Laser (Introduction and Indication in Posterior Segments Diseases

By Mutahir ShahResident M Phil VSPakistan Institute of Community OphthalmologyHMC Peshawar.Laser (Introduction and Indication in Posterior Segment Diseases

LASER:the word laser was initially an acronym for Light Amplification by Stimulated Emission of Radiation (LASER) .It means that when an electron receive energy in the form of photon it jumps to its outer most shell or high energy level.These electrons are unstable in higher state so they loose their energy and jump down to its lower energy state.An electron may stay in a metastable state for minutes or longer.A photon of appropriate frequency passing near such an electron will stimulate the electron immediately to drop to a lower state and radiate an identical photon.

Although the total energy in laser light may be slight, it can be focused on a very small area to produce a very high energy density.Laser light is also highly directionalLasers may operate continuously ( eg, an argon laser photocoagulator) or in pulses ( eg, a YAG laser for capsulotomy). Mode locking and Q-switching are 2 common methods of producing a pulsed output.

Properties of LaserCoherency (inphase)MonochromatismDivergenceCollimated (perfectly align and parallel)sAbility to be concentrated in short time interval

Pulsed and Contineous LaserPulsed energy delivered in brief bursts, more powerExamples: Nd YAG, Excimer lasers

Continuous Argon, krypton lasers, diode lasers, and dye lasers

MODES OF LASER OPERATION Continuous Wave (CW) Laser: It deliver their energy in a continuous stream of photons.Pulsed Lasers: Produce energy pulses of a few tens of micro to few mili second. Q Switches Lasers: Deliver energy pulses of extremely short duration (nano second).A Mode-locked Lasers: Emits a train of short duration pulses (picoseconds).Fundamental System: Optical condition in which only one type of wave is oscillating in the laser cavity. Multimode system: Large number of waves, each in a slight different direction ,oscillate in laser cavity.

Wavelengths of Laser Light

Electromagnetic Spectrum

Basic Components of LaserA Laser Medium e.g. Solid, Liquid or GasExciting Methods for exciting atoms or molecules in the medium e.g. Light, Electricity Optical Cavity (Laser Tube)around the medium which act as a resonator

Laser Tissue Interaction:Laser surgery involves l of 4 light-tissue interactions: photocoagulation photoablation,photodisruption, or photoactivation.Usually - Visible Wavelength:PhotocoagulationUltraviolet Yields:Photoablation Infrared:Photodisruption Photocoagulation

PhotocoagulationPhotocoagulation is the process by which heat generated by the absorption of light denatures proteins.Pigmented tissue absorbs light and converts it to heat, which denatures (coagulates) the pigmented and adjacent tissues.Retinal photocoagulation was first performed by focusing sunlight onto the retina using a heliostat. Sunlight was replaced by a xenon light source, which was ultimately replaced by a variety of lasers. During retinal photocoagulation, laser light is absorbed bythe retinal pigment epithelium (RPE), and the heat produced denatures (coagulates) the retinal proteins.

The outer retinal layers are more affected than are the inner layers, a fact that has several clinical implications.The more edematous the retina, the less heat reaches the inner layers and the less visible the laser burn. Accordingly, when photocoagulating an edematous retina, it is important to look for signs of photocoagulation occurring in the deeper retinal layers. The difficulty with coagulating the inner retinal layers is the reason laser photocoagulation is often ineffective in preventing the progress of retinoschisis, especially when only the innermost layers split. Controlling laser spot size and duration is crucial.

(A) Head-mounted binocular indirect ophthalmoscopy laser under general anaesthesia; (B) appearance immediately following laser photocoagulation for type 1 disease

PhotoablationIt uses high-energy ultraviolet photons to break covalent chemical bonds.An excimer laser, for example, generates photons at a wavelength of 193 nm; these photons are absorbed by and break the covalent bonds in corneal collagen, thereby vaporizing the collagen molecules. Because the energy of photoablation is used only to break bonds, no heat is produced and the technique does not scar adjacent tissue. Presently, photoablation is used only for keratorefractive procedures.

Fig. 7.16 Corneal (Photo ablation ) during photorefractive keratectomy

PhotodisruptionThe posterior capsule is transparent to visible and near-infrared light. This type of laser is pulsed, so the energy it produces is released in a very short time, producing a large momentary power.Also, the laser beam is focused, concentrating the power into a small area. In the vicinity of the focus, electrons are stripped from their atoms by ionization, but they quickly recombine, which produces a spark and an acoustic wave that mechanically disrupts the posterior capsule.

(A) Vacuolated or pearl-type; (B) Elschnig pearl formation (arrow) following laser capsulotomy; C: laser pitting of an IOL

PhotoactivationPhotoactivation is the conversion of a chemical from one form to another by light. Vision itself depends on the photoactivation (cis-trans isomerization) of rhodopsin in photoreceptor outer segments. A clinical application of photoactivation includes the use of verteporfin, a drug that remains chemically inert until activated by light, after which it destroys neovascular tissue e.g. Photo Dynamic Therapy in New Vascular AMD.

What is PDT ? Visudyne (Verteporfin)+ Laser 689 nm after 15 minutes of the start of infusion. Selective Damage of SRNVM. Costly.

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LASER TREATMENT OF FUNDUS DISORDERS Diabetic Retinopathy Retinal Vascular DiseasesChoroidal Neovascularization (CNV)Clinical Significant Macular Edema (CSME)Central Serous Retinopathy (CSR)Retinal Break/DetachmentTumor

ARMDRetinal Vein Occlusion Eales DiseaseCoats DiseasePeripheral Retinal Lesion Retinopathy of prematurity

CLASSIFICATION OF CHORIORETINAL BURN INTENSITYLight :Barely visible retinal blanchingMild :Faint white retinal burnModerate:Opaque dirty white retinal burnHeavy :Dense white retinal burn

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Pathogenesis of diabetic macular edema

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Retinal hemorrhage

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Retinal breaks and tears

Laser settingsWavelength :argon green, Duration :0.1-0.2seconds.Retinal spot size: 200-500microns.Intensity : moderate retinal whitening

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Choroidal melanoma

Indication: Photocoagulation technique.Initial destruction of the surrounding choroidal blood supply-1-2rows -200-500 microns 0.5-1sec-intense burn.Direct tumour photocoagulation-low energy burns long duration5-30sec.

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Retinoblastoma before treatment

Retinoblastoma after thermotherapy

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Diode laser cycloablation;

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Thank You

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