Laser Principles in Ophthalmology

Function

Mechanisms of Lasers

The mechanisms of lasers can be separated into five types based on the power density of the laser and its exposure time with the tissue.[1] The categories are (in ascending order of power density) as follows:

• Photochemical interaction
• Thermal interaction
• Photoablation
• Plasma-induced ablation
• Photodisruption

Photochemical interaction, otherwise known as photoactivation, is the mechanism of using long exposure times at lower power density to chemically catalyze a photosensitive dye (photocatalysis).[5] The long exposure time allows for the absorption of a large number of photons, and the low power density guarantees an insignificant rise in temperature. By utilizing a laser at the specific wavelength of the dye, a photochemical reaction occurs only in the tissue where the dye is located. 

Thermal interactions usually use shorter exposure times and higher power density levels than photochemical interactions. This allows the laser to be absorbed by tissue pigment, which causes the tissue to become hot enough to coagulate and denature.[6][7] 

The amount of heating depends on the properties of the laser and the tissue targeted, but usually, an increase of 10 to 20 degrees C is enough to cause coagulation, with the effect predominately occurring at the 60-70 C mark.[8]

Photoablation, also known as photodecomposition, allows for the splitting of molecules by breaking their covalent chemical bonds without heat transfer to nearby tissue (ultraviolet dissociation).[9] This is accomplished by using excimer lasers in the UV range that deliver around 6 electron voltage in short pulses that are a few nanoseconds in duration (short duration to avoid heat and enough energy).[10] The name 'excimer' laser is short for "excited dimer" laser as they are comprised of two gases, such as the argon fluoride excimer laser, which emits a laser of 193 nm wavelength.[10]

Plasma-induced ablation, like photoablation, breaks down molecules but also leads to plasma formation through a process called cascade ionization. Essentially, it ionizes atoms by stripping them of their electrons and speeds electrons up (optical breakdown) so that they can collide with other atoms and ionize those atoms as well (plasma ionization).[11] This is possible with infrared femtosecond lasers that are 1053 nm in wavelength by using power density up to 10^13 W/cm^2 range while using exposure times in the pico to femtosecond range (higher power density than photoablation and shorter pulse duration). The plasma formation, in turn, allows for precise and clean tissue removal with minimal thermal damage to surrounding tissue and inflammation. Unlike photoablation, UV lasers are not needed to transfer energy to transparent tissues like the cornea as the plasma created can absorb non-UV laser photons. However, there are some side effects, such as cavitation bubbles and the creation of a shock wave.[11] However, these are ultimately insignificant to the overall effect of the laser on the tissue, unlike in photodisruption, where these side effects are the main method of breaking down tissue. 

Photodisruption uses even higher energy power densities than plasma-induced ablation using pulses that are a few nanoseconds long.[12] As such, it can also induce "optical breakdown," resulting in plasma formation. The plasma then expands, producing a focal mechanical shockwave at the site, which is used to cut ocular tissue, unlike plasma-induced ablation which relies on laser light to break down tissue.[12] 

When the pulse duration is reduced, the power (energy per second) increases for a given energy. Thus, ultrashort pulses (femtosecond laser) can generate higher peak power with lower energy when compared to nanosecond pulses.

Issues of Concern

Complications

The most likely cause of complications from lasers is accidental exposure to reflected beams.[13] Lasers will be reflected by any equipment or item that is reflective in nature. For instance, many items in the surgical field are metal and will reflect the laser. Even the cornea and contact lenses are reflective. Additionally, many lasers operate in the infrared and ultraviolet ranges and are not perceptible to the human eye. Findings on examination of people with laser damage include corneal scarring, retinal hemorrhage, or in severe cases, corneal perforation.[14]

Outside of accidental exposure to reflective beams, complications of laser damage can depend on the procedure used and the type of laser used. For example, adverse effects of PRP (pan-retinal photocoagulation) include macular edema, Bruch membrane ruptures, subretinal or vitreous hemorrhage, retinal or choroidal scarring, secondary angle-closure glaucoma due to choroidal and ciliary effusion, and exudative retinal detachments.[15][16] However, lasers used in LASIK (Laser-assisted in situ keratomileuses) would have adverse effects such as flap complication, post-LASIK corneal ectasia, or corneal scarring.[17] 

Finally, another common source of laser injuries is those that pertain to laser pointers. There has been a significant increase in laser pointer-related injuries. It has been hypothesized that due to the increase in accessibility of laser pointers, even high-end lasers, over the internet. Patients have been shown to have symptoms ranging from focal photoreceptor defects to loss of visual acuity and central scotoma.[18] 

If the patient has symptoms or findings suspicious of accidental laser exposure or procedural complications, a thorough dilated ophthalmoscopy exam should be done to visualize and characterize the damage done to the eye. It may be necessary to use imaging such as OCT (optical coherence tomography), fluorescein angiography, or fundus autofluorescence to document the injury.[19][20][21] In general, there can be irreversible damage to the eye if lasers are not handled properly.

