Two types of contact lenses are available to assist in slit-lamp delivery of photocoagulation:
negative-power planoconcave lenses
The planoconcave lenses provide an upright image with superior resolution of a small retinal area. Most clinicians favor use of these lenses for macular treatments. Mirrored planoconcave lenses facilitate viewing and photocoagulation of more peripheral retina; the exact location in view depends on the angle of the mirror used. Planoconcave lenses generally provide the same retinal spot size as that selected on the slit-lamp setting.
High-plus-power lenses provide an inverted image with some loss of fine resolution, but they offer a wide field of view, which facilitates efficient treatment over a broad area. The macula may be kept in view while the midperiphery of the retina is being treated, making these lenses ideal for panretinal photocoagulation. High-plus-power lenses provide a spot size that is magnified over the laser setting size; the magnification factor depends on the lens used (Table 19-1).
Parameters and indications
Selection of laser setting parameters depends on the treatment goals, the clarity of the ocular media, and the fundus pigmentation. As a general rule, smaller spot sizes require less energy than larger spot sizes, and longer-duration exposures require less energy than shorter-duration exposures to achieve the same intensity effects. The following sections present guidelines for conventional laser treatments.
Macular laser Although its use is declining, laser photocoagulation still has a role in the management of some forms of macular edema, extrafoveal CNV, and focal retinal pigment epithelium (RPE) abnormalities with leakage, such as those seen in central serous chorioretinopathy. To avoid causing central scotomas and perifoveal capillary dropout, treatment should not be administered within 500 μm of the foveal center. Macular laser treatment for edema generally employs a smaller spot size (50–200 μm) and shorter duration (≤0.1 second) to achieve smaller, less-intense burns. For diabetic macular edema, green or yellow direct laser therapy should be applied to all leaking microaneurysms located between 500 μm and 3000 μm from the center of the macula. For more diffuse leakage or zones of capillary nonperfusion, a light-intensity grid pattern using a green or yellow laser is applied to all areas of diffuse leakage more than 500 μm from the center of the macula and 500 μm from the temporal margin of the optic nerve head. A similar strategy is employed in treating macular edema caused by branch retinal vein occlusion. In the treatment of CNV or RPE leakage spots, the aim is to achieve a more intense burn of the entire lesion or area of leakage.
Table 19-1 Magnification Factors for Common Laser Lenses
Peripheral retinal photocoagulation Peripheral retinal photocoagulation may be delivered with either a slit-lamp or indirect ophthalmoscopic delivery system. The latter system offers less control over precise spot size or burn intensity. In panretinal photocoagulation (PRP) or laser retinopexy, the aim is to achieve somewhat intense, larger laser burns. For PRP, the laser spot size typically is 200–500 μm, and laser power is adjusted to achieve gray or light cream-colored burns. Applications, or burns, are usually one-half to one burn width apart (see Chapter 5, Fig 5-8) and should spare the macula. For initial PRP treatment, some practitioners leave approximately 1–2 disc diameters of retina outside of the macula and optic nerve head untreated, whereas others treat up to the arcade vessels and up to the optic nerve (Fig 19-2). The initial treatment pattern can be concentrated more densely in the inferior retina to help minimize temporal and inferior visual field loss and to facilitate retreatment in the case of vitreous hemorrhage. The long ciliary nerves in the 3- and 9-o’clock meridians should be avoided, if possible, and any coexisting macular edema should be treated beforehand to avoid exacerbation after PRP. Typically, 1200– 1400 laser applications of 500-μm spot size—the equivalent of smaller burns—are placed.
Figure 19-2 Illustration of the posterior extent of full panretinal photocoagulation (peripheral treatment not shown).
(Illustration by Mark Miller.)
Laser retinopexy is used to create a chorioretinal adhesion around retinal tears or for demarcation of a (small) retinal detachment; it uses spot sizes of 200–500 μm, and its goal is creating whitish, but not snow-white, burns. Usually, 2–3 rows of photocoagulation around the break are considered appropriate to achieve the desired adhesion.
Laser ablation of retinal vascular lesions Vascular lesions are often treated using a large spot size (≥500 μm), lower power, and long duration so each lesion slowly heats up and coagulates “from the inside out.” High-intensity burns whiten the surface of the lesions, after which visible-light laser does not penetrate well, making it difficult to achieve the treatment goal.
Alternative laser delivery systems and strategies
New technologies and concepts have led to novel treatment systems and strategies. Recent innovations in slit-lamp delivery systems include pattern scanners that deliver an entire array of laser applications with each foot-pedal depression; the high-intensity laser pulses are ultrashort (20–50 milliseconds) and are delivered in rapid succession. This approach possibly increases the efficiency of treatment, but it may not achieve an effect equivalent to traditional laser treatment on a spot-number-to-spot-number comparison.
Some systems incorporate real-time retinal image overlay and registration with the laser delivery system. This configuration allows for computer-assisted planning and precise targeting of the retinal lesions during treatments.
Delivery systems that apply subthreshold (barely visible to invisible) laser spots administer laser in micropulses (≤0.1 millisecond) that can confine heat conduction to the RPE while limiting thermal damage to the photoreceptors and choriocapillaris. These systems have been shown to be effective in treating diabetic macular edema and have the potential to reduce the number and size of scotomas. Titration of burn intensity and monitoring of the area of placement of invisible laser spots during laser delivery is enhanced in commercial laser systems using endpoint software technology.
Baumal CR, Ip M, Puliafito CA. Light and laser injury. In: Yanoff M, Duker JS. Ophthalmology. 4th ed. Philadelphia: Saunders/Elsevier; 2013:461–466.
Chappelow AV, Tan K, Waheed NK, Kaiser PK. Panretinal photocoagulation for proliferative diabetic retinopathy: pattern scan laser versus argon laser. Am J Ophthalmol. 2012;153(1):137–142.e2.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.