Lasers Used in Ophthalmology - A Comprehensive Guide

1. Physical Basis: How Lasers Work on Ocular Tissue

Key principle: wavelengths 400-700 nm are absorbed by haemoglobin and melanin. Wavelengths above 500 nm are preferred near the macula because yellow xanthophyll pigment absorbs blue light (450-500 nm) and can cause inner retinal damage. - Kanski's Clinical Ophthalmology, Bailey & Love's Surgery

2. The Major Laser Types

A. Argon Laser (Blue-Green, 488 nm / 514 nm)

• Type: Gas laser (ionised argon gas)
• Wavelength: 488 nm (blue) and 514 nm (green); the green wavelength is preferred clinically
• Mechanism: Photocoagulation - absorbed by melanin and haemoglobin
• Key uses:
  • Panretinal photocoagulation (PRP) - proliferative diabetic retinopathy, to ablate ischaemic peripheral retina and reduce neovascular drive
  • Focal laser - leaking microaneurysms in diabetic macular oedema (spot 50-100 µm, 0.02-0.1 s)
  • Retinal tear/break photocoagulation - sealing breaks to prevent retinal detachment
  • Argon laser trabeculoplasty (ALT) - treating open-angle glaucoma by applying burns to trabecular meshwork
  • Argon laser iridoplasty - peripheral iris burns to pull open a narrowed angle
• Note: Blue-green (488 nm) should NOT be used at the macula (xanthophyll absorption causes inner retinal damage). Use pure green (514 nm) instead.

B. Nd:YAG Laser (Neodymium: Yttrium-Aluminium-Garnet)

• Type: Solid-state crystal laser
• Wavelength: 1064 nm (infrared); frequency-doubled version = 532 nm (green)
• Mechanism: Photodisruption at 1064 nm - plasma formation and acoustic shockwave break non-transparent or transparent tissue mechanically
• Key uses:
  1. Posterior capsulotomy (YAG capsulotomy) - treatment of posterior capsule opacification (PCO), which occurs in 5-10% of post-cataract cases. The Nd:YAG creates an opening in the opaque posterior capsule, instantly restoring vision
  2. Laser peripheral iridotomy (LPI / laser iridotomy) - creates a hole in the iris in acute angle-closure glaucoma (and prophylactically in fellow eye). Both affected and fellow eye are treated. Pre-treatment with argon laser is an option for thick/dark irides to pre-coagulate tissue and reduce bleeding
  3. Anterior laser vitreolysis - for symptomatic vitreous floaters
  4. Anterior hyaloid disruption - in pseudophakic macular oedema
• Frequency-doubled Nd:YAG (532 nm):
  • Used for retinal photocoagulation (same role as argon green)
  • Used in the PASCAL pattern scan laser system

C. Excimer Laser (ArF - Argon Fluoride)

• Type: Gas laser (argon fluoride mixture)
• Wavelength: 193 nm (ultraviolet)
• Mechanism: Photoablation - UV photons break C-C and C-N molecular bonds in corneal collagen with extreme precision; no thermal damage to adjacent tissue; each pulse removes ~0.25 µm of stroma
• Key uses (refractive surgery):
  1. Photorefractive keratectomy (PRK) - epithelium removed, then excimer ablates Bowman layer and anterior stroma to reshape corneal curvature. Heals in 48-72 h. Risk of subepithelial haze; mitomycin C applied intraoperatively to reduce this
  2. LASIK (Laser in situ keratomileusis) - a flap is created (microkeratome or femtosecond laser), excimer ablates the stromal bed, flap is replaced. Most common refractive procedure
  3. LASEK / Epi-LASIK / Trans-PRK - surface ablation variants minimising discomfort and post-laser haze

