Retinal Laser Therapy Explained

What is Laser?
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 anew form
of light energy which is more useful.

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

• The concept of ocular therapy using light first was publicized by Meyer-
Schwickerath, who took patients to the roof of his laboratory in 1949 and
focused sunlight on their retinas to treat melanomas.
• Carbon arcs were used; by the mid-1950s, the xenon arc photocoagulator had
been developed and was made commercially available by Zeiss.
• Large size,
• poorly collimated beam,
• tendency to produce intense retinal burns.
1960 - Theodore Maiman : Built first laser by using a ruby crystal medium
.

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 WattJ/s)

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

Absorption
Energized electron in
higher orbit
Electron in orbit
A photon of the “right” energy gets absorbed and
“bumps” an electron into a higher energy level.
photon

Spontaneous Emission
photon
Electron in
orbit
An excited electron falls back to its lower energy level,
releasing a photon in a random direction
Energized electron in higher
orbit

Stimulated Emission:
photon
photon
Electron in orbit
photon
A photon strikes an excited electron. The electron falls to its lower energy
level, releasing a photon that is going in the same direction and in exact
phase with, the original photon. Note that only one photon strikes the atom
but two photons leave it—the original photon plus the emittedphoton
Energized electron in higher
orbit

• Now consider this ‘stimulated emission’.
• The two photons that have been produced can then generate 4 photons,
and the 4  16 etc… etc… cascade of intense monochromatic radiation.
• Thus Stimulated emission is the basis of the laser action.

Laser Construction
1
2
3
• A pump source or exciting medium
• A gain medium or laser medium.
• An optical resonator or laser tube.

Pump Source
• Provides energy to the laser system.
• Examples: electrical discharges, flash lamps and chemical
reactions.
• Ex. Excimer lasers use an electrical discharges.

Gain Medium
• It is a major determining factor of the wavelength of the
laser.
• It is Excited by the pump source.
• Here spontaneous and stimulated emission of
photons takes place.
• Gain medium can be solid, liquid, and gas.

• The energy of the emitted laser beam from gain medium is
increased(amplified) still further by causing the light beam to
traverse through the same material multiple times.
• This is accomplished by placing a mirror over each end of the
crystal or gas tube so that the distance between them is an even
multiple of the laser light’s wavelength.
• The coherent light beam is reflected back and forth
becoming more and more intense.
LASER tube or Optical Resonator

2
Laser Output
Continuous Output
(CW)
Pulsed Output
(P)
Energy
(Watts)
Time
Energy
(Joules)
Time

MODES OF LASER OUTPUT
• Continuous Wave (CW) Laser: Delivery of energy in a
continuous stream of photons.
• Pulsed Lasers: Produce energy pulses of a few tens of micro to
millisecond.
• Q Switches Lasers: Delivery of energy pulses is of extremely
short duration (nanosecond).
• A Mode-locked Lasers: Emits a train of short duration pulses
(picoseconds).

LASER INSTRUMENTATION
LASER Components are –
• Console: It contain laser medium and tube,
power supply and laser control system.
• Control Panel: It contain dials or push buttons or
touch screen for controlling various parameters.
• Aiming Beam
• Laser Switch
• Safety Filter
• Delivery System:
 Slit Lamp Microscope
 Indirect Ophthalmoscopes
 Endolaser probes

ACCESSORY COMPONENT
Corneal Contact Lenses for Laser used-
• Single mirror goniolens for goniotomy
• Abraham lens - iriditomy
• Goldman style 3-mirror lens for photcoagulation
(PRP) lenses
• Volk-Superquad and pan 165 for PRP
• Mainster and Area centralis for focal and grid laser
• Indirect Fundus Lenses (20 D) for Indirect laser
delivery

• Laser light transmitted by optical fibre
Optical fiber typically consists of a core, cladding, and a jacket (buffer coating). Light
launched into the fiber with its acceptance cone is trapped within a core due to the
total internal reflection at the core/cladding interface.

1- Optical fiber and electronic cable connecting laser with a slit lamp system.
2- Optical coupler projecting the beam exiting from the fiber onto the retina.
3- Contact lens.