Hazard Risk

As outlined in the ANSI Z136.1, lasers can be grouped into classes based on their hazard risk. This is based on if the laser's maximum permissible emission (MPE), which determines how long an unprotected eye can be exposed to a laser before damage occurs, is shorter or longer than the human aversion response.[22]

The aversion response is the autonomic blinking and moving away of the eye from the stimulus. Lasers are placed into four classes, with the lower classes generally do not need protective eyewear because the eye can avert from the light before damage occurs, but higher classes, such as 3 and 4, require eye protection at all times.

There are several classes for lasers.[22] They are as follows:

• Class 1: safe under all conceivable uses, 1M: safe but potentially hazardous with magnification
  • CD-ROM players, diagnostic lasers such as those used in OCT
• Class 2: laser must emit a visible beam and are safe if viewed for less than 0.25 seconds, 2M: same as for Class 2 but not safe with optical viewing aids
  • Supermarket scanners
• Class 3R: potentially hazardous under direct and reflective beams but does not pose either a fire hazard or diffuse-reflection hazard, 3B: hazardous under direct and reflective beams and not normally a fire hazard
  • Laser pointers
• Class 4: laser systems whose output exceeds class 3 and are either a fire or skin hazard
  • Medical and research lasers include the double frequency Nd: YAG laser 532 nm.

Clinical Significance

Each of the five mechanisms of lasers has clear and separate clinical applications.

Thermal lasers are the most commonly used in ophthalmology, and photocoagulation is used to treat a wide array of pathology, including retinal detachments, laser barricade of retinal tears, treatment of retinopathy of prematurity and proliferative diabetic retinopathy, laser trabeculoplasty in glaucoma, focal laser in diabetic macular edema and central serous chorioretinopathy, and transpupillary thermotherapy for choroidal melanoma.[23][24][25] 

The laser's wavelength is important as absorption is heavily dependent on it. Common targets in the retina, such as hemoglobin and melanin, generally absorb wavelengths of light between 400 to 700 nm.[26][27] Additionally, lasers above 500 nm are preferred in the macula as yellow xanthophyll pigment in the macula absorbs light at wavelengths between 450 to 500 nm, and the macular damage can be avoided.[28] 

It is no surprise that common lasers used are frequency-doubled yttrium-aluminum-garnet (Nd: YAG) green laser at 532 nm, green argon lasers at 514 nm, Krypton red lasers at 647 nm, and yellow semiconductor at 577 nm and diode laser at 810 nm with the two most common being frequency-doubled Nd: YAG and argon.[1] 

Additionally, there is the PASCAL (PAtterned SCAnning Laser), which uses the frequency-doubled Nd: YAG laser that incorporates patterned, short, multiple shots at a very short duration which results in reduced discomfort and collateral retinal damage during procedures such as pan-retinal photocoagulation (PRP).[29] 

Thermal laser (photocoagulation) is also used in laser thermal keratoplasty [LTK, holmium: yttrium aluminum garnet (YAG), 2060 nm (contact LTK) or 2130 nm (noncontact LTK)], which causes corneal shrinkage to treat hyperopia.[30]

Photodisruption is another widely used laser mechanism in ophthalmology, being only second to thermal lasers. It commonly uses the Nd: YAG laser at 1064  nm in posterior capsulotomy, anterior laser vitreolysis, and peripheral laser iridotomy.[31][32][33]

Photochemical interaction is the basis of a number of techniques in ophthalmology, such as photodynamic therapy (PDT) in central serous chorioretinopathy or age-related macular degeneration (AMD), corneal collagen cross-linking in keratoconus, and prevention of graft rejection in corneal grafts by reducing corneal neovascularization.[34][5][35][36] 

In AMD, by injecting verteporfin into the patient's circulation, the blood vessels can be thrombosed by irradiating the blood vessels with the low-energy red laser at 689 nm.[37] In the case of keratoconus, riboflavin is activated using ultraviolet-A (UV-A, 365 to 370 nm) light, and in corneal neovascularization, a green argon laser is used.[38][39] For corneal photodynamic therapy for infective keratitis or corneal cross-linking with rose Bengal, a green laser (532 or 518, or 525 nm) is used.[40][41]

Photoablation is used in refractive surgeries, such as laser in situ keratomileuses (LASIK) and photorefractive keratectomy (excimer 193 nm), eliminating injury or dystrophy caused by superficial corneal opacities and in phototherapeutic keratectomy which is used in the treatment of recurrent corneal erosions as the cornea absorbs UV light under 315nm.[42][43][44]

Plasma-induced ablation has been used for refractive surgery, such as the formation of LASIK stromal flaps (femtosecond laser 1053 nm). Further research is being done into its use in other laser procedures that use photoablation.[45]

