LASERS IN OPHTHALMOLOGY PART -1

LASERS IN OPHTHALMOLOGYPART -1DR. BHARTI AHUJAINTRODUCTIONA LASER (from an acronym,LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION) is an optical source that emits photons in a coherent beam.The backformed verb to lase means to produce laser light or to apply laser light to.HISTORYIn 1916, ALBERT EINSTEIN laid the foundation for invention of laser and its predecessor ,the MASER.The first working laser in ophthalmology was made by THEODORE.H MAIMAN,1960.It utilized a pulsed ruby laser coupled with a monocular direct ophthalmoscopic delivery system.

Principle of laser HOW LASERS WORK??

PROPERTIES OF LASERSMONOCHROMATICDIRECTIONALCOHERENCE-spatial and temporalCOLLIMATION/IN PHASEAbility to be concentrated in short time interval.

Laser tissue interactionsphotothermalphotochemicalMechanical /photoionizationphotodisruptionPhotocoagulation/PhotovaporizationphotocarbonizationPhotoradiation/photoablation9PHOTOTHERMAL EFFECTSPhotocoagulation-absorption of light by target tissue results in a temperature rise which causes denaturation of proteins.

Visible and infrared wavelengths.

Argon green(514nm)Argon blue-green(488nm)Krypton red(647 nm)Ruby red(694nm)Diode(810nm)Frequency doubled Nd:YAG(532nm)Key pigments in ocular tissuesMelanin(RPE ,choroid)-green,yellow,red and infrared wavelengths.Xanthophyll(macula)-blue(minimal absorption of yellow ,red wavelengths)Hemoglobin(blood vessels)-blue,greenand yellow(minimal absorption of red wavelengths)PHOTOVAPORIZATION AND PHOTOCARBONIZATIONHigher energy laser light absorbed by target tissue resulting in Vaporization of both intra and extracellular water.

When the local store of water is vaporized, then carbonization Occurs.

CO2 LASER (IR wavelength)PHOTOCHEMICAL EFEECTS photoradiation./PHOTODYNAMICphotoablationIntravenous administration of hematoporphyrin derivative, taken up by target tissue causes sensitization of target tissue.exposure of this tissue to red laser light induces the formation of cytotoxic free radicals.Exposure time is longer (minutes)Tunable dye lasers

High energy laser wavelength in far ultraviolet region(less than 350 nm )of spectrum are used to break long chain tissue polymers into smaller volatile fragments.

Exposure time is shorter(nanoseconds)Excimer lasersPHOTOIONIZATION/MECHANICAL EFFECTSPhotodisruption-high energy laser light deposited over a short interval to target tissue stripping electrons from molecules of that tissue.this cloud of electrons and ionized molecules constitute a plasma. Rapid expansion of it causes an acoustic wave ,disrupting the tissue.

Infra red wavelengths,1064 nm(Nd:YAG)

SOLID STATEGASEXCIMERDYELASING materialCrystals containing rare earthNoble gases(kr/Ar)Ionized formReactive gases(Cl,Fl)+inert gases(Ar, kr/Xe)Complex organic dyes(rhod-amine 6G, in liquid/sus-pensionWAVE-LENGTHInfra red(1.064 micron)Visible red/far IR(10.6 micrometer)Ultra violet Tunable over a broad range of wavelengthsEXAMPLEND:YAG/Ruby laserHe,He-Ne,CO2 laserArf(193)xeclTunable dye lasersSEMICONDUCTOR /DIODE LASERS-Small and less power.IR and deep red wavelengths.

Laser outputContinuous/constant amplitude/cwSingle pulsedOutput is relatively constant with respect to time.Delivers overall more total energy.Less power.Delivers Energy over a relatively long period(fraction of a sec to a sec) Argon,krypton,dye ,diode laserOutput varies with respect to time(alternating off and on periods) Q and Modelocking/gain switchingModest amount of energy.Each pulse has relatively high power.Energy is concentrated into very brief periods(micro to milliseconds)Nd:YAGexcimerLaser parametersExposure time(0.1-0.2 sec)PowerSpot size(100micron-macular area, 500micron-periphery)Laser contact lenses

Laser lenses in photocoagulationPlanoconcaveHigh plus powerUpright imageHigh resolution of small retinal area. Magnify the laser spot size(1.08)Prototype-goldman 3 mirror lens,angulated at degrees- 59-ora serrata67-equator73 posterior poleInverted imageWide field of view(mild loss of fine resolution)Magnify the spot size(depending on the lenses) Panfundoscope-1.41Mainster wide angle-1.47Quadr Aspheric lens-1.92

Therapeutic indications of lasers in ophthalmologyLid lesions Conjunctival tumoursDacryocystitisRefractive errorsCataractGlaucomaDiabetic retinopathyVenous occlusions-CRVO/BRVOARMDTumours(melanomas,retinoblastomas)Miscellaneous(orbitotomies.suturolysis)

Diagnostic applicationsScanning laser ophthalmoscopy/SLO Laser light with its coherent properties,produces the retinal images,that are having higher image resolution than fundus photography. High resolution,real time motion images of the macula without patient discomfort.

Optical coherence tomographyIt uses diode laser light in the near infra red spectrum(810nm) to produce high resolution cross-sectional images of the retina using coherence interferometry.

Wavefront analysis It is the study of the shape of the group of photons that leave an object at any point in time and how they are affected by optical media.Lasers are used in the measurement of complex optical aberrations of the eye using wavefront analysis.

