Blood Flow Stasis Induced by cw-Nd: YAG Laser Irradiation: Comparative Morphology of Mesenteric and Choroidal Vessels in Pigmented Rabbits

Theoretical and practical aspects relating to the photothermal therapy of tumors of the retina and choroid: A review

Photothermal treatment of tumors of the retina and choroid such as retinoblastomas, malignant melanomas, benign tumors as well as of vascular malformations can be performed by using laser radiation. A number of basic physical laws have to be taken into account in this procedure. Of particular importance thereby are: Arrhenius' law to approximate the kinetics of protein denaturation and photocoagulation, furthermore the electromagnetic radiation field, the distribution of both radiant and thermal energy induced in tumors and vascular structures, the influence of the wavelength and laser pulse duration (exposure time), as well as of the optical properties of the tissue. Strict confinement of the extent of the photothermal damage is critical since such pathological entities are frequently located close to the macula or optic nerve head.The conditions for tumor destruction are best fulfilled when using radiation in the near-infrared range of the electromagnetic spectrum such as that emitted from the diode (810 nm) and the Nd: YAG (1064 nm) laser, because of the good optical penetration properties of these radiations in tissue. Short wavelength sources of radiation, such as the argon ion (488, 514 nm) or the freqeuency-doubled Nd: YAG (532 nm) laser are less well suited for the irradiation of large vascular structures due to their poor penetration depths. However, for vascular formations with a small thickness (1 mm or less), short wavelength sources appear to be the most appropriate choice. Optical coupling of radiant energy to the eye by means of indirect ophthalmoscopic systems or positive contact lenses is furthermore of importance. Strong positive lenses may lead to severe constrictions of the laser beam within the anterior segment, that leads to high irradiance increasing the probability for structures to be damaged; with negative contact lenses, such as the -64 D Goldmann type lens, this danger is largely absent.

In vivo veritas: Thrombosis mechanisms in animal models

Experimental models have enhanced our understanding of atherothrombosis pathophysiology and have played a major role in the search for adequate therapeutic interventions. Various animal models have been developed to simulate thrombosis and to study in vivo parameters related to hemodynamics and rheology that lead to thrombogenesis. Although no model completely mimics the human condition, much can be learned from existing models about specific biologic processes in disease causation and therapeutic intervention. In general, large animals such as pigs and monkeys have been better suited to study atherosclerosis and arterial and venous thrombosis than smaller species such as rats, rabbits, and dogs. On the other hand, mouse models of arterial and venous thrombosis have attracted increasing interest over the past two decades, owing to direct availability of a growing number of genetically modified mice, improved technical feasibility, standardization of new models of local thrombosis, and low maintenance costs. To simulate rupture of an atherosclerotic plaque, models of arterial thrombosis often involve vascular injury, which can be achieved by several means. There is no animal model that is sufficiently tall, that can mimic the ability of humans to walk upright, and that possesses the calf muscle pump that plays an important role in human venous hemodynamics. A number of spontaneous or genetically engineered animals with overexpression or deletion of various elements in the coagulation, platelet, and fibrinolysis pathways are now available. These animal models can replicate important aspects of thrombosis in humans, and provide a valuable resource in the development of novel concepts of disease mechanisms in human patients.

Fluorescein and indocyanine green angiography after transpupillary thermotherapy of choroidal neovascularization. Early vascular changes

Transpupillary thermotherapy (TTT) is currently being evaluated in clinical trials for the treatment of choroidal neovascularization (CNV). Preliminary results show that TTT achieves closure of CNV and reduction in subretinal exudation while preserving visual acuity. Fluorescein and indocyanine green angiography reveal increased leakage activity from CNV and collateral choroid within 1 hour after TTT and absence of dye leakage from CNV at 1 week after treatment. Optical coherence tomography confirms the angiographic findings. The early vascular changes after TTT of CNV are similar to those observed after PDT of CNV. Transpupillary thermotherapy and PDT might share common mechanisms of action. A placebo-controlled, multi-center trial (TTT4CNV) evaluating the long-term efficacy and visual implications of TTT in occult CNV is underway.

