- Email: [email protected]
Safety of Neodymium-Yag Lasers
LETTERS TO THE JOURNAL SAFETY OF NEODYMIUM-YAG LASERS J O H N T . THOMPSON,
M.D.
Wilmer Ophthalmological Institute, Johns Hopkins Hospital, 601 N. Broadway, Baltimore, MD 21205
The neodymium-YAG (yttrium, alumi num, garnet) laser is fundamentally dif ferent from the commercially available argon, ruby, carbon dioxide, and krypton lasers. All of those lasers work by the generation of heat because the energy of the laser is absorbed in the form of heat in a tissue. This produces a thermal coagula tion of tissue in a focal area. The heat is dissipated by the surrounding tissue. The YAG laser, however, works by a different mechanism. The YAG laser produces a shorter energy pulse with a much higher peak power. The instantaneous electric field generated by the YAG laser is so strong that it literally rips electrons from the target atoms, leading to ionization of the tissue. A spark appears at the point of tissue ionization. An ionized cloud of charged particles called a "plasma" is formed by this process. This plasma ab sorbs most of the energy locally. There are two ways to generate t h e brief pulse from the YAG laser. The first method is the mode-locked pulse in which the various waves are synchro nized into phase. The second method is the Q-switched pulse in which a gate
allowing a controlled loss of the laser light is suddenly closed. This creates an in tense burst of light. The importance of the plasma and the advantages of Qswitched vs mode-locked YAG lasers are hotly contested. Some of the energy is transmitted to the retina as a relatively diffuse beam. The exact action of the diffuse laser energy on the retina is not known. There is a secondary production of heat and acoustic waves, which must be dissipated by the surrounding tissue. We do not know whether the actual tissue destruction by t h e YAG is secondary to the ionization process or t h e acoustic waves, or both. There are few data from animal and human studies regarding the safety of the YAG laser. Both the short-term and longterm effects of the YAG laser must be studied in an unbiased fashion. The effect of t h e YAG laser on nearby intraocular lenses is another source of concern. The YAG laser has a highly convergent beam with a narrow depth of focus. The energy density is maximal near the point of focus but structures just anterior or posterior to the point of focus can also be ionized. Another complication is that, as the ener gy of the YAG laser is increased, the point of optical breakdown moves anterior to the point of focus. 1 Use of the YAG laser for posterior capsulotomy has been asso ciated with visible pits in polymethylmethacrylate intraocular lenses. Most of the posterior capsulotomies performed by Aron-Rosa and associates 2,3 have been
THE JOURNAL invites letters that describe unusual clinical or pathologic findings, experimental results, and new instruments or techniques. The title and the names of all authors appear in the Table of Contents and are retrievable through the Index Medicus and other standard indexing services. These brief reports must not duplicate data published or submitted for publication elsewhere. Each letter must be accompanied by a signed statement transferring copyright to THE AMERICAN JOURNAL OF OPHTHALMOLOGY.
Letters must be typed, double-spaced, on 8V2 x 11-inch bond paper with lVk-inch margins on all four sides. They should be no more than two typewritten pages in length. A maximum of two black and whitefiguresmay be used; they should be cropped to a width of 2V3 inches (one column). Color figures cannot be used. References should be limited to five. Letters may be referred to outside editorial referees for evaluation or may be reviewed by members of the Editorial Board. All letters are published promptly after acceptance. Authors do not receive galley proofs, but if the editorial changes are extensive, the typescript is sent for approval.
393
394
AMERICAN JOURNAL OF OPHTHALMOLOGY
MARCH, 1983
on iris-plane or anterior chamber intraoc ular lenses. Posterior chamber intraocu lar lenses are becoming more popular, and it is likely that the damage to the intraocular lens will b e greater when the intraocular lens is much closer to the posterior capsule. Riggins 4 has shat tered a glass intraocular lens in vitro by focusing the YAG laser on the glass intra ocular lens at moderate energy levels. The relatively diffuse beam of the YAG pulse is, it is claimed, safe to the retina and corneal endothelium when the pulse is focused on the posterior capsule. These data are based, however, on extrapolation of retinal damage thresholds and a failure to find ophthalmoscopically visible dam age in the patients treated with the YAG laser. Preliminary reports· have shown no loss of endothelial cells for as much as six months after YAG laser use. 2 The longterm effects of the YAG pulses on the retina and corneal endothelium are not known.
studies must be completed to determine the safety and potential uses of the YAG laser.
