Lasers are tools, which deliver energy (typically in the vicinity of the light spectrum) in a precisely targeted manner, in order to achieve an effect. Their uses are diverse, from passive range-finding equipment and scanning, through to their active use effecting tissue changes in a surgical role.
This discussion is an overview of what lasers can do in the world of eyes. If you are undergoing such a laser procedure, the patient experience, side effects and complications will be discussed with you on a case-by-case basis, since it is too big a topic to cover comprehensively here.
Retina and Optic nerve head
OCT (Ocular Coherence Tomography) produces ultra high resolution 3D images of the retina. This is a very useful tool for the diagnosis and management of macular problems, such as diabetes and Age Related Macular Disease. The image on the right shows the 3D macular landscape as affected by diabetic swelling or oedema.
HRT (Heidelberg Retina Tomograph) is a scanning laser attached to some sophisticated software, which creates 3D images of the retina and the optic nerve head. These applications are useful in diagnosing and monitoring the optic nerve head (particularly the objective volume of the cup and thickness of the nerve fibre layer. The advantages of these scanning lasers is that they do not emit harmful ionising radiation as with X-Rays, whilst producing detailed microscopic images that would have been unthinkable 20 years ago.
The HRT uses a confocal scanning laser to derive accurate 3D images of the optic nerve head. Monitoring of objective volume changes in the optic nerve cup and nerve fibre layer is a useful tool in following the progression of optic nerve damage in glaucoma.
Axial length measurements for Cataract surgery
The IOL Master uses a scanning laser to measure the length of the eye with extraordinary precision. Computer software uses this data to calculate an intraocular lens power, which is optimised for the best visual results. The scan on the right indicates to the surgeon the type of lens that should be inserted, the power of the lens and the orientation of the lens in the eye in order to correct astigmatism.
Therapeutic Applications – Laser Surgery
When the laser is delivered in a focused and intense form, it can have a destructive effect upon tissue. How the tissue responds depends upon the type of laser energy and the intensity of the delivery.
The middle picture shows a large area of panretinal photocoagulation used to treat a diabetic eye, and the picture on the right shows some laser spots around a peripheral horseshoe tear in order to prevent the retina from detaching
Laser energy (in the green/yellow light spectrum) is focused on a tiny area of the deeper retinal layers via a microscope delivery system which is controlled by the surgeon. This causes a coagulative effect (thermal burn) on the tissues which contain pigment; the result initiates an inflammatory and subsequent scarring response. This results in the sealing of blood vessels and is effective in the treatment of diabetic maculopathy. It is also used in a broader sense when large areas of the retina are ablated in order to control new blood vessel formation in diabetic disease. By preventing friable new blood vessels from forming subsequent haemorrhage can be prevented. Due to the scars that are formed between the retina and the outer coats of the eye, the laser can be very useful in the treatment of retinal tears, holes and detachments literally sticking the retina down like glue.
SLTP (Selective Laser TrabeculoPlasty)
This is an office laser procedure, where the drainage meshwork in the front of the eye is treated. This stimulates a tissue response, which opens up the aqueous fluid drainage in this area, thus reducing pressure in the eye. This has important applications in the treatment of glaucoma.
The image bottom left, shows a hazy lens capsule (which was affecting vision) and will be treated with the YAG laser. The image on the right shows a hole in the capsule where the hazy area has been removed, thereby clearing the vision.
If laser energy in the ultraviolet spectrum is delivered in a highly focused and targeted way, it’s effects can be different, in that it causes photodisruption. This is where tissues are ionised or blown apart creating a mechanical shockwave. The energy levels required to do this are massively concentrated, but highly localised. The application is very useful when clearing the cloudy posterior capsule behind a lens implant, thus restoring clear vision. It is also useful in forming a hole in the iris (YAG Iridotomy) in order to prevent angle closure glaucoma.
Excimer Laser (PRK and LASIK – both forms of laser refractive surgery)
The extraordinary precision with which this laser can shave tissue is seen in the notches created on a human hair.
The Excimer laser is a pulsed laser in the ultraviolet light spectrum, which delivers a precise amount of energy to the corneal tissue, resulting in a process called ablative photodecomposition. The result is that tissue is shaved away in a precise manner with minimal damage to surrounding tissues. This has revolutionised refractive surgery as the corneal shape can be altered in order to achieve a desired refractive outcome in a highly predictable manner.
Scanning lasers are used to make a 3D map of the front of the eye. Technology has advanced to the stage where the Excimer laser can be used to perform all of the main steps involved with cataract surgery. The theoretical advantages of undergoing surgery of this type are that the incisions are the same every time and hence the results are more stable, predictable and potentially safer. It remains to be seen whether the added cost of such a procedure ($1000) can be justified, based upon the use of technology for its own sake rather than the more rigorous standard of proving that the outcomes are actually better; competition in the marketplace (driven by doctors and patients alike) may cynically ignore the latter test.
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