Ocular effects of various laser wavelengths

Current Therapeutics and Future Perspectives to Ocular Melanocytic Neoplasms in Dogs and Cats

Neoplasms of melanocytic origin are diseases relevant to dogs and cats' ophthalmic oncology due to their incidence, potential visual loss, and consequent decrease in life quality and expectancy. Despite its non-specific clinical presentation, melanocytic neoplasms can be histologically distinguished in melanocytomas, which present benign characteristics, and malignant melanomas. The diagnosis often occurs in advanced cases, limiting the therapeutic options. Surgery, cryotherapy, radiotherapy, photodynamic therapy (PDT), and laser are currently available therapeutic strategies. As no clinical guidelines are available, the treatment choice is primarily based on the clinician's preference, proficiency, and the owner's financial constraints. While surgery is curative in benign lesions, ocular melanomas present a variable response to treatments, besides the potential of tumour recurrences or metastatic disease. This review presents the currently available therapies for ocular melanocytic neoplasms in dogs and cats, describing the therapeutic, indications, and limitations. Additionally, new therapeutics being developed are presented and discussed, as they can improve the current treatment options.

Role of 532 nm transfoveal subthreshold micropulse laser in non-resolving central serous chorioretinopathy with subfoveal leaks

Purpose:

The purpose of this study was to evaluate the role of 532 nm transfoveal subthreshold micropulse laser in non-resolving central serous chorioretinopathy with subfoveal leak.

Methods:

A retrospective chart analysis of 23 eyes of 21 patients with central serous chorioretinopathy was performed. Inclusion criteria include vision loss ⩾3 months and focal subfoveal leak on fluorescein angiography. Exclusion criteria include prior treatment for central serous chorioretinopathy and chronic central serous chorioretinopathy. All eyes were treated with 532 nm subthreshold micropulse laser (5% duty cycle). Visual acuity score, contrast sensitivity, autofluorescence, spectral domain optical coherence tomography, and fundus fluorescein angiography were assessed at baseline, 1, 3, 6 months.

Results:

Average visual acuity score (letters) improved from 66.0 ± 8.51 (baseline) to 71.35 ± 8.48 (1 month, p < 0.01), 77.30 ± 11.34 (3 months, p < 0.01), 80.17 ± 9.30 (6 months, p < 0.01). Contrast sensitivity improved from 0.75 ± 0.30 to 1.30 ± 0.37 (p < 0.01) at 6 months. Two eyes needed rescue laser at 3 months followed by photodynamic therapy at 6 months; two eyes needed rescue laser at 6 months.

Conclusion:

The 532 nm subthreshold micropulse laser is safe in non-resolving central serous chorioretinopathy with subfoveal leaks.

A Model to Study Thermal Energy Delivery to the Choroid: A Comparison of Surgical Devices

Purpose

We measure and compare surgical devices using an ex vivo, temperature-controlled, choroidal incision model during thermal energy transfer with a high-resolution infrared camera.

Methods

Ex vivo porcine choroidal tissue specimens (n = 516) were isolated and placed on a temperature-regulated (37°C) perfusion platform. We tested the pulsed electron avalanche knife (PEAK), micropulse laser (MpL), continuous laser (CL), and bipolar cautery (BpC) at three energy settings (11 [low], 45 [medium], and 134 [high] mJ/mm). Each device was clamped to a stationary mechanical arm. Movement of tissue specimens beneath the surgical device was achieved using a stepping motor-driven x-y table. An infrared video camera measured orthogonal temperature variation in the surrounding tissue.

Results

Increased power resulted in greater lateral thermal spread using all modalities (P < 0.001). Mean (standard deviation) lateral thermal spread at low energy was smallest for the MpL at 0.0 (0.01) mm (P < 0.001), whereas BpC had the least collateral tissue damage at medium and high energies (0.02 [0.08] and 0.34 [0.22] mm, respectively; P < 0.001). Fluidics of the ex vivo system may limit thermal spread. The PEAK had the greatest thermal spread across all energy groups (P < 0.001), with clinically relevant variation between disposable blades.