Many other uses for the YAG laser have been suggested and tried in small groups of patients. These include using the YAG laser for iridectomy and laser trabeculoplasty, to lyse anterior synechiae, to cut vitreous strands, to create a central pupil lary opening, to cut a pupillary mem brane, and to perform an anterior capsulotomy before cataract extraction. 2,4 Clinical experience with these tech niques is minimal compared with the experience with posterior capsulotomy by the YAG laser and a greater degree of caution must be used. There has been a flurry of interest in the YAG laser; reportedly, 200 orders were registered for various commercially produced YAG lasers at the 1982 com bined meeting of the American Academy of Ophthalmology and the International Congress of Ophthalmology. The YAG laser has an exciting therapeutic poten tial, but the short-term and long-term effects of the YAG laser on the eye have not been adequately investigated. Animal studies and carefully monitored human
OBLITERATION O F ANTERIOR C H A M B E R CAUSED BY M E D I U M - S I Z E D CHOROIDAL MALIGNANT MELANOMA
REFERENCES 1. van der Zypen, E., Bebie, H., and Fankhauser, F. : Morphological studies about the efficiency of laser beams upon the structures of the angle of the anterior chamber. Facts and concepts related to the treatment of the chronic simple glaucoma. Int. Ophthalmol. 1:109, 1979. 2. Aron-Rosa, D., Aron, J. J., Griesemann, M., and Thyzel, R. : Use of the neodymium YAG laser to open the posterior capsule after lens implant surgery. A preliminary report. J. Am. Intraoc. Implant Soc. 6.352, 1980. 3. Aron-Rosa, D., Griesemann, J. C., and Aron, J. J.: Use of a pulsed neodymium YAG laser (picosec ond) to open the posterior capsule in traumatic cataract. A preliminary report. Ophthalmic Surg. 12:496, 1981. 4. Riggins, J.: Investigation of High-Power, Pulsed, Neodymium YAG Lasers for Correcting Opacities of the Human Eye, publication AFIT/ GEO/PH 82J-1. Air Force Institute of Technology, June 1982.
ABRAHAM SPIERER, M.D., NAVA N A V E H - F L O M A N , M.D., AND M I C H A E L BLUMENTHAL, M.D.
Department of Ophthalmology, Maurice and Gabriela Goldschleger Eye Institute, Tel-Aviv University School of Medicine, Chaim Sheba Medical Center, Tel Hashomer, 52621 Israel (Dr. Naveh-Floman)
We treated an unusual case of unilateral choroidal malignant melanoma accompa nied by complete obliteration of the ante rior chamber and associated with in creased intraocular pressure. Most previous reports1 " 5 stressed that in patients with malignant melanoma intraocular pressure is most likely to increase when the tumors are large 1 (measuring more than 1,000 mm 3 ). This increase is closely related to the retinal detachments ob-
M.D.
Wilmer Ophthalmological Institute, Johns Hopkins Hospital, 601 N. Broadway, Baltimore, MD 21205
The neodymium-YAG (yttrium, alumi num, garnet) laser is fundamentally dif ferent from the commercially available argon, ruby, carbon dioxide, and krypton lasers. All of those lasers work by the generation of heat because the energy of the laser is absorbed in the form of heat in a tissue. This produces a thermal coagula tion of tissue in a focal area. The heat is dissipated by the surrounding tissue. The YAG laser, however, works by a different mechanism. The YAG laser produces a shorter energy pulse with a much higher peak power. The instantaneous electric field generated by the YAG laser is so strong that it literally rips electrons from the target atoms, leading to ionization of the tissue. A spark appears at the point of tissue ionization. An ionized cloud of charged particles called a "plasma" is formed by this process. This plasma ab sorbs most of the energy locally. There are two ways to generate t h e brief pulse from the YAG laser. The first method is the mode-locked pulse in which the various waves are synchro nized into phase. The second method is the Q-switched pulse in which a gate
allowing a controlled loss of the laser light is suddenly closed. This creates an in tense burst of light. The importance of the plasma and the advantages of Qswitched vs mode-locked YAG lasers are hotly contested. Some of the energy is transmitted to the retina as a relatively diffuse beam. The exact action of the diffuse laser energy on the retina is not known. There is a secondary production of heat and acoustic waves, which must be dissipated by the surrounding tissue. We do not know whether the actual tissue destruction by t h e YAG is secondary to the ionization process or t h e acoustic waves, or both. There are few data from animal and human studies regarding the safety of the YAG laser. Both the short-term and longterm effects of the YAG laser must be studied in an unbiased fashion. The effect of t h e YAG laser on nearby intraocular lenses is another source of concern. The YAG laser has a highly convergent beam with a narrow depth of focus. The energy density is maximal near the point of focus but structures just anterior or posterior to the point of focus can also be ionized. Another complication is that, as the ener gy of the YAG laser is increased, the point of optical breakdown moves anterior to the point of focus. 1 Use of the YAG laser for posterior capsulotomy has been asso ciated with visible pits in polymethylmethacrylate intraocular lenses. Most of the posterior capsulotomies performed by Aron-Rosa and associates 2,3 have been
THE JOURNAL invites letters that describe unusual clinical or pathologic findings, experimental results, and new instruments or techniques. The title and the names of all authors appear in the Table of Contents and are retrievable through the Index Medicus and other standard indexing services. These brief reports must not duplicate data published or submitted for publication elsewhere. Each letter must be accompanied by a signed statement transferring copyright to THE AMERICAN JOURNAL OF OPHTHALMOLOGY.
