Laser therapy has become the method of choice for treating many forms of glaucoma, especially when medical modalities prove inadequate or patient compliance with medical regimens is poor. In angle-closure glaucoma with pupillary block, for example, neodymium:yttrium-aluminum-garnet (Nd:YAG) or argon laser iridotomy has replaced surgical iridectomy, which is generally reserved for situations in which a laser approach is not possible. Laser trabeculoplasty has advanced the treatment of primary open-angle glaucoma by providing a safe and usually effective means of achieving intraocular pressure (IOP) reductions with fewer complications than invasive surgical procedures such as trabeculectomy. In cases of medically and surgically uncontrollable glaucoma, diode, argon laser cyclophotocoagulation, and Nd:YAG of the ciliary body have proven effective. Laser techniques are generally less invasive, carry fewer risks, and result in fewer complications than their surgical counterparts. Because laser procedures involve risks of their own, however, a thorough understanding of the principles of clinical laser therapy is essential.
GENERAL ASPECTS OF LASER THERAPY
Laser energy can be delivered in a variety of wavelengths and time durations, from extremely short-burst lasers to continuous-wave lasers. The effect of laser energy on target and surrounding tissue depends on the amount and duration of energy absorbed, which are dependent on tissue pigmentation, wavelength, amount of energy delivered, exposure time, and size of the laser spot.
TISSUE EFFECTS OF LASER
THERMAL EFFECTS (PHOTOCOAGULATION, PHOTOVAPORIZATION)
In general, long exposures by lasers at relatively low energy densities produce a coagulative effect that shrinks collagen ( Box 29-1 and Fig. 29-1 ). Higher energy densities can vaporize tissue. If this vaporization occurs rapidly, the steam expansion effect is explosive. Thus high energy for short exposures can cause tissue explosions. These effects are dependent on the tissue’s chromophore components, which determine the relative absorption, transmission, reflection, and scattering of incident laser radiation. The frequency of the laser can then be matched to the absorption characteristics of the target tissues. Continuous-wave Nd:YAG laser energy, in contrast to q-switched Nd:YAG laser energy, is absorbed primarily by pigmented tissues and penetrates deeply. It can thus be directed with minimal absorption through the unpigmented sclera. The energy generated by the erbium-YAG laser (2.94 μm), however, is absorbed by the water in high water content tissues with a short absorption depth. This type of laser can be employed to create sclerostomies but not be used for procedures such as cyclophotocoagulation or iridectomy, in which transmission through the sclera or cornea is necessary.
Gonioplasty (iridoplasty) (argon)
Reopening filtering blebs (both)
Cutting sutures (argon)
Filtration surgery (Nd:YAG)
Rupture cysts of iris or ciliary body (both)
Closing cyclodialysis cleft (argon)