A comparative study of argon and krypton laser. Photocoagulation in the treatment of presumed ocular histoplasmosis syndrome

MANAGEMENT OF NEOVASCULAR OCULAR HISTOPLASMOSIS: Past and Present

PURPOSE

To review the wide variety of treatment modalities available for choroidal neovascularization secondary to the presumed ocular histoplasmosis syndrome.

METHODS

A literature search was performed to review the multitude of studies conducted to investigate the efficacy and safety of treatment modalities available for choroidal neovascularization secondary to the presumed ocular histoplasmosis syndrome.

RESULTS

Each treatment modality is reviewed, with the studies summarized and presented to support or refute the method of treatment. Two case reports are presented to demonstrate the treatment regimens.

CONCLUSION

This is a comprehensive review of the treatment modalities available to address choroidal neovascularization secondary to the presumed ocular histoplasmosis syndrome. Investigators will continue to strive toward higher efficacy and safety with future innovations in the field.

Animal experiments for the determination of an optimal wavelength for retinal coagulations

The retina of rabbits was coagulated with different wave-lengths (570-630 nm) using a tunable dye laser with Rhodamin 6G. To achieve comparable ophthalmoscopic appearance the intensity of the laser beam was varied with neutral filters of varying absorption. Histologic examination of fresh coagulation effects and 3-week-old coagulation scars showed no difference in relation to the wave-length used. Theoretically, 577 nm seems to be a particularly useful wavelength for the treatment of a wide variety of retinal lesions because it is not absorbed in xanthophyll and penetrates opacities of the optic media better than argon blue green. Furthermore, it is absorbed maximally in hemoglobin, making it particularly suitable for the coagulation of blood vessels.

Wavelength selection in macular photocoagulation. Tissue optics, thermal effects, and laser systems

The therapeutic effects of macular photocoagulation result from focal heating of the retina and choroid. The magnitude, spatial extent, and duration of temperature increases produced by laser exposures are influenced by light scattering in intraocular and intraretinal transit; light absorption by melanin, hemoglobin, and xanthophyll in target tissues; and beam parameters, such as wavelength, spot size, and exposure duration. Tissue optics and thermodynamics provide a useful guide for selecting new laser systems of potential value in macular photocoagulation, but laser-tissue interactions and subsequent chorioretinal responses are poorly understood, and therapeutic efficacy can be established only by controlled clinical trials.

Krypton red laser photocoagulation for subretinal neovascularization

One hundred fifty-seven patients with subretinal neovascularization (SRN) were treated in a prospective, nonrandomized, consecutive study with the krypton red laser (KRL). The patients were analyzed with respect to age, etiology of the SRN, location of the SRN, and pre- and post-treatment visual acuity in patients with at least three months follow-up. Previous studies have not investigated KRL photocoagulation within the foveal avascular zone (FAZ). The results of this study compare favorably with other large series in which patients with SRN located outside the FAZ were treated with the argon blue-green laser (ABGL).

Treatment of recurrent juxtafoveal subretinal neovascular membranes with krypton red laser photocoagulation

Thirty-seven eyes initially treated with argon blue-green, argon green, or krypton red laser photocoagulation demonstrated a recurrent subretinal neovascular membrane (SRNVM) at the edge of the treatment scar extending into the foveal avascular zone and were retreated with krypton red laser photocoagulation. The subretinal neovascular membrane was due to the presumed ocular histoplasmosis syndrome (POHS) in 10 eyes, aging macular choroidal degeneration (AMCD) in 22 eyes, and miscellaneous causes in five eyes. Anatomic success, defined as complete obliteration of the recurrent SRNVM, was achieved in 26 eyes (70%). When compared to pretreatment visual acuities, 23 of 37 eyes (62.2%) had stable or improved postoperative visual acuities during the follow-up period. Twenty-six eyes were retreated one time; nine eyes required two retreatments; and two eyes required three retreatments. Final visual results showed visual acuity greater than or equal to 20/40 in 7 eyes (18.9%); visual acuity greater than or equal to 20/80 in 16 eyes (43.8%); visual acuity greater than or equal to 20/100 in 19 eyes (51.4%); visual acuity greater than or equal to 20/200 in 27 eyes (73%); and visual acuity greater than or equal to 20/400 in 34 eyes (91.9%).

Important points in management of patients with choroidal neovascularization

The efficacy of argon blue-green laser treatment for extrafoveal choroidal neovascularization (CNV) in aging macular degeneration, presumed ocular histoplasmosis syndrome and idiopathic choroidal neovascularization has recently been proven by the Macular Photocoagulation Study. These results are applicable only to eyes managed according to certain principles of therapy and post-treatment follow-up. These include the use of a recent fluorescein angiogram and retrobulbar anesthetic, aiming for complete obliteration of the CNV. Meticulous post-treatment follow-up is critical. This includes daily patient monitoring of the Amsler grid, and prompt examinations if distortion is noted. Excellent quality fluorescein angiography is mandatory. This is repeated frequently and studied promptly. Residual or recurrent extrafoveal CNV requires prompt re-treatment.

