Laser

LASER IN OPHTHALMOLOGY

Eranda Wannigama

Objectives

• What is Laser ?• LASER history..• LASER Properties.• How LASER is produced ?• Effects of laser.• Application of LASERs in

Ophthalmology.• LASER Safety.

What is Laser?

LASER is an acronym for:

L : Light

A : Amplification (by)

S : Stimulated

E : Emission (of)

R : Radiation

Term coined by Gordon Gould.

Lase means to absorb energy in one form and to emit a new form of light energy which is more useful.

LASER history

• 1917 -Sir Albert Einstein created the foundations for the laser.

• 1958 - C.H. Townes, A.L. Schawlow: Theoretical basis for lasers.

1960 - Theodore Maiman : Built first laser by using a ruby crystal medium .

• 1963 - C. Zweng: First medical laser trial (retinal coagulation).

• 1965 - W.Z. Yarn: First clinical laser surgery.

• 1970- The excimer laser was invented in  by Nikolai Basov

• 1971 -Neodymium yttrium

aluminum garnet

(Nd.YAG) and

Krypton laser developed.

Lasers have many important applications.

• They are used in common consumer devices such as DVD players, laser printers, and barcode scanners.

• They are used in medicine for laser surgery and various skin treatments,

• And in industry for cutting and welding materials.

• They are used in military and law enforcement devices for marking targets and measuring range and speed.• Laser lighting displays use laser light as an entertainment medium (in DJ).

PROPERTIES OF LASER LIGHT

• Monochromatic (emit only one wave length)

• Coherence (all in same phase-improve focusing

)

• Polarized (in one plane-easy to pass through

media)

• Collimated (in one direction & non spreading )

• High energy (Intensity measured by Watt J/s)

LASER Vs. LIGHT

LASER LIGHT

Simulated emission Monochromatic. Highly energized Parallelism Coherence Can be sharply

focussed.

Spontaneous emission.

Polychromatic. Poorly energized. Highly divergence Not coherent Can not be sharply

focussed.

How LASER is produced ?

Light is a form of energy at which the human eye is sensitive

LASER PHYSICS

• Light as electromagnetic waves, emitting radiant

energy in tiny package called ‘quanta’/photon.

Each photon has a characteristic frequency and its

energy is proportional to its frequency.

• Three basic ways for photons and atoms to

interact:

Absorption

Spontaneous Emission

Stimulated Emission

3 Mechanisms of Light Emission

Atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energyThere is another process whereby the atom in an upper energy level can be triggered or stimulated in phase with the an incoming photon. This process is:

Stimulated emission

Is an important process for laser action

1. Absorption2. Spontaneous Emission3. Stimulated Emission

Therefore 3 process of light emission:

Absorption

E1

E2

Spontaneous Emission

Stimulated Emission

Background Physics

• Consider the ‘stimulated emission’ as shown previously.

• Stimulated emission is the basis of the laser action. • The two photons that have been produced can then

generate more photons, and the 4 generated can generate 16 etc… etc… which could result in a cascade of intense monochromatic radiation.

CLASSIFICATION OF LASER

Solid StateRubyNd.YagErbium.YAGMolmium.YAG

GasIonArgonKryptonHe-NeonCO2

Metal VapourCuGold

TYPES OF OPHTHALMIC LASERS

Nd:YAG laser• (neodymium-doped yttrium aluminum garnet) is a crystal that

is used as a lasing medium for solid-state lasers.• Nd:YAG lasers typically emit light with a wavelength of 1064nm

, in the infrared.

Applications

• Correct posterior capsular opacification• Peripheral  iridotomy in patients with 

acute angle-closure glaucoma.• Frequency-doubled Nd:YAG lasers (wavelength 532 nm) are

used for pan-retinal photocoagulation in patients with diabetic retinopathy.

Excimer laser

• Is a form of ultraviolet laser

• Used indelicate surgeries  such as eye surgery  eg ;LASIK.

LASER TISSUE INTERACTION

LASER VARIABLE:

Wavelength

Spot Size

Power

Duration

TISSUE VARIABLE:

Transparency

Pigmentation

Water Content

THREE TYPE OF OCULAR PIGMENT

Haemoglobin:absorbs blue, green and yellow with minimal red

wavelength absorption, useful to coagulate the blood vessels.

