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

Monitoring and Titration During

Extensive Photocoagulation

In cases where extensive photocoagulation is required, the ophthalmologist should constantly monitor the retinal reaction since this can vary markedly from one spot to the next depending on the amount of tissue absorption. Titration for the correct amount of energy is critical.

Pearls in the Treatment of

Proliferative Diabetic Retinopathy

When an advanced stage of proliferative diabetic retinopathy is present, large numbers of laser application spots are usually necessary. It is recommended to deliver these in multiple stages to avoid exudative choroidal and retinal detachment not infrequently found after extensive treatment. In patients where neovascularization at the disc or elsewhere remains with recurrent bleeding in spite of adequate photocoagulation, further laser treatment should not be insisted. Vitrectomy with endolaser photocoagulation (Figure 9) should be applied without delay to avoid permanent damage.

Timing for Vitrectomy

In proliferative diabetic retinopathy, it is preferable to intervene early with vitrectomy when indicated rather than later. This avoids the production of a small visual field as a result of increased laser scaring following extensive photocoagulation. Ophthalmologists must remain flexible in their laser applica-

tion to the retina is important to obtain the desired results with the least application of energy possible. The goal is to spare retina rather than to destroy retina.

Comparative Tissue Effects of

Different Lasers

The Blue Laser

The argon laser that incorporated blue and green wavelengths was used for many years in the treatment of chorioretinal diseases. The majority of commercial argon laser photocoagulators available during the 70’s produced a light beam of 70% blue (488 nm) and 30% green (515 nm). Treatment with the blue wavelength has been discontinued for use in retinal photocoagulation in favor of many others, especially the green wavelength.

Disadvantages of the Blue Laser Light in the Treatment of the Retina

Photochemical (non-thermal) retinal damage is higher with lasers of shorter wavelengths (blue) than those having longer wavelengths (green, yellow, red and infrared). This is because shorter wavelengths create more energy per photons. Blue is scattered many times more in the media than the green, yellow or red. Therefore, higher energies are needed to obtain the desired absorption by the lesion to be treated. Scattering in the ocular media (Figure 2) increases with changes from aging so higher power levels

at the cornea are necessary to obtain the desired retinal burn. Also, it is possible that scattered blue light could damage normal retina next to the treatment area (Figure 1). These were some of the reasons why the blue part of the argon spectrum was eliminated for retinal treatment.

Blue light is absorbed by the yellow pigment present in the inner layers of the macula (Figure 1) producing damage to these vital tissues during macular photocoagulation. This may increase visual field defects from the treatment of macular lesions.

Also, the yellowed lens in aging eyes and cataract opacities increase absorption of blue light. This produces higher energy uptake by the crystalline lens with subsequent risk of damage (Figure 2).

The Green Laser

The green argon laser light has a wavelength of 515 nm. This laser is the most widely available and popular laser for retinal photocoagulation. It can be found in the following types of lasers: 1) lasers made exclusively for pure green output or 2) a blue-green laser with filter to provide the pure green wavelength.

Advantages of the Green Laser

Compared With Red

The green wavelength has the advantage of being absorbed by the hemoglobin of the blood in a subretinal neovascular membrane (NM) (Figure 3). The disadvantage is when

a small layer of blood (B) is present in the inner layers of the retina (intraretinal blood), the green light (G) will be absorbed by the hemoglobin. This absorption of energy will damage the inner retinal layers (Figure 3). Red light (R) will penetrate deeper due to the lack of absorption by hemoglobin.

Advantages of the Yellow Laser

Compared With Red

Yellow (Figure 4), as well as green, laser light is maximally absorbed by hemoglobin. This allows direct treatment of superficial retinal vascular lesions and subretinal neovascular membranes. The absorption of yellow

and green light by hemoglobin becomes a disadvantage when the subretinal neovascular membrane (NM) lies under a thin layer of subretinal hemorrhage (H). The yellow and green laser energy is first absorbed by the layered blood (H) before affecting the deeper structures. On the other hand, red laser light can penetrate these hemorrhages.

