23

Lasers in Ophthalmology

James McHugh, FRCOphth, and Edward Pringle, MRCP, FRCOphth

LASER TECHNOLOGY

“Laser” is an acronym for light amplification by stimulated emission of radiation. Most light sources radiate energy in all directions, with waves that are out of phase (incoherent), and with multiple wavelengths. By contrast, laser light has a single wavelength (monochromatic) and waves that are in phase (coherent) with very little tendency to spread out (collimated), so they can illuminate with extremely high power (irradiance). A 1-watt laser produces a retinal irradiance approximately 100 million times greater than a 100-watt light bulb.

Laser light is generated from a “gain” medium such as a transparent crystal rod, a semiconductor diode (solid-state laser), a gas, or a liquid dye (Figure 23– 1). The gain medium is housed in a resonator cavity with a fully reflective mirror at one end and a partially reflective mirror at the other. An optical or electrical source “pumps” energy into the gain medium, raising the energy level of the atoms to a high and unstable level.

Figure 23–1. Laser components.

When a high-energy electron returns to a lower energy level, the excess energy is released as a photon of light (Figure 23–2). If this photon encounters another atom in the nonexcited ground state, it will be absorbed, and an electron of the recipient atom will be promoted to a higher energy level. If the photon encounters another atom that is already in a high-energy state, the photon will

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not be absorbed, but instead will stimulate the release of a second photon. Critically, the new photon will have the same wavelength, phase, and direction as the first photon.

Figure 23–2. Photon absorption resulting in spontaneous or stimulated emission according to the level of electron excitation.

If the gain medium is excited to the point where more atoms are in an excited than a nonexcited (absorbing) state, “population inversion” is said to have occurred. In this unnatural state, photons encountering an atom are more likely to stimulate further photon emission than to be absorbed, resulting in an amplification cascade of exponentially increasing photon release. The presence of mirrors at either end of the resonator cavity, positioned a whole number of wavelengths apart, allows a standing wave of stimulated photon emission in the gain medium between the mirrors. A proportion of photons exits the resonator cavity through the partially reflective mirror, giving an output of laser light.

Pulsed Laser

Laser energy can be emitted continuously or in pulses, which usually have pulse durations of nanoseconds (1 ns = 10–9 s) or less.

Q-switching is a method of pulse generation in which the quality (Q) of the resonator is decreased by closing an optical switch between the mirrors of the resonator cavity, preventing the establishment of a standing wave of stimulated emission. Energy losses are limited to spontaneous emission alone, so that pumped energy accumulates in the gain medium. When the optical switch is opened, the stimulated emission of radiation is able to resume, and the energy stored in the gain medium is released in a giant pulse lasting a few nanoseconds.

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Mode locking pulse generation relies on the ability of many laser devices to support multiple “axial modes,” or slightly different wavelengths of laser light. When the modes are synchronized (locked), constructive interference between their waves results in peaks of very intense amplitude that oscillate within the resonator cavity. Mode locking typically causes extremely brief low-power laser pulses of 1 picosecond or less (1 ps =10–12 s), repeated at several megahertz (MHz). A second gain medium is usually needed to amplify output power while decreasing repetition to manageable rates (hundreds of kHz).

LASER-TISSUE INTERACTIONS

Light of wavelengths of 315 to 1400 nanometers (1 nm = 10–9 m) penetrates into the eye, whereas light of other wavelengths is absorbed by the cornea with some transmission to the lens (Table 23–1). Laser light’s interaction with tissue can be grouped into categories depending on the intensity and duration of interaction (Figure 23–3).

Table 23–1. Ocular Penetration of Different Wavelengths of Light

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Figure 23–3. Categories of laser tissue interaction.

Photochemical

Exposure of ocular tissue to visible or near ultraviolet (UV) light for durations of 10 seconds or more can cause damage via the creation of oxygen free radicals, which are cytotoxic. Toxicity is increased by the use of a topical or systemic photosensitizing agent, which accumulates in the target tissue and produces free radicals when excited by laser. Treatments based on photochemical interaction include corneal crosslinking and retinal photodynamic therapy (PDT).

