We provide laser therapy to the needy patients. LASER is acronym of Light Amplification by Stimulated Emission of Radiation and it has made a remarkable impact on all branches of medical sciences but it has become a new dimension in the field of ophthalmology because of transparent nature of ocular tissues. Besides, being an important treatment modality for various ocular pathologies, it is now used in more sophisticated diagnostic studies like Scanning Laser Ophthalmoscopy, Optical Coherence Tomography and Wavefront analysis which promise to significantly enhance our understanding and treatment of many disease processes in the eye.
EVOLUTION OF OPHTHALMOLOGIC LASER THERAPY:
The concept of laser photocoagulation was derived from macular burn caused by looking at sun during solar eclipse. The process of Laser generation was first predicted by Albert Einstein in 1917. However, Meyer Schwikererath in 1946 first got the idea of using light as photocoagulation source for treating ocular disorder which benefited from direct heat treatment like tumor, retinal hole etc. During 1946 – 1949, he designed a system comprising lenses which focused the sun rays on the area of retina which needed thermal treatment. His treatment was carried on the open roof only on the bright sunny afternoon and had to be postponed often due to vagaries of weather. To get rid of this problem he switched over to another powerful light source Xenon Arc Lamp, which was used by many ophthalmologist, till the beginning of practice of laser in ophthalmology. In 1961 first laser system used in ophthalmology was pulsed Ruby laser. In 1962 Infra Red diode laser was developed. In 1968, L’Esperance developed the Argon laser which was superior to Ruby laser. 1981, tunable Dye laser was introduced.
LASERS are generated from following sources
- GAS LASER
- SEMICONDUCTOR DIODE LASER
- DYE LASER
In Gas Laser, photons having energy equal to energy difference between electron orbit passes in vicinity of high energy electron of inert gases. Photon stimulate the electron to drop into lower orbit and release of Laser photon coherent to stimulating photon, which is amplified by the two semitransparent reflecting mirror present at both end of gas cylinder.
In Semiconductor Diode Laser Gallium and Arsenic crystals that have been doped oppositely (n-type & p-type) are joined to create recombination region at p-n junction. At external electrical potential is applied across p-n junction and Laser photon are emitted as excess electron in conduction band. The Gallium and Arsenic crystal acts as semitransparent mirror and light are internally reflected and stimulate additional Laser photon emission in recombination region.
In Dye Laser, energy is produced by interaction of jet of liquid organic dye and high power Argon laser beam. The liquid dye is squirted across the path of Laser beam at a pressure of 40-50psi.The output beam is approx. 25-30% of power of pumping laser beam and the wavelength can be tuned very accurately with rotating birefringent mirror at micrometer setting that allow only photon of certain wavelength to resonate within Laser cavity.
The Laser color depends upon wavelength of Laser light and it emits light either continuously or in pulses.
In pulsed Laser, modest amount of energy concentrated into very brief period, so each pulse has a relatively high power. Nd-YAG Laser is an example of pulsed Laser.
Continuous Laser modality delivers more overall energy to the target tissue over relatively long time, thus power is lower. Most ophthalmic Lasers operate continuously with a shutter to control exposure time. Argon, Krypton, Diode, & Dye Laser are example of this.
Laser is delivered to the target tissue through fiber-optic cable or mirror. Slit lamp bio-microscopy is the most common method which delivers the Laser transcorneally. Other methods include endolaser and exolaser probes, in which treatment is delivering by fiber optics probes used within the eye through pars plana or trans-sclerally respectively.
Theory of Laser effects:
Tissue effect produced by light may be classified as mechanical, thermal or photochemical. These effects are determined by irradiance, wavelength of irradiating light, duration of exposure, & absorption of target tissue.Laser exerts its effect by one of the following mechanisms
- PHOTOCOAGULATION: In this process Laser energy is absorbed by various pigments like Hemoglobin, Xanthophylls, or Melanin and is converted into thermal energy which raises the temperature of target tissue high enough to coagulate and denature the cellular element. The effect of Argon green, Krypton red, & Diode Laser are based on this mechanism. Choice of optimal wavelength depends upon absorption spectrum of the target tissue. Retinal lesion size is dependent on laser power, spot size, exposure time, and scattering of light by contact lens. Scattering of light depends upon wavelength; smaller wavelength scatters more. Trans-corneal photocoagulation is usually accomplished by the help of contact lens system like Goldmann lens which delivers the light in posterior segment.
