boyd - practical aspects of laser photocoagulation - ch 6

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6

Fundamental Principles ofLaser Energy

Laser Energy as Applied to RetinalDiseases

Laser photocoagulation is the transfer oflight energy into heat energy that denaturesproteins and produces tissue coagulation.Laser light can be more concentrated thannormal light leading to more efcient heatproductioninthetargettissue.Thespectrumof laser wavelengths that will be discussedin this section as applied to laser treatmentof the retina extends from blue light around400 nm (Figures 1 - 2) to red light around780 nm (Figures 5 - 7). Between those are

puregreen(Figure3)andyellowlight(Figure4).Theinfraredwavelengthof800nmusedin the diode laser is longer than the visible780 nm of red (Figures 8 - 9).

Wavelengths shorterthan 400nmare theultraviolet, x-rays and gamma rays. Shorterwavelengths (blue) providemore energy perphotonthanthelongerones(red).Therefore,lasers with shorter wavelengths are moredamaging to the retinal tissues.

Types of Lasers Used for RetinalTherapy

Theoutput of alaser canbe classiedascontinuous-wave or pulsed. Although theseterms are used throughout ophthalmology,their significance may not be quite clearto those colleagues who do not use lasersoften. For practical understanding, retinalphotocoagulation is usually performed witha continuous-wave laser. In continuous-

wave lasers, a pumping source constantlyexcites the lasering material and radiationis continuously emitted. The output forretinal photocoagulation is usually delivered

NelsoN sabates, MD,

savak teyMooriaN, MD, Mba,

Felix sabates, MD

Practical Aspectsof Laser Photocoagulation



70

Retinal and Vitreoretinal Diseases and Surgery

during an interval of 0.1 to 1.0 seconds. Incontrast, pulsed laser operation occur whena ash lamp or other pumping source turnson and off causing pulses of laser light to be generated. Pulses are usually less than1 millisecond.

Evolution of Photocoagulationfor Retinal Diseases

Photocoagulation of the retinahasunder-gone rapid and steady development sincethe first Xenon arc instrument developed by Meyer-Schwickerath and produced com-

mercially by Zeiss in 1956. Laser technol-ogy has provided better and more reliableinstrumentation (Figures1-9). Photocoagula-tion has evolved from intense polychromaticwhite light sources like the xenon lamp, togas lasers (argon blue-green and green, andkrypton yellow and red), and most recentlyto solid-state diode lasers.

Therstdecadeof laserphotocoagulationof the retina was marked by steady rene-ment in the quality of the spectral delivery.This resulted in the elimination of the blue

portion ofthe spectrumbecause itwasmoredamaging to the retina (Figure 1). It also

Figure 1: Disadvantages of ArgonBlue Laser in Retinal Treatment.

Retinal damage occurs with the blue

light of the argon laser (AR) through

scattering (arrow) within the retina.

The blue light is also absorbed by

the yellow pigment present in the

inner layers of the macula (F) which

produces damage to the retina dur-

ing macular photocoagulation. Blue

light is also absorbed at the pigment

epithelium level (P) which is anterior

to the choroid (C). (Art from Jaypee

Highlights Medical Publishers).



71

Practical Aspects of Laser Photocoagulation

made available specific wavelengths thatmaximize absorption while requiring lesspower (Figures 3 - 9). Thesemodicationshelpedachievethedesiredtherapeuticresultswhilesimultaneouslygeneratinglessdamageto the surrounding normal retinal tissue. Apure green wavelength was attained by theaddition of a greenlter that only transmitsthemonochromaticgreenbandthattheargonlaser emits.

