A number of different lasers are used in treating the eye. Lasