How do lasers work in the eye?
LASER stands for light amplification by stimulated emission of radiation. A photon of the right frequency passing a metastable excited electron stimulates it to drop to a lower energy level and emit an identical photon. The result is light that is monochromatic and coherent and highly directional, with a high energy density. Ophthalmology was the first medical specialty to use lasers (a ruby laser on ocular lesions, about a year after the laser was invented) and still performs more laser procedures than any other specialty.
The laser medium sets the wavelength: gas (argon, argon fluoride), liquid (dye), solid (neodymium:YAG) or semiconductor (diode). What the beam does to tissue then depends on its power density and exposure time, which is why the same eye can be treated with very different lasers.
What are the five mechanisms of laser–tissue interaction?
StatPearls groups ophthalmic lasers into five mechanisms, listed here in ascending order of power density:
| Mechanism | How it acts | Laser examples | Ophthalmic uses |
|---|---|---|---|
| 1. Photochemical (photoactivation) | Long exposure, low power; activates a photosensitive dye; no significant heating | Red laser 689 nm with verteporfin; UV-A 365–370 nm with riboflavin | Photodynamic therapy (AMD, central serous), corneal collagen cross-linking in keratoconus |
| 2. Thermal (photocoagulation) | Absorbed by tissue pigment; heats and coagulates; coagulation mostly at 60–70 °C | Argon 514 nm, frequency-doubled Nd:YAG 532 nm, krypton 647 nm, yellow 577 nm, diode 810 nm | Panretinal and focal photocoagulation, retinal tears and detachments, ROP, trabeculoplasty, central serous chorioretinopathy |
| 3. Photoablation | UV light breaks covalent bonds without heating nearby tissue | Excimer (argon fluoride) 193 nm | PRK, LASIK, phototherapeutic keratectomy |
| 4. Plasma-induced ablation | Ultrashort pulses ionise tissue (optical breakdown) with minimal thermal damage | Femtosecond infrared 1053 nm | Creating the LASIK corneal flap |
| 5. Photodisruption | Highest power, nanosecond pulses; plasma expansion produces a mechanical shockwave that cuts tissue | Nd:YAG 1064 nm | Posterior capsulotomy, peripheral iridotomy, anterior vitreolysis |
Excimer is short for excited dimer: the laser is made of two gases, such as argon fluoride, which emit at 193 nm in nanosecond pulses delivering about 6 eV, short enough to avoid heat and strong enough to break bonds. Thermal lasers are the most commonly used in ophthalmology; photodisruption is second.
Which wavelength goes with which laser and target?
| Laser | Wavelength | Typical role |
|---|---|---|
| Argon fluoride (excimer) | 193 nm | Corneal photoablation (PRK, LASIK) |
| Argon (green) | 514 nm | Retinal photocoagulation, argon laser trabeculoplasty |
| Frequency-doubled Nd:YAG (green) | 532 nm | Photocoagulation, selective laser trabeculoplasty (Q-switched) |
| Yellow semiconductor | 577 nm | Retinal photocoagulation |
| Krypton (red) | 647 nm | Retinal photocoagulation |
| Diode | 810 nm | Photocoagulation; cyclophotocoagulation in painful blind eyes |
| Nd:YAG (infrared) | 1064 nm | Photodisruption: capsulotomy, iridotomy |
| Femtosecond | 1053 nm | LASIK flap creation |
Wavelength matters because absorption depends on it. Retinal targets such as haemoglobin and melanin absorb light between about 400 and 700 nm. Lasers above 500 nm are preferred near the macula because the yellow xanthophyll pigment absorbs 450 to 500 nm light, so shorter wavelengths risk macular damage. The two most commonly used retinal lasers are frequency-doubled Nd:YAG and argon. The PASCAL patterned scanning system fires short multiple shots to reduce pain and collateral retinal damage during PRP.
How are thermal lasers used for the retina and glaucoma?
Panretinal photocoagulation (PRP) has been the standard treatment of proliferative diabetic retinopathy (PDR) for decades. It destroys ischaemic peripheral retina, a source of VEGF release. High-risk PDR without macular oedema needs PRP; PDR with diabetic macular oedema may need combined anti-VEGF injection and PRP. Focal or grid laser guided by ETDRS is the treatment of choice for non-centre-involving macular oedema, whereas centre-involving oedema is now treated first with anti-VEGF injections, with laser as an adjunct. The DRCR Protocol S found ranibizumab non-inferior to PRP in PDR.