Indications and Availability of Laser Equipment

Indications for laser therapy have beenexpanded through landmark multicenterstudies that have proved to create benecialeffect on diabetic retinopathy and subretinalneovascular membranes inmacular diseases.With the availability of less expensive andsmaller instruments, laser technology isnowwidelyavailable.Thisincludesnotonlyretinalcenters in academic institutions that employretinaspecialistsbutalsomanyprivateofcesthroughout the world that are managed byhighlytrainedgeneral ophthalmologists. Al-though there arewell knownadvantages forthedifferentwavelengths,greenhasbeenthemost popular due to its availability in lowcost instrumentation and the wide range ofits applications (Figures 3 - 6).

How to Improve Your Results

Importance of Power DensityDelivery

It is importanttounderstandthe conceptofpowerdensity when applying a coagulat-

ing beam to diseased tissue. The spot size,power setting and exposure time determinethepowerdensityofthelaser.Byconvention,the spot size is selected prior to treatmentwhile the power setting and exposure timeare adjusted throughout the laser treatment.Protocols forcontrolling thesevariableshave beenestablished fordifferentapplicationsandindications.

Maintaining the correct power densityrequires careful attention to the relationshipofspotsize,exposuretimeandpower.Once

a good power density has been found, thepowerand exposure interval should be keptconstant as long as the spot size does notchange.Anydecreaseinspotsizeshouldbeaccompanied by a decrease of input power.However, in practice there are multiple fac-tors that will affect the size of the spot andpower density such as wavelength, mediaopacity, and the absorption quality of thetissue to be treated.

Theophthalmologisttreatingpatientswithlaserphotocoagulationshouldbecomefamiliarwith a limited number of laser wavelengthand contact lens combinations to developexpertise with the factors that will affectthe correct power density delivered by thedifferent lasers used. Several good qualitycontact lenses are available, each with itsown advantages and disadvantages.

Pearls for Treatment in the Macular Area Close to the Fovea

Mostproceduresareperformedunderanes-theticdropsalthoughoccasionallyretrobulbaranesthesia is required. When treating closeto the foveal area, special care needs to be



72


taken to avoid injury to thisimportantarea.Patientcoop-erationiscritical.Theyshould bemadeawareofanypossibledistractionthatmayoccurdur-ing treatment. For example,theshutternoiseoftheinstru-ment and anticipation of thelaserapplicationmayproducea slightmovement of theeyecausingdamagetothecentralfoveal area.

Other considerationswhen t reat ing near thefovea include the laser set-tings used. An example ofpossiblesettingsbeginswitha100micronspot,ashortexpo-sure of 0.1 - 0.2 seconds, anda low power intensity of 100milliwattsorless.Thepowercan then be slowly increaseduntil the desired reaction isobtained.

Selectionoftheappropriatewavelength is also important.For instance the use a redwavelengthwillallowforbet-ter penetration through earlyopacication of the lens (Figure 2). This de-creases the need for greater power density.

Figure 2: Disadvantages of Argon Blue Laser in Pres-

ence of Yellow Lens from Aging. The blue light of the

argon laser (AR) is absorbed by the yellow lens of an

aging eye with risk of damage at this level. (Below)

The red light of the Krypton laser (KR) is absorbed less

by a yellow lens and thus more energy reaches the

retina with little effect on the lens. (Art from Jaypee




73


Monitoring and Titration During Extensive Photocoagulation

In cases where extensive photocoagula-tion is required, the ophthalmologist shouldconstantly monitor the retinal reaction sincethis can vary markedly from one spot tothe next dependingon the amount of tissueabsorption. Titration for the correct amountof energy is critical.

Pearls in the Treatment of

Proliferative Diabetic Retinopathy When an advanced stage of proliferativediabeticretinopathyispresent,largenumbersof laser application spots are usually neces-sary. It is recommended to deliver these inmultiple stages to avoid exudative choroidalandretinaldetachmentnotinfrequentlyfoundafter extensive treatment. In patients whereneovascularization at the disc or elsewhereremains with recurrent bleeding in spite ofadequatephotocoagulation,furtherlasertreat-

mentshouldnotbeinsisted.Vitrectomywithendolaserphotocoagulation(Figure9)should beappliedwithoutdelaytoavoidpermanentdamage.