Letters must be typed, double-spaced, on 8V2 x 11-inch bond paper with lVk-inch margins on all four sides. They should be no more than two typewritten pages in length. A maximum of two black and whitefiguresmay be used; they should be cropped to a width of 2V3 inches (one column). Color figures cannot be used. References should be limited to five. Letters may be referred to outside editorial referees for evaluation or may be reviewed by members of the Editorial Board. All letters are published promptly after acceptance. Authors do not receive galley proofs, but if the editorial changes are extensive, the typescript is sent for approval.
393
394
AMERICAN JOURNAL OF OPHTHALMOLOGY
MARCH, 1983
on iris-plane or anterior chamber intraoc ular lenses. Posterior chamber intraocu lar lenses are becoming more popular, and it is likely that the damage to the intraocular lens will b e greater when the intraocular lens is much closer to the posterior capsule. Riggins 4 has shat tered a glass intraocular lens in vitro by focusing the YAG laser on the glass intra ocular lens at moderate energy levels. The relatively diffuse beam of the YAG pulse is, it is claimed, safe to the retina and corneal endothelium when the pulse is focused on the posterior capsule. These data are based, however, on extrapolation of retinal damage thresholds and a failure to find ophthalmoscopically visible dam age in the patients treated with the YAG laser. Preliminary reports· have shown no loss of endothelial cells for as much as six months after YAG laser use. 2 The longterm effects of the YAG pulses on the retina and corneal endothelium are not known.
studies must be completed to determine the safety and potential uses of the YAG laser.
Many other uses for the YAG laser have been suggested and tried in small groups of patients. These include using the YAG laser for iridectomy and laser trabeculoplasty, to lyse anterior synechiae, to cut vitreous strands, to create a central pupil lary opening, to cut a pupillary mem brane, and to perform an anterior capsulotomy before cataract extraction. 2,4 Clinical experience with these tech niques is minimal compared with the experience with posterior capsulotomy by the YAG laser and a greater degree of caution must be used. There has been a flurry of interest in the YAG laser; reportedly, 200 orders were registered for various commercially produced YAG lasers at the 1982 com bined meeting of the American Academy of Ophthalmology and the International Congress of Ophthalmology. The YAG laser has an exciting therapeutic poten tial, but the short-term and long-term effects of the YAG laser on the eye have not been adequately investigated. Animal studies and carefully monitored human
OBLITERATION O F ANTERIOR C H A M B E R CAUSED BY M E D I U M - S I Z E D CHOROIDAL MALIGNANT MELANOMA
REFERENCES 1. van der Zypen, E., Bebie, H., and Fankhauser, F. : Morphological studies about the efficiency of laser beams upon the structures of the angle of the anterior chamber. Facts and concepts related to the treatment of the chronic simple glaucoma. Int. Ophthalmol. 1:109, 1979. 2. Aron-Rosa, D., Aron, J. J., Griesemann, M., and Thyzel, R. : Use of the neodymium YAG laser to open the posterior capsule after lens implant surgery. A preliminary report. J. Am. Intraoc. Implant Soc. 6.352, 1980. 3. Aron-Rosa, D., Griesemann, J. C., and Aron, J. J.: Use of a pulsed neodymium YAG laser (picosec ond) to open the posterior capsule in traumatic cataract. A preliminary report. Ophthalmic Surg. 12:496, 1981. 4. Riggins, J.: Investigation of High-Power, Pulsed, Neodymium YAG Lasers for Correcting Opacities of the Human Eye, publication AFIT/ GEO/PH 82J-1. Air Force Institute of Technology, June 1982.
ABRAHAM SPIERER, M.D., NAVA N A V E H - F L O M A N , M.D., AND M I C H A E L BLUMENTHAL, M.D.
Department of Ophthalmology, Maurice and Gabriela Goldschleger Eye Institute, Tel-Aviv University School of Medicine, Chaim Sheba Medical Center, Tel Hashomer, 52621 Israel (Dr. Naveh-Floman)
We treated an unusual case of unilateral choroidal malignant melanoma accompa nied by complete obliteration of the ante rior chamber and associated with in creased intraocular pressure. Most previous reports1 " 5 stressed that in patients with malignant melanoma intraocular pressure is most likely to increase when the tumors are large 1 (measuring more than 1,000 mm 3 ). This increase is closely related to the retinal detachments ob-