Photothermal (Vaporization and Coagulation)

Light energy is converted to heat if its wavelength is within the absorption spectrum of the target and if the exposure is longer than a few microseconds. The absorption spectrums of ocular pigments differ (Figure 23–4). Melanin, which is located in retinal pigment epithelium, absorbs across the spectrum including infrared light; hemoglobin absorbs blue, green, and yellow and weakly absorbs red and infrared light; oxyhemoglobin absorbs blue, green, and particularly yellow light; and the macular pigment xanthophyll particularly absorbs blue light. The variation between the absorption spectra has led to “tuning” of lasers to a specific wavelength, eg, yellow to target oxyhemoglobin, but the clinical value is uncertain.

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Figure 23–4. Light absorption by ocular pigments.

A rise of 10–20°C within the retina or choroid will cause photocoagulation (tissue burn). If the temperature reaches 100°C, water vaporizes, causing localized disruption.

The time required for peak heat to be conducted from laser-absorbing tissue to adjacent tissues is known as the thermal relaxation time, typically measured in microseconds for micrometer distances. When laser pulses have a duration that is much shorter than the tissue’s thermal relaxation time, they cause thermal damage to laser-absorbing pigmented cells without any significant rise in the temperature of adjacent nonabsorbing tissue (selective thermolysis). Micropulse diode laser and selective laser trabeculoplasty, using pulsed FD (frequency doubled) YAG laser, use nanosecond pulses to achieve selective photothermal effects.

Photoablation

Photons of shorter wavelength light have higher energy. Short-wavelength lasers, such as the 193-nm argon-fluoride excimer (“excited dimer”) laser, have sufficient energy to break molecular bonds. Biological polymers subjected to excimer laser will degrade to small molecules, while water is explosively evaporated. The duration of photoablative excimer laser pulses is much shorter than the thermal relaxation time of corneal tissue. The superficial cornea is therefore ablated with extreme precision, without any significant thermal collateral damage.

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Photomechanical (Photodisruption/Plasma-Mediated

Ablation)

In practice, laser light rays are slightly divergent, and a beam of laser light has points of greater intensity called transverse modes. The point of greatest intensity is called the fundamental mode. Apertures within the laser cavity can be used to eliminate nonfundamental modes, so that a single point of focus of a few micrometers in diameter can be treated with maximum laser irradiance, while tissues outside the target plane are not affected.

High-energy laser causes photomechanical disruption by means of very large temperature gradients at the point of focus and an intense electrical field that is able to strip electrons from atoms, creating a plasma of ionized atoms and highenergy free electrons (“optical breakdown”). These effects cause a shock wave that expands with supersonic speed and a subsequent microscopic cavitation bubble. The pulse durations of photomechanical lasers are far shorter than the thermal relaxation time of ocular tissues, so there is no significant heat transfer to adjacent tissues.

Photomechanical interactions are the basis of Q-switched Nd:YAG lasers, which are used to perform capsulotomy and iridotomy, and mode-locked femtosecond lasers, which are used for precise computer-controlled cutting of the cornea or lens.

LASER SAFETY

The International Electrotechnical Commission (IEC) laser classification system (Table 23–2) ranks lasers according to their relative risk; almost all ophthalmic lasers are in class 4, indicating high risk. Rules concerning safe use of lasers vary between jurisdictions. Typically, designated laser safety officers are responsible for the safety of laser equipment, procedure for laser use, and staff training. Laser rooms should have clear warning signs, and doors should be locked during treatment. Interlocks may be used, cutting power to the laser when the door is open. Laser output should be directed away from doorways. Windows and reflective surfaces must be covered.