Experiments on the absorption of argon and krypton laser by blood

Experiments were performed to determine the absorption by and transmission through blood of argon blue-green (ABG) and green (AG) and krypton red (KR) laser light. Krypton was transmitted better through intact erythrocytes than argon. The difference between krypton and argon was greater if the erythrocytes contained oxygenated hemoglobin (Hb) than deoxygenated Hb. None of the wavelengths penetrated well a confluent blood specimen only 4 cells thick. Branch retinal vein occlusions were created in monkeys causing scattered areas of retinal hemorrhage. Argon burns placed in areas of retinal hemorrhage caused much inner or full thickness retinal damage. Krypton passed through the inner retinal hemorrhage without causing damage and created a burn at the level of the retinal pigment epithelium and inner choroid. Preretinal hemorrhages were created in rabbits by perforating Bruch's membrane with a high intensity krypton burn. Argon burns placed over the preretinal hemorrhages were absorbed much more than krypton burns. Neither argon or krypton however could penetrate even moderately thick preretinal hemorrhages to cause an underlying retinal burn.

Effects of the CW YAG Laser on the Human Iris and Retina

A CW YAG laser was used to coagulate retina and to produce an optical iridotomy in a human eye that was to be enucleated for malignant melanoma at the optic nerve head. Moderate coagulation with the CW YAG laser produced a deep choroidal reaction involving the outer portion of the retina. The inner structure of the fovea remained relatively unaffected. The nerve fiber over the coagulated vessels was unchanged. An optical iridotomy also was produced with the CW YAG. At least ten times more energy was needed to produce retinal photocoagulation with the CW YAG than is generally needed with the argon laser. Retrobulbar anesthesia should be used to prevent pain if extensive coagulation is done.

Effect of CW YAG and argon green lasers on experimentally detached retinas

We evaluated the effects of argon-green (514.5 nm) and CW neodymium YAG (1060 nm) wavelengths on experimentally detached retinas of primates. Neither laser produced damage to the sensory retina of the fovea. The argon green wavelength, which was absorbed by haemoglobin in the vessel or by extravasated red blood cells, created vasospasm and nerve fiber layer damage. The beam of the CW YAG was not absorbed by haemoglobin; therefore, no vasospasm could be produced on experimentally detached retinas.

Ocular effects of various laser wavelengths

Lasers have become an indispensable tool in management of many ocular disorders. This review describes the principles of lasers and the effects various laser modalities and wavelength modalities can produce on ocular structure. Guidelines are provided for appropriate selection of various wavelengths and prevention of complications of the laser treatment.

Transpupillary CW YAG laser coagulation. A comparison with argon green and krypton red lasers

The authors have developed a CW YAG laser for transpupillary coagulation. The effects of CW YAG coagulation on the retina, retinal vessels, and fovea were compared with those produced by the krypton red and argon green lasers. To produce threshold coagulative lesions in monkeys and rabbits, we needed five to ten times more energy with the CW YAG than with the krypton red or argon green lasers. Nerve fiber damage was observed only when coagulating retinal vessels with the argon green laser. At the parameters used, none of the lasers damaged the sensory retina of the fovea. The CW YAG may be used as a new mode of laser coagulation in the treatment of retinal diseases.

Management of juvenile disciform lesions of the macula

Fourteen eyes with a juvenile disciform lesion of unknown aetiology in the macula were followed up for 1-6 years. Eyes with inflammatory signs, pre-existing chorioretinal scars in the macula, or myopia of more than -5.0 dioptres were not included. Seven eyes in which the subretinal neovascular membrane ws within 200 mu from the foveola were followed-up without treatment. Of these, in 4 the new vessels underwent regression, and the result was good central vision in 3, whereas in 3 eyes the lesion progressed. Seven eyes were treated with photocoagulation: 5 with argon, 2 with krypton laser. In the treated eyes, the neovascular membrane was 200-500 mu from the foveola. New vessels were occluded by photocoagulation in 6 eyes. In the 3 treated eyes in which the new vessels were 350 mu or more from the foveola, the visual acuity improved or remained good (0.8-1.0), whereas in the remaining 4 the vision deteriorated. In the eyes with good initial result the vision has remained stable in spite of remarkable late pigmentary changes in and around the scar.

The effects of red krypton and green argon laser on the foveal region. A clinical and experimental study

The destruction of senile neovascular membranes in the central avascular region of the macula by argon laser is difficult and associated with complications. Yellow pigment in this region absorbs the blue component of the conventional argon laser radiation. In this study, the authors used lasers of different wavelengths: red krypton and green argon. Optical and electron microscopic studies on nonhuman primates were performed at 1 hour and 6 weeks. Maximum damage was seen at the level of the internal choroid with krypton laser and at the level of the pigment epithelium with green argon laser. A clinical and angiographic study showed that the destruction of the juxtafoveal neovascular membranes by both lasers was possible. Our clinical experience suggests that the red krypton laser offers a better alternative for treatment of neovascular membranes located near the foveola than the green argon laser.

Photocoagulation for choroidal neovascularization

Nearly 90% of severe visual loss among patients with senile macular degeneration can be ascribed to the neovascular form of the disease. The Senile Macular Degeneration Study demonstrated that argon laser photocoagulation can reduce the risk of severe visual loss by more than 60% among patients with choroidal new vessel membranes 200 microns or greater from the center of the foveal avascular zone (FAZ). The Krypton Photocoagulation Study, also supported by the National Eye Institute, is evaluating whether krypton red laser photocoagulation can reduce the risk of visual loss in SMD patients with new vessels within 200 microns of the center of the FAZ.