CLASSIFICATION OF LASER
• Solid State
Ruby
Nd. Yag
Erbium. YAG
• Gas Ion
Argon
Krypton
He-Neon
CO2
• Metal Vapour
Cu
Gold
• Dye
Rhodamine
• Excimer
Argon Fluoride
Krypton Fluoride
Krypton Chloride
• Diode
Gallium-Aluminum-
Arsenide (GaAlAs)

• Light entering the eye can be reflected, absorbed , transmitted or scattered.
• Absorption depend on ocular characterstics chromophores
melanin - retinal and iris pigmented epithelia, choroid, uvea, trabecular meshwork;
 haemoglobin - red blood cells;
 xanthophyll - plexiform layers of the retina, especially in the macula;
rhodopsin and cone photopigments – photoreceptors
 lipofuscin located primarily in the RPE layer

lasers Wavelength
Diode 810 nm
Krypton red 647 nm
Krypton yellow 568 nm
Frequency
doubled Nd YAG
532 nm
Argon green 514 nm
Argon blue 485 nm
TYPES OF OPHTHALMIC LASERS

Light - tissue interactions
light
Photochemical effects
Thermal effects Ionizing effects
Photoradiation
eg. Dye laser
Photoablation
eg. Excimer laser
A
Photocoagulation argon,
krypton,dye,Nd:YAG
Photovapourization
CO2 laser
Photodisruption
eg. Nd:YAG laser

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 occurswhen
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..co2 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.
Usually -
Visible Wavelength
Ultraviolet Yields :
Photocoagulation
Photoablation
Infrared Photodisruption,
Photocoagulation

PHOTOCHEMICAL EFFECT
• Photoradiation (PDT):
 Also called photodynamic therapy.
 e.g. Treatment of Ocular tumours andCNV
Photon + Photo sensitizer in ground state(S)
Molecular Oxygen Free Radical
S + O2 Cytotoxic Intermediate
Cell Damage, Vascular Damage , ImmunologicDamage

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

CLASSIFICATION OF
CHORIORETINAL BURN INTENSITY
• Light
• Mild
: Barely visible retinal blanching
: Faint white retinal burn
• Moderate : Dirty white retinal burn
• Heavy : Dense white retinal burn

SELECTION OF OPTICAL WAVELENGHT FOR COAGULATION
• Wavelengths that are highly absorbed by macular yellow (such as 488 nm) are
relatively contraindicated when treating in or near the macula.
• Absorption of these wavelengths in macular pigments may cause heating and
destruction of the nerve fiber layer, resulting in loss of vision.
• Double ND:YAG or green Argon

• Scattering loss in cataract or in vitreous opacities can be minimized using longer
wavelengths: yellow (577 nm) or red (640–680 nm)
• Large quantity of hemoglobin- wavelengths between 520 and 580 nm are best
suited

How focal laser works?
Several theories
• 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.

How does panretinal photocoagulation work?
• PRP reduces retinal ischemia and hypoxia to anoxia thus decreases expression of
VEGF.
• Enhanced oxygen diffusion into the inner retina and vitreous reduces inner
retina ischemia and the stimulus for neovascularization.
• Number of sittings: 3
ꟷ PRP I: nasal retina
ꟷ PRP II: inferior retina
ꟷ PRP III: Superior retina

Uses
1. Diabetic Retinopathy – Pan-retinal photocoagulation.
Indications:
• High riskPDR
• Early PDR or very severe NPDRin
 Patients with poor compliance
 During pregnancy
 Patients with systemic diseases
 Pending cataract surgery
 One-eyed patients

• Type of laser: PRP with Argon(green-514nm wavelength)
• Laser delivery system: Indirect ophthalmoscope and+20 D lens, Slit lamp
laser
• Laser parameters:
ꟷ Spot size: 200 μ
ꟷ Pulse duration: 100 ms
ꟷ Power: 200-250 mW (goal is to produce greyburn)
ꟷ Spacing: 1-1.5 burn width apart

• Complications
• Transient: headache, blurring of vision, macular edema
• Persistent: nyctalopia, accomodative defect, reduced contrast
sensitivity, photophobia, reduced visual fields
• PVD induction
• Inadvertent foveal burns

2. Diabetic maculopathy:
• Indication: Clinically significant macular edema
Basic guidelines
• All areas of macular thickening must be treated
• FFA is done to look for points of leakage
• Focal leak →focal laser photocoagulation
• Diffuse leak → grid photocoagulation
• Laser delivery system: Slit lamp

Focal laser
Direct laser to microaneurysm
>500 μm from centre of fovea
ꟷ Spot size: 50-100 μ
ꟷ Duration: 50-100 ms
ꟷ Power- titrated to whiten
microaneurysm
Grid laser
• Laser to area of diffuse leakage & capillary
non-perfusion on FFA
ꟷ Spot size: 50-200 μ, Duration: 50-100 ms
ꟷ Power: titrated to achieve mild burn
ꟷ Laser is done in C-shaped manner within the
vascular arcade & avoiding area of
papillomacular bundle

Focal or grid laser treatment
Grid laser in dme
Laser to ischemic areas in
ROP
Posterior
hyloidotom
y
Laser barrage arouind retinal
tear. 3
rows of laser burns
given .

