useful in opening transparent tissue as they do not depend on tissue absorption of the energy.

Photoablation

Photoablation is typified by excimer lasers (excited dimer), which generate intense ultraviolet energy beams. At 193  m, the ultra­ violet argon fluoride excimer laser energy breaks molecular bonds, disintegrating tissue into molecular fragments that are ejected from target sites at high velocities. Adjacent tissues are usually not affected. This approach has been used in photorefractive keratec-

tomy to sculpt the corneal surface with excisions of precise shape and depth with perfectly smooth and regular edges.7,10–13 In a

similar way, at 308 nm, the xenon chloride excimer laser is used to precisely excise the meshwork tissue via a fiber-optic probe to remove obstructions to outflow without provoking inflammation in the adjacent tissues.The depth at which most of the energy is absorbed by the tissues at this wavelength is very short, preventing damage to internal structures within the eye. In a similar manner, the short tissue absorption depth of the infrared 2.94- m erbium:YAG laser, although slightly more thermal than UV wavelengths, allows precise tissue removal with minimal adjacent tissue effects.

Photochemical effects

Laser energy can cause chemical reactions in tissues. Photodynamic therapy involves the reaction of a photochemical sensitizer to a wavelength-appropriate laser energy (often red light, 625–635 nm) to create oxygen free radicals in order to create a targeted tissue response. Medical use currently includes destruction of tumors previously sensitized by hematoporphyrin derivatives and precise chorioretinal thermal damage for subretinal neovascularization.

In addition, just as sunlight can cause mutations in skin cells, ultraviolet lasers may have direct mutagenic capabilities and can crosslink proteins to DNA.

General preparation of the patient

Patients should have a clear understanding of the planned laser surgery procedure, including its purpose and rationale. Many patients are fearful of lasers. Fearful patients should be reminded that laser modalities are safer and less invasive than their surgical counterparts. Many laser procedures require slit-lamp delivery systems. The patient should be seated comfortably at the slit lamp so that movement and discomfort are minimized. The patient should also understand that there may be sounds, bright flashes, and possibly slight sensations with each laser pulse. The patient should be told that the surgeon will assist in stabilizing the eye.The patient should be instructed to warn the surgeon immediately if he or she feels uncomfortable.The combination of anxiety, pressure on the globe, and possible side effects of drugs being used can result in a syncopal episode during laser therapy.The surgeon and the room should be prepared for this possibility so that the patient does not fall and sustain injury. Resuscitative equipment should be available.

A skilled laser surgeon can perform the planned procedure quickly with the fewest required laser applications. If prolonged sessions are necessary, consideration should be given to allowing the patient occasional breaks.

Lasers have brought increased safety and simplicity to glaucoma management. As technology improves, so will our abilities to use these new tools.

Basic laser safety

The laser surgeon’s eyes must be protected against reflected laser energy by using filters in the slit-lamp operating biomicroscope or protective goggles or safety glasses. Contact lenses used for laser surgical procedures generally have antireflective coatings to minimize the intensity of reflected laser light, but unshielded eyes in the operating room are still vulnerable to reflected light. Access

to the operating room should be restricted to persons directly involved in the laser procedure; the eyes of individuals assisting the surgeon must also be protected with appropriate goggles that are wavelength matched to the laser being used.The room containing the laser should have appropriate signage so that accidental entry is avoided, both for the safety of the intruder and so that the surgeon or patient is not startled.

Lasers often operate at high voltages; therefore, laser device enclosures must be secure and all lasers must be grounded electrically to avoid inadvertent exposure to these high voltages. Periodic checks should be made of the electrical connections to ensure continued reliability. For those lasers requiring cooling systems, these should have periodic examinations to be sure that flow continues at a safe level and filters are clean.

References

1.Hillenkamp F: Interaction between laser radiation and biological systems. In: Hillenkamp. F, Pratesi. R, Sacchi CA, editors: Lasers in biology and medicine, NewYork, Plenum Press, 1980.

2.L’Esperance FA Jr: Ophthalmic lasers: photocoagulation, photoradiation, and surgery, 2nd edn, St, Louis, Mosby, 1983.

3.Mainster MA: Finding your way in the photoforest: laser effects for clinicians, Ophthalmology 91:886, 1984.

4.Mainster MA: Ophthalmic laser surgery: principles, technology, and technique: symposium on the laser in

ophthalmology and glaucoma update, St Louis, Mosby, 1985.

5.Mainster MA, et al: Laser photodisruptors: damage mechanisms, instrument design and safety, Ophthalmology 90:973, 1983.

6.March WF: Ophthalmic lasers: current clinical uses, Thorofare, NJ, Slack, 1984.

7.Trokel SL, Srinivasan R, Braren B: Excimer laser surgery of the cornea,Am J Ophthalmol 96:710, 1983.

8.Worthen DM:The laser: how it works, Ophthalmic Forum 1:22, 1983.

9.Gaasterland DE:Thresholds for beneficial and harmful effects during use of high-power, pulsed lasers to treat problems of the anterior segment of the eye,Trans Am Ophthalmol Soc 84:1004, 1986.

10.Krueger RR,Trokel SL: Quantitation of corneal ablation by ultraviolet laser light,Arch Ophthalmol 103:1741, 1985.

11.Krueger RR,Trokel SL, Schubert HD: Interaction of ultraviolet laser light with the cornea, Invest OphthalmolVis Sci 26:1455, 1985.

12.Marshall J, et al:An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm, Ophthalmology 92:749, 1985.

13.Puliafito CA, et al: Excimer laser ablation of the cornea and lens, experimental studies, Ophthalmology 92:741, 1985.