Table 23–2. International Electrotechnical Commission 60825-1 Laser Safety

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Categories

Slitlamp laser delivery systems use inbuilt filters within the microscope to prevent the surgeon from being harmed by reflected laser light. Surgeons using handheld lasers and observers of all types of laser treatment must wear goggles filtering the wavelength in use. Users must check that the goggles have a high optical density (OD) for the wavelength of the laser to be used (Figure 23–5). OD has a logarithmic scale, so that material with OD1 at a given wavelength would transmit 10% of light, whereas OD2 would transmit 1% and OD3 0.1%. Most laser safety goggles have an OD of 7 for the wavelength of their intended laser (ie, transmission of 0.00001% of energy at that wavelength).

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Figure 23–5. Laser safety glasses (A), each are marked with their optical densities for different wavelengths of light (B).

THERAPEUTIC APPLICATIONS OF LASERS

A wide variety of lasers are used in ophthalmology (Table 23–3).

Table 23–3. Lasers Used in Ophthalmology

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CORNEAL REFRACTIVE SURGERY

Corneal Surface Ablation

Excimer laser allows extremely precise ablation of ocular surface tissue without significant damage to adjacent tissue. Photorefractive keratectomy (PRK) uses excimer laser to treat myopia by ablating the central corneal surface so that it becomes flatter (Figure 23–6), or to treat hypermetropia by ablating the periphery so that the central cornea becomes steeper. PRK is uncomfortable for several days until the epithelium has healed and usually produces subepithelial haze lasting several months.

Figure 23–6. Diagram of photorefractive keratectomy (PRK).

Laser epithelial keratomileusis (LASEK) is a development of PRK in which the corneal epithelium is loosened with alcohol and detached and then

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repositioned after excimer laser ablation of the stroma. This causes less pain, less haze, and faster visual recovery than PRK. Epi-LASIK likewise removes the epithelium prior to laser ablation, using a mechanical microkeratome rather than alcohol.

Phototherapeutic keratectomy (PTK) uses excimer ablation of the anterior corneal stroma to treat conditions such as recurrent corneal erosion syndrome, superficial scarring, some corneal dystrophies, nodular and spheroidal degeneration, and band keratopathy.

Laser In Situ Keratomileusis (LASIK)

Laser in situ keratomileusis (LASIK) is the most widely used laser refractive procedure. A flap of anterior corneal stroma is cut with a femtosecond laser or an automated keratome (Figures 23–7 and 23–8). The flap is reflected, and the underlying stroma is treated with excimer laser. The corneal flap is then repositioned.

Figure 23–7. Planning of femtosecond laser in situ keratomileusis (LASIK) flap using the Victus system. (Used with permission from Bausch & Lomb Incorporated.)

Figure 23–8. Diagram of laser in situ keratomileusis (LASIK). Superficial stromal flap has been reflected (right) allowing ablation of underlying stroma.

LASIK can be used to treat a broader range of refractive errors than PRK or LASEK. It is essential that at least 250 μm of corneal stroma is left after ablation to avoid iatrogenic corneal ectasia, and occult (forme fruste) keratoconus is a contraindication to LASIK. Wavefront custom ablation improves the accuracy of treatment, reduces spherical aberration, and may cause fewer night-vision

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problems.

LASIK offers minimal discomfort and very rapid visual recovery, but the use of a corneal flap introduces risks such as epithelial ingrowth, diffuse lamellar keratitis, flap buttonholing, or amputation. Transection of corneal nerves often causes dry eye symptoms after laser refractive surgery and particularly after LASIK.

Small Incision Lenticule Extraction (SMILE)

Femtosecond laser allows extremely precise cutting within the cornea and lens, in which cavitation bubbles separate lamellae without significant thermal or mechanical damage. Small incision lenticule extraction (SMILE) uses femtosecond laser to correct myopia by cutting a convex lenticule within the corneal stroma, which is mechanically removed via a laser-cut incision (Figure 23–9). Early studies have shown that SMILE gives broadly comparable results to LASIK and LASEK, but may be slightly superior in treating higher degrees of myopic astigmatism, with lower spherical aberration postoperatively. SMILE may cause less postoperative dry eye symptoms than excimer refractive procedures.

