light on Current Endoillumination
Brighter Light can be Safe Light
DAVID R. CHOW, MD, FRCSC

For the past decade, most retina surgeons have relied on the light sources integrated into the vitrectomy systems for their endoillumination requirements. For the Alcon Accurus users this has meant dual-output halogen illumination, while for the Bausch and Lomb (B&L) Millennium users this has meant dual-output metal halide illumination. Clinically, these sources of illumination provided a safe viewing environment with adequate illumination for most tasks using a standard 20 g light probe. Unfortunately, these illumination sources lacked the power to provide adequate viewing for lighted instrumentation, wide-angle probes, and chandelier systems, which is why they failed to gain mainstream popularity.

Figure 1. Synergetics Photon Xenon light source.From Illinois Retina Associates, Chicago, IL; Assistant Professor, Rush University, Chicago, IL. Dr. Chow is a consultant to Synergetics and Alcon.
Figure 2. Synergetics Photon "bullseye technology": 25 g lighted laser probe allows surgeon to scleral depress in 25 g cases.
Figure 3. Bimanual diabetic dissection using Synergetics 25 g Awh chandelier as sole source of illumination.

In a study I completed earlier this year, which is pending publication, I showed that the power output in lumens of wide-angle probes and lighted instruments is roughly 30%­50% of the typical 10 lumens of output from a standard 20 g light probe (illumination set to maximum on the Accurus or Millennium). Most of you were probably aware that the illumination capabilities of these instruments suffered, but were probably unaware it was a 50%­70% drop in illumination! No wonder many surgeons have refused to adapt these illumination modalities. Another evolving area of weakness created by the limited power capabilities of our current halogen and metal halide light sources is 25 g vitrectomy procedures. The current 25 g light probes with their smaller light fibres are only capable of roughly 3 lumens of output (illumination set to maximum on the Accurus or Millennium). Many surgeons trying to make the conversion to 25 g vitrectomy systems have often complained about the inadequate illumination, and some have altogether turned away from performing these procedures for this very reason.

Obviously, we would all like a brighter light source!

POWERFUL LIGHT SOURCES

This year, multiple companies have begun to release more powerful light sources, which are driven by a Xenon light source. The first Xenon light source to come to market was the Synergetics Photon, which features a Xenon dual-output illumination source with an integrated laser pathway capability referred to as "bullseye technology" (Figures 1 and 2). Its increase in power capabilities is significant with 25 g probes and lighted instruments that are capable of higher illumination levels than those previously achievable with 20 g probes on maximum illumination. In addition, because of its tremendous power capabilities, very high levels of illumination can be driven through smaller gauge fibres, making 20 g and 25 g illuminated infusion cannulas and 25 g chandeliers clinically useful. The ora-to-ora viewing provided by these chandeliers has allowed me to perform bimanual 20 g and 25 g surgery more than comfortably (Figures 3, 4, and 5). In addition to the Synergetics Photon, Alcon laboratories (Figure 6) and DORC are also shortly releasing more powerful Xenon light sources.

IS MORE POWER SAFE?

More power is great but the question, of course, is can it be given to us safely? The safety of an endoilluminator light source is usually determined by measuring its aphakic retinal hazard function. Phototoxicity created by exposure to an endoilluminator can be either thermal or photochemical in nature. Thermal phototoxicity is usually not a concern with endoilluminators, but more of a concern with endoscopes. Essentially when we worry about phototoxicity we are worrying about ultraviolet (UV) or blue light toxicity. Through the work of Ham and colleagues on rhesus monkeys, we know the action spectrum or relative risk of UV or blue light toxicity when the retina is exposed to various wavelengths of UV or blue light.¹ The action spectrum that Ham and colleagues defined was then used to create an aphakic hazard curve, which is a relative risk of phototoxicity associated with a given wavelength of light. To determine the aphakic retinal hazard function of a light source, its chromatic curve (or output at each wavelength) is measured and then the amount of light it generates under the aphakic retinal hazard curve is multiplied and summed together (Figure 7). In the study I performed earlier this year I tested most of the currently available light sources against the new Xenon illuminators. After repeated testing with different equipment, as well as 3rd party, independent, unbiased verification I found the Synergetics Photon Xenon light to be essentially equivalent to the Alcon Accurus halogen and slightly superior to the B&L Millennium metal halide light from a safety standpoint. The clinical implication of this is that, at least for the Photon Xenon light source, the power of any light probe (20 g or 25 g, a lighted instrument, or a wide field probe) can be optimized to achieve a level compatible with the 10­12 lumens of output we are currently comfortable with from a safety standpoint. As such the increased power capabilities of the Xenon light source can be used to provide illumination levels with ancillary lighting options without a compromise in power. From a practical standpoint, the Synergetics Photon Xenon light incorporates a "smart card" into each light probe's packaging, which directs the surgeon to the recommended levels that the power dial should be set to, in order to achieve these levels intraocularly.

