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Laser
Charles Townes (1915-) an American physicist was awarded the Nobel Prize in physics for his pioneering work on laser.
Pituitary ablation was used to treat proliferative diabetic retinopathy but the side effects were significant.
With the advent of photocoagulation and vitrectomy which are effective with fewer risks and complications, pituitary ablation for PDR is no
longer performed.
Meyer-Schwickerath and Schott first used light photocoagulation to treat diabetic retinopathy in 1955 but this gives rise to extensive visual field defect. The problem was resolved 
with the use of laser that produces smaller lesions. Beetham was the 
first to use ruby laser to treat 
diabetic retinopathy in 1967. 
Section 5 Lasers and lenses
5.1 Contact lenses
Laser therapy can be carried out either through the dilated pupil using a 
contact lens or the indirect ophthalmoscope, or externally through the 
sclera. Trans-pupillary laser is normally applied using the slit lamp 
biomicroscope and a contact lens. The modern wide angle contact lenses 
are superior to the three mirror contact lens. Lenses such as the Volk, 
Mainster or Rodenstock lenses give a good view of the macula d the 
peripheral retina. These lenses give an inverted image but provide easy 
access to the post-equatorial region which is difficult to visualize with 
a three mirror lens. Both the wide angle indirect lens system 
(Panfunduscope, Mainster, Volk lens) and the laser indirect ophthalmoscope 
allow laser to be delivered to otherwise inaccessible areas of the retina. It 
is important to remember that the spot size may vary with different types 
of lenses and the operator should be familiar with the lens used.
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5.2 Lasers 
Optical radiation produced by gas or solid lasers are unique in that they 
are emitted at effectively one wavelength. Dye lasers are produced in 
organic dyes and have varying wavelengths. Gas laser (argon, krypton) 
produce optical radiations in the visible spectrum, while the newer diode 
lasers produce energy in the infrared band; argon, krypton and newer 
diode lasers are used in the treatment of diabetic retinopathy.
Lasers act by inducing thermal damage after absorption of the energy by 
tissue pigments. Blue/green lasers are absorbed by haemoglobin pigment 
and may therefore act at the level of the retinal vessels, and particularly 
microaneurysms which are one target for direct laser therapy in cases of 
maculopathy with focal retinal damage. Most of the energy from lasers of 
all types, including the long wavelength lasers, is absorbed by the melanin 
in the retinal pigment epithelium where much of the tissue destruction is 
induced.
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A variety of different lasers have been developed for retinal use. The 
wavelengths range from 488-810nm through the spectrum of colours 
from blue at 488nm, green at 532nm, yellow at 577nm, red at 640nm and 
infrared at 810nm. As far as the effect of treatment is concerned, all of 
these lasers have the effect of producing a coagulation at the level of the 
pigment epithelium radiation both into the choroids and into the retina. The 
only wavelength to be avoided is the blue wavelength (488nm) as this has 
been shown to be reflected off the surface of the contact lens into the 
operatorís eye causing reduction in blue vision which may be cumulative over 
many years. This is also more likely to cause loss of blue colour perception by 
the patient due to reflection within the eye. Because of the possible potential 
hazard with shorter wavelengths, coaxial red aiming beams are now commonly 
used on modern laser machines.
The yellow laser (577nm) is becoming more popular because of the 
ease with which red lesions can be directly coagulated. This is because 
red and infra-red wavelengths (dye lasers, krypton laser and diode lasers) 
are better transmitted through haemoglobin pigments and will frequently 
allow better uptake by the retinal pigment epithelium. Burns with these 
lasers can also be produced with a lower power than blue/green lasers. 
The diode laser in the infrared or invisible spectrum is a highly popular source 
of laser because it is delivered via a portable machine and there is no flash 
accompanying the laser application, thus being favoured by many patients. 
However, with the diode laser the end-point is more difficult to appreciate, 
being a greyish lesion at the level of the pigment epithelium rather than the 
more obvious white lesion produced by other wavelengths. If the laser surgeon 
is unaware of this difference, there may be the tendency to raise the power of 
the diode laser to produce a white lesion which may cause pain and excessive 
damage to the retina.
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5.3 Side effects of lasers
5.3.1 Pain 
Delivery of laser energy to the ocular fundus may in some cases be associated 
with significant pain. Diode lasers may be more painful than conventional lasers. 
The cause of the pain is unclear but may be due to direct thermal damage to 
branches of the posterior ciliary nerves. Pain may be prevented with the use of 
simple analgesia but on occasion may require retro-bulbar anaesthesia or even 
less frequently general anaesthesia, to achieve a satisfactory full PRP particularly 
in patients with florid proliferative retinopathy in whom delayed therapy may be 
risky.
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5.3.2 Vitreous haemorrhage
Excessive laser therapy in patients with forward new vessels may be sufficient 
to cause marked regression of vessels which separate from the posterior hyaloid 
face and produce vitreous and subhyaloid haemorrhage. In most cases this is a 
rare event but patients require information concerning this risk prior to initiation 
of therapy.
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5.3.3. Effect on visual function
There is now good evidence that the risk of causing reduction in visual field is 
around 40% to 50% after full PRP (see later chapter). This has implications 
fitness to drive and should form part of the information provided to patients 
prior to treatment. There may also be other more subtle effects of PRP on 
visual function such as some degree of loss of contrast sensitivity and reduction 
in the ERG. Finally it must be remembered that visual function may be lost 
through inadvertent laser application to the foveal parafoveal regions, or 
through the development of secondary neovascular membranes after focal 
treatment of microaneurysms. 
Epidemiology Clinical features Risk factors Screening
Lasers and lenses. NVD,, NVE.. Maculopathy
Vitrectomy. Cataract Special problems Counselling
References.. AAO guidelines Atlas of Retinopathy Contact lenses
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