Conclusions

Our ex vivo model enabled direct comparison of threshold thermal tissue injury across four devices. MpL and BpC showed the least thermal damage. PEAK had a higher variation in energy delivery, but also has the advantage of more effective tissue cutting.

Translational Relevance

Our ex vivo surgical device analysis provides thermal tissue injury predictions for choroidal surgery.

Non-Therapeutic Laser Retinal Injury

BACKGROUND:

As lasers have become an increasingly important component of commercial, industrial, military, and medical applications, reported incidents of non-therapeutic laser eye injuries have increased. The retina is particularly vulnerable due to the focusing power and optical transparency of the eye. Continued innovations in laser technology will likely mean that lasers will play an increasingly important and ubiquitous role throughout the world. Critical evaluation should thus be paid to ensure that non-therapeutic injuries are minimized, recognized, and treated appropriately.

METHODS:

A comprehensive literature review on the PubMed database was conducted to present case reports and case series representative of the variety of laser eye injuries in different injury circumstances, tissue types, and biological damage mechanisms.

RESULTS:

A general summary of non-therapeutic laser retina injuries is presented, including information about growth of the industry, increasingly accessible online markets, inconsistent international regulation, laser classifications, laser wavelengths, and laser power, mechanisms of tissue injury, and a demonstration of the variety of settings in which injury may occur. Finally, 68 cases found in the literature are summarized to illustrate the presentations and outcomes of these patients.

CONCLUSIONS:

As non-therapeutic laser eye injuries increase in frequency, there is a greater need for public health, policy, diagnosis, and treatment of these types of injuries.

A Review of Subthreshold Micropulse Laser for Treatment of Macular Disorders

Micropulse laser treatment is an alternative to the conventional continuous-wave laser for the treatment of retinal or macular diseases. In contrast to the conventional laser, the therapeutic effect of the subthreshold micropulse laser is not accompanied by thermal retinal damage. This fact is of particular importance when a treatment near the fovea is required. Micropulse treatment is applied in indications such as central serous chorioretinopathy (CSC), diabetic macular edema (DME), or macular edema due to retinal vein occlusion (RVO). This review outlines and discusses the published literature of subthreshold micropulse laser treatment for CSC, DME, and macular edema after RVO.

Effects of focal/grid laser treatment on the central visual field in diabetic macular oedema: a 2-year follow-up study

Purpose

To assess the long‐term effects of focal/grid laser treatment on the central visual field in subjects with clinically significant diabetic macular oedema (CSMO).

Methods

Twenty‐six subjects with CSMO were included in the study. Fundus photography, assessment of retinal thickness by time‐domain optical coherence tomography (OCT), measurement of visual acuity (VA) and standard automated perimetry of the central 10° of the visual field were performed at baseline before focal/grid laser treatment and thereafter at four follow‐up visits 6 months apart, when the laser treatment was also repeated in subjects with persistent oedema. Changes in VA and retinal thickness between baseline and study end at 24‐month follow‐up were calculated. Trends in changes in the visual field were assessed by linear regression of number of significantly depressed test points over time.

Results

On average after the laser treatment, retinal thickness decreased by 17% and VA decreased by three letters. In most eyes, visual fields were stable over time: the mean of the trend slopes (0.05 points/month) did not differ significantly from 0 (p = 0.63). The number of depressed test points at follow‐up was not correlated with either the total number of laser effects or the number of laser sessions.

Conclusion

Focal/grid laser treatment for CSMO did not affect retinal sensitivity in the central visual field in the subjects included in this 2‐year follow‐up study.

