Micropulse modulation in retinal laser therapy was intended to confine tissue heating around the light-absorbing layers, such as RPE and choroid, while the transparent retina is heated less as a result of slow heat diffusion. Current implementations use micropulses of 100-300μs at 500Hz, with overall pulse envelope of 100-300ms. The effect of such modulation compared to continuous-wave (CW) is not well characterized and misleading comparisons are made in the literature between exposures of different average power or overall duration. In this study, we modeled and measured the retinal tissue response to pulse trains with duty cycles from 4% (80μs pulse at 500Hz) to CW at overall envelope of 200ms and 20ms. Three thresholds of tissue response were measured in Dutch-belted rabbits: immediate (<3s after laser delivery) and delayed (1-5min) ophthalmoscopic visibility of lesions corresponding to photoreceptor damage, as well as fluorescein angiography visibility indicating RPE damage. Both the model and experimental results show that tissue response to micropulse modulation with long pulse envelope (200ms) is not significantly different from CW exposures at the same average power and duration. Heat confinement is improved with lower duty cycle (2%) and shorter pulse envelope (20ms), however further decrease in exposure duration raises the temperature dangerously close to vaporization. Pulse modulation cannot improve the therapeutic range of non-damaging thermal therapy since it is defined by the Arrhenius integral, regardless of the time course of hyperthermia. However, it does allow greater thermal stress to the RPE and underlying choroid while avoiding damage to neural retina.
Unlike conventional photocoagulation, non-damaging retinal laser therapy (NRT) limits laser-induced heating to stay
below the retinal damage threshold and therefore requires careful dosimetry. Without the adverse effects associated with
photocoagulation, NRT can be applied to critical areas of the retina and repeatedly to manage chronic disorders. Although
the clinical benefits of NRT have been demonstrated, the mechanism of therapeutic effect and width of the therapeutic
window below damage threshold are not well understood. Here, we measure activation of heat shock response via laser-induced
hyperthermia as one indication of cellular response. A 577 nm laser is used with the Endpoint Management (EpM)
user interface, a titration algorithm, to set experimental pulse energies relative to a barely visible titration lesion. Live/dead
staining and histology show that the retinal damage threshold in rabbits is at 40% of titration energy on EpM scale. Heat
shock protein 70 (HSP70) expression in the retinal pigment epithelium (RPE) was detected by whole-mount
immunohistochemistry after different levels of laser treatment. We show HSP70 expression in the RPE beginning at 25%
of titration energy indicating that there is a window for NRT between 25% and 40% with activation of the heat shock
protein expression in response to hyperthermia. HSP70 expression is also seen at the perimeter of damaging lesions, as
expected based on a computational model of laser heating. Expression area for each pulse energy setting varied between
laser spots due to pigmentation changes, indicating the relatively narrow window of non-damaging activation and
highlighting the importance of proper titration.
Transparent ocular tissues, such as the cornea and crystalline lens, can be ablated or dissected using short-pulse lasers. In refractive and cataract surgeries, the cornea, lens, and lens capsule can be cut by producing dielectric breakdown in the focus of a near-infrared (IR) femtosecond laser, which results in explosive vaporization of the interstitial water, causing mechanical rupture of the surrounding tissue. Here, we compare the texture of edges of lens capsule cut by femtosecond lasers with IR and ultraviolet (UV) wavelengths and explore differences in interactions of these lasers with biological molecules. Scanning electron microscopy indicates that a 400-nm laser is capable of producing very smooth cut edges compared to 800 or 1030 nm at a similar focusing angle. Using gel electrophoresis and liquid chromatography/mass spectrometry, we observe laser-induced nonlinear breakdown of proteins and polypeptides by 400-nm femtosecond pulses above and below the dielectric breakdown threshold. On the other hand, 800-nm femtosecond lasers do not produce significant dissociation even above the threshold of dielectric breakdown. However, despite this additional interaction of UV femtosecond laser with proteins, we determine that efficient cutting requires plasma-mediated bubble formation and that remarkably smooth edges are the result of reduced thresholds and smaller focal volume.