Figure 23–9. Small incision lenticule extraction (SMILE) surgery. Femtosecond laser is used to cut an intrastromal lenticule, as well as an incision for its removal.

CATARACT SURGERY

Femtosecond Cataract Surgery

Femtosecond laser has been used to cut clear corneal incisions and limbal relaxing incisions (keratotomies); perform capsulorhexis; and fragment the lens nucleus, reducing the required phacoemulsification energy (Figure 23–10).

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Femtosecond laser is safe and highly repeatable, in particular allowing optimization of capsulorhexis size. This may be particularly valuable in pediatric cataract surgery. However, at present, there is no evidence of better visual outcomes when compared with conventional cataract surgery performed by an experienced surgeon. A barrier to widespread use is the high cost of femtosecond laser.

Figure 23–10. Planning femtosecond laser cataract surgery using the Victus system. (A) Main incision and paracentesis. (B) Arcuate keratotomy. (C) Lens disruption patterns. (Used with permission from Bausch & Lomb Incorporated.)

Postoperative Capsulotomy

It is common for the posterior lens capsule to opacify after cataract surgery, due to proliferation and metaplasia of lens epithelial cells. Q-switched Nd:YAG laser is used to cut a posterior capsulotomy in either a cross, circular, or inverted U pattern, clearing the opacified capsule from the visual axis (Figure 23–11). An Abraham or Peyman lens helps focusing on the capsule to minimize the power required. Low laser power, starting at 0.8 mJ and increasing until sufficient to breach the capsule, may help to minimize the risk of retinal detachment and

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postlaser rise of intraocular pressure. Some surgeons advocate routine use of topical antihypertensives (eg, single dose of apraclonidine 1%). Laser applied too anteriorly will pit the lens, and the use of posterior defocus limits this risk. If a circular capsulotomy is cut, any lens pits will be away from the center of the visual axis, but the circular technique may cause a large floater in some patients. The capsulotomy tends to enlarge by 20–30% over the first 3 months after laser due to capsular tension.

Figure 23–11. Posterior capsule opacification showing outline of laser capsulotomy using (A) cross, (B) circle, and (C) inverted U patterns. Red dots show positions of intended laser burns with closer spacing in the sector of denser opacification.

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Anterior capsulotomy is required if phimosis of the anterior capsule encroaches upon the visual axis. Nd:YAG laser is used to make radial incisions in the anterior capsulotomy margin (Figure 23–12), using a capsulotomy lens and anterior defocus to avoid lens pitting. Higher power is generally required for anterior than posterior capsulotomy.

Figure 23–12. YAG laser anterior capsulotomy for capsular phimosis. Red dots show lines of intended laser burns.

Anterior Vitreolysis

Incomplete clearance of vitreous from the anterior chamber during the management of vitreous loss secondary to trauma or cataract surgery may result in pupillary distortion, chronic uveitis, and cystoid macular edema. The vitreous bands can be cut with the Nd:YAG laser, using a capsulotomy contact lens. Topical pilocarpine constricts the pupil, tightening the vitreous strands to allow easier cutting. Multiple low-power burns limit concussion of the cornea and iris.

GLAUCOMA

Iris Laser Treatment

In primary angle-closure glaucoma, contact between the lens and the iris impedes flow of aqueous through the pupil (pupil block). Increased pressure in the posterior chamber results in forward bowing of the peripheral iris (iris bombé) that occludes the trabecular meshwork leading to increased intraocular pressure (see Chapter 11). Laser iridotomy creates a small hole in the peripheral iris to overcome pupil block. In acute angle-closure glaucoma, it is undertaken to treat and prevent recurrence in the affected eye and for prophylactic treatment of

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the fellow eye. It is also undertaken in chronic and subacute primary angleclosure glaucoma and in secondary angle-closure glaucoma due to posterior synechiae.