The yellow laser wavelength is not frequently used due to the cost of instrumentation and equipment. It still remains, however, the best wavelength to treat vascular lesions due to the increased absorption by oxyhemoglobin. This requires less power to obtain the tissue reaction needed to coagulate the vascularized tissue.

The red laser uses a wavelength around 647 nm. It continues to be used in some retinal diseases such as age-related macular degeneration (ARMD) (Figure 5), but it is not as popular now as the green wavelength.

The red laser is particularly effective when coagulating tissue or subretinal neovascular vessels that lies under a thin layer of subretinal hemorrhage (Figure 6). Red light produces less scatter irradiation and heat into the retina from the blood. This preserves the desired retinal tissue, in particular when treating near the fovea (Figure 7).

Figure 5: Advantages of Red Krypton Laser with Subretinal Neovascular Membrane in ARMD. Above is shown a cross section of the retina and choroid emphasizing the area of a subretinal neovascular membrane

(M) that lies between the pigment epithelium layer (E) and choriocapillaris (C). This area of fibrous growth is vascularized by outgrowths from the choroid and is a very important complication of exudative ARMD. Note that the retina (R) is detached in this area. The red Krypton light (Kr) travels through the vitreous (V) with very little involvement of the nerve fiber layer seen at area 1. There is less absorption of laser light within the inner retina at area 2. Lack of absorption in the inner layer results in decreased intraretinal fibrosis at area 3. Here the surgeon aims at occlusion of choroidal blood vessels that is the possible source of the subretinal neovascular membrane (M). Other anatomy: Photoreceptors (P) and sclera (S).(Art from Jaypee

Figure 6: Location of Krypton Red Laser Absorption in Treatment of Subretinal Vascular Membrane. This anatomical cross section of the retina shows that red laser light (KR) is mainly absorbed by the melanin (blue arrow) of choroid (C) and retinal pigment epithelium (P-red arrow). The retina is shown detached in the area of the subretinal vascular membrane (M). Other retinal anatomy: inner limiting membrane (I), ganglion layer (G), inner nuclear layer (A), outer nuclear layer (D), outer limiting membrane (O), rod and cone layer (R), Bruch’s membrane (B) and choriocapillaris (H). (Art from Jaypee Highlights Medical Publishers).

There are also other advantages of the red krypton laser. It provides deeper tissue penetration leading to coagulation of the subretinal neovascularization or subretinal neovascular membrane (Figure 5). There is less energy absorption by the inner retina (Figures 6 - 7). This leads to less involvement of the nerve fiber layer and decreased intraretinal fibrosis. There is less absorption of the laser light by the macular yellow pigment or blood in the macula. This is critical as it limits the damage to the fovea and thus minimizes the decrease in visual acuity immediately following treatment.

Disadvantages of the Red Laser

The main disadvantage of red krypton laser is that its use may lead to choroidal bleeding. The best way to avoid this complication is to abstain from using short exposures with a small spot and high intensity.

The Pure Monochromatic Green Laser Compared to Red Krypton

If red krypton equipment is available as shown in Figure 7, it is better to use red in cases with intraretinal blood. In all other instances as shown in Figures 5 and 6, a pure green wavelength is as good as red krypton. For treatment of subretinal neovascular membranes, a key complication of ARMD, the red wavelength has not been demonstrated to be better than pure green unless there is intraretinal blood, as shown in Figure 7.

If dealing with superficial retinal neovascularization such as in diabetes and vascular tumors, the krypton red laser is not indicated because it is not absorbed by hemoglobin. Those cases are better treated with green or yellow wavelengths.

The Diode Laser

The diode laser produces an infrared light with long wavelengths in the range of 700-820 nm. The efficiency of semiconductor diode lasers makes it possible for them to have minimal electrical or cooling needs. They can be made small, portable and even be mounted on existing slit lamps. Their solid-state design allows them to be made economically and reliably.

Main Uses for Diode Laser

This laser is used for direct retinal photocoagulation either transclerally for treating retinal pathology such as retinal tears or holes, diabetic macular edema, and proliferative diabetic retinopathy; or for use in endophotocoagulation. It can be utilized in photodynamic therapy for subretinal neovascular membranes in ARMD (Figure 8). The diode laser can also be used effectively in non-retinal diseases particularly for cyclodestructive procedures in glaucoma.