Preoperative laser work upHistory and visual acuityEvaluation of anterior segmentIOP recordingVisual fieldsMacular function testsRetinal and vitreous examinationFlourescein/ICG angiographyOCT

Nd:YAG LASERSolid state(PHOTODISRUPTION)Wavelength (1064nm,IR region)Invisible pulses,red aiming beam.

Q switchMode lockingpower10(6) to 10(9)wattsless10(12)wattsmorePulse durationOne millionth of a secondFemto to pico secondscostinexpensiveexpensiveReliable

Difficult to maintain

Indications of ND:YAG laserCapsulotomyIridotomy laser trabeculoplasty(532nm)Selective laser trabeculoplasty(532nm)Management of diabetic retinopathy(532 nm)Perilenticular membrane disruptionRemoval of deposits from IOL surface(YAG sweeping)cyclophotocoagulation

Laser suturolysis,in trabeculectomyBleb remodelling in overfiltering blebGoniopunctureDeep sclerectomyTreatment of Persistent pupillary membranePupilloplastyAnterior hyloidotomyVitreous strand lysis in cataract woundIridolenticular synaecholysis

Excimer laserArgon flouride laser(193nm,UV region)Emits photons with an energy of 6.5 eV,capable of breaking intramolecular C-C (3.6eV),C-N(3.1eV),C-H(4.3eV)bonds. When the concentration of the photons exceeds a critical value, the broken intramolecular bonds can no longer recombine.To make accurate corneal lesions.INDICATIONS OF EXCIMER LASER LASIK-myopia(-1to -15D),hypermetropia(+1 to +8 D) astigmatism(-5 to 10 D)Femto-LASIKCustomized wavefront guided LASIKPRKLASEKLASER surgery for presbyopiaPTK(Phototherapeutic keratectomy)Excimer laser assisted deep sclerectomyExcimer laser trabeculectomy

Argon laserGas laser(PHOTOCOAGULATION)514 nm wavelength(visible)

INDICATIONSArgon laser trabeculoplastyArgon laser peripheral iridoplastyDiabetic retinopathyMacular edemaCRVO/BRVO

Eales diseaseCentral serous chorioretinopathyARMDMiscellaneous-sickle cell retinopathy,coats disease,retinal breaks,tumoursTreatment of trichiasisPunctal occlusion in dry eyeEndoscopic DCR

CO2 laser10,6oonm(IR)photovapourization

Intraocular photocautery-malignant intraocular tumoursFiltering procedures(especially for neovascular glaucoma)-pulsed focussed beam,400-500micron spotAdnexal uses-removal of capillary hemangiomas,lid lesions(seborrhoeic keratitis)Blepharoplasty(6watt,0.1 mm spot),orbitotomiesEndonasal DCR

ADVANTAGEAdjacent blood vessels are also treated resulting in a bloodless surgical field.

Diode laser750-950 nm(near infrared)

INDICATIONS :

Retinal photocoagulationTranspupillary thermotherapyTrabeculoplastyCyclophotocoagulationDCR

Advantages of diode laserLack of laser flash,Portable,No special electrical/cooling system,Less breakdown of blood retinal barrier,No blue light hazard,Ability to penetrate through cataractous lens,Ability to penetrate through haemorrhage.Disadvantage of diode laserBleeding stumps cannot be coagulated since hemoglobin is not absorbed in infrared region.

Complications of lasers in ophthalmologyBleeding

Increased in pressure within the eye

Clouding of the cornea

Inadvertent corneal burns

Cataract

Scarring of the retina

Accidental foveal burns . Laser induced retinal edema .

Choroidal neovascularisation

Exudative retinal detachment .

Contraindications to lasers in ophthalmologyCognitive difficulties /unreliable patient Areas of haemorrhageIncreased sensitivity to light(Pts of porphyria)Epileptic patientsPatients on immunosuppressive drugsUncontrolled diabetes/vascular disease/autoimmune diseasePregnancy/nursingProgressive refractive errorHerpetic infection in the pastSevere dry eyeActive/residual systemic disease affecting wound healing

ISAFELow

CD players

IINot intended to be viewed1 mw

Laser pointers

IIa1MW,1000sec

Laser scanners

IIIaHazard if collected and focussed in eye

Less than or equal to 5 mwfirearms

IIIbHazard if the direct or reflected beam is viewed

5-500 mwIVDirect and reflected exposureMore than 500 mw,1/10th of asecondMedical,scientific, industrial,military lasers

Safety check listAppropriate warning signs posted.Access to laser and treatment area is secure and controlled.Inspect laser for proper functioning.Visually inspect and clean safety goggles.Laser injury management protocol.

NEVER UNDER ANY CIRCUMSTANCES LOOK INTO ANY LASER BEAMLaser procedures( under evaluation)Penetrating keratoplasty with non contact trephiningLamellar keratoplastyLaser asepsis(healing of corneal ulcer)-argon laserRemoval of lens-photoablation and photofragmentation(Nd: YAG laser)Laser scleral buckling-holmium yag laserVitrectomy-pulsed erbium yag laserDrainage of subretinal fluid-argon laserPhototherapy-neovascular stimulation,aseptic phototherapy

Advantages of lasers Painless procedureDry surgical fieldNo risk of infectionLess timeOutpatient basis ConvenientPreciseSafeDisadvantages of lasersExpensiveEquipmentRepeatability

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