- PHOTOABLATION: In it, Laser energy breaks the chemical bond of biologic tissue converting them into small molecules that diffuses away. This effect is produced by Excimer Laser. Most common Excimer Laser used is ArF producing wavelength of 193nm. Others are KrF (248), XeCl (308). This effect is used to treat corneal pathologies & Keratorefractive surgery like LASIK.
- PHOTODISRUPTION: In this process Laser light creates extremely high electromagnetic field which strips electrons from the nuclei producing a state of PLASMA. In this state chemical nature of material destroyed and molecules are broken off into random mixture of electron and proton which is called as optical breakdown. Nd-YAG Laser works on this principle and used for capsulotomy in posterior capsular opacification after ECCE and peripheral iridectomy in Angle closure Glaucoma.
- PHOTO CHEMICAL REACTION: Photo Dynamic therapy is based on this principle. These reactions occur with low to moderate irradiance below photocoagulation threshold and with short wavelength particularly visible light. Absorption of photon by outer electron produce excited molecular state which can drive a chemical reaction. Because energy per photon is inversely proportional to its wavelength hence, short wavelengths produces high energy and induce photochemical changes. Long wavelength visible light can induce photochemical changes when tissues are sensitized by exogenous photosensitisers.
USE OF LASER IN RETINAL DISORDERS
1. Scanning Laser Ophthalmoscopy (SLO): In it a narrow laser beam of single wavelength illuminates the retina one spot at a time and amount of reflected light at each point is measured which depends upon reflective, refractive and absorptive property of tissue. The advantage of SLO over conventional Ophthalmoscopy is better resolution of image in case of media opacity (Vitreous Hemorrhage, Cataract) and the patient is not affected by bright illumination of conventional Ophthalmoscopy which allow better high resolution real time motion image of macula without patient discomfort. SLO Angiography can be performed after intravenous Fluorescein or Indocyanine green dye to study choroid and retinal blood flow.
2. Optical Coherence Tomography (OCT): Laser light in near infra red spectrum (810nm) is used to produce high resolution cross sectional image of retina using coherence inferometry. In coherence inferometry partial reflective mirror is used to split a single laser beam into measuring and reference beam. Measuring beam pass from neurosensory retina upto choriocapillaris. At each optical interface some of light reflected back to OCT photodetector. Reference beam is reflected off the reference mirror at a known distance of beam splitter back to photo detector. Reference mirror can be adjusted to make the path traversed by reference and measuring beam equal, and creates wave pattern in constructive interference. However some of the lights through retinal surface are reflected off deeper layer in the retina. This light will have traversed a longer distance than reference beam and create some degree of destructive interference when the two beams come at photodetector. The amount of destructive interference measured by OCT and translated into measurement of retinal depth and graphically displayed as retinal cross-section.
Retinal disorders need only photocoagulation effect of laser because laser treatment is needed for prevention and treatment of neovascularization, leakage, edema or sealing of retinal breaks. Laser therapy usually performed to stabilise the existing vision rather to improve the vision.
Before considering for laser photocoagulation following evaluation must be done.
- Visual acuity
- Slit lamp biomicroscopy for assessment of ocular media and anterior segments disorders(Rubeosis iridis, Neovascularization of angle)
- Intraocular pressure measurement
- Gonioscopic evaluation of angles
- Indirect ophthalmoscopy
- Fundus Fluorescein angiography(FFA)
- Careful explanation about the procedure to the patient because patient expectations towards the laser treatment is tremendous.
Indications of laser Photocoagulation in Retinal diseases:
1. Diabetic Retinopathy (DR):
The treatment of choice for diabetic retinopathy is laser photocoagulation besides, the control of blood sugar level.
