Lasers in Ophthalmology

Laser Facts

Brief History

1917 - Einstein: Laser possible.
1958 - H. Townes, A.L. Schawlow: Theoretical basis for lasers.
1960 - Maiman: Built first laser.
1963 - Zweng: First medical laser trial (retinal coagulation).
1965 - Z. Yarn: First clinical laser surgery.

Instrument Wavelengths

(Chronological order in nm)

SWAP: 440, blue light
Retinal Thickness Analyser RTA: green, 540-nm HeNe laser
Hertmann Shack’s Aberrometer: 575 nm
HRT- CSLO: 670 diode
GDX-SLO: 780 diode
IOL Master: 780 diode
Lenstar: 820, SLD
Macular OCT (Posterior Segment OCT): 830
RNFL OCT: 810/850, SLD
AS-OCT: 1310, SLD
Swept Source OCT: 1050

Therapeutic Wavelengths

(nm)

Excimer laser: 193, Argon Fluoride (ultraviolet)
Excimer laser: 308, Xenon Cloride (ultraviolet)
Excimer laser trabeculostomy ELT: 308, Xenon Cloride
Argon blue-green: 488
Df Nd YAG: 512 (green)
ND YLF laser: 527 (in the treatment of diabetic macular edema)
Argon green: 514
Yellow dye laser: 577 for DME
Krypton red or Dye red: 620-630
He Ne Gas laser: 632, for Laser interferometry
Rostaporphin PDT: 664 nm
Diode: 689, PDT (its verteporphin’s peak absorption)
Ruby laser: 694, red
Infrared diode laser: 780, IOL Master
Transpupilary thermotherapy, DLCP: 810
subthreshold micropulse STMP diode 810nm: for DME
Intralase: Nd: YLF, wavelength is 1053 nm
Nd YAG: 1064
Ho YAG: 2100, used for laser thermokeratoplasty
Erbium YAG: 2940, for laser phacoemulsification
CO2 laser: 10600

Laser settings for glaucoma

ND YAG PI: 500u, 5-15mJ, 12ns pulse, 1-3 pulse/sec
Argon PI: 50u, 1000mW, 0.1-0.2s,
Laser pupilloplasty: 200u, 0.2s, 400mW
Laser sphincterotomy: 50u, 0.01-0.05s, 1.5W
ALT: 50u, 100ms, 1000mW
SLT: 400u, 0.5-1.2mJ
DLCP: 810 nm, 2 mm from limbus, 8 spots per quadrent, 2000 ms, 1200-2000 mW

Laser settings in Retina

PRP: df NdYAG
- 200-500 micron spot size
- 200-500 ms
- 200-500 mW
Macular Grid Laser: df NdYAG
- 50-100 micron spot size
- 50-100 ms
- 50-100 mW

Laser Parameters for PTK

Fluence: 160 ± 10 mJ/cm2
Repetition rate: 5 Hz
Ablation rate: 0.20-0.35 mm per pulse
Ablation diameter: 5.5–6 mm including a 0.5 mm transition zone
Ablation depths:
- Epithelium 40 mm (default value) or as determined by pachymetry
- Stroma: Depth of scar or opacity (postoperative corneal thickness should be at least 250 mm)

Modes of Laser

There are three modes of laser: continuous, Q-switched and mode-locked.

Laser from continuous mode has a constant power and is measured in watts.
Q-switched and mode-locked increases the energy by compressing the energy in time and the energy is best measured in joules.
Mode-locked laser compresses the laser more than Q-switched laser and therefore produces more energy.

Laser Mediums

Solid state lasers have lasing material distributed in a solid matrix, e.g., the ruby or neodymium-YAG (yttrium aluminum garnet) lasers. The neodymium-YAG laser emits infrared light at 1.064 micrometers.
Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of a visible red light. CO2 lasers emit energy in the far-infrared, 10.6 micrometers, and are used for cutting hard materials.
Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudomolecule or dimer is produced and when lased, produces light in the ultraviolet range.
Dye lasers use complex organic dyes like rhodamine 6G in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.
Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, e.g., the writing source in some laser printers or compact disk players.

Laser Safety Classification

The International Safety Classification of Lasers divides the lasers into 4 groups. Group 3 is subdivided into 3a and 3b. Class 3b and above is damaging to the eye and their powers are 5MW and above. All lasers used in ophthalmology are classed as 3b and above. Safety goggles should always be worn by people in the vicinity.

Class I: Do not emit hazardous levels.

Class II: Visible light lasers that are safe for momentary viewing but should not be stared into continuously; an example is the aiming beam of ophthalmic lasers, or laser pointers.

Class III: Unsafe for even momentary viewing, requiring procedural controls and safety equipment.

Class IV: Also pose a significant fire and skin hazard; most therapeutic laser beams used in ocular surgery are in this class

- compiled & published by Dr Dhaval Patel MD AIIMS