Timing for Vitrectomy

In proliferative diabetic retinopathy, it ispreferable to intervene earlywithvitrectomywhenindicatedratherthanlater.Thisavoidsthe production of a small visual eld as aresult of increased laser scaring following

extensivephotocoagulation.Ophthalmologistsmust remain exible in their laser applica-

tion to the retina is important to obtain thedesired results with the least application ofenergypossible. The goal is to spare retinarather than to destroy retina.

Comparative Tissue Effects ofDifferent Lasers

The Blue Laser

The argon laser that incorporated blueand green wavelengths was used for manyyears in the treatment of chorioretinal dis-eases. The majority of commercial argonlaser photocoagulators available during the70’s produced a light beam of 70% blue(488 nm) and 30% green (515 nm). Treat-ment with the blue wavelength has beendiscontinued foruse in retinal photocoagula-tion in favor of many others, especially thegreenwavelength.

Disadvantages of the Blue Laser

Light in the Treatment of the Retina

Photochemical (non-thermal) retinaldamage is higher with lasers of shorterwavelengths (blue) than thosehaving longerwavelengths(green,yellow,redandinfrared).This is because shorter wavelengths createmore energy per photons. Blue is scatteredmanytimesmoreinthemediathanthegreen,yellow or red. Therefore, higher energiesare needed to obtain the desired absorption by the lesion to be treated. Scattering in

the ocular media (Figure 2) increases withchanges from aging so higher power levels



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at the cornea are necessary to obtain thedesired retinalburn. Also,it ispossiblethatscattered blue light could damage normalretina next to the treatment area (Figure 1).Theseweresomeofthereasonswhythebluepart of the argon spectrumwas eliminatedfor retinal treatment.

Blue l ight is absorbed by the yel lowpigment present in the inner layers of themacula(Figure1)producingdamagetothesevitaltissuesduringmacularphotocoagulation.This may increase visual eld defects from

the treatment of macular lesions.

Also,theyellowedlensinagingeyesandcataract opacities increaseabsorptionofbluelight. This produces higher energy uptake by the crystalline lens with subsequent riskof damage (Figure 2).

The Green Laser The green argon laser light has awave-length of 515 nm. This laser is the mostwidelyavailableandpopularlaserforretinalphotocoagulation. It can be found in thefollowing types of lasers: 1) lasers madeexclusively for pure green output or 2) a blue-green laser with filter to provide thepure green wavelength.

Advantages of the Green Laser

Compared With RedThegreenwavelengthhastheadvantageof being absorbed by the hemoglobin of the blood in a subretinal neovascular membrane(NM) (Figure 3). Thedisadvantage iswhen

Figure 3: Advantages of Green Laser

Wavelength - Disadvantages with Intra-

retinal Blood. (1) The green wavelengthhas the advantage of being absorbed

by the hemoglobin of the blood vessels

of a subretinal neovascular membrane

(M). (2) However, when a small layer

of blood is present in the inner layers

of the retina (intraretinal blood), the

green light will be absorbed by the

hemoglobin thereby producing damage

(green arrows) to the inner retinal layers.

On the other hand, red light (3) will

penetrate deeper (red arrow) due to

the lack of absorption by hemoglobin.

Choroid (C) and sclera (S). (Art from

Jaypee Highlights Medical Publishers).



75


a small layer of blood (B) is present in theinner layers of the retina (intraretinal blood),the green light (G) will be absorbed by thehemoglobin. This absorption of energy willdamage the inner retinal layers (Figure 3).Red light (R) will penetrate deeper due tothe lack of absorption by hemoglobin.