- Proliferative DR with presence of new vessels at the disc, raised peripheral new vessel, pre retinal and vitreous hemorrhage needs pan retinal photocoagulation.
- Clinically Significant Macular Edema
- Pre ischemic obstruction or Advanced Non Proliferative DR are advised pan retinal photocoagulation.
Pan Retinal Photocoagulation (PRP):
Pan Retinal Photocoagulation is applied to the retina from nasal side of disc towards outside or beyond the equator so that the available retinal blood improves nourishment of the central retina. Aims of treatment are to protect foveal vision at the cost of peripheral retinal. In PRP about 2000 to 3000 spots of 200µ to 500µ size set at 0.1 to 0.2 second duration and 250 – 600 mW power are beamed. Smaller spots are used for central retinal and larger for periphery. Treatment is usually carried out in 3 – 4 sessions.
Advanced Diabetic Retinopathy with partial tractional detachment:
In the presence of traction detachment and if macula is still attached the latter is first protected with heavy continuous row of small 100 – 200 µ laser spots. After 4 to 6 weeks, when this treatment has firmly healed proceed with further photocoagulation of retinal periphery.
It shows clear benefits from laser photocoagulation if it is done before severe visual loss. Gross ischemic changes with disruption of perifoveal capillaries shows little benefit. In Diabetic Maculopathy grid or focal laser treatment should be done on the basis of FFA.
In Focal maculopathy area of leak usually lateral to macula as shown by FFA. This is treated with small number of 100 µ -200µ spots outside the leak.
Ischemic maculopathy is usually associated with Proliferative retinopathy and is treated with PRP but ischemic maculopathy does not respond to Laser treatment.
In Diffuse maculopathy Grid photocoagulation is usually performed. It does not improve visual acuity rather prevent further visual loss.
In diffuse capillary leakage causing Cystoid macular edema or diffuse exudative maculopathy, all the leaks are coagulated.
If the patient has minimal PDR with macular edema, macular edema should be treated first and stabilized. PRP should be done later if Proliferative disease progresses.
If the patient has moderate to advance PDR with diffuse macular edema one of the two procedures can be followed. Proliferative phase can be treated first with PRP in two or three session followed by treatment of macular edema 3-4 months after the Proliferative disease stabilized. Alternatively both PRP and modified Grid treatment for macular edema can be done in selected patient
In patient with macular edema with cataract, macular edema should be treated before cataract extraction.
2. Central Retinal Venous Obstruction (CRVO):
CRVO has two major complications; Neovascularization & macular edema. In partial CRVO neovascularization is rare complication while in complete CRVO anterior segment neovascularization and neovascular Glaucoma is significant. If an extensive area of capillary non perfusion is suggested by FFA, the eye may have significant risk of developing neovascular complication. If a definite sign of anterior segment neovascularization is present, PRP is performed.
Macular Edema is another significant complication associated with poor visual prognosis. In non ischemic CRVO increased retinal venous pressure is transmitted to perifoveal capillaries resulting in macular edema seen in FFA. Para-macular grid laser treatment may produce remission in it and improvement in central visual acuity.
In Ischemic CRVO macular edema is the more serious problem and patient is less likely to be benefited from paramacular grid treatment.
3. Branch Retinal Venous Obstruction (BRVO):
When central vision is affected in BRVO due to hemorrhage edema or ischemia and vision is 6/60 or less, laser treatment should be started immediately. However, in few cases spontaneous improvement occurs but late treatment may be started after gross swelling and hemorrhage subside. FFA carried out at 3 to 4 weeks when the hemorrhage partly clears and if area of non perfusion is revealed grid pattern laser photocoagulation should be applied.
If the central vision is not affected, laser treatment is not required but if there is capillary non perfusion in a sector then scatter pattern of laser is performed in affected sector. Fill-in PRP may be applied if neovascularization progresses or if vitreous hemorrhage
4. Eale’s Disease (Retinal Vasculitis): In Eales disease there is inflammation of small size capillaries resulting in weakening of capillaries prone to occlusion & rupture of blood vessels. FFA demonstrates peripheral retinal vessel occlusion associated with non perfusion, retinal ischemia followed by neovascularization. The neovascularization is treated by laser photocolgulation.