The usual site for laser iridotomy is within an iris crypt between the 10 and 2 o’clock position, so that the upper lid prevents glare from light passing through the iridotomy (Figure 23–13). Following treatment with pilocarpine to constrict the pupil and apraclonidine to reduce any elevation of intraocular pressure due to the laser treatment, with an Abraham or Wise contact lens, the iridotomy is created with a Q-switched Nd:YAG laser, using a few high-power (2.0–8.0 mJ) burns or a greater number of moderate power (0.8–2.0 mJ) burns. In dark irises, the iridotomy site can be pretreated with argon or FD-YAG laser, using 20–40 low-power (120 mW, 50 ms, 50 μm) burns followed by 20 high-power (700 mW, 100 ms, 50 μm) burns to form a crater, which is easier to breach with the Nd:YAG laser. If the view of the iris is obscured by pigmented debris, the treatment is suspended for a few minutes to let it clear. Successful breach of the iris results in a gush of aqueous and pigmented cells through the iridotomy into the anterior chamber. The iridotomy is enlarged to around 200 μm diameter to ensure its patency. Bleeding from iris vessels is arrested by gentle pressure on the contact lens. The intraocular pressure is checked at least 1 hour later, and any pressure spike is treated with topical and/or systemic treatment. A short course of topical steroid is used to reduce intraocular inflammation.

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Figure 23–13. Patent iridotomy at 2 o’clock position seen by (A) direct illumination and (B) retroillumination, with the edge of the intraocular lens being visible through the iridotomy. A location of 10 to 2 o’clock is generally preferred for iridotomy because the upper lid then prevents glare.

Argon laser peripheral iridoplasty (ALPI) can be used when an acute angleclosure glaucoma does not respond to medical treatment such that laser iridotomy cannot be performed and also for angle closure not due to pupil block, such as in plateau iris. Using an iridotomy lens, a ring of argon or FD-YAG laser burns (1–2 per clock hour, 500 μm diameter, 500 ms duration, power increasing from 200 mW until the iris fibers visibly contract) is applied to the peripheral iris stroma to cause the iris to pull away from the anterior chamber angle, leading to reduction of intraocular pressure and potentially allowing iridotomy. An alternative treatment for acute angle-closure glaucoma unresponsive to medical therapy is surgical peripheral iridectomy (see Chapter 11).

Trabecular Meshwork Laser Treatment

Laser trabeculoplasty can be used to improve trabecular outflow in open-angle glaucoma. It probably attracts macrophages that clear debris from the trabecular

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meshwork and may also cause mechanical opening of Schlemm’s canal and untreated trabecular spaces.

In argon laser trabeculoplasty (ALT), using a gonioscopy lens, approximately 100 burns (50 μm diameter, 100 ms duration) are placed circumferentially at the junction of the pigmented and nonpigmented trabecular meshwork. The desired end point is transient blanching or a tiny bubble. The effectiveness of ALT declines over time, and most patients will return to baseline pressure within 4 years. ALT causes focal scarring of the trabecular meshwork, so that repeat ALT treatments are much less effective, and may also cause peripheral anterior synechiae.

Selective laser trabeculoplasty (SLT) has largely replaced ALT. It uses the Q- switched FD-YAG laser to apply photothermal treatment to pigmented cells within the trabecular meshwork. The 3-nanosecond pulse duration is much shorter than the thermal relaxation time of pigmented trabecular tissue, preventing damage to nonpigmented trabecular cells. The spot size is 400 μm, significantly larger than ALT, and covers the entire trabecular meshwork. Starting at 0.8 mJ, the power is titrated to 0.1 mJ below the level at which “champagne bubbles” are seen. Either 180° or 360° is treated, with 50 or 100 nonoverlapping burns, respectively.

Histopathologic studies have shown less damage to the trabecular meshwork following SLT than following ALT (Figure 23–14), and peripheral anterior synechiae are much rarer following SLT. Intraocular pressure reduction from initial treatment with ALT and SLT has been shown to be equivalent at 12 months. SLT retreatment is much more effective than ALT retreatment.

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Figure 23–14. Electron micrographs of trabecular meshwork following (A) argon laser trabeculoplasty (ALT) and (B) selective laser trabeculoplasty (SLT).