Advantages of the Yellow Laser Compared With Red

Yellow(Figure4),aswellasgreen,laser

light is maximally absorbed by hemoglobin.This allows direct treatment of superficialretinalvascularlesionsandsubretinalneovas-cularmembranes. The absorption of yellow

and green light by hemoglobin becomes adisadvantagewhenthesubretinalneovascularmembrane (NM) lies under a thin layer ofsubretinal hemorrhage (H). The yellow andgreen laser energy is rst absorbed by thelayeredblood(H)beforeaffectingthedeeperstructures.Ontheotherhand,redlaserlightcan penetrate these hemorrhages.

The yellow laser wavelength is not fre-quently useddue tothe cost ofinstrumenta-tionandequipment.Itstillremains,however,thebest wavelength to treat vascular lesions

duetotheincreasedabsorptionbyoxyhemo-globin. This requires less power to obtainthe tissue reaction needed to coagulate thevascularized tissue.

Figure 4: Advantages and Disadvan-

tages of Green and Yellow Lasers.

Yellow, along with green laser light,is maximally absorbed by hemoglo-

bin. This allows direct treatment of

supercial retinal vascular lesions (1)

and subretinal neovascular membranes

(2). This absorption of yellow and

green light by hemoglobin becomes

a disadvantage when the subretinal

neovascular membrane (M) lies under

a thin layer of subretinal hemorrhage

(3). The yellow and green energy are

rst absorbed by the blood in layer

(3) before having the desired effect in

deeper structures. On the other hand,

the red laser light can penetrate these

hemorrhages. Other anatomy: Choroid

(C) and sclera (S). (Art from Jaypee




76


The Red Krypton Laser Theredlaserusesawavelengtharound647 nm. It continues to be used in someretinal diseases such as age-related maculardegeneration(ARMD)(Figure5),butitisnotas popular now as the green wavelength.

Advantages of the Red Laser

The red laser is particularly effectivewhencoagulatingtissueorsubretinalneovas-cular vessels that lies under a thin layer ofsubretinal hemorrhage (Figure 6). Red lightproduceslessscatterirradiationandheatintothe retina from the blood. This preservesthe desired retinal tissue, in particular whentreating near the fovea (Figure 7).

Figure 5: Advantages of Red Krypton Laser with

Subretinal Neovascular Membrane in ARMD. Above is

shown a cross section of the retina and choroid em-

phasizing the area of a subretinal neovascular membrane

(M) that lies between the pigment epithelium layer (E)

and choriocapillaris (C). This area of brous growth is

vascularized by outgrowths from the choroid and is a

very important complication of exudative ARMD. Note

that the retina (R) is detached in this area. The red

Krypton light (Kr) travels through the vitreous (V) with

very little involvement of the nerve ber layer seen at

area 1. There is less absorption of laser light within

the inner retina at area 2. Lack of absorption in the

inner layer results in decreased intraretinal brosis atarea 3. Here the surgeon aims at occlusion of cho-

roidal blood vessels that is the possible source of the

subretinal neovascular membrane (M). Other anatomy:

Photoreceptors (P) and sclera (S). (Art from Jaypee

Figure 6: Location of Krypton Red Laser Absorption

in Treatment of Subretinal Vascular Membrane. This

anatomical cross section of the retina shows that red

laser light (KR) is mainly absorbed by the melanin (blue

arrow) of choroid (C) and retinal pigment epithelium

(P-red arrow). The retina is shown detached in the

area of the subretinal vascular membrane (M). Other

retinal anatomy: inner limiting membrane (I), ganglion

layer (G), inner nuclear layer (A), outer nuclear layer

(D), outer limiting membrane (O), rod and cone layer

(R), Bruch’s membrane (B) and choriocapillaris (H). (Art

from Jaypee Highlights Medical Publishers).



77


There are also other advantages ofthe red krypton laser. It provides deepertissue penetration leading to coagulation ofthesubretinalneovascularizationorsubretinalneovascularmembrane (Figure 5). There is

less energy absorption by the inner retina(Figures 6 - 7). This leads to less involve-ment of the nerve ber layer and decreasedintraretinal brosis. There is less absorptionof thelaser lightby themacularyellow pig-mentorbloodinthemacula.Thisiscriticalasitlimitsthedamagetothefoveaandthusminimizes the decrease in visual acuity im-mediately following treatment.

