LASERS IN OPHTHALMOLOGY Dr Rashmi Amarnath Minto Regional Institute of Ophthalmology

Lasers in Ophthalmology 1917- Einstein proposed the theory of stimulated emission Light Amplification by Stimulated Emission of Radiation Ruby laser in 1967- Theodore Maiman

Terminology Stimulated Emission Light Amplification Population Inversion

Laser Physics When electrons from higher energy fall back to lower orbital energy levels, they emit photons- stimulated emission.

Components of Laser Activated medium Energy pumping source Resonant cavity for oscillation

Properties of Laser Monochromacity- allows selection of particualr wavelength Coherence- Spatial and temporal coherence Directionality- highly collimated beam for focussing Polarization- max energy transmission without loss due to reflection Brightness- Power/unit area /solid angle

Laser output Continous mode: Continous output, externally controlled, has uniform power Pulse mode: Concentrated energy, delivered over short period Q switched lasers: Intracavitary shutter allows rapid energy release, short high power pulse Mode locking: Ultra short pulses in pico seconds

Laser tissue interaction Photochemical reactions: PDT, Photoablation by EXIMER Mechanical effect: Photodisruption Nd-YAG. Photothermal: Photocoagulation , Photovaporization

Types of lasers Gas : Semiconductor diode laser/Solid state: Nd-YAG Noble gases- argon, krypton Molecular gases- CO2, N2 Dimer- EXIMER Semiconductor diode laser/Solid state: Nd-YAG Dye tunable:

Absorption spectrum in ocular tissues

Absorption spectrum of ocular tissues Melanin absorption in 400-700nm Deoxyhemoglobin 555nm Oxyhemoglobin 542nm, 577nm Xanthophyll <500nm

Lasers in posterior segment diseases Argon green Krypton red Frequency doubled Nd Yag laser Diode laser, Dye laser,

Indications for Laser Diabetic retinopathy Retinal breaks PDR, macular edema Retinal breaks Neovascularization- CRVO, Sickle cell retinopathy Macular edema CSR SRNVM Leaking aneurysm- Coats, VHL, retinal macroaneurysm

PRP in diabetic retinopathy Inferior quadrant covered first Focal laser first if associated macular edema present From arcades to equator 1DD nasal to disc Completed in 2-3 sittings. Fill in laser for periphery if NVE does not regress

Mechanism of action in PRP >1200 spots, 300-500um, 200-300mW Produces photocoagulation of proteins resulting in adhesion of chorioretinal lasers Decreases release of hypoxic factors Produces decreased consumption of nutrients in peripheral retina

Mechanism in macular edema Focal laser: 50-100um, Duration : 0.1s Coagulation of all leaking microaneurysm 500-3000u from center Enhances the RPE pump Grid Laser: 100um, Duration: 0.1s Decreases edema

Mechanism in prophylaxsis of retinal breaks Barrage laser: 100-200um, 200-300mW 3-4 rows around break Produces chorioretinal adhesion

CNVM

PDT 689 nm solid state laser Spot size- GLD+1000 microns(max spot size is 6400 microns) Intensity-600Mw/sq cm Power-50 J/sq cm Duration-83 seconds

Mechanism in PDT Verteporfin dye is absorbed by LDL receptors present in the CNVM These porphyrins absorb Laser light and are promoted to their excited triplet state (free radicals, singlet oxygen) Free radicals lead to local molecular and cellular injury

Wet AMD - TTT Treatment of Subfoveal Occult CNV Choroid RPE Retina Bruch’s Membrane Eyes with symptomatic subfoveal occult CNV Occult defined as fibrovascular RPE detachments and late leakage of undetermined source

Mechanism in TTT Low energy, longer duration laser acts on chorioretinal layer producing photochemical reaction 810nm, large spot light adapter

Complications of Laser Decreased color vision Decreased contrast Decreased field of vision (PRP) Accidental foveal burns CNVM Vitreous hemorrhage Retinal holes

Laser Hazards Ophthalmic Lasers are grade IV biological hazard Indicator board Antireflection coating of lens, slit lamp Use of protective glasses by surrounding personnel

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