Ocular phototherapy

Phototherapy can be translated to mean 'light or radiant energy-induced treatment.' Lasers have become the exclusive source of light or radiant energy for all applications of phototherapy. Depending on the wavelength, intensity, and duration of exposure, tissues can either absorb the energy (photocoagulation, thermotherapy, and photodynamic therapy (PDT)) or undergo ionization (photodisruption). For phototherapy to be effective, the energy has to be absorbed by tissues or more specifically by naturally occurring pigment (xanthophyll, haemoglobin, and melanin) within them. In tissues or tumours that lack natural pigment, dyes (verteporphin, Visudyne) with narrow absorption spectrum can be injected intravenously that act as focal absorbent of laser energy after they have preferentially localized within the tumour. Ocular phototherapy has broad applications in treatment of ocular tumours. Laser photocoagulation, thermotherapy, and PDT can be delivered with low rates of complications and with ease in the outpatient setting. Review of the current literature suggests excellent results when these treatments are applied for benign tumours, particularly for vascular tumours such as circumscribed choroidal haemangioma. For primary malignant tumours, such as choroidal melanoma, thermotherapy, and PDT do not offer local tumour control rates that are equivalent or higher than those achieved with plaque or proton radiation therapy. However, for secondary malignant tumours (choroidal metastases), thermotherapy and PDT can be applied as a palliative treatment. Greater experience is necessary to fully comprehend risks, comparative benefits, and complication of ocular phototherapy of ocular tumours.

Retinal Capillary Hemangiomas and von Hippel-Lindau Disease

von Hippel-Lindau disease is a multisystem familial cancer syndrome that commonly manifests in the eye as retinal capillary hemangiomas. As the earliest manifestation of the disease, these benign hamartomas can lead to secondary visual loss. Their typical clinical characteristics can lead to accurate diagnosis and appropriate treatment with either laser photocoagulation or cryotherapy enhancing one's probability of preserving vision before becoming symptomatic. Accurate diagnosis can also lead to surveillance of the various other organ systems involved in the disease such as the CNS, kidneys, adrenal glands and pancreas. This will also enable the clinician to provide genetic counseling for patients regarding the lifelong manifestations of this disease.

The effect of ultraviolet radiation on choroidal melanocytes and melanoma cell lines: cell survival and matrix metalloproteinase production

BACKGROUND

Ultraviolet radiation (UVR) can induce DNA damage and regulate the expression of factors important for tumour growth and metastasis, including matrix metalloproteinases (MMPs). Epidemiological studies suggest that chronic UVR exposure, especially during early adulthood, may be a risk factor in patients with choroidal melanoma. However, the effects of UV(R)-B on human choroidal melanocyte survival and growth are unknown. In this study, we investigated if UV(R)-B affected the in vitro survival, growth and MMP production of choroidal melanocytes and melanoma cells.

METHODS

Cultures of primary choroidal melanocytes and melanoma cell lines (OCM-1 and OCM-8) were exposed to UV(R)-B (0-30 mJ/cm(2)). The cell morphology and growth were examined, and cell viability was assessed using an MTT assay. Gelatin zymography was used to assess the enzymatic activity for MMP-2 and -9 in conditioned media following UV(R)-B treatment.

RESULTS

UV(R)-B > or =20 mJ/cm(2) was cytotoxic for choroidal melanocytes. Cytotoxic doses of 5 to 10 mJ/cm(2) were found for OCM-8 and OCM-1 melanoma cell lines. Low levels of UV(R)-B (2.5 and 3.5 mJ/cm(2)) significantly reduced melanoma cell viability after 48 h, although melanocyte viability was not affected by doses of UV(R)-B <10 mJ/cm(2). Conditioned media from melanoma cells and melanocytes displayed pro-MMP-2 activity independent of UV(R)-B. Control and UV(R)-B-treated OCM-1 cells secreted active MMP-2 up to 72 h. Pro-MMP-9 activity was seen from 36 h for control and UV(R)-B-treated OCM-1 and OCM-8 cells.

CONCLUSIONS

Melanocytes appeared more resistant to physiological doses of UV(R)-B than melanoma cells; the potential of melanocytes to initially survive DNA damage following UV(R)-B exposure may be relevant to the subsequent transformation of melanocytes to melanomas. Although UV(R)-B did not induce the production and/or activation of MMP-2 and -9 in melanocytes or melanoma cells, we are currently investigating whether DNA damage-response genes such as p53 and p21 can be regulated following UVR exposure, and whether they are important for choroidal melanoma development.