Disadvantages of the Red Laser

Themaindisadvantageofredkryptonlaser is that its use may lead to choroidal bleeding.Thebestwaytoavoidthiscomplica-tion is toabstain fromusingshortexposureswith a small spot and high intensity.

The Pure Monochromatic GreenLaser Compared to Red Krypton

If red krypton equipment is availableas shown in Figure 7, it is better to use

red in cases with intraretinal blood. In allother instances as shown in Figures 5 and6, a pure green wavelength is as good asred krypton. For treatment of subretinalneovascular membranes, a key complicationof ARMD, the red wavelength has not beendemonstrated to be better than pure greenunless there is intraretinal blood, as shownin Figure 7.

Ifdealingwithsupercialretinalneovas-cularization suchas indiabetes andvascular

tumors,thekryptonredlaserisnotindicated because it is not absorbed by hemoglobin.Those cases are better treated with green oryellow wavelengths.

Figure 7: Red Laser Light with Intraretinal Blood

Near Fovea. Red light (Kr) is the method of choice

when treating subretinal neovascular membranes

(N) near the fovea (F) when blood (B) is in the

center of the fovea, where it is commonly found.

One gets less scatter irradiation and heat into

the fovea via the blood such as found with the

green laser (green arrow), thus avoiding the de-

struction of the fovea. (Art from Jaypee Highlights

Medical Publishers).



78


The Diode Laser The diode laser produces an infraredlight with longwavelengths in the range of700-820 nm. The efciency of semiconduc-tor diode lasers makes it possible for themto have minimal electrical or cooling needs.They can be made small, portable and even be mounted on existing slit lamps. Theirsolid-state design allows them to be madeeconomically and reliably.

Main Uses for Diode Laser This laser is used for direct retinalphotocoagulationeithertransclerallyfortreat-ing retinal pathology such as retinal tearsor holes, diabetic macular edema, and pro-liferative diabetic retinopathy; or for use inendophotocoagulation. It can be utilized inphotodynamic therapy for subretinalneovas-cular membranes in ARMD (Figure 8). Thediode laser can also be used effectively innon-retinal diseases particularly for cyclode-

structive procedures in glaucoma.

Advantages of the Diode Laser

In the presently available commerciallasers,thediodelaserhasseveraladvantages.Because of decreased scatter and absorption,the infrared diode laser penetrates vitreoushemorrhage and nuclear sclerotic cataracts better than the shorterwavelength laser suchasgreenandyellow.Thedeeperpenetrationspares the inner sensory retina. The laser

can be delivered through diabetic preretinalmembranes without contracting them. The

absence of xanthophyll absorption alongwith the lower absorption for melanin andoxyhemoglobin provides safe delivery to themacula (Figure 7). The lack of hemoglobinabsorption allows penetration through thinlayersof preretinalor subretinalhemorrhagewithout excessive laser energy uptake(Figure 8).

Other Advantages of the DiodeLaser

Itsportabilityhasbeenveryusefulgiventhemanylocationswherelasertreatmentcan be delivered. This is particularly importantin the operating rooms of many hospitalsthroughout theworld. Despite the inevitabletrauma to the equipment that comes frommoving it, there is no damage to the func-tion of the laser.Without the need for cool-ingor triphase 220-voltpower, laser therapycan be performed in any room containing aHaag-Streitslitlamporendophotocoagulationsystem.Thediodelasercanbeconnectedtoan AC source of electrical power or can bepowered by batteries if needed. The solid-state design of the laser makes it resistantto extremes of humidity and temperature.