- Complications of PRP: macular oedema, Bruch membrane rupture, subretinal or vitreous haemorrhage, retinal or choroidal scarring, secondary angle-closure glaucoma from choroidal and ciliary effusion, exudative retinal detachment if too many shots are given in one sitting.
- Visual cost of PRP: reduced contrast sensitivity, peripheral field and night vision.
- Other thermal indications: laser barricade of retinal tears and detachments, retinopathy of prematurity, central serous chorioretinopathy, transpupillary thermotherapy for choroidal melanoma.
- Delivery: slit-lamp (commonest), indirect ophthalmoscope with fibre-optic cable (peripheral breaks, ROP), or an intraocular endolaser probe at vitrectomy.
Laser trabeculoplasty lowers intraocular pressure in open-angle glaucoma by increasing aqueous outflow through the trabecular meshwork, which carries about 90% of aqueous drainage. Argon laser trabeculoplasty (ALT) uses green argon light and causes coagulative necrosis of the meshwork; selective laser trabeculoplasty (SLT) uses a Q-switched frequency-doubled Nd:YAG laser that cracks intracellular pigment granules and is less destructive. ALT and SLT lower IOP about equally, but the effect wanes with time. The most common complication is a transient IOP spike, which prophylactic alpha-adrenergic agonists reduce by about two-thirds.
| Feature | Detail |
|---|---|
| Indications | OAG uncontrolled on maximal tolerated medical therapy; poor adherence or tolerance; pseudoexfoliation or pigmentary glaucoma; angle-closure glaucoma with a patent iridotomy; when surgery needs deferring |
| Contraindications | Inflammatory glaucoma, advanced glaucoma, poor view of the trabecular meshwork (e.g. synechiae) |
| Lens | Gonioscopy mirror lens (Goldmann three-mirror, Ritch, Latina for SLT) |
| Complications | IOP spike (commonest), low-grade iritis, peripheral anterior synechiae, hyphaema, corneal oedema from HSV reactivation |
What does the Nd:YAG laser do in the eye?
The Nd:YAG laser at 1064 nm is a photodisruptive laser — it cuts rather than burns. Its main uses are posterior capsulotomy, peripheral iridotomy and anterior laser vitreolysis, delivered through special lenses (Abraham lens for capsulotomy and iridotomy).
Posterior capsule opacification (PCO) is the commonest long-term complication of cataract surgery, affecting up to one-fifth of patients within 5 years. A Nd:YAG capsulotomy opens a clear central window in the opacified posterior capsule. Risks include transient IOP spikes, cystoid macular oedema and, rarely, retinal detachment.
Laser peripheral iridotomy (LPI) treats pupillary block, the main mechanism of primary angle-closure glaucoma (PACG). The iridotomy lets aqueous pass directly from the posterior to the anterior chamber, bypassing the pupil. It is performed with the Nd:YAG laser; a small (about 150–200 micron), peripheral, fully patent hole is ideal, and the 11 to 1 o'clock position under the upper lid is commonly used. Typical complications are a transient IOP spike (most common), hyphaema (because Nd:YAG is photodisruptive, not thermal, it can bleed) and iridotomy closure, which occurs after argon treatment (21–30%) but not after Nd:YAG in the trials quoted.
How do excimer lasers correct refractive error?
The 193 nm excimer laser reshapes the anterior corneal stroma by photoablation. The cornea provides about two-thirds of the eye's refractive power (about 43 dioptres), so small changes in its curvature change the refraction. Flattening the central cornea corrects myopia; steepening it corrects hyperopia.