5. Sickle Cell Disease & Proliferative Vitreo-Retinopathy:
Sickle cell disease causes retinal ischemia and neovascularization due to increase in viscosity which is treated by Laser photocoagulation.
6. Coat’s Disease and Retinal Telangiectasia:
Treatment is contemplated when exudation is extensive and progressively threating the central acuity or produces significant Retinal Detachment. Photocoagulation is treatment of choice for mild to moderate case of exudation and is done by application of large (200-500µ) spot of moderate intensity of light at the site of leak.
7. Retinopathy of Prematurity (ROP):
The treatment’s goal in ROP is to destroy the retina that is deprived of retinal vessels. This helps to shrink the new vessels and prevents the formation of dense scars. Laser photocoagulation is the treatment of choice because complications are less common than cryotherapy. A laser is directed to a designated spot to destroy abnormal vessels and to seal the leaks.
8. Age Related Macular Degeneration (ARMD):
Although anti-VEGF therapy is treatment of choice for ARMD, laser treatment is also performed in most cases. In dry form, laser photocoagulation has shown to prevent progression into wet form and resorption of drusen and improvement visual acuity.
For the treatment of wet form of ARMD photodynamic therapy is preferred over conventional laser photocoagulation. The goal of photodynamic therapy (PDT) is to spoil target neovascular tissue while sparing surrounding and overlying retina.
Photosensitizing agent (Verteporfin) used in PDT has affinity for proliferating neovascular tissue due to its increased expression of LDL receptor on neovascular endothelium and localized within the Choroidal neovascularization. Activation of Verteporfin with specific non thermal light produces triplet state which reacts with O2 ultimately producing singlet O2. The resulting local cytotoxicity causes an acute inflammatory response with production of cytokines. Occlusion of vascular bed occurs from endothelial damage, platelet activation and subsequent thrombus formation. Photodynamic therapy has shown to preserve the contrast sensitivity and limitation of lesion progression, leakage from the lesion and final lesion size.
9. Central Serous Chorio-Retinopathy (CSCR):
CSCR is characterized by an idiopathic serous neural retinal detachment in the macular region. Although majority of cases resolve spontaneously, treatment option for CSCR include either laser photocoagulation or photodynamic therapy. Laser photocoagulation is applied to the site of fluorescein leakage in the form of 6 -12 laser burns 50 – 200µ spot size with power of 75 – 200mW for 0.1sec. Photodynamic therapy has been used to treat chronic CSCR (i.e. > 6 month duration) with diffuse decompensation of RPE and lacking focal leaks. The only definite benefit from laser therapy is decrease in duration of neurosensory detachment.
10. Retinal Detachment (RD):
Laser is used for the prevention of RD and retinal tear following retinal detachment surgery. The tear should be surrounded completely by 3 to 4 rows of Diode Laser burn with 200 – 500mm spot size for 0.1 – 0.2 seconds. It is often used to treat degenerative retinal changes including lattice degeneration. Retinal tears develop at edge of lattice degeneration and so it is important to treat the normal surrounding retinal rather than thin lattice area itself. At least 2 to 3 rows of 500 µ spot should be placed on normal retina adjacent to degeneration.
Endolaser photocoagulation is carried out at the end of the retinal detatachment surgery to seal the deliberate retinal break, created to drain sub retinal fluid or, to treat the edges of a deliberate retinopathy made in the case of fibrotic contracted retina to allow it to reattach.
11. Retinoblastoma & Hemangiomas:
There are two methods of laser therapy for the treatment of selected intra retinal retinoblastoma. In photocoagulation Argon green laser of appropriate wavelength is employed at sufficient power to produce instantaneous pronounced whitening of target tissue. Alternatively in trans-pupillary thermotherapy an infrared Laser beam is directed to retinal tumor with large spot size to produce dull white discoloration of the tumor portion of retina. The aim of Laser therapy in retinoblastoma is complete chorio-retinal atrophy corresponding to the site of prior intra retinal tumor. Retinal Hemangiomas are also treated by laser photocoagulation.
