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Pan Retinal Photocoagulation (PRP), Focal Laser & Photodynamic Therapy (PDT) – Technique & Tips.

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Correspondance:Department of Retina & Vitreous, Aravind Eye Hospital, Avinashi Road, Coimbatore – 641014, Tamilnadu. E-mail: drshahpk2002@yahoo.com

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Introduction
‘LASER’ is acronym for “Light Amplification by Stimulated Emission Radiation.” This describes the emission process by which an intense beam of electromagnetic radiation is generated

Basic Properties of Laser
Always monochromatic (one wavelength)
Collimated (all photons run parallel) & focused to a small point
Coherent (always in same phase)
Highest possible speed

Physics
Certain substances have property to lase i.e, absorb energy in one form & emit a new form of energy
On pumping these lasing substances electrons are transfered from a lower orbit to higher orbit (Bohr’s theory)
Excited atoms in turn decay back to their original orbit of lower energy, emitting photons (packets of energy)
This emission can be spontaneous or induced (stimulated)

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Photocoagulation (PHC)

Photothermal reaction
Temperature elevation 10 to 20 deg C
Protein denaturation
Pigment dependent
Argon, krypton dye, diode, frequency double Nd Yag

Tissue effects
Induces moderate sterile inflammation- Creates bio adhesion
Collagen shrinkage (Beneficial)
Membrane shrinkage (Harmful effect)

Ocular pigment absorption characteristics
Melanin – Entire visible range, 300 to 1300 nm
Hemoglobin – Blue & Green range 480 – 520 nm
Xanthophyll – Blue 488 nm

Hence, when treating macular area avoid blue lasers to avoid inadvertent damage to macula. Argon green, doubled frequencies NdYag, 577dye are lasers of choice for macular photocoagulation

Laser delivery systems

  1. Slit lamp
  2. Laser indirect ophthalmoscope (LIO)
  3. Endo laser

Chorioretinal burn intensity classification

  1. Light – barely visible retinal blanching
  2. Mild – faint white retinal burn
  3. Moderate – opaque dirty white retinal burn
  4. Heavy – dense white retinal burn

Prerequisites for retinal PHC
Informed consent
Dilated pupil 4mm
Anaesthesia-Topical / retrobulbar
Fundus contact lens / 3 or 4 mirror contact lens/ panfudoscopic lenses

Panfundoscopic contact lenses most commonly used

  1. Rodenstock panfundoscope
  2. Mainster wide field
  3. Goldmann 3 mirror

Spot selector set at 200μm

 

Goldmann 3 mirror

216

Rodenstock panfundus

286

Mainster wide field

294

Pan retinal photocoagulation (PRP)

Indications

  1. Proliferative diabetic retinopathy (PDR)
  2. Retinal vascular obstructions (CRVO)
  3. Retinal vasculitis
  4. Ocular ischemic syndrome

Laser parameters
Spot size 200-500 microns
Duration 0.2 to 0.5 seconds
Power 140 – 200 mW depending on media clarity. Aim is to create a moderate intensity burn. Each burn should be at least 1 burn width apart. 900 burns are required for each half of the retina. Total of 1800 to 2200 burns for complete treatment PRP

Technique (slit lamp delivery)
Under topical anaesthesia, place lens by asking patient to look up at the same time retracting lower lid. Once lens is placed, focus to obtain clear view of retina.
Treatment completed in 2 sessions. Inferior half of retina is treated in first session (because if vitreous hemorrhage occurs then it would be difficult to do inferior half) and then superior half after 15 days.

Temporally start 3000 microns away form the foveal center (Figure 1).

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Figure 1: Fundus photo of RE showing the temporal extent of PRP laser

Nasally do not go closer than 500 microns along the margin of the optic disc (Figure 2)

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Figure 2: Fundus picture of RE showing nasal extent of PRP laser.

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Video 1 shows technique of PRP laser.

Laser indirect ophthalmoscope (LIO) delivery

Indications
Media opacities like dense cataracts, vitreous hemorrhage
Treat peripheral areas like retinal holes /tears
Patients who cannot sit for long duration (morbid)

Parameters used is similar to that of slit lamp delivery system except higher power may be required in dense media opacities

Endolaser in direct laser inside the eye with an endolaser probe during parsplana vitrectomy.

Scatter photo coagulation is 360 degree PRP given with each burn should be at least 2 burn width apart. Total of 800 to 1000 burns are required for complete scatter PRP (Figure 3)

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