Reporter: 曾 千芳 Retinal Light Damage Mechanism 1

大綱 Introduction of light hazard to eyes Retinal light damage  Background  Mechanisms of light damage  Acute light exposure /Chronic light exposure( 不討論 ) Laser–Tissue Interaction  Photocoagulation  Photodisruption  Photoablation 2

Introduction of Light Hazards to Eyes 3 Cornea and lens --Cornea : Ultraviolet-B+C ( nm) (photochemical damage) Infrared-B and Infrared-C (1400 nm to 1.0mm) (thermal damage) -- Lens: Ultraviolet -A( nm) (photochemical damage) Retina -- susceptible to photochemical injury from blue light in wavelength nm ( nm for aphakic eye) -- susceptible to thermal injury from optical radiation in wavelength nm

Retina Light Damage-Background 4 Cornea absorption : below 295nm Lens absorption : strongly in UV-B( nm), full UV- A( nm) Cornea/Lens absorption : infrared radiation- at water bands(980,1200,1430nm) Vitreous absoption:1400nm-1mm Visible perception result from a response to visible radiation between 380nm~780nm reaching retina.(visible component)

5 Retina absorption: -- selectivity is achieved by different light absorption characteristics of rods and three types of cones found in human retina. -- absorption maximum wavelength(nm) --absorption increase with decreasing wavelength >> blue light damage Melanin and lipofuscinBroad band absorber Haemoglobin and other protiensAround 400nm Flavins and flavoproteinAround 450nm Macular pigment400~530nm

Mechanisms of Light Damage 6 Three types of tissue damage: 1.Mechanical damage (ionisation damage) 2.Thermal photocoagulation damage 3.Photochemical damage

7 Mechanical (or ionisation) damage : Result from extremely short exposures at high irradiance level which cause shock waves that mechanically disrupt the tissue. Thermal damage : Short-intense exposures(100ms-10s) is trapped or absorbed in a substrate molecule resulting in temperature quoted as 10 ˚ C or more for retinal damage. Such damage tends to occur in the blue/green and green regions of the visible spectrum and is depend on the absorption spectrum of chromophore. Photochemical damage : Take place in normal ambient conditions and involve a reaction between energetic photons and an absorbing molecule. In the presence of oxygen this reaction leads, via a number of intermediary steps, to the production of reactive oxygen species (ROS) including singlet oxygen, superoxide, hydrogen peroxide and hydroxyl radicals.

Acute Light Exposure Studies 8 *Photochemical damage -- the most probable cause of hazards by ophthalmic instrumentation and may be responsible for solar retinitis. -- two types of photochemical damage: (dependent on the action spectra, duration of exposure and irradiance energy) One associated with short exposures operating at the level of the RPE and the other associated with longer, relatively less- intense exposures, at the level of the photoreceptor outer segments.

9 TYPE Ⅰ (Blue light harzard) TYPE Ⅱ Exposure timeShort (up to 12 hr)Long(12-48 hr) Damage locationOriginated in RPE cellsPhotorecepter outer segment Action spectraNot coincide with the absorption spectrum of melanin Corresponds well to the absorption spectrum of the visual pigment ReactionROS(reactive oxygen species) ROS Causing damageAMD(age-related macular degeneration) Cones/Rod photorecepter degeneration

Laser–Tissue Interaction 10 Photocoagulation(thermal damage) -- selective absorption of light energy and conversion of that energy to heat -- subsequent thermally induced structural change in the target -- therapeutic results depend on laser wavelength and laser pulse duration -- A variety of photocoagulating lasers are currently in clinical use: argon, krypton, dye, holmium, and the solid-state gallium arsenide lasers

Laser–Tissue Interaction 11 Photodisruption(machanical damage) -- uses high-peak-power pulsed lasers to ionize the target and rupture the surrounding tissue -- uses laser light as a pair of virtual microsurgical scissors -- reaching through the ocular media to open tissues such as lens capsule, iris, inflammatory membranes, and vitreous strands without damaging surrounding ocular structures -- Currently, the Nd:YAG and Er:YAG lasers are the principal photodisruptive lasers used in clinical ophthalmology

Laser–Tissue Interaction 12 Photoablation(thermal damage) high-powered ultraviolet laser pulses can precisely etch the cornea in the same manner that they etch synthetic polymers high energy of a single photon of 193-nm ultraviolet light exceeds the covalent bond strength of corneal protein high absorption of these laser pulses precisely removes a submicron layer of cornea without opacifying adjacent tissue, owing to the relative absence of thermal injury excimer laser photoablation to clinical use in refractive surgery and corneal therapeutics