D. Diode Laser (Semiconductor Laser)

• Type: Semiconductor diode
• Wavelength: 810 nm (near-infrared) or 689 nm (PDT version)
• Mechanism: Infrared photocoagulation - well-absorbed by melanin in RPE and ciliary body pigment epithelium; passes through haemoglobin with less scattering
• Key uses:
  1. Diode laser cycloablation / Cyclodiode laser - trans-scleral photocoagulation of the ciliary body to reduce aqueous production in refractory glaucoma (uncontrolled secondary glaucoma, end-stage disease). Also called "cyclodiode"
  2. Retinal photocoagulation - PRP and focal macular laser (alternative to argon/frequency-doubled YAG). Well-tolerated in the clinic
  3. Retinopathy of prematurity (ROP) - preferred laser for peripheral retinal ablation in premature infants
  4. Transpupillary thermotherapy (TTT) at 810 nm - subthreshold thermal treatment for choroidal melanoma, choroidal haemangioma (see below)
  5. Photodynamic therapy (PDT) at 689 nm - used to activate verteporfin

E. Selective Laser Trabeculoplasty (SLT) - Frequency-Doubled Nd:YAG 532 nm

• Type: Q-switched, frequency-doubled Nd:YAG
• Wavelength: 532 nm
• Mechanism: Short nanosecond pulses selectively target melanin-containing trabecular meshwork cells without coagulative damage to adjacent tissue ("selective" = spares collagen, non-pigmented cells)
• Key uses:
  • Open-angle glaucoma - first-line or adjunct treatment to lower IOP. Can be repeated (unlike argon laser trabeculoplasty which causes scarring). Used as an alternative to argon laser trabeculoplasty (ALT)
• Advantage over ALT: SLT is repeatable, causes no coagulation scarring, and leaves the trabecular meshwork intact for potential future surgical filtration

F. Femtosecond Laser (Ultrafast Infrared)

• Type: Solid-state ultrafast pulsed laser
• Wavelength: ~1053 nm (infrared)
• Mechanism: Photodisruption via plasma formation with femtosecond (10⁻¹⁵ s) pulses - extreme precision, minimal collateral thermal damage. Each pulse disrupts a tiny volume of tissue by ionisation
• Key uses:
  1. LASIK flap creation - replaces the mechanical microkeratome, creating a precise, planar flap with adjustable thickness, diameter, and hinge angle; reduces risk of flap complications
  2. Small incision lenticule extraction (SMILE) - all-femtosecond, flapless refractive procedure: a lenticule is cut within the stroma and extracted through a small incision
  3. Femtosecond laser-assisted cataract surgery (FLACS) - capsulotomy, nuclear fragmentation, and limbal relaxing incisions performed with femtosecond precision before phacoemulsification
  4. Corneal transplant (keratoplasty) trephination - laser-cut donor and recipient corneas for improved wound architecture

G. Krypton Laser (Red, 647 nm)

• Type: Gas laser
• Wavelength: 647 nm (red)
• Mechanism: Photocoagulation - absorbed by melanin but NOT by haemoglobin; penetrates through retinal haemorrhage and vitreal blood
• Key uses:
  • Subretinal neovascular membrane treatment (historically, before PDT/anti-VEGF era)
  • Photocoagulation in patients with vitreous haemorrhage (where red penetrates blood better)
  • Macular photocoagulation - safer near vessels as haemoglobin absorption is low
• Note: Largely replaced by frequency-doubled Nd:YAG and diode lasers in modern practice

H. Photodynamic Therapy (PDT) - Diode Laser 689 nm + Verteporfin

• Mechanism: Photochemical - intravenous verteporfin (a photosensitiser) preferentially accumulates in abnormal choroidal neovascular tissue. Activation by 689 nm laser generates reactive oxygen species → vascular thrombosis and closure of the CNV
• Key uses:
  • Classic subfoveal CNV in wet AMD - historically first-line, now largely superseded by anti-VEGF therapy but still used for certain CNV subtypes
  • Juxtafoveal CNV, polypoidal choroidal vasculopathy (PCV) - PDT ± anti-VEGF
  • Central serous chorioretinopathy (CSCR) - half-dose/reduced-fluence PDT is now a recognised treatment
  • Choroidal haemangioma, choroidal melanoma - as part of management