Disadvantages of the Diode Laser

Vascular abnormalities such as retinalangiomas or retinal telangiectasia cannot bedirectly treatedwith the800 nmwavelength becauseit isnotabsorbedbyhemoglobin. Itsusemaybeinadequateinsubretinalneovascu-

larmembranesinlight-coloredfundibecauseof low laser light absorption. Broad-field



79


contact lenses are suitable for photocoagu-lation with the diode laser. These produce

inverted and real images. Lensesthatworkwell with the diode laser include the VolkCentralis, Trans-Equator, andQuadraspheric;and theMainster Standard and Wideeld.

SYSTEMS TO DELIVERLASER ENERGY

After the c linician has decided whichwavelength to use, the next question iswhich system to use to deliver the laser

energy. Delivery systems include the tradi-tional slit-lamp system, endoberoptics foruse intraocularly such as in endolaser pho-tocoagulation, the indirect ophthalmoscope,

Figure 8: Uses of the Diode Laser. The

diode laser is a solid-state, infrared laser

of long wavelength 700-820 nanometers

in the present commercial models. It

represents the most recent technology

that is the solid-state laser. It can be

made small and portable. The laser is

well transmitted by the ocular media, and

absorption by melanin and oxyhemoglo-

bin is lower. The diode laser may be

used in direct retinal photocoagulation

either through the traditional slit lamp

system or through endophotocoagula-

tion, transcleral irradiation for retinal

pathology such as retinal holes, and

cyclodestructive procedures in glaucoma

(not shown). (Art from Jaypee Highlights

Medical Publishers).

andcontactprobes.Allophthalmologistsarefamiliar with the slit-lamp delivery system

which is the most commonly used. Con-sequently, single spot treatment will not bediscussed here except for the relatively newPASCAL treatment. The rest of the focuswill be about the endolaser and binocularindirect ophthalmoscopic delivery system.

PASCAL Photocoagulation

PASCAL Coagulation Background

The PASCAL (Pattern Scan Laser) co-agulation system by OptiMedica is a recentdevelopment intended to expand upon thecurrent single laser spot used in coagulation



80


therapy. Thismodied slit lamp coagulatoruses a 532 nm laser that provides multiplespot therapy ofup to56in number that areapplied in pre-arranged congurations suchas squares and arc arrays. These arrayscan be adjusted to provide faster and moreefcient laser applications depending uponthe desired treatment.

Indications for PASCALPhotocoagulation

The PASCAL coagulation therapy hasindications for both posterior and anteriorsegment ocular pathology. Retinal uses in-cludepanretinal,focal,ormaculargridtreat-ment in patients with proliferative diabeticretinopathy, retinal tears and detachments,choroidal neovascularization, age-relatedmacular degeneration, and branch retinalvein occlusions. The anterior segment usesinclude trabeculoplasty and iridectomy, butfurther discussion about these applicationsis beyond the scope of this chapter.

Advantages of PASCAL Coagulation

This treatmentmethod provides efcientlaser therapy over large areas of the retinausing multiple spots in a rapid successiveorder. The pattern and number of spotscan be adjusted depending on the desiredlocation. It is also versatile in itsuses fromlarge panretinal therapy requiring hundredsof spots to localized single spot focal treat-ment. This rapid therapy is believed to

provide less patient discomfort by shorting

bothlasertimebetweeneachspotapplicationand total time at the slit lamp.

Disadvantages of PASCALCoagulation

There are some disadvantages to thiscoagulation treatment. Patients need to beabletoattheslitlamp for the therapy. Oncesituated,theircooperationiscriticalasmultiplespots are delivered in successive order afteractivation. Sudden movements by patients

can result incoagulation ofunintended loca-tions.

