Ординатура / Офтальмология / Английские материалы / Retinal and Vitreoretinal Diseases and Surgery_Boyd, Cortez, Sabates_2010

The disadvantage of the OFFISS system is the fact that it is mounted only one microscope (OMS-800), therefore it is most expensive for that.

Thechandelier-styleendoilluminator(Chan- delier; Synergetics Inc, St Challes, Missouri, USA) produces homogeneous and wide-angle endoillumination, making it suitable for use in combination with this system for obtaining a glare-free and panoramic viewing (Figure 12).10 Because of its hands-free nature bimanual

manipulation is possible during removal of strongly adherent preretinal membranes in patients with diabetic retinopathy and proliferative vitreoretinopathy (Figure 10). However, this system is not suitable for delicate surgical procedures in the posterior pole. We have used magnifying quartz contact lens with HHV silicon holder (HOYA health care, Tokyo, Japan) for delicate procedures such as internal limiting membrane and epiretinal membrane peeling (Figure 11). With OFFISS, 120-D and wide-angle endoillumination, all requirements

forwide-angle observation, panoramicviewing vitreoretinal surgery and constant bimanual vitreoretinal surgery are met. The surgeon does not need an assistant, operation time

is shortened, the risk of complications is minimized and complicated cases with severe vitreoretinal pathology are easier to handle.

References

1)Shah VA, Chalam KV : Autoclavable two-pieces non-cased wide-angle contact lens system for vitrectomy. Ophthalmic Res 36:94-97, 2004

2)Peyman GA, Canakis C, Livir –Rallatos C et al: Small-size pediatric vitrectomy wide-angle contact lens. Am J Ophthalmol 135:236-237,2003

3)Nakata K, Ohji M, Tano Y et al: Wide-angle viewing lens for vitrectomy. Am J Ophthalmol 137:760762,2004

4)Horiguchi M, Kojima Y, Shimada Y: New system for fiberoptic-free bimanual vitreous surgery. Arch Ophthalmol 120: 491-494,2002

5)Spitznas M, Reiner J:Astereoscopic diagonal inverter (SDI) for wide-angle vitreous surgery. Graefes Arch Clin Exp Ophthalmol 225: 9-12,1987

6)Nakata K, Ohji M, Ikuno Y: Wide-angle viewing lens for vitrectomy. Am J Ophthalmol 137: 760762,2004

7)Oshima Y: Advancements in Endo-Illumination and Wide-Angle Viewing System. J of the Eye

25:1345-1353, 2008

8)Landers MB, Peyman GA, Wessels IF et al: A new, noncontact wide field viewing system for vitreous surgery. Am J Ophthalmol 136: 199-201,2003

9)Spitznas M: A binocular indirect ophthalmomicroscope (BIOM) for non-contact wide-angle vitreous surgery. Graefes Arch Clin Exp Ophthalmol 225 : 13-15,1987

10)Oshima Y, Awh C, Tano Y: Self-retaining 27-gauge transconjunctival chandelier illumination for panoramic viewing during vitreous surgery. Am J Ophthalmol 143: 166-167,2007

Fundamental Principles of

Laser Energy

Laser Energy as Applied to Retinal Diseases

Laser photocoagulation is the transfer of light energy into heat energy that denatures proteins and produces tissue coagulation. Laser light can be more concentrated than normal light leading to more efficient heat production in the target tissue. The spectrum of laser wavelengths that will be discussed in this section as applied to laser treatment of the retina extends from blue light around 400 nm (Figures 1 - 2) to red light around 780 nm (Figures 5 - 7). Between those are pure green (Figure 3) and yellow light (Figure 4). The infrared wavelength of 800 nm used in the diode laser is longer than the visible 780 nm of red (Figures 8 - 9).

Wavelengths shorter than 400 nm are the ultraviolet, x-rays and gamma rays. Shorter wavelengths (blue) provide more energy per photon than the longer ones (red). Therefore, lasers with shorter wavelengths are more damaging to the retinal tissues.

Types of Lasers Used for Retinal Therapy

The output of a laser can be classified as continuous-wave or pulsed. Although these terms are used throughout ophthalmology, their significance may not be quite clear to those colleagues who do not use lasers often. For practical understanding, retinal photocoagulation is usually performed with a continuous-wave laser. In continuouswave lasers, a pumping source constantly excites the lasering material and radiation is continuously emitted. The output for retinal photocoagulation is usually delivered

during an interval of 0.1 to 1.0 seconds. In contrast, pulsed laser operation occur when a flash lamp or other pumping source turns on and off causing pulses of laser light to be generated. Pulses are usually less than 1 millisecond.

Evolution of Photocoagulation for Retinal Diseases

Photocoagulation of the retina has undergone rapid and steady development since the first Xenon arc instrument developed by Meyer-Schwickerath and produced com-

mercially by Zeiss in 1956. Laser technology has provided better and more reliable instrumentation (Figures 1-9). Photocoagulation has evolved from intense polychromatic white light sources like the xenon lamp, to gas lasers (argon blue-green and green, and krypton yellow and red), and most recently to solid-state diode lasers.

The first decade of laser photocoagulation of the retina was marked by steady refinement in the quality of the spectral delivery. This resulted in the elimination of the blue portion of the spectrum because it was more damaging to the retina (Figure 1). It also

made available specific wavelengths that maximize absorption while requiring less power (Figures 3 - 9). These modifications helped achieve the desired therapeutic results while simultaneously generating less damage to the surrounding normal retinal tissue. A pure green wavelength was attained by the addition of a green filter that only transmits the monochromatic green band that the argon laser emits.

Indications and Availability of

Laser Equipment

Indications for laser therapy have been expanded through landmark multicenter studies that have proved to create beneficial effect on diabetic retinopathy and subretinal neovascular membranes in macular diseases. With the availability of less expensive and smaller instruments, laser technology is now widely available. This includes not only retinal centers in academic institutions that employ retina specialists but also many private offices throughout the world that are managed by highly trained general ophthalmologists. Although there are well known advantages for the different wavelengths, green has been the most popular due to its availability in low cost instrumentation and the wide range of its applications (Figures 3 - 6).

How to Improve Your Results

Importance of Power Density

Delivery

It is important to understand the concept of power density when applying a coagulat-

ing beam to diseased tissue. The spot size, power setting and exposure time determine the power density of the laser. By convention, the spot size is selected prior to treatment while the power setting and exposure time are adjusted throughout the laser treatment. Protocols for controlling these variables have been established for different applications and indications.

Maintaining the correct power density requires careful attention to the relationship of spot size, exposure time and power. Once a good power density has been found, the power and exposure interval should be kept constant as long as the spot size does not change. Any decrease in spot size should be accompanied by a decrease of input power. However, in practice there are multiple factors that will affect the size of the spot and power density such as wavelength, media opacity, and the absorption quality of the tissue to be treated.

The ophthalmologist treating patients with laser photocoagulation should become familiar with a limited number of laser wavelength and contact lens combinations to develop expertise with the factors that will affect the correct power density delivered by the different lasers used. Several good quality contact lenses are available, each with its own advantages and disadvantages.

Pearls for Treatment in the Macular Area Close to the Fovea

Most procedures are performed under anesthetic drops although occasionally retrobulbar anesthesia is required. When treating close to the foveal area, special care needs to be

taken to avoid injury to this important area. Patient cooperation is critical. They should be made aware of any possible distraction that may occur during treatment. For example, the shutter noise of the instrument and anticipation of the laser application may produce a slight movement of the eye causing damage to the central foveal area.

Other considerations when treating near the fovea include the laser settings used. An example of possible settings begins with a 100 micron spot, a short exposure of 0.1 - 0.2 seconds, and a low power intensity of 100 milliwatts or less. The power can then be slowly increased until the desired reaction is obtained.

Selection of the appropriate wavelength is also important. For instance the use a red wavelength will allow for better penetration through early

opacification of the lens (Figure 2). This decreases the need for greater power density.

Figure 2: Disadvantages of Argon Blue Laser in Presence of Yellow Lens from Aging. The blue light of the argon laser (AR) is absorbed by the yellow lens of an aging eye with risk of damage at this level. (Below) The red light of the Krypton laser (KR) is absorbed less by a yellow lens and thus more energy reaches the retina with little effect on the lens. (Art from Jaypee Highlights Medical Publishers).