A recent study showed that 355-nm nanosecond lasers cut cornea with similar precision to infrared femtosecond lasers. However, use of ultraviolet wavelength requires precise assessment of ocular safety to determine the range of possible ophthalmic applications. In this study, the 355-nm nanosecond laser was evaluated for corneal and iris damage in rabbit, porcine, and human donor eyes as determined by minimum visible lesion (MVL) observation, live/dead staining of the endothelium, and apoptosis assay. Single-pulse damage to the iris was evaluated on porcine eyes using live/dead staining. In live rabbits, the cumulative median effective dose (ED50) for corneal damage was 231 J/cm2, as seen by lesion observation. Appearance of endothelial damage in live/dead staining or apoptosis occurred at higher radiant exposure of 287 J/cm2. On enucleated rabbit and porcine corneas, ED50 was 87 and 52 J/cm2, respectively, by MVL, and 241 and 160 J/cm2 for endothelial damage. In human eyes, ED50 for MVL was 110 J/cm2 and endothelial damage at 453 J/cm2. Single-pulse iris damage occurred at ED50 of 208 mJ/cm2. These values determine the energy permitted for surgical patterns and can guide development of ophthalmic laser systems. Lower damage threshold in corneas of enucleated eyes versus live rabbits is noted for future safety evaluation.
Transparent ocular tissues such as cornea and crystalline lens can be precisely ablated or dissected using ultrafast ultraviolet, visible, and infrared lasers. In refractive or cataract surgery, cutting of the cornea, lens, and lens capsule is typically produced by dielectric breakdown in the focus of a short-pulse laser which results in explosive vaporization of the interstitial water and mechanically ruptures the surrounding tissue. Here, we report that tissue can also be disrupted below the threshold of bubble appearance using 400 nm femtosecond pulses with minimal mechanical damage. Using gel electrophoresis and liquid chromatography/mass spectrometry, we assessed photodissociation of proteins and polypeptides by 400 nm femtosecond pulses both below and above the cavitation bubble threshold. Negligible protein dissociation was observed with 800 nm femtosecond lasers even above the threshold of dielectric breakdown. Scanning electron microscopy of the cut edges in porcine lens capsule demonstrated that plasma-mediated cutting results in the formation of grooves. Below the cavitation bubble threshold, precise cutting could still be produced with 400 nm femtosecond pulses, possibly due to molecular photodissociation of the tissue structural proteins.
Application of femtosecond lasers to cataract surgery has added unprecedented precision and reproducibility but ocular safety limits for the procedure are not well-quantified. We present an analysis of safety during laser cataract surgery considering scanned patterns, reduced blood perfusion, and light scattering on residual bubbles formed during laser cutting. Experimental results for continuous-wave 1030 nm irradiation of the retina in rabbits are used to calibrate damage threshold temperatures and perfusion rate for our computational model of ocular heating. Using conservative estimates for each safety factor, we compute the limits of the laser settings for cataract surgery that optimize procedure speed within the limits of retinal safety.
Non-compressible hemorrhages are the most common preventable cause of death on battlefield or in civilian traumatic
injuries. We report the use of sub-millisecond pulses of electric current to induce rapid constriction in femoral and
mesenteric arteries and veins in rats. Extent of vascular constriction could be modulated by pulse duration, amplitude
and repetition rate. Electrically-induced vasoconstriction could be maintained at steady level until the end of
stimulation, and blood vessels dilated back to their original size within a few minutes after the end of stimulation. At
higher settings, a blood clotting could be introduced, leading to complete and permanent occlusion of the vessels. The
latter regime dramatically decreased the bleeding rate in the injured femoral and mesenteric arteries, with a complete
hemorrhage arrest achieved within seconds. The average blood loss from the treated femoral artery was about 7 times
less than that of a non-treated control. This new treatment modality offers a promising approach to non-damaging
control of bleeding during surgery, and to efficient hemorrhage arrest in trauma patients.
Femtosecond lasers have added unprecedented precision and reproducibility to cataract surgery. However, retinal safety limits for the near-infrared lasers employed in surgery are not well quantified. We determined retinal injury thresholds for scanning patterns while considering the effects of reduced blood perfusion from rising intraocular pressure and retinal protection from light scattering on bubbles and tissue fragments produced by laser cutting. We measured retinal damage thresholds of a stationary, 1030-nm, continuous-wave laser with 2.6-mm retinal spot size for 10- and 100-s exposures in rabbits to be 1.35 W (1.26 to 1.42) and 0.78 W (0.73 to 0.83), respectively, and 1.08 W (0.96 to 1.11) and 0.36 W (0.33 to 0.41) when retinal perfusion is blocked. These thresholds were input into a computational model of ocular heating to calculate damage threshold temperatures. By requiring the tissue temperature to remain below the damage threshold temperatures determined in stationary beam experiments, one can calculate conservative damage thresholds for cataract surgery patterns. Light scattering on microbubbles and tissue fragments decreased the transmitted power by 88% within a 12 deg angle, adding a significant margin for retinal safety. These results can be used for assessment of the maximum permissible exposure during laser cataract surgery under various assumptions of blood perfusion, treatment duration, and scanning patterns.