Advantages of the Diode Laser

In the presently available commercial lasers, the diode laser has several advantages. Because of decreased scatter and absorption, the infrared diode laser penetrates vitreous hemorrhage and nuclear sclerotic cataracts better than the shorter wavelength laser such as green and yellow. The deeper penetration spares the inner sensory retina. The laser can be delivered through diabetic preretinal membranes without contracting them. The

absence of xanthophyll absorption along with the lower absorption for melanin and oxyhemoglobin provides safe delivery to the macula (Figure 7). The lack of hemoglobin absorption allows penetration through thin layers of preretinal or subretinal hemorrhage without excessive laser energy uptake (Figure 8).

Other Advantages of the Diode

Laser

Its portability has been very useful given the many locations where laser treatment can be delivered. This is particularly important in the operating rooms of many hospitals throughout the world. Despite the inevitable trauma to the equipment that comes from moving it, there is no damage to the function of the laser. Without the need for cooling or triphase 220-volt power, laser therapy can be performed in any room containing a Haag-Streit slit lamp or endophotocoagulation system. The diode laser can be connected to an AC source of electrical power or can be powered by batteries if needed. The solidstate design of the laser makes it resistant to extremes of humidity and temperature.

Disadvantages of the Diode Laser

Vascular abnormalities such as retinal angiomas or retinal telangiectasia cannot be directly treated with the 800 nm wavelength because it is not absorbed by hemoglobin. Its use may be inadequate in subretinal neovascular membranes in light-colored fundi because of low laser light absorption. Broad-field

contact lenses are suitable for photocoagulation with the diode laser. These produce inverted and real images. Lenses that work well with the diode laser include the Volk Centralis, Trans-Equator, and Quadraspheric; and the Mainster Standard and Widefield.

SYSTEMS TO DELIVER

LASER ENERGY

After the clinician has decided which wavelength to use, the next question is which system to use to deliver the laser energy. Delivery systems include the traditional slit-lamp system, endofiberoptics for use intraocularly such as in endolaser photocoagulation, the indirect ophthalmoscope,

and contact probes. All ophthalmologists are familiar with the slit-lamp delivery system which is the most commonly used. Consequently, single spot treatment will not be discussed here except for the relatively new PASCAL treatment. The rest of the focus will be about the endolaser and binocular indirect ophthalmoscopic delivery system.

PASCAL Photocoagulation

PASCAL Coagulation Background

The PASCAL (Pattern Scan Laser) coagulation system by OptiMedica is a recent development intended to expand upon the current single laser spot used in coagulation

therapy. This modified slit lamp coagulator uses a 532 nm laser that provides multiple spot therapy of up to 56 in number that are applied in pre-arranged configurations such as squares and arc arrays. These arrays can be adjusted to provide faster and more efficient laser applications depending upon the desired treatment.

Indications for PASCAL

Photocoagulation

The PASCAL coagulation therapy has indications for both posterior and anterior segment ocular pathology. Retinal uses include panretinal, focal, or macular grid treatment in patients with proliferative diabetic retinopathy, retinal tears and detachments, choroidal neovascularization, age-related macular degeneration, and branch retinal vein occlusions. The anterior segment uses include trabeculoplasty and iridectomy, but further discussion about these applications is beyond the scope of this chapter.

Advantages of PASCAL Coagulation

This treatment method provides efficient laser therapy over large areas of the retina using multiple spots in a rapid successive order. The pattern and number of spots can be adjusted depending on the desired location. It is also versatile in its uses from large panretinal therapy requiring hundreds of spots to localized single spot focal treatment. This rapid therapy is believed to provide less patient discomfort by shorting

both laser time between each spot application and total time at the slit lamp.

Disadvantages of PASCAL

Coagulation

There are some disadvantages to this coagulation treatment. Patients need to be able to at the slit lamp for the therapy. Once situated, their cooperation is critical as multiple spots are delivered in successive order after activation. Sudden movements by patients can result in coagulation of unintended locations.

