Lasers in Ophthalmology

Physics & Tissue Interactions

Classification of Ophthalmic Lasers

1. Argon Laser (488 nm / 514 nm – blue-green)

• Mechanism: Thermal photocoagulation
• Applications:
  • Panretinal photocoagulation (PRP): First-line for proliferative diabetic retinopathy — ablates ischaemic peripheral retina, reducing VEGF drive
  • Focal/grid laser: For diabetic macular oedema (focal leaky microaneurysms or grid for diffuse oedema); largely superseded by anti-VEGF but still used
  • Retinal tears/breaks: Seals tears before progression to detachment
  • Argon laser trabeculoplasty (ALT): Burns to trabecular meshwork in open-angle glaucoma; thermal damage limits repeatability

2. Diode Laser (810 nm – near-infrared)

• Mechanism: Thermal photocoagulation; better choroidal penetration
• Applications:
  • Retinal photocoagulation (alternative to argon)
  • Trans-scleral cyclophotocoagulation (TSCPC): Targets ciliary body through the sclera to reduce aqueous production in refractory/secondary glaucoma with uncontrolled IOP — used when trabeculectomy has failed or is contraindicated
  • Endocyclophotocoagulation (ECP): Intraocular delivery via endoscope during vitreoretinal surgery

3. Nd:YAG Laser (1064 nm; frequency-doubled 532 nm)

• Mechanism: Photodisruption (plasma-mediated)
• Applications:
  • Posterior capsulotomy (PCO): Photodisrupts the opacified posterior capsule ("after-cataract") — this occurs in 5–10% of cases after cataract surgery and is the most common use of Nd:YAG in routine practice
  • Laser peripheral iridotomy (LPI): Creates a full-thickness hole in the peripheral iris for acute angle-closure glaucoma; treats both the affected and the fellow eye
  • Vitreolysis: Disruption of vitreous floaters (off-label but practised)

4. Selective Laser Trabeculoplasty (SLT) — Frequency-doubled Nd:YAG, 532 nm

• Mechanism: Selectively targets melanin in trabecular meshwork (TM) cells without thermal damage to non-pigmented structures
• Clinical significance: Based on the LiGHT trial (6-year data), SLT is now recommended as first-line treatment for ocular hypertension and primary open-angle glaucoma — patients who had SLT first showed less disease progression and were less likely to require trabeculectomy. IOP reductions of 10–40% can be expected; ~80% of patients will be drop-free at 3 years
• Because there is no thermal tissue damage, treatment can be repeated even after initial failure
• Complications: transient mild inflammation, transient IOP spike (especially in heavily pigmented angles), rare endothelial decompensation

5. Excimer Laser (ArF, 193 nm – ultraviolet)

• Mechanism: Photoablation — breaks molecular bonds with negligible thermal spread; ablates corneal stroma to a precise depth
• Applications (Refractive Surgery):
  • Photorefractive keratectomy (PRK): Epithelium removed; stroma ablated directly. Corrects myopia up to ~6 D, astigmatism up to ~3 D, low–moderate hypermetropia. Main disadvantage vs LASIK: slower recovery, more stromal haze
  • LASER subepithelial keratectomy (LASEK): Epithelium chemically separated then replaced after ablation
  • LASIK (Laser-Assisted in Situ Keratomileusis): Corneal flap created (by microkeratome or femtosecond laser) → stromal ablation by excimer → flap repositioned. Corrects myopia up to 6–8 D, hypermetropia up to 3–4 D, astigmatism up to 5 D. Advantages: faster recovery, better comfort, wider correction range. Risk: flap-related complications
  • Wavefront-guided LASIK: Excimer ablation pattern based on wavefront aberrometry measurements to minimise induced higher-order aberrations

6. Femtosecond Laser (1053 nm – infrared, ultrashort pulses)

• Mechanism: Photodisruption at extremely precise focal points
• Applications:
  • LASIK flap creation: Produces uniform, precise flap thickness (100–120 μm) — significantly reduces flap-related complications vs mechanical microkeratome
  • Small incision lenticule extraction (SMILE): Entirely femtosecond-based; lenticule of stroma removed through small incision — no flap
  • Femtosecond-assisted cataract surgery (FLACS): Capsulotomy, lens fragmentation, corneal incisions
  • Corneal transplantation: Precise lamellar cuts for DALK, DSAEK

7. Photodynamic Therapy (PDT) — Verteporfin + 689 nm Diode Laser

• Mechanism: Verteporfin (photosensitiser) is infused IV → accumulates in abnormal choroidal neovascular tissue → activated by 689 nm laser → singlet oxygen generation → selective vascular occlusion
• Applications:
  • Wet AMD with predominantly classic subfoveal choroidal neovascularisation (CNV)
  • Polypoidal choroidal vasculopathy (PCV) — often combined with anti-VEGF
  • Chronic central serous chorioretinopathy (CSC) — half-dose or half-fluence PDT

8. Micropulse Laser

• Mechanism: Delivers thermal energy in extremely short pulses with off-time periods; subthreshold — no visible tissue reaction
• Applications:
  • Micropulse laser trabeculoplasty (MLT): Comparable IOP lowering to SLT/ALT; no visible burns, no repeat limitations
  • Subthreshold micropulse laser (SML): For diabetic macular oedema and central serous chorioretinopathy — as effective as conventional photocoagulation with considerably less retinal damage; onset slower

Summary Table

Retina: Panretinal Photocoagulation Post-Treatment Appearance

• Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed. — Chapters on Glaucoma (LTP), Refractive Surgery, Macula
• Bailey & Love's Short Practice of Surgery, 28th ed. — "Lasers in Ophthalmology," p. 757
• The Wills Eye Manual, 8th ed. — Glaucoma, Retina, Cornea sections
• Goodman & Gilman's Pharmacological Basis of Therapeutics — Verteporfin/PDT
• Harrison's Principles of Internal Medicine, 22nd ed. — Diabetic retinopathy laser treatment