Endolaser Photocoagulation

Endolaser Coagulation Background

Endolaser coagulation is a method bywhich the laser light is brought directlyinside the eye through a beroptic to applytreatment to theretina (Figure 9). This is incontrast to conventional laser photocoagula-tion that is performed through the clearcornea also known as the “transpupillary”method. The endolaser is essentially usedonly during vitrectomy. When the surgeonis working inside the eye and a need forcoagulation exists, the laser light is directeddirectly toward that area through a 1 mmdiameter probeand photocoagulation isper-formed.Also,ifahemorrhageoccursduringsurgery, the media can turn too cloudy fortranspupillary application. Since the surgeoncannot bring the patient to the slit lamp,photocoagulation can be completedwith the

endolaser (Figure 9).



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Figure 9: Endolaser Does Not Touch the

Surface of Retina. The proper wattage to

use for endophotocoagulation should result

in a faint whitish reaction on the retina

(1). These threshold lesions should be

obtained with the tip of the laser probe

(P) about 2 disc diameters from the retinal

surface as shown at (2). (3) A stronger

laser reaction on the retina can be ac-

complished by increasing exposure time

or bringing the probe (P) slightly closer

to the retina (R). The instrument never

touches the surface of the retina, saving

adjacent structures from damage. (Art

from Jaypee Highlights Medical Publishers).

Indications for Endolaser Photocoagulation

What i s now done now with an en-dolaser was previously performed throughintraocular diathermy, external cryocoagula-tion, or endocryotherapy. These methodshave been almost abandoned and replaced by the endolaser.

Theindicationsfor the use of endolaserduring vitrectomy are: 1) to coagulate pre-existing, posteriorly located retinal tears oriatrogenically produced retinal tears; 2) to

assistwiththeinternaldrainageofsubretinaluid in retinal detachment; 3) to coagulate bleeding retinal surface neovascularization;

4) to perform panretinalphotocoagulation indiabeticpatientsimmediatelyaftervitrectomy;

and 5) to manage penetrating injuries andintraocular foreign bodies.

Comparison with Other Methods Previously Used

Whenusinganintraoculardiathermyprobe,the probe needs to be close to the retinanearlytouchingthetissue.Duringcoagulationthe tissue can adhere to the probe resultingin the instrument itself inicting damage tothe retina and choroid. The surgeon canactually create a choroidal hemorrhage byaccidentally penetrating the choroid and notcoagulating it.



82


When intraocularcryotherapyisused,theprobe has to be held motionless inside theeye on the retina. The effect of coagulationstartsontheretinalsideandthenpenetratesdeeper into the choroid. This produces alarger reaction in the sensory retina thanin the pigment epithelium and choroid. Ifthe probe is not held still, the retina can befractured at the edge of the cryocoagulationand could create a new tear.

The disadvantageof external cryotherapyversusendolaserintreatingposteriorlylocated

retinal tears is that a largeareaof theretinahastobecoagulatedthatmayleadtodamagein the nearby fovea and optic nerve. In ad-dition, thesealing of tears close to the foveaorto theoptic nerve isa more complexpro-ceduretechnicallywhenexternalcryotherapyis used because of their location.

Binocular Indirect Ophthalmo-scopic Laser Photocoagulation(BIOLP)

BIOLP is an essential tool for those whowanttotreatperipheralretinalneovasculariza-tion.Theadventofthislaserdeliverysystemallows the surgeon tovisualize and treat theretinalperipheryeasily,animportantadvance-ment. Laser treatment delivered by meansof BIOLP has made possible the treatmentof peripheral retinal neovascularization.

Indications and Advantages of theBIOLP

The BIOLP has several indications. Itcanbeusedforpanretinalphotocoagulationin

patientswithproliferativediabeticretinopathywhocannotsitataslitlamp.Otherindicationsinclude treatment for peripheral retinal tearsand demarcation of localized retinal detach-ments. BIOLOP can also be used in retinalvasculardiseasesaffectingtheperipheryasinsome casesofbranch retinal vein occlusions,central retinal vein occlusions, retinopathyof prematurity, and for inflammatory andretinal diseases. This techniquealsopermitstreatmentofinfantsundergeneralanesthesiaand children without anesthesia if they arecooperative.