Sunlight exposure and pathogenesis of uveal melanoma

Uveal melanoma is the most frequent primary malignant intraocular tumor of adults. Among various non-modifiable risk factors, Caucasian race seems to be the most significant with light skin color, blond hair, and blue eyes being specific risk factors. The racial predisposition to uveal melanoma have been explained on the basis of susceptibility of Caucasian race to oncogenic effects of sunlight. Although there is ample evidence in support of this hypothesis in regard to skin melanoma, the evidence in regard to uveal melanoma is insufficient and contradictory. In the following review, we examine physiologic, epidemiological, and genetic data in order to determine the role of sunlight exposure in the pathogenesis of uveal melanoma.

Glutamate-induced excitotoxicity in retina: neuroprotection with receptor antagonist, dextromethorphan, but not with calcium channel blockers

The purpose of our studies was to evaluate different strategies for possible neuroprotection in glutamate-induced neurotoxicity in the retina. In a first set of experiments we attempted to determine if dextrorphan antagonism of glutamate action on NMDA receptors would protect against excitotoxic injury associated with secondary damage seen after surgical laser treatment in retina. In a second set of experiments, the effects of different calcium channel blockers in an in-vitro model of N-methyl-D-aspartate (NMDA)-induced retinal ganglion cell excitotoxicity that utilized rabbit retinal explants were evaluated. Dextrorphan infusion prior to laser treatment of rabbit retina produced a significant decrease in the area of neural retinal damage. We attribute the apparent dextrorphan protection to attenuation of glutamate mediated excitotoxicity secondary to laser induced cell death. Preincubation of rabbit retinal explants with verapamil, nimodipine or omega-conotoxin MVIIA did not cause a significant change in NMDA induced cell death in the ganglion cell layer.

von Hippel-Lindau disease

In recent years advances have been made in the clinical and genetic aspects of von Hippel-Lindau disease (VHL). Retinal capillary hemangioma is the most common manifestation of VHL disease and, therefore, ophthalmologists are frequently involved in the care of patients with this disease. The incidence of VHL disease is approximately 1 in 40,000 live births. It is estimated that there are approximately 7000 patients with VHL disease in the USA. The inheritance of VHL disease is autosomal dominant with high penetrance. Depending on the clinical circumstances, retinal capillary hemangioma may be managed by observation, laser photocoagulation, cryotherapy, and plaque radiotherapy. Typical extraocular lesions associated with VHL disease are central nervous system hemangioma, renal cyst, renal carcinoma, pancreatic cysts and adenoma, pancreatic islet cell tumors, pheochromocytoma, endolymphatic sac tumor of the inner ear, and cystadenoma of the epididymis and the broad ligament. The life expectancy of affected individuals may be improved by early detection and treatment of varied manifestations with the use of surveillance protocols. Identification of the VHL gene on chromosome 3p25-26 has now made it possible for suspected individuals to undergo genetic testing with a high degree of accuracy. We review herein the ophthalmic manifestations and treatment of retinal capillary hemangioma and systemic findings of the VHL disease.

Intraocular and interocular symmetry in normal retinal capillary perfusion

PURPOSE

To investigate retinal capillary perfusion in different areas of the posterior pole (intraocular symmetry) and to compare capillary perfusion between the right and left eyes (interocular symmetry).

METHODS

The study consisted of 52 clinically healthy patients between the ages of 45 and 74 years. Good quality images were obtained in 30 patients. By using scanning laser Doppler flowmetry with the Heidelberg Retina Flowmeter, three 10 degree scans were obtained from the fovea and superior and inferior retina of the study eye (for intraocular symmetry) and from the fovea of the fellow eye (for interocular comparison). Values of volume, flow, and velocity were recorded from each of these areas. Repeated measures analysis of variance, with Bonferroni correction, was used for groupwise comparison between areas in the study eye. Paired t tests, with Bonferroni correction, were used to compare nonfoveolar perfusion between the right and left eyes for each patient.

RESULTS

Capillary volume in the foveola was significantly less than that in areas superior and inferior and 3 degrees nasal and temporal, and it was found to increase with eccentricity in the macular region. Only 2 of 30 patients had significant interocular differences between the extrafoveolar areas.