(Used with permission from Lumenis.)

Both SLT and ALT may cause a high pressure spike within 1–2 hours of treatment. Prophylactic topical antihypertensives should be used (eg, apraclonidine 1% immediately before laser), and intraocular pressure should be checked at least an hour after laser. A short course of topical steroids is sometimes used following trabeculoplasty.

Ciliary Body Laser Treatment

Aqueous production can be decreased by photothermal laser treatment to the ciliary body (Figure 23–15). Transscleral cyclophotocoagulation (TCP) (cyclodiode) is performed under sub-Tenon’s, peribulbar, or general anesthesia. An oblique light source reveals the anterior edge of the ciliary body, which does not transilluminate (Figure 23–16). The laser probe is applied over the ciliary body, and 8–10 burns (1.5 W and 1.5 s for diode laser; 7–9 W and 0.7 s for Nd:YAG) are applied per quadrant around the circumference of the ciliary body, avoiding the 3 and 9 o’clock positions to prevent ciliary nerve damage. A “pop”

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sound indicates tissue vaporization requiring reduction of power.

Figure 23–15. Diagram of transscleral (cyclodiode) and endoscopic cyclophotocoagulation (ECP) of the ciliary body to reduce aqueous production.

Figure 23–16. During cyclodiode, the anterior edge of the ciliary body is silhouetted by oblique illumination. (Used with permission from I. Rodrigues.)

TCP requires less intensive postoperative management than trabeculectomy or shunt surgery but may hasten phthisis. Intense inflammation is common, and postoperative steroid drops are essential. TCP greatly diminishes aqueous production and has traditionally been reserved for patients with painful blind eyes due to uncontrollable pressure. However, there is evidence that it can be used safely earlier in the natural history of glaucoma.

Laser endocyclophotocoagulation (ECP) uses an endoscopic diode laser probe to visualize and treat the ciliary processes (Figure 23–15). This can be performed via the pars plana during vitrectomy or via corneal incisions at the time of cataract surgery. Laser treatment is applied to each ciliary process for approximately 2 seconds, with power titrated to produce visible blanching and shrinkage (300–900 mW) (Figure 23–17). ECP causes more localized coagulation of the ciliary epithelium than TCP, requiring much less laser energy and with lower risks of hypotony, choroidal effusion, and loss of vision.

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Figure 23–17. Endoscopic view of the ciliary processes during endoscopic cyclophotocoagulation (ECP). The laser aiming spot is red. The treated ciliary processes are white. (Used with permission from I. Rodrigues and S. Goyal.)

Laser Suture Lysis

Following trabeculectomy, aqueous drainage into the conjunctival bleb can be increased by the use of laser to break the sutures securing the scleral flap. A Hoskins or Blumenthal suture lysis lens is used to compress the overlying conjunctiva and Tenon’s capsule, and argon or FD-YAG laser pulses cut the sutures.

RETINAL LASER TREATMENT

Pan-Retinal Photocoagulation (Scatter Laser)

Retinal ischemia may lead to retinal or iris neovascularization. The most common causes are diabetic retinopathy and retinal vein occlusion. Other causes are sickle cell disease, ocular ischemic syndrome, uveitis, Coats’ disease, and retinopathy of prematurity. The definitive treatment for retinal and iris neovascularization is laser pan-retinal photocoagulation (PRP), which decreases the oxygen demand of the peripheral retina and hence the production of vascular endothelial growth factor (VEGF).

PRP may be achieved with argon laser or now, more commonly, with FDYAG pattern scanning laser. Pattern lasers use briefer laser pulses of higher power. They are less uncomfortable and deliver multiple spots at each activation so that treatment can be performed more quickly. A wide angle contact lens is

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used to treat the entire retina, apart from the macula and around the disk (Figure 23–18). Burns of 200–500 μm in diameter are placed 0.5–1 burn widths apart. Shot duration is typically 100 milliseconds with a conventional argon or YAG laser or 20 milliseconds with a pattern laser. Power is adjusted to produce a gently blanching burn. It needs to be readjusted during treatment as more peripheral retina requires lower power. Most patients find the procedure uncomfortable but tolerable; peribulbar or sub-Tenon’s anesthesia is occasionally required.