Xanthophyll:Macular area, Lens Maximum absorption is blue. minimally absorbs yellow

or red wavelengths

Melanin:RPE, Choroidabsorbs green, yellow, red and infrared wavelengths Pan Retinal Photocoagulation, and Destruction of RPE

Effective retinal photocoagulation depends on how well light penetrates the ocular media and how well the light is absorbed by pigment in the target tissue

LASER TISSUE INTERACTIONLASER

TISSUE

Thermal Effect

Photo-chemical

Ionizing Effect

Photocoagulation

Photoradation

Photodisruption

Photoablation

Photovaporization

Thermal Effects

(1) Photocoagulation:

Laser Light

Target Tissue

Generate Heat

Denatures Proteins (Coagulation)

Rise in temperature of about 10 to 20 0C will cause coagulation of tissue.

Thermal Effects(2) Photodisruption:• Mechanical Effect: Laser Light

Acoustic Shockwaves Tissue Damage

Contd. …

Thermal Effects(3)photovaporization Vaporization of tissue to CO2 and water occurs

when

its temperature rise 60—100 0C or greater.

Commonly used CO2

Absorbed by water of cells

Visible vapor (vaporization) Heat Cell disintegration Cauterization Incision eg..Femtosecond laser

Photochemical effcts

Photoablation:• Breaks the chemical bonds that hold tissue

together essentially vaporizing the tissue, e.g. Photorefractive Keratectomy, Argon Fluoride (ArF) Excimer Laser.

Contd. …

PHOTOCHEMICAL EFFECT

Photoradiation (PDT):

• Also called photodynamic therapy.

E.G. Treatment of Ocular tumours and CNV

Photon + Photo sensitizer in ground state (S)

Molecular Oxygen Free Radical S + O2 (singlet oxygen) Cytotoxic

Intermediate

Cell Damage, Vascular Damage , Immunologic Damage

LASER INSTRUMENTATION

Three Main Components –

• Console: It contain laser medium and tube, power supply and laser control system.

• Control Panel: It contain dials or push buttons for controlling various parameters. Contain a standby switch as a safety measure.

• Delivery System:

Slit Lamp Microscope

Indirect Ophthalmoscopes

ACCESSORY COMPONENT

• Aiming Beam

• Laser Switch

• Safety Filter

• Corneal Contact Lenses for Laser use

Single mirror gonio lens

Abraham or wise iriditomy lens

Goldman style 3-mirror lens

Panretinal lenses

e.g. Rodenstock, Mainster, Volk-Quadri spheric

• Indirect Fundus Lenses for Indirect Ophthalmoscopes

USING THE OPHTHALMIC LASER

PREPARATION OF THE PATIENT FOR laser:

Local Anaesthetic

Position of the patient at Slit Lamp

THE SURGEON:

Comfortable position at Slit Lamp

Semi-darkened Room

Appropriate Contact Lens

LASER IN ANTERIOR SEGMENT

CORNEA:

Laser in Keratorefractive Surgery:• Photo Refractive Keratectomy (PRK)• Laser in situ Keratomileusis (LASIK)• Laser Subepithelial Keratectomy (LASEK)• Epi Lasik

Laser Thermal Keratoplasty

Corneal Neovascularization

Retrocorneal Pigmented Plaques

Laser Asepsis

LASER IN GLAUCOMA

Laser Iridotomy, Laser Iredectomy

Laser Trabeculoplasty (LT)

Selective Laser Trabeculoplasty

• always pre-treat with argon prior to doing a Yag. • a small spot size (~50microns) with a relatively high power (500

or so). Place about 10-15 spots in a flower petal type pattern in a iris crypt in the supero-nasal quadrant in the far peripheral iris.

• This hopefully coagulates any iris stromal vessels and prevents bleeding when doing the Yag portion.

• Start Yag power at about 3 to 4 mJ, only take about 5 - 10 shots to get that wonderfully rewarding gush of pigment and fluid.

• Remember to check IOP about 1 hour post op, warn them about signs/symptoms of IOP spike and keep on pred forte qid for about a week to prevent inflammation.