	Figure 4. Synergetics 20 g illuminated infusion cannula.
	Figure 5. Panoramic chandelier view of subretinal hemorrhage in AMD.
	Figure 6. Alcon Xenon light source vs. Alcon Accurus halogen on high 3.

CHANDELIER ILLUMINATION

In my opinion, one of the greatest advances to come with the power capabilities of the new Xenon Photon light source is the development of chandelier illumination with adequate light levels to act as the sole source of illumination intraoperatively. These chandeliers are easy to insert and can be particularly useful for performing bimanual surgery in diabetic dissections, and for working anteriorly in the vitreous base. The chandeliers allow me to dissect anterior proliferative vitreoretinopathy (PVR) membranes bimanually and to perform surgeon controlled vitreous base removal.

I suspect as we gain increasing experience with these chandeliers the list of true bimanual techniques that we will be able to perform will be limited only by our imagination. From a safety standpoint, chandeliers benefit from their extreme distance from the retinal surface. When theoretic retinal threshold times are calculated using the aphakic retinal hazard function of a given light source, a threshold level for retinal damage and a known working distance are established; the greatest increase in safety comes from increasing the working distance. Chandeliers obviously maximize the working distance to the posterior pole and theoretic calculations show increases in working times from minutes to hours. For those of you unable to convince your hospital to budget for a new Xenon light source this year, there are some new chandelier systems available for use on the Alcon Accurus halogen or B&L metal halide. The current options include the Tornambe 25 g minilight from Insight Instruments and the DORC Neptune chandelier, which is a dual fibre chandelier (Figure 8). Having used both of these chandeliers, I have found both to be highly effective, but limited by the power capabilities of halogen and metal halide light.

Another aspect of safety is in determining the best use of filters. Unfiltered Xenon light is unsafe because it contains high levels of UV illumination. As such, all the new Xenon lights will have a series of integrated filters to maximize safety. The color of the new Xenon light sources will depend on the filter system integrated into the light source. The color of an endoilluminator light source as perceived by the surgeon is a function of the chromatic curve of the light source modulated by the obstacles in the pathway from the retinal-viewing environment up through the cornea and microscope to the eyepieces and the surgeon's retina. A halogen light bulb has a chromatic curve that creates a light with a slightly yellowish hue. On the other hand a metal halide light bulb has a chromatic curve that creates a light source with a bluish tinge. I have recently completed a study assessing the concept of monochromatic illumination or the use of various filters to maximize the safety of the Photon Xenon light source thus allowing increased illumination levels. These results will be presented at upcoming meetings.

CONCLUSIONS

The development of powerful, safe Xenon light sources has come at an opportune time for us as retinal surgeons to take advantage of 25 g surgical techniques. It has also allowed those of us who like lighted instruments to use these instruments without sacrificing illumination levels. Chandeliers driven by the Xenon light sources provide a spectacular panoramic viewing environment within which "true" bimanual surgery can be performed with the ultimate in safety threshold working times. For the future we are continuing to work on ways of maximizing our illumination levels while at the same time reducing our risk of phototoxicity.

Figure 7. Aphakic retinal hazard curve and chromatic curve for the Alcon Accurus halogen at high 3.	Figure 8. Tornambe 25 g minilight from Insight Instruments.

Address correspondence to: David Chow, MD, FRCSC, Assistant Professor, Rush University, Illinois Retina Associates, 71 W 156th St, Suite 400, Harvey, IL, 60426. Telephone: (708) 596-8710, Fax: (708)596-9820, E-mail: davidrchow@mac.com.

REFERENCE

1. Ham WT Jr, Mueller HA, Ruffolo JJ Jr, Guerry D 3rd, Guerry RK. Action spectrum for retinal injury from near-ultraviolet radiation in the aphakic monkey. Am J Ophthalmol. 1982;93:299-306.