Figure 23–18. Pan-retinal photocoagulation. A: Diagram showing planned distribution of laser burns separated by 0.5–1 burn diameter, avoiding main blood vessels. B: Wide angle fundus photograph showing retinal scars.

PRP is usually split into sessions of approximately 1000–1500 burns 1–2 weeks apart to reduce the incidence of uveitis, macular edema, and exudative retinal detachment. The inferior retina is often treated first as any subsequent vitreous hemorrhage is more likely to obscure this area. At least 2000 and sometimes 6000 or more burns are required to cause regression of new vessels. If retinal ischemia is localized, as in branch retinal vein occlusion, sectoral PRP may be performed, treating only areas shown to be ischemic on fluorescein angiography. If there is significant preexisting macular edema, macular laser

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treatment is performed before or at the same time as PRP to avoid exacerbating it.

If there is vitreous hemorrhage dense enough to prevent slitlamp PRP, PRP may be performed during vitrectomy surgery using diode laser delivered via a fiberoptic intraocular probe (endolaser). PRP can also be delivered by diode laser using an indirect ophthalmoscope via the pupil (eg, when treating retinopathy of prematurity) or by transscleral diode laser.

In diabetic patients, PRP should be performed urgently if there is high-risk proliferative diabetic retinopathy (see Chapter 10). PRP may also be performed in patients with low-risk proliferative retinopathy, as it reduces the risk of progression to high-risk retinopathy by 50%. Patients with severe nonproliferative diabetic retinopathy are sometimes treated, for example, in an only eye where the first eye was lost to proliferative disease, if patients are difficult to examine, or if patients are at high risk of missing follow-up appointments.

Ischemic central retinal vein occlusion (CRVO) carries a high risk of iris rubeosis, and patients are reviewed frequently to check for its development so that urgent PRP can be performed before rubeotic glaucoma occurs. Intravitreal anti-VEGF agents may be used as an adjunct to laser treatment. Proliferative retinopathy occurs less frequently than rubeosis in CRVO and also is treated with urgent PRP. Areas of retinal hemorrhage should be avoided during PRP unless with a diode laser, which is minimally absorbed by blood. Patients with extensive ischemia sometimes undergo prophylactic PRP in the absence of neovascularization if regular follow-up by an ophthalmologist is not possible.

Laser for Macular Edema

Retinal laser is the standard treatment for macular edema due to retinal vascular disease. Macular laser requires lower power than PRP and is thought to decrease edema by photostimulatory effects on the retinal pigment epithelium. In recent years, it has been supplemented and to some extent replaced by intravitreal corticosteroid and anti-VEGF injections. However, it still has an important role, providing long-lasting treatment at relatively low cost.

Burns of 50–100 μm in diameter are applied as a grid, spaced 1–2 burn widths apart across the superior, temporal, and inferior macula (full grid treatment), or in a modified grid limited to areas of retinal thickening, with additional direct burns to leaking microaneurysms (Figure 23–19). No laser is used within 500

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μm of the center of the fovea (foveal avascular zone). The papillomacular bundle, between the disk and fovea, should be treated with caution. Power is titrated to cause faint blanching of the retina close to the vascular arcades or to be just below the threshold for blanching (subthreshold treatment).

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Figure 23–19. Macular grid laser for diabetic macular edema. A: Fundus color photograph showing intended distribution of laser burns for full grid. Modified grid: B: Optical coherence tomography (OCT) scan showing retinal thickening inferotemporal and superior to the fovea. C: Fluorescein angiogram showing corresponding areas of fluorescein leakage. D: Fundus color photograph showing intended distribution of laser burns.