I. Transpupillary Thermotherapy (TTT) - Diode 810 nm

• Mechanism: Subthreshold photothermal - large spot (2-3 mm), long duration (60 s), low power diode laser produces a mild temperature rise (~10°C) within the tumour/lesion without visible burn; distinct from photocoagulation
• Uses:
  • Choroidal melanoma (small/medium, juxtapapillary)
  • Occult subfoveal CNV (historical)
  • Choroidal haemangioma
  • Retinoblastoma (as an adjunct)

J. Subthreshold Micropulse Laser

• Wavelength: 577 nm (yellow) or 810 nm (diode)
• Mechanism: Train of very short "on" pulses (100-300 µs) with longer "off" intervals (1700-1900 µs) - allows energy dissipation between pulses; stimulates RPE without causing visible burns or chorioretinal scarring. Subfoveal treatment is possible
• Key uses:
  • Diabetic macular oedema (DMO) - no coagulative scarring, safe for foveal/subfoveal use
  • Central serous chorioretinopathy (CSCR)
  • Branch/central retinal vein occlusion macular oedema
• Advantage: No permanent retinal scar, can be retreated, safer at macula

3. Summary Table

4. Classification by Ocular Target

Retina

• PRP (proliferative diabetic retinopathy): Argon green, frequency-doubled Nd:YAG, diode 810 nm
• Focal macular laser (DMO): Argon green, frequency-doubled Nd:YAG, micropulse 577 nm
• Retinal tears/detachment prophylaxis: Argon, diode
• ROP: Diode 810 nm (preferred)
• CNV (wet AMD, other): PDT 689 nm + verteporfin; historically krypton red

Glaucoma

• Open-angle glaucoma: ALT (argon 514 nm) or SLT (532 nm Nd:YAG)
• Angle-closure / pupillary block: Nd:YAG 1064 nm laser iridotomy; argon pre-treatment for thick irides
• Peripheral angle opening: Argon laser iridoplasty
• Refractory/end-stage glaucoma (ciliary body ablation): Diode cyclodiode (810 nm trans-scleral)

Lens / Posterior Segment

• Posterior capsule opacification: Nd:YAG 1064 nm capsulotomy
• Vitreous floaters: Nd:YAG vitreolysis
• Anterior hyaloid disruption: Nd:YAG

Cornea / Refractive

• Myopia, hypermetropia, astigmatism correction: Excimer 193 nm (PRK/LASIK/LASEK), femtosecond (SMILE)
• LASIK flap creation: Femtosecond laser

Tumours

• Choroidal melanoma, retinoblastoma: TTT (diode 810 nm), PDT (diode 689 nm)
• Choroidal haemangioma: PDT or TTT

5. Clinical Pearls

1. Blue light (488 nm) is contraindicated at the macula - xanthophyll pigment absorbs it, causing inner retinal/nerve fibre layer damage
2. SLT is preferred over ALT for repeatable glaucoma treatment - no thermal scarring, trabecular architecture preserved
3. Nd:YAG capsulotomy is the most common laser procedure after cataract surgery (PCO incidence 5-10%)
4. Diode laser is the laser of choice for ROP in premature infants - portable, well-tolerated
5. Femtosecond LASIK has largely replaced microkeratome LASIK due to superior flap precision and safety
6. Pre-treatment with argon laser before Nd:YAG iridotomy is recommended for thick, darkly pigmented irides to coagulate tissue and minimise bleeding
7. Micropulse laser allows subfoveal treatment of DMO and CSCR without visible burn - a major clinical advantage over conventional photocoagulation
8. PDT is now second-line for wet AMD (anti-VEGF is first-line) but remains relevant for polypoidal choroidal vasculopathy and CSCR