CONCLUSION

Significant differences in retinal capillary perfusion were found across the macular region and corresponded with the increase in capillary beds. The Heidelberg Retina Flowmeter also measured the expected decrease in volume at the foveolar avascular zone. There were no significant interocular differences in perfusion. Eye movements, media opacities, and poor tear film quality had a deleterious effect on image quality.

Laser eye injuries

Laser instruments are used in many spheres of human activity, including medicine, industry, laboratory research, entertainment, and, notably, the military. This widespread use of lasers has resulted in many accidental injuries. Injuries are almost always retinal, because of the concentration of visible and near-infrared radiation on the retina. The retina is therefore the body tissue most vulnerable to laser radiation. The nature and severity of this type of retinal injury is determined by multiple laser-related and eye-related factors, the most important being the duration and amount of energy delivered and the retinal location of the lesion. The clinical course of significant retinal laser injuries is characterized by sudden loss of vision, often followed by marked improvement over a few weeks, and occasionally severe late complications. Medical and surgical treatment is limited. Laser devices hazardous to the human eye are currently in widespread use by armed forces. Furthermore, lasers may be employed specifically for visual incapacitation on future battlefields. Adherence to safety practices effectively prevents accidental laser-induced ocular injuries. However, there is no practical way to prevent injuries that are maliciously inflicted, as expected from laser weapons.

Lasers in ophthalmology

This article reviews the principle uses of ophthalmic lasers, providing historical background with an emphasis on new applications and areas of investigation. Ophthalmic photocoagulation was the first medical laser application and has restored or maintained vision in millions of people. More recently, photodisruption and, increasingly, ablation have gained prominence for treating a wide range of ocular pathology. The unique properties of lasers have also been harnessed for diagnostic purposes, with optical coherence tomography representing a significant improvement over existing imaging methods. Many ophthalmic applications of lasers have been developed, but the field is a dynamic one which continues to evolve along with laser technology itself.

Systemic immunostimulation after retinal laser treatment in retinitis pigmentosa

Systemic immunostimulation followed an experimental treatment trial of scatter argon laser photocoagulation directed to the retina of one eye of 10 patients with heredo-degenerative retinitis pigmentosa (RP). Significantly increased RP lymphocyte CD25, CD26, and CD4/CD26 activation epitope expressions over prelaser values and controls were found with a normalization of soluble interleukin-2 receptor secretion after laser treatment. Serum interferon-gamma was low both pre- and postlaser. Interestingly, when a panel of viral antibodies was tested, only those to rubella virus were elevated in the early postlaser period. The character of RP immunostimulation after laser-induced inflammation could be consistent with an antigenic stimulus from laser-released retinal proteins which might be of autoimmune or latent infectious origin. Enhanced immune responses may be a common but unrecognized sequellae of retinal laser.

Gamma-aminobutyric acid (GABA) immunocytochemistry of laser coagulations in the rabbit retina

Rabbit retinas were treated with laser coagulations of low intensity, using five different wavelengths. After coagulation (1, 4 and 28 days), the retinas were stained for GABA, using a direct biotin-avidin-horseradish peroxidase immunocytochemical method. These time intervals were chosen with regard to the existing knowledge of retinal repair. The staining for GABA was increased in the inner plexiform layer at the centre of the lesions at days 1 and 4 after coagulation. At the margins of the lesions, cells in the ganglion cell layer showed strong staining and cells in the amacrine cell layer showed cuffing, independent of the wavelength used. On day 28, a regression of the hyperstaining was noted. Some possible pathophysiological mechanisms are discussed.

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.

Atrophic creep of the retinal pigment epithelium after focal macular photocoagulation

To determine if enlargement of laser scars occurs in nonmyopic individuals, the authors retrospectively reviewed 126 consecutive patients with age-related macular degeneration who had been successfully treated with focal macular laser photocoagulation for subretinal neovascularization. Of the 174 laser scars in the study, 122 (70%) increased in size from 50 to 1016 microns (mean, 290 microns) as determined photographically on serial examination ranging from 2 to 81 months. There was no statistically significant difference in the number of scars which increased in size among the three laser wavelengths used. Four (3%) patients lost vision as a result of the scar extending into the fovea. Enlargement of retinal pigment epithelial (RPE) atrophy after focal macular laser photocoagulation may cause significant, delayed visual loss after successful treatment of subretinal neovascularization.