Sincemostofthesediseasesweretreatedin the pastwith cryopexy, it is important topointout that laserburnsappear toproducefaster adhesions and less breakdown of the blood-retina barrier. BIOLP is also of greatvalueintraoperativelybecauseitallowsawideview that is helpful for applying treatmentto the peripheral retina.

Disadvantages of the BIOLP

In traditional slit-lamp delivery systems,the operator controls the spot size, powerandduration. Spotsizeisdifcultto controlwith theBIOLP. This requires special train-ing to use it adequately and safely.

Duration and power are controlled in amanner similar to that for slit-lampdeliverysystemsandaretitratedtoachievethedesired burn. Care should be taken as the treatmentmoves farther to the periphery because theretinal spot may become smaller. Either the

laser spot needs to be further defocused orthe power decreased. It is best to deliverless power over a longer duration becausethe lesion producedcanbebettermonitored.



83


Finally,itisdifculttotreatwithinthemacula,especially when rst using the instrument.This is because small movements will shiftthe placement of the lesion and that spotsize is difcult to determine precisely. TheBIOLP is therefore best suited for patientswith peripheral disease.

Thedurationandpowerneededdependsonmultiplefactorsincludingthewavelengthofthelaser,theclarityofthemedia,andthepigmentationoftheretinalpigmentepithelium.Itisbesttouseatleast200msecburns,be-

cause slower burns can beobserved as theyoccurand breaks inBruch’smembranemay be prevented by stopping the treatment iftheburns are becoming too intense. LowerpowerisneededwiththeargonBIOLPthanwith the infrared diode BIOLP if themediaare clear. Conversely, in the presence ofmediaopacity,the infrareddiodeBIOLPmayneed lower power than the argon BIOLP.Another note is that pigmented races needlowerpoweranddurationtoachieveawhite burn because the retinal pigment epitheliumis more absorbent.

Availability of BIOLP Equipment

The latest versions are available as at-tachments to the argon laser, argon-kryptonlaser, frequency-doubled YAG laser, andinfrared diode laser.

Anesthesia With BIOLP

Retinalburnswiththisinstrumentcanbepainful.Subconjunctivalanesthesiashouldbeused.Retrobulbaranesthesiaishighlyuseful

torelievepainbut ithasdisadvantages. Thepatient cannot move the eye to the side ofthelesiontofacilitatevisualizationandtreat-ment. When this happens, a cotton swab orotherdepressorcanbeusedtomovetheeyeor push the peripheral retina into view.

Adjusting the Aiming Beam

Once the eye is moved in the directionof the area requiring treatment, the aiming beam is adjusted so that it is in the middle

oftheretinalimage.Poweranddurationarethen titrated to achieve the burn required.If using a green wavelength, duration is setto 0.2 to 0.5 sec and power is increased asnecessary. With the infrared diode laser,duration begins at 0.4 sec and power at200 mW in a patient with well-pigmentedretinal pigment epithelium. These changeto 0.5 sec and 300 mW, respectively, in apatient with hypopigmented retinal pigmentepithelium.TheBIOLPdeliverysystemhasagreaterpotentialofcausingbreaksinBruch’smembrane than does the slit lamp becausekeeping a consistent burn size is difcult.

Precautions Using the BIOLP

Otherpeopleinthetreatmentroomshouldwear safety goggles. Windows should becovered to avoid exposing people outsidethe room to stray laser light, and a signmandating the use of safety goggles should be placed on the door.

Because the eyelashes may absorb thelaser energy and burn, a lid speculum can be used to hold the eyelids open since there



84


is no contact lens to do so. The corneashould be kept well lubricated because if itdries the epithelium becomes opaque.

Reference

PASCAL. Available at: http://www.optimedica.com/default.aspx. Accessed May 19, 2008.

boyd - practical aspects of laser photocoagulation - ch 6

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