Laser Iridotomy, Laser Iredectomy

LASER IN LENS

• Posterior capsulotomy(YAG)

• Laser phacoemulcification

• Phacoablation

LASER IN VITEROUS• Viterous membranes

• Viterous traction bands

Posterior capsulotomy(YAG)

LASER TREATMENT OF FUNDUS DISORDERS

Diabetic Retinopathy

Retinal Vascular Diseases

Choroidal Neovascularization (CNV)

Clinical Significant Macular Edema (CSME)

Central Serous Retinopathy (CSR)

Retinal Break/Detachment

Tumour

panretinal photocoagulation

• PRP place laser spots in the peripheral retina for 360 degrees sparing the central 30 degrees of the retina.

 POWER, SIZE, NUMBER, AND SESSIONS Recommendations in the ETDRS for an initial treatment

consisted of 1,200 to 1,600 burns of moderate intensity, 500-μm size, one-half to one-spot diameter spacing at 0.1-second duration, divided over at least two sessions.

typical starting power setting for a 300-μm spot of 0.1-second duration might be around 250 mW, but this is highly dependent on the operator's laser, the status of the ocular media, and the pigmentation of the retina.

Panretinal photocoagulation (PRP) ctd

1. Proliferative diabetic retinopathy with high risk characteristics

2. Neovascularisation of iris 3. Severe non proliferative diabetic retinopathy

associated with-poor compliance for follow up-before cataract surgery-renal failure-one eyed patient and-pregnancy

4. central retinal vein occlusion, branch retinal vein occlusion,

5. sickle retinopathy, 6. Eales disease and IRVAN (idiopathic retinal vasculitis,

aneurysms, and neuroretinitis )

Indications

How does panretinal photocoagulation work?

• Sublethally injured RPE cells that surround areas of photocoagulation necrosis and produces significant thinning of the outer retina.

• By decreasing the oxygen consumption at the photoreceptor–RPE complex, more oxygen is available to diffuse into the inner retina and vitreous.

• Enhanced oxygen diffusion into the inner retina and vitreous reduces inner retina ischemia and the stimulus for neovascularization.

•  PRP reduces retinal ischemia and the hypoxia-induced expression of VEGF.

Focal or grid photocoagulation

• Macular edema from diabetes or branch vein occlusion .

• Retinopathy of prematurity(ROP)• Closure of retinal microvascular abnormalities such as

microaneurysms, telangiectasia and perivascular leakage

• Focal ablation of extrafoveal choroidal neovascular membrane

• Creation of chorioretinal adhesions surrounding retinal breaks and detached areas.

• Focal treatment of pigment abnormalities such as leakage from central serous chorioretinopathy

• Treatment of ocular tumors.

Focal or grid laser settings• 50-100 micron spot size, 0.05-0.1 sec( for focal spot size

50micron, for grid 100-200 micron)

• Spots must be atleast one burn width apart

• seal specific leaking blood vessels in a small area of the retina, usually near the macula.

Pathophysiology of focal Laser

• laser energy removes unhealthy RPE cells which are then replaced by more viable RPE cells.

• photocoagulation stimulates the existing RPE cells to absorb more fluid.

• laser treatment may stimulate vascular endothelial proliferation and improve the integrity of the inner blood-retinal barrier.

several theories

FEMTOSECOND LASER

Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12s) or femtoseconds (10−15s).

Indications

1. Clear Corneal Incisions in LASIK it replaces a mechanical device (

microkeratome) to create a precise corneal flap, also in cataract surgery to create the incision

2.  Capsulotomy3.  Phacofragmentation

LASER HAZARDS

EYE• Small lesion to extensive haemorrhage• Disruption of retina and choroid• Immediate loss of vision • Epiretinal membrane formation• Macular hole,gliosis SKIN• Erythema• Carcinogenesis

COMPLICATION OF LASER TREATMENT

Increased IOP

Corneal Damage

Iris Burn

Cataract

InternalOphthalmoplegia

Pain

Seizure

CD & RD

Foveal Burn

PREVENTION OF LASER HAZARDS

Engineering Control Measure: laser housing

filters and shutter for safe observer viewing

Personal protective devices, like protective eye wear or goggles with side shields, protective clothes may be included.

New Developments

Pattern Scan Laser:(pascal)

PASCAL

PATTERN SCAN LASER:(Pascal)

Offering multiple, patterned burns in a single-

session procedure.

Improved precision

Safety

Patient comfort

Significant reduction in treatment time.