Modified grid pattern treatment of diabetic perifoveal edema by orange dye laser photocoagulation

Diabetic perifoveal edema is a potentially dangerous condition. A consecutive series of 58 diabetic eyes with fluorescein angiographically verified perifoveal edema and decreasing visual acuity and/or metamorphopsia was treated by orange dye laser photocoagulation in a perifoveal grid pattern. Observation periods ranged from 3 to 74 weeks (median 23.5 weeks). Post-treatment fluorescein angiography showed 51 dry foveas, 5 had discrete residual edemata and 2 were not controlled by fluorescein angiography, but were clinically dry. Metamorphopsia decreased simultaneously in most patients. Two eyes deteriorated more than 2 lines on the Snellen chart. Both developed chronic maculopathy, maybe induced by treatment. Except for these two, we found only minor changes in visual acuities and central visual fields. We find the method to be a relatively safe procedure to dry up diabetic foveal edema.

Laser interactions with the cornea

Principles of laser-tissue interactions in the eye are reviewed. Corneal structure and function are summarized, with particular regard for features related to laser treatment. A summary of argon and carbon dioxide laser techniques in the cornea is presented, followed by a review of studies on corneal response to ultraviolet radiation. A detailed description is then given of the characteristics of excimer laser tissue ablation. Potential applications of this process in corneal and keratorefractive surgery are reviewed.

Histology of macular photocoagulation

The histology of macular photocoagulation is reviewed. This information may be helpful in selecting the best available laser for retinal vascular and choroidal disorders in the macular region, as providing an experimental method to better understand the effects of photocoagulation on retinal disease processes.

Management of large filtering blebs with the argon laser

Complications attributable to large conjunctival blebs after glaucoma filtering surgery range from annoying foreign-body sensation to devastating endophthalmitis. We used the argon laser to shrink large complicated blebs in four eyes of four patients who had previously undergone glaucoma filtering surgery. The eye is first anesthetized with a topical agent. The bleb surface is lightly abraded with a sterile cotton swab and then "painted" with a tissue-staining dye. Diffuse laser burns applied over the surface resulted in sufficient shrinkage to allow resolution of the respective symptoms or secondary complications in all four cases. The first two patients developed small leaks that we attribute to delivering laser burns to extremely thin areas of conjunctiva. In both cases the leaks gradually sealed over several weeks. None of the eyes developed increases in intraocular pressure or anterior segment inflammation. In each case, filtering capability was maintained.

Clinical applications of the organic dye laser

Clinical research utilizing the yellow, orange and red wavelengths of the dye laser (Rhodamine 6G and MD-631) indicates that the liquid organic dye laser may have considerable applications in ocular photodynamic and photocoagulation therapy. The effectiveness of the dye laser as a photodynamic (photoradiation) and photocoagulation source is due to the large numbers of wavelengths of relatively high output power that can be produced in the visible spectrum. Therefore, a target tissue can be selectively destroyed with minimal laser energy necessarily transmitted through the refractive media. The dye laser, as a photoradiation or photocoagulation system, should prove valuable in the therapy of ocular disease.

Alterations in the blood-aqueous barrier of the rabbit eye after neodymium: YAG photodisruption

We studied the effects of the Nd:YAG laser on the blood-aqueous barrier following photodisruption of the anterior capsule, posterior capsule, and mid-vitreous of the albino rabbit eye with fluorophotometric techniques using albumin-bound fluorescein. After photo-disruption of the anterior capsule, fluorescence appeared in the anterior chamber at 30 minutes, peaked at two hours, and decreased almost to baseline values by 24 hours. No fluorescence was noted at any time in contralateral control eyes or in eyes receiving photo-disruption of the posterior capsule or mid-vitreous.

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.