| Feature | PRK (surface ablation) | LASIK |
|---|---|---|
| Step one | Epithelium removed down to Bowman's layer | Flap made with a femtosecond laser (1053 nm) |
| Ablation | Excimer on the exposed anterior stroma | Excimer on the stromal bed under the flap |
| Healing | Re-epithelialisation in about 3–5 days with a bandage contact lens; slower visual recovery and more early discomfort | Faster recovery |
| Specific risks | Corneal haze (reduced by intra-operative mitomycin C), especially with higher corrections | Flap complications, ectasia, scarring; more corneal nerves cut, more dry eye |
| Preferred when | Thin corneas, epithelial basement membrane dystrophy, history of recurrent erosions, risk of flap trauma (contact sports, certain occupations), irregular surface | Otherwise suitable corneas where a fast recovery is wanted |
- Absolute contraindications to PRK: keratoconus, pellucid marginal degeneration or other corneal ectasia, corneal thinning or oedema, neurotrophic or interstitial keratitis, extensive vascularisation, significant cataract, unstable glaucoma, uncontrolled external disease (blepharitis, dry eye, atopy).
- Relative contraindications: pregnancy and breastfeeding (refraction shifts), functional monocularity, very steep or flat corneas.
- Wavefront-guided or topography-guided ablation profiles reduce higher-order aberrations.
- Phototherapeutic keratectomy (PTK) uses the same excimer laser for superficial corneal opacities and recurrent corneal erosions.
- Laser thermal keratoplasty with a holmium:YAG laser (2060 or 2130 nm) shrinks corneal collagen to treat hyperopia — a thermal, not an excimer, procedure.
What are photochemical laser treatments?
Photochemical interaction needs long exposure at low power, so tissue does not heat. A photosensitive dye absorbs a specific wavelength and triggers a local reaction.
- Photodynamic therapy (PDT): verteporfin is injected into the circulation, then irradiated with a low-energy red laser at 689 nm, thrombosing the abnormal vessels (age-related macular degeneration, central serous chorioretinopathy).
- Corneal collagen cross-linking: riboflavin activated by UV-A at 365–370 nm in keratoconus, to stiffen the cornea.
- Rose Bengal or green-light cross-linking and photodynamic therapy (518–532 nm) for infective keratitis; green argon laser for corneal neovascularisation.
How are lasers used for diagnosis, and how is laser safety managed?
Lasers are central to diagnostic imaging too. OCT uses superluminescent diodes or ultrashort pulsed lasers (about 840–1050 nm, depending on the system) and low-coherence interferometry to image the retina and anterior segment; OCT angiography images vessels from the movement of red cells; scanning laser ophthalmoscopy uses confocal laser scanning to image the retina, cornea and optic nerve head. Biometry instruments such as the IOL Master use partial-coherence or swept-source OCT principles.
- Commonest cause of laser injury is accidental exposure to reflected beams — from metal instruments, even the cornea or a contact lens. Many therapeutic lasers are infrared or ultraviolet and invisible.
- Hazard classes: Class 1 (safe, e.g. OCT) to Class 4 (fire and skin hazard; medical lasers including the 532 nm Nd:YAG). Class 3 and 4 need eye protection at all times.
- Protection: wavelength-specific safety glasses for everyone in the room, filters in slit lamps, indirect ophthalmoscopes and operating microscopes, a warning sign on the door, and no reflective objects.
- Laser pointer injuries range from focal photoreceptor damage to loss of acuity and central scotoma.
What are the common exam traps in ophthalmic lasers?
- Posterior capsule opacification → Nd:YAG capsulotomy (1064 nm).
- Acute or chronic angle closure with pupillary block → Nd:YAG laser peripheral iridotomy; plateau iris → iridoplasty.
- Proliferative diabetic retinopathy → PRP (argon or frequency-doubled Nd:YAG); centre-involving DME → anti-VEGF first.
- Open-angle glaucoma → ALT (argon) or SLT (Q-switched frequency-doubled Nd:YAG).
- Myopia correction → excimer 193 nm (PRK, LASIK); LASIK flap → femtosecond laser; thin cornea → PRK.
- Keratoconus → cross-linking (riboflavin + UV-A); keratoconus is an absolute contraindication to PRK and LASIK.
- Neovascular age-related macular degeneration → PDT with verteporfin, 689 nm.
- Photocoagulation is thermal; photodisruption (Nd:YAG) is not thermal and can cause bleeding; photoablation (excimer) is cold.
For the diseases these lasers treat, see primary glaucoma, cataract types and surgery, diabetes mellitus and its complications and cornea and keratoplasty.