3. Retinal vein occlusion
• Indication:
• Macular edema (non responsive)
• Large segment of capillary non-perfusion(>10 DD)
• Neovascularization
• Contraindication:
• Macular ischemia

Macular edema
ꟷ Spot size: 50-200 μm
ꟷ Power: titrated to achieve
mild burn
Sectoral photocoagulation
for neovacularization
• Laser parameters
ꟷ Power: 200-250 mW
ꟷ Area: beyond 2 DD from centre of
macula upto equator

4. Retinopathy of prematurity
• Indications:
ꟷ Stage I, Zone I with plus disease
ꟷ Stage II, Zone I with plus disease
ꟷ Stage III, Zone I with plus disease
ꟷ Stage III, Zone I without plus disease
ꟷ Stage II, Zone II with plus disease
ꟷ Stage III, Zone II with plus disease
• Type of laser: PRP (withLIO)

ꟷ Spot size:200-500 μm
ꟷ Aim is to ablate the entire avascular retina from the ridge
upto the ora serrata in a near confluent burn pattern
getting as close to the ridge as possible.

• Complications:
ꟷ Premature infants are prone to develop apnoea.
ꟷ Conjunctival chemosis.
ꟷ Subconjunctival hemorrhage due to excessive scleral
indentation.
ꟷ Intense photocoagulation may lead to anterior segment
ischemia and necrosis resulting in hypotony and phthisis
bulbi.

5. CNVM-Photodynamic therapy:
Dosage: 6 mg/m2 of verteporfin infused intravenously
The amount of dye calculated is given over 10 minutes
(infusion phase) and then a further 5 minutes are allowed for
the dye to accumulate (accumulation phase).

Verteporfin injected intravenously
selectively accumulates in neovascular tissue which is rich in low-density lipoprotein
receptors, while it is rapidly cleared from the surrounding normal tissues
Upon absorption of a photon at the proper wavelength the porphyrin molecule
undergoes a transition from the ground state into the singlet excited state

The singlet excited porphyrin can decay back to the ground state with release of energy in the
form of fluorescence, which enables optical identification of tumor tissue
Such energy transfer produces toxic singlet oxygen
Singlet oxygen is very reactive and therefore it has a very short diffusion path length – less
than 20 nm, so all its interactions are highly localized.

• The activation by laser is typically performed 15–20 min after the intravenous
injection of the dye.
• Red laser light (689 nm diode laser)
• Closure of the abnormal (leaking) blood vessels occurs within approximately 6–12
weeks in most patients.

• Complications:
ꟷ Visual disturbances
ꟷ Photosensitivity reactions
ꟷ Overdosing- macular infarction
ꟷ In case of extravasation- pain at injection site and
allergic reactions

6. Retinal lesions predisposing to detachment and retinal tear
Purpose: Toinduce a sterile inflammationwhich stimulate
proliferation of the RPE →indirectly improves adhesion
between the RPE and the neurosensory retina.
Laser delivery system: Slit lamp with contact lensorLIO
Principle:
ꟷ The entire perimeter of the break should be
surrounded by laser application.
ꟷ Particular attention to the anterior margin and horns of a
tear should be paid.

ꟷ In the presence of a rim of fluid or in subclinical
detachment, laser is applied to the attached retina
immediately around the detachment.
ꟷ If applied to the area of detachment, it may cause
further progression of the detachment.
ꟷ Laser treatment of an inflamed retina is avoided as there
is a risk of producing a retinal break
• Laserparameters:
ꟷ Two-three rows of confluent burns
ꟷ Mild to moderate burn intensity

7. Eales’ disease: It is an idiopathic, inflammatory peripheral
vasculitis characterised by retinal periphlebitis and
capillary non-perfusion leading to hypoxia
• Indications:
ꟷ Neovascularization elsewhere
ꟷ Neovascularization disc
ꟷ Neovascularization iris
• Laser delivery system:
LIO
Slit lamp

• NVD → PRP
• Single NVE → sectoral scatter photocoagulation
• NVI → PRP
Laser parameter:

8. Central serous chorioretinopathy
Focal laser photocoagulation (extra-foveal leakage)
• Indications:
ꟷ Non-resolving or recurrent CSCR with V/A: <6/12
ꟷ Well defined leakage on FFA, atleast 500 μ away from
centre of fovea
ꟷ Spot size: 100- 200 μ

9. Retinal artery macroaneurym:
• These are solitary, saccular or fusiform dilation (diameter:125 -250μ) of
the retinal arteriole involving usually, the first three divisions.
• Two forms: acute &chronic
• Acute form: sudden loss of vision due to retinalorvitreous
hemorrhage
• Chronic form: gradual loss of vision due toleakage and exudation into
the macular area
• Laser photocoagulation is requiredfor chronic forms

ꟷ Power: 200mW
• Direct treatment: laser is focused directly onthe macroaneurysm so as
to obtain slow and gentle whitening
• In indirect treatment: laser burns are placedaround the aneurysm