Diode micropulse laser uses either 810-nm (infrared) or 577-nm (yellow) light, delivered in very brief bursts—100 micropulses each 0.1 millisecond in duration during an “on” period of 200 milliseconds, giving a 5% duty cycle. The micropulses are shorter than the thermal relaxation time of retinal tissue, minimizing heat buildup, and the bursts are separated by longer (1800 ms) “off” periods to allow heat to dissipate. Yellow laser has the advantage of low absorption by xanthophyll, minimizing collateral heat damage to the macula. Unlike continuous wave macular laser treatments, micropulse retinal laser is applied in a confluent grid to thickened areas of macula, with power adjusted to half the power needed to cause visible blanching. The lack of a visible end point can make administration difficult, but confluent diode laser is slightly more effective in decreasing edema than conventional macular grid laser, with a lower risk of retinal scarring.

In diabetic macular edema, the Early Treatment Diabetic Retinopathy Study (ETDRS) criteria are used to identify patients likely to benefit from macular laser treatment (see Chapter 10). If edema is limited to the fovea, intravitreal steroid or anti-VEGF injections may be more appropriate.

Retinal laser is ineffective for macular edema due to central retinal vein occlusion. However, modified grid laser is effective for macular edema due to

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branch retinal vein occlusion and should be considered if acuity is 20/40 or worse and the edema has persisted for 3 months after the onset of symptoms. Macular laser is avoided in patients with extensive macular hemorrhage due to retinal vein occlusion, because hemorrhage limits laser effectiveness and increases the risk of retinal burns. Intravitreal anti-VEGF injections are better.

Laser for Retinal Tear (Retinopexy)

Peripheral retinal tears, usually due to vitreous traction during posterior vitreous detachment, can lead to retinal detachment. If retinal tears are detected prior to the accumulation of subretinal fluid, they can be encircled by retinal laser to create an adhesion of the adjacent neural retina to the pigment epithelium. Using either pattern or single-shot FD-YAG or argon laser and a wide-angle or threemirror contact lens, two or three rings of confluent 200–500 μm 100-millisecond burns are placed around the tear. The power is titrated to achieve moderate blanching. Tears in the extreme retinal periphery cannot be treated with a slitlamp laser but can be treated with indirect laser with scleral indentation (Figure 23–20).

Figure 23–20. Indirect retinal diode laser. (Used with permission from G. Bowler.)

Retinopexy is indicated for almost all U-shaped retinal tears and for any retinal breach with a small associated area of subretinal fluid. It is not indicated for fully operculated round holes, except for round holes in lattice in high-risk situations, such as recent symptoms of posterior vitreous detachment or history of retinal detachment in the fellow eye.

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Laser for Choroidal Neovascular Membrane

Choroidal neovascular membrane (CNVM) growing through a break in Bruch’s membrane is most commonly found in wet macular degeneration, but also occurs in high myopia, traumatic choroidal rupture, and presumed ocular histoplasmosis syndrome (POHS). Photodynamic therapy (PDT) was the treatment of choice for CNVM prior to intravitreal anti-VEGF injections. Verteporfin, a photosensitizing agent, is administered intravenously. The lesion is treated with 689-nm laser (spot size 1000 μm greater than the diameter of the lesion, for 83 seconds). PDT is still useful in patients who are allergic to anti-VEGF treatments and in polypoidal choroidal vasculopathy (PCV). CNVM can also be treated with confluent, strongly blanching argon or FD-YAG laser burns covering the entire lesion. This causes an immediate positive scotoma. Laser is also an option for extra-foveal CNVM threatening vision if repeated anti-VEGF injections are not possible.

Laser for Retinal Macroaneurysm

There is a high rate of spontaneous resolution of retinal artery macroaneurysm, especially following hemorrhage. However, if exudation from the macroaneurysm threatens or involves the central macula, laser treatment may be indicated. A single ring of confluent, lightly blanching, 200-μm diameter FDYAG or argon laser burns is applied around the aneurysm. Laser may also be applied directly to blanch the aneurysm, but this carries a small risk of occluding the distal arteriole.

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