Diagnostic Use of Laser In Ophthalmology

Although lasers are used in various therapeutic techniques, they are also an integral part of diagnostic imaging in ophthalmology. For instance, scanning laser ophthalmoscopy (SLO) uses confocal laser scanning microscopy for diagnostic imaging of the retina or cornea. It uses lasers to rapidly scan a sample whose reflected light is imaged through a pinhole that suppresses reflections outside those in the focal plane.[46]

This process allows a sample to be scanned at multiple levels, which then is reconstructed in 3-dimensional images using computer software. This technique is used to track glaucoma-induced changes in the optic nerve head, along with imaging structures of the eye such as the cornea, lens, and macula. Additionally, there is optical coherence tomography (OCT) which creates high-resolution images of parts of the eye such as the retina or the anterior segment.[47] 

It typically uses either superluminescent diodes or ultrashort pulsed lasers to scan the eye and creates the image using the principles of low-coherence interferometry. OCTs are used to manage many pathologies such as glaucoma, AMD, and diabetic macular edema (DME). Finally, there is OCT angiography (OCT-A) which uses OCT to image vasculature in the eye by using the reflection of light off of moving red blood cells.[48] 

Common wavelengths for diagnostic lasers are 850-1050 nm laser in OCT, 840 nm or 1050 nm laser in OCT-A, and SLO uses a combination of lasers at 532 nm, 658 nm, and 840 nm.[49][50][51] Common laser wavelengths used in ophthalmic diagnostic machines include Optovue (RTVue) OCT (superluminescent diode, 840 nm), Cirrus OCT (superluminescent diode, 840 nm), Spectralis OCT (broadband superluminescent diode, central wavelength 880 nm, 40 nm spectral bandwidth), swept-source OCT (DRI-OCT-1, Topcon, tunable laser, median wavelength 1050 nm), IOL Master 700 (1055 nm, swept-source OCT), IOL Master 500 (partial coherence interferometry, 780 nm), and Lenstar LS-900 (optical low coherence reflectometry, superluminescent diode, 820 nm).[52][53][54]

Other Issues

Methods of Delivery

Using lasers at a slit lamp is the most commonly used method to deliver treatment since changing and controlling parameters such as power, spot size, and exposure time is easy. Another commonly used method is using an indirect ophthalmoscope via a fiber optic cable. It is commonly used to treat peripheral retinal break and retinopathy of prematurity. Finally, there is the endolaser, a small fiber-optic probe inserted into the eye for vitreoretinal surgical procedures.[55]

For focal laser of the macular lesions or leaks, the lenses used usually have a laser spot magnification factor of around 1 (including the Volk Area Centralis lens). Thus, the laser spot size at the retina is approximately 50 microns when a spot size of 50 microns is set on the laser machine. The lenses used for PRP usually have a laser spot magnification factor of 2 (including Volk Super Quad 160 and Ocular Mainster PRP 165). Thus, a laser spot set at the machine at 250 microns would yield a retinal spot of around 500 microns. Other common lenses used in Ophthalmic lasers include:

• Abraham lens: Different lenses are available for laser (YAG) capsulotomy and laser peripheral iridotomy.
• Singh Mid-Vitreous for treatment of symptomatic vitreous floaters, and
• Blumenthal lens: Different lenses are available for suture lysis (usually after trabeculectomy) and laser peripheral iridotomy).

Personnel

The administration and usage of lasers require an advanced understanding of their use and safety risks. Only trained and qualified physicians should use the laser device. Additional help to improve patient comfort may require trained assistants who can help facilitate safe and effective treatment.

Preparation

Before any laser procedure, informed consent is an essential part. This includes discussing benefits, risks, and alternatives before the procedure starts. The laser device should be turned on and set to the appropriate settings for the procedure. If there are any issues with the laser, maintenance and calibrations should only be performed by a trained technician before the laser device can be operated again.[55]

Safety

Instructions and safety precautions should be reviewed when using a laser as outlined in the manual, and equipment should be inspected. A warning sign should be placed on the door to the treatment team to inform people outside that laser therapy is ongoing and should not be entered. Inside the room, any mirror or reflective hazards should be removed or moved as they could reflect the laser and cause accidental laser exposure previously discussed.

Physicians performing laser therapy should have safety filters in their devices to protect them from back-scattered light. These filters should be implemented in all slit lamps and indirect ophthalmoscopes for lasers. Operating microscopes should have a separate eye filter implemented in each viewing path of the microscope. To enable long-term viewing of the laser, these filters need to have an optical density specific to the laser wavelength used. Any personnel or bystanders must wear laser safety glasses designed to filter the specific power and wavelength of the laser.

Enhancing Healthcare Team Outcomes

Lasers are essential to ophthalmology treatment and diagnostics, but if used improperly can cause unnecessary injury. Understanding the use and mechanism of lasers is crucial for all team members associated with care. Lasers are best managed by an interprofessional team that includes an ophthalmologist trained in the specific use of that laser, nurses, and technicians, all contributing from their areas of expertise and engaging in open communication among team members, so that patient care is the top priority. Proper setup and safety precautions are important to ensuring patient and staff safety while maximizing outcomes and comfort.