A Primer on Therapeutic Ophthalmic Lasers
With the advancement of refractive surgery, we've all learned a good deal about the excimer laser. However, therapeutic lasers--namely the argon and krypton lasers and the neodymium and holmium YAG lasers--have not received much attention of late. These devices are instrumental in the management of glaucoma, retinal disease and post-cataract care. It's important to understand how clinicians use these lasers.
This laser-tissue interaction is accomplished with the argon and krypton lasers in the performance of trabeculoplasty, iridoplasty and iridotomy for glaucoma, as well as fundus photocoagulation for retinal vascular disease and choroidal neovascular membrane obliteration. This process is pigment-dependent and will not occur without melanin in treated tissues.
During photocoagulation, the doctor focuses laser light on the pigmented tissue, which absorbs the light and converts it into heat. When tissue is warmed 10-20° C, photocoagulation occurs. This is considered a thermal event, so these lasers are referred to as thermal or "hot" lasers.
The effects may take hours to appreciate. Photocoagu-lation involves the denaturing of proteins. It coagulates blood and produces inflammation. These actions create the desired scarring and adhesions, sealing leakages in retinal vascular disease.
Tissue atrophy arises surrounding each photocoagulation scar (the goal in pan-retinal photocoagulation). Photocoagulation warms collagen, stimulating it to contract. This changes the microanatomy of the tissue (e.g. the trabecular meshwork in trabeculoplasty and the iris in iridoplasty). Hemoglobin absorbs light, becomes heated, and forms a thrombus. This, along with blood vessel wall (collagen) contraction, seals leaking vessels.
Adverse effects include IOP rise with a possible glaucomatous vision loss, and intraocular inflammation with attendant anterior uveitis. These adverse effects arise in the majority of procedures, and are effectively managed by pre- and post-treatment with medications such as apraclonidine and topical corticosteroids.
This laser-tissue interaction forms the basis of iridotomy for angle closure glaucoma and capsulotomy for post-cataract capsular opacification. The Nd:YAG laser is used in this procedure, and it is non-pigment dependent; that is, it can be performed on tissues with little or no melanin.
The photodisruption procedure involves the delivery of large amounts of energy into very small focal spots in very brief durations (nanoseconds to picoseconds). The result is an instantaneous, highly localized temperature increase (15,000° C). However, heating is so quick that the heat dissipates outwardly and the procedure is considered non-thermal. Molecules and atoms are stripped of their electrons and ionization occurs. The light energy causes tissue to be reduced to a form of matter called plasma. This results in hemodynamic waves--the generation of fluid forces.
Photodisruption also produces acoustic pulses (sound waves)--forces that propagate outward in all directions (but especially back toward the direction from which the laser was fired). These propagating forces incise tissues.
Adverse effects include more inflammation than with thermal lasers, and a greater incidence of hemorrhage. Bleeding is not a serious complication because mild digital pressure to the eye will increase IOP and cause the bleeding vessel to seal. The main difference is that photodisruptive lasers can cause acoustic insult to the vitreous (during capsulotomy), which may induce retinal detachment. Patients need to know there is approximately a 2% incidence of retinal detachment during this procedure.1
In photodynamic therapy, subfoveal choroidal neovascular membranes are treated using a non-thermal laser and a newly developed light activated chemical, Visudyne (verteporfin, Novartis Ophthalmics). In this therapy, the patient's retina is infused with verteporfin 15 minutes prior to treatment. At that time, the retina is irradiated for 83 seconds with low-intensity, non-thermal light from a diode laser. The choroidal neovascular membrane, which accumulates the light-activated drug, is irradiated at the absorption peak of this dye (689nm). The light-activated dye transfers energy to molecular oxygen, resulting in endothelial cell damage and vessel thrombosis. This obliterates the neovascular membrane and then closes it without damaging the overlying fovea.
This treatment heralds great hope for patients with exudative, age-related macular degeneration, ocular histoplasmosis, pathological myopia and angioid streaks.
1. Koch DD, Liu JF, Gill EP, Parke DW. Axial myopia increases the risk of retinal detachment complications after Nd:YAG posterior capsulotomy. Arch Ophthalmol 1989;107:986-90.
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