- Author: Edmin Michael G Santos, MD, DPBO; Chief Editor: Hampton Roy, Sr, MD more...
Photocoagulation uses light to coagulate tissue. When energy from a strong light source is absorbed by tissue and is converted into thermal energy, coagulation necrosis occurs with denaturation of cellular proteins as temperature rises above 65 degrees C.
Since the Diabetic Retinopathy Study, technology evolved from using a diffuse Xenon arc to using well-focused laser in photocoagulating retinal tissue in high risk proliferative diabetic retinopathy. Presently, laser retinal photocoagulation is a therapeutic option in several retinal and eye conditions.[2, 3, 4]
Effective retinal photocoagulation depends on how well light penetrates the ocular media on its way to the retinal tissue and how well the light is absorbed by pigment in the target tissue. In retinal tissue, light is absorbed by melanin, xanthophyll or hemoglobin. Melanin absorbs green, yellow, red and infrared wavelengths; xanthophyll (in the macula) absorbs blue but minimally absorbs yellow or red wavelengths; hemoglobin absorbs blue, green and yellow with minimal red wavelength absorption.
Indications for retinal photocoagulation include the following:[6, 7, 8, 9, 10]
Treatment of threshold and high-risk prethreshold retinopathy of prematurity
Closure of retinal microvascular abnormalities such as microaneurysms, telangiectasia and perivascular leakage
Focal ablation of extrafoveal choroidal neovascular membrane
Creation of chorioretinal adhesions surrounding retinal breaks and detached areas 
Focal treatment of pigment abnormalities such as leakage from central serous chorioretinopathy 
Treatment of ocular tumors
Patients require anesthesia for the procedure. Most patients undergo retinal laser procedures under topical anesthesia such as proparacaine eye drops. Other patients will require peribulbar or retrobulbar injections of lidocaine.
General anesthesia is usually employed for children and patients with problems in compliance.
A laser medium is connected via a fiberoptic cable to different types of delivery systems. Laser is delivered to the retina externally either through the cornea (transcorneal) or the sclera (transscleral). Transcorneal delivery employs a slit lamp (see first image below) or a Laser Indirect Ophthalmoscope (LIO). With the slit lamp delivery system, the laser is fired onto the retina using a contact lens which is placed on the corneal surface of the patient. Using the LIO delivery system, a noncontact binocular indirect ophthalmoscope condensing lens such as 28 D or 20 D lens (see second image below), is used to focus the laser onto the retina.
Transscleral delivery uses a diode transscleral laser probe applied onto the sclera.
Laser can also be delivered internally (inside the eye), usually during vitrectomy procedures.
An endolaser probe is introduced into the vitreous cavity, and laser is fired directly to the retina. The procedure is viewed using vitrectomy lens under an operating microscope.
When using the slit lamp delivery system, the procedure is performed with the patient in a sitting position. With the endolaser and transscleral delivery systems, the patient is supine. With the LIO, the patient may be sitting or supine.
Proper laser protection goggles are required for all staff assisting the procedure. The laser safety filter on the delivery system should always be activated upon performing the procedure.
The patient should be well positioned and instructed prior to the procedure. Retrobulbar block or general anesthesia may be done for compliance problems.
The procedure should be performed or supervised by an experienced ophthalmologist to avoid technical errors resulting in complications from the procedure.
When using the slit lamp delivery system, a slit lamp contact lens is used to focus a beam of laser light onto the retina. With the indirect ophthalmoscope system, an indirect lens is used to focus the laser light onto the retina. With the endolaser, a laser probe is introduced into the vitreous cavity (usually during vitrectomy surgery) and the laser light is directly applied to the retina.
Conventional laser delivery systems for retinal photocoagulation deliver spots individually on the retina. Newer semiautomatic laser delivery systems like the pattern scanning laser (PASCAL) have been designed to produce multiple spots on the retina in the same amount of time as conventional laser delivery systems. This makes the procedure less tedious and time consuming, allowing for better patient comfort.
Although proven safe, like any other surgical procedure, retinal photocoagulation may occasionally be associated with complications. Before undergoing retinal photocoagulation, the patient should be fully informed of these, which include the following:[16, 17]
Anterior segment complications such as corneal or lenticular opacification
Transient visual loss
Photocoagulation of the fovea
Color vision alterations
Visual field defects and night vision problems
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