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 

operator’s 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.