COMPLICATIONS & LIMITATIONS:
Complications can occur in any procedure but with careful evaluation and prevention they can all be eliminated. In laser photocoagulation these occur due to excessive treatment or misdirected laser burn over the retina or other tissue. The following complication can occur:
- The duration of the adverse effects may be
- Transient (7-10 days) Blurred vision, raised intraocular pressure (occasionally glaucoma), headache
- Medium term (3-6 months) Blurred vision
- Long term (permanent) Reduced vision, near and distance, poor night vision, poor colour vision, light sensitivity (commoner in fair skinned & those with cataracts), reduced peripheral vision preventing driving
TRANSIENT SIDE EFFECTS
- Blurring of vision
- Choroidal detachment
- Macular oedema
- BLURRING OF VISION
- persistent effects of dilating drops
- anterior chamber activity because of released pigment or inadvertent damage to the iris
- CHOROIDAL DETACHMENT
- may be accompanied by myopic shift of up to 4D
- associated shallowing of anterior chamber may precipitate angle closure if pre-existing shallow anterior chamber angle
- usually lasts for up to 10 days responds to steroids and acetozolamide
- MACULAR OEDEMA
- laser burns disrupt blood-retina barrier
- fluid leaks from choroid into sensory retina and thus to the fovea along the nerve fibres
- blood-retina barrier restored after 7-10 days
- vision usually recovers in type 1 diabetes
- in type 2 diabetes prognosis poorer because of degenerative changes in retinal pigment epithelium and bruch’s membrane.
- Dull and throbbing
- Cause obscure
- Treated with rest and simple analgesia
- If persistent and severe consider angle closure glaucoma
MEDIUM TERM SIDE EFFECTS
- May persist for up to 3 months in type 1 diabetes mellitus
- Severity does not predict outcome
- In type 2 diabetes mellitus may not recover
PERSISTENT SIDE EFFECTS
- Loss of visual acuity
- Accommodative defects
- Loss of colour vision
- Loss of visual field
- Foveal Burn
- Macular Edema
- Epiretinal membrane formation
- Pupillary Dilatation and Loss of Accommodation
- Accidental Burn of Lens Iris and Cornea.
- LOSS OF VISUAL ACUITY
- After full scatter panretinal photocoagulation a small proportion of patients lose one or more lines of visual acuity
- May be result of photochemical damage to macular from light scatter
- often without macular oedema
- often associated with nyctalopia
- ACCOMODATIVE DEFECTS
- Reduced near vision
- Secondary to failure of accommodation
- May be result of damage to long ciliary nerves
- Reading spectacles required
- DIMNESS OF VISION
- Likened to wearing sunglasses permanently
- Usually dose related.
- Rarely seen with <2000 burns
- Poor night vision
- Not entirely secondary to reduced rod photoreceptors
- longer duration burns
- COLOUR VISION
- May be pre-morbid finding
- May be secondary to direct cone destruction
- duration and intensity of burn important
- Dose related
- Commoner with fair skinned patients and those with posterior subcapsular cataracts
- Result of hallation (scatter of intraocular light off a reflective surface)
- Symptoms reduced by shading their eyes with brimmed hat
- Sunglasses ineffective
- LOSS OF VISUAL FIELD
- Greater the more confluent the laser
- Loss depends on number and intensity of burns
- Rare even with full scatter threshold burns Seen with “fill in” burns Negative scotoma Arcuate field defect
Persistent side effects are not uncommon but do vary in severity and appear to depend on the extent and intensity of laser therapy. They have a tendency to improve over months or years
The major limitation of laser photocoagulation is small pupil, as in Diabetes. Other limitations are as follow:
1. Opacity in Media
2. Elevated New Vessel
3. White Background
4. Retinal Detachment
5. Retinal Traction and Fibrous Tissue
Despite many complications and limitations, laser has provided a new treatment modality for many retinal disorders. But ophthalmologic research and development is in progress with continuous refinement in existing laser and introduction of new laser types.