Early vascular changes induced by transpupillary thermotherapy of choroidal neovascularization

OBJECTIVE

To study early direct effects of transpupillary thermotherapy (TTT) on choroidal neovascularization (CNV) and choroid.

DESIGN

Retrospective, noncomparative interventional case series.

PARTICIPANTS

Sixty-four eyes with subfoveal CNV.

INTERVENTION

TTT was delivered using a diode laser at 810 nm through a contact lens. Exposure time was 60 seconds with a power/diameter ratio of 247 mW/mm. The end point was an invisible treatment with no color change at the retina level.

MAIN OUTCOME MEASURES

Fluorescein and indocyanine green angiographic findings within 1 hour, and at 1, 2, and 4 weeks after TTT.

RESULTS

Fluorescein angiography (FA) and indocyanine green angiography (ICGA) performed within 1 hour after TTT showed increased leakage of CNV and choroidal vessels. Follow-up at 1 and 2 weeks demonstrated a hypofluorescent area corresponding to the laser spot and absence of angiographic leakage seen on FA and ICGA. At 4 weeks after TTT, FA showed mottled hypofluorescence-hyperfluorescence of the TTT-treated area and absence of angiographic leakage.

CONCLUSIONS

TTT induces a characteristic dynamic sequence of vascular changes. Treatment with TTT can lead to absence of angiographic (FA and ICGA) leakage for 4 weeks. Determination of the long-term efficacy and visual implications awaits the results of clinical trials.

Irradiation of rabbit retina with diode and Nd:YAG lasers

AIMS

This study was carried out to compare the effects of continuous wave infrared laser radiation on pigmented and albino rabbit retinas at two wavelengths: 810 nm (diode) and 1064 nm (Nd:YAG).

METHODS

Transpupillary laser pulses were applied with a spot size of 200 microns and durations of 200 ms (pigmented rabbits) and 0.5-1 s (albino rabbits). Light and electron microscopic analyses were performed immediately after exposure.

RESULTS

In pigmented rabbits, threshold lesions were induced using a power of 100 mW with the diode and 200 mW with the Nd:YAG lasers. Damage was incurred by the retinal pigment epithelium with extension into the superficial and mid choroid posteriorly and into the outer retina anteriorly. In albino rabbits, lesions of comparable anteroposterior extension were identified using a power of 10 W with the Nd:YAG laser. Using diode laser irradiation, a maximum power output of 1.2 W failed to produce discernible lesions.

CONCLUSIONS

The observed patterns of morphological damage are produced by complex tissue radiation interactions. In pigmented animals, this was primarily related to absorption of radiant energy by melanin within the retinal pigment epithelium and the choroidal melanocytes. In albino rabbits, laser induced effects occurred as a consequence of multiple scattering, together with absorption within haemoglobin and possibly also within tissue water. The data obtained provide further insight into the biological mechanisms arising from retinal photocoagulation with near infrared lasers.

Induction of vascular haemostasis by Nd:YAG laser light in melanin-rich and melanin-free tissue

Haemostasis was effected in vessels of melanin-rich (MR: choroid) and melanin-free (MF: mesentery) rabbit tissue irradiated with a cw-Nd:YAG laser. The following parameters were employed: - pulse duration: 200 ms (MR) and 100 ms (MF); focal spot diameter: 200 microns (MR) and 80 microns (MF); pulse energies: 100-250 mJ (MR) and 0.5-1 J (MF); irradiances: 1.6-4.0 kW cm-2 (MR) and 1-2 x 10(2) kW cm-2 (MF). In melanin-rich tissue, laser energy is absorbed principally by melanin granules contained within the stromal melanocytes. The heat generated in these structures radiates into the surrounding tissue where it is dissipated. The damage thus incurred by the endothelium of blood vessels encompassed within this field triggers the haemostatic mechanism whereby blood flow is arrested. This effect is realized by the formation of an occluding plug of platelets, which is stabilized by the deposition of fibrin, particularly in capillaries, and to a lesser degree in larger vessels of the vascular lamina. In melanin-free tissue, haemoglobin serves as the primary site of energy absorption, which is thus shifted from the stroma to the vessel lumen. Irradiation of vessels in such tissue leads to thermocoagulation of plasma proteins and consequent stasis of blood flow.