10. Coats’ disease: Idiopathic retinal telangiectasia associated with
intraretinal and subretinal exudation and frequently exudative
retinal detachment, without signs of vitreoretinal traction
• Indication:
ꟷ Severe vascular anomalies with macular exudation
ꟷ Exudative retinal detachment
ꟷ Vascular anomalies posterior to equator
ꟷ Neovascularization
• Type of laser: LIO or Slitlamp

ꟷ Power: 200 mW
ꟷ End point: whitening of lesion

11. Retinal capillary hemangioma: Vascular hamartoma
• Indication:
ꟷ All capillary hemangiomas except those touching the optic nerve head
• Type of laser: Argon green or frequency doubledYAG
ꟷ Duration: 0.2- 1.0 s
ꟷ Power: titred to produce mild-moderate whitening of lesion.
ꟷ Small lesion → directtreatment
ꟷ Largelesion→ treatment of feeder vessel

12. Choroidal hemangioma: Vascular hamartoma
Manifest in two forms: diffuse & circumscribed
• Indication:
Serous retinal detachment
• Aim of treatment: achieve resolutioon ofserous retinal
detachment and not tumor obliteration
• Conventional laser: entire tumor surface iscovered with
laser spots

PDT: 6 mg/m2 of verteporfin dye is injected intravenously
• Laser parameters
ꟷ Spot size: 6000 μ(maximum)
ꟷ Laser used 689 nm
ꟷ Type of laser delivery: Slit lamp
ꟷ Lens used: Mainster wide field lens
ꟷ In peripapillary choroidal hemangioma, laser spot is applied at a
distance of 200 μ from the optic disc edge
ꟷ Large lesion(>2 mm) radiant exposure of 100 J/cm2 with
exposure of 186 seconds
ꟷ Small lesion(<2 mm) radiant exposure of 75 J/cm2 with
exposure of 125 seconds

13. YAGLaser hyloidotomy:
• Indications:
ꟷ Premacular haemorrhage secondary to diabetic retinopathy,
aplastic anemia, Terson’s syndrome, Valsalva retinopathy, vasculitis,
ruptured retinal artery macroaneurysm
ꟷ Persistent premacular haemorrhage beyond 4 weeks
ꟷ Size of harmorrhage: >3 DD
ꟷ Absence of retinal proliferation(If present PRP is done first)

• Type of laser: Slitlamp
• Technique:
ꟷ laser energy is focused above the inferior extent of the haemorrhage to
facilitate gravity-aided drainage of blood into the vitreous cavity
ꟷ Begin with an energy of 5mJ using single pulse.
ꟷ each shot ,increase energy by 1mJ ,maximum 8 shots to be given
• Complications:
Non-resolving vitreous haemorrhage
Creation of retinal holes
Retinal detachment

14. Optic disc pit:
• Indication:
Associated serous macular detachment
• Laser photocoagulation along the temporal marginofoptic disc.

Recent advances
1. PASCAL(Pattern scanning laser)
• Semi-automated laser delivering device
• Allows for a pattern of 4-56 burns to be applied in <1 sec using a scanning
laser with shorter pulse duration
• Indications:
ꟷ Diabetic retinopathy (PDR &NPDR)
ꟷ Choroidal neovascularization (CNVM & SRNVM)
ꟷ Age-related macular degeneration
ꟷ Retinal vein occlusion (BRVO & CRVO)
ꟷ Retinal tear, holes

• Laser source: Nd:YAGlaser
• Delivery system: Slit lamp orLIO
• Laserparameters
ꟷ Spot size: 200 μ
ꟷ Duration: 20 ms

• Advantages:
ꟷ Safe
ꟷ Relatively painless
ꟷ Less time consuming
ꟷ Well tolerated
ꟷ More number of spots in single sitting
ꟷ Requires less number of sitting

2. Navigational lasers
• 532-nm pattern-type eye-tracking laser integrateslive colour fundus
imaging, red-free and infra-red imaging, FFA with photocoagulator system.
• After image acquisition and making customized treatment plans by
physicians including marking areas which will be coagulated the treatment
plan is superimposed onto the live digital retina image during treatment
• The physician controls laser application and the systems assist with
prepositioning the laser beam.

• Advantages over conventionallasers:
ꟷ Fast
ꟷ Painless
ꟷ Better documentation
ꟷ Wide field viewing system allows for better accuracy

3.SELECTIVE RPE THERAPY (SRT)
• Light is strongly absorbed by melanosomes in the RPE
• Application of microsecond laser pulses allows for confinement of the thermal
and mechanical effects of this absorption within the RPE layer, thus sparing the
photoreceptors and the inner retina.
• Application of repetitive pulses of microsecond and sub-microsecond duration
results in selective damage to RPE due to the formation of small cavitation
bubbles around melanosomes.
• Subsequent RPE proliferation and migration restores continuity of the RPE layer
• Lack visible changes in retina
• CSR, DME

Retinal Laser Therapy Explained

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