Endolaser Photocoagulation

Endolaser Coagulation Background

Endolaser coagulation is a method by which the laser light is brought directly inside the eye through a fiberoptic to apply treatment to the retina (Figure 9). This is in contrast to conventional laser photocoagulation that is performed through the clear cornea also known as the “transpupillary” method. The endolaser is essentially used only during vitrectomy. When the surgeon is working inside the eye and a need for coagulation exists, the laser light is directed directly toward that area through a 1 mm diameter probe and photocoagulation is performed. Also, if a hemorrhage occurs during surgery, the media can turn too cloudy for transpupillary application. Since the surgeon cannot bring the patient to the slit lamp, photocoagulation can be completed with the endolaser (Figure 9).

Indications for Endolaser

Photocoagulation

What is now done now with an endolaser was previously performed through intraocular diathermy, external cryocoagulation, or endocryotherapy. These methods have been almost abandoned and replaced by the endolaser.

The indications for the use of endolaser during vitrectomy are: 1) to coagulate preexisting, posteriorly located retinal tears or iatrogenically produced retinal tears; 2) to assist with the internal drainage of subretinal fluid in retinal detachment; 3) to coagulate bleeding retinal surface neovascularization;

4) to perform panretinal photocoagulation in diabetic patients immediately after vitrectomy; and 5) to manage penetrating injuries and intraocular foreign bodies.

Comparison with Other

Methods Previously Used

Whenusinganintraoculardiathermyprobe, the probe needs to be close to the retina nearly touching the tissue. During coagulation the tissue can adhere to the probe resulting in the instrument itself inflicting damage to the retina and choroid. The surgeon can actually create a choroidal hemorrhage by accidentally penetrating the choroid and not coagulating it.

When intraocular cryotherapy is used, the probe has to be held motionless inside the eye on the retina. The effect of coagulation starts on the retinal side and then penetrates deeper into the choroid. This produces a larger reaction in the sensory retina than in the pigment epithelium and choroid. If the probe is not held still, the retina can be fractured at the edge of the cryocoagulation and could create a new tear.

The disadvantage of external cryotherapy versus endolaser in treating posteriorly located retinal tears is that a large area of the retina has to be coagulated that may lead to damage in the nearby fovea and optic nerve. In addition, the sealing of tears close to the fovea or to the optic nerve is a more complex procedure technically when external cryotherapy is used because of their location.

Binocular Indirect Ophthalmoscopic Laser Photocoagulation (BIOLP)

BIOLP is an essential tool for those who want to treat peripheral retinal neovascularization. The advent of this laser delivery system allows the surgeon to visualize and treat the retinal periphery easily, an important advancement. Laser treatment delivered by means of BIOLP has made possible the treatment of peripheral retinal neovascularization.

Indications and Advantages of the

BIOLP

The BIOLP has several indications. It can be used for panretinal photocoagulation in

patients with proliferative diabetic retinopathy who cannot sit at a slit lamp. Other indications include treatment for peripheral retinal tears and demarcation of localized retinal detachments. BIOLOP can also be used in retinal vascular diseases affecting the periphery as in some cases of branch retinal vein occlusions, central retinal vein occlusions, retinopathy of prematurity, and for inflammatory and retinal diseases. This technique also permits treatment of infants under general anesthesia and children without anesthesia if they are cooperative.

Since most of these diseases were treated in the past with cryopexy, it is important to point out that laser burns appear to produce faster adhesions and less breakdown of the blood-retina barrier. BIOLP is also of great value intraoperatively because it allows a wide view that is helpful for applying treatment to the peripheral retina.

Disadvantages of the BIOLP

In traditional slit-lamp delivery systems, the operator controls the spot size, power and duration. Spot size is difficult to control with the BIOLP. This requires special training to use it adequately and safely.

Duration and power are controlled in a manner similar to that for slit-lamp delivery systems and are titrated to achieve the desired burn. Care should be taken as the treatment moves farther to the periphery because the retinal spot may become smaller. Either the laser spot needs to be further defocused or the power decreased. It is best to deliver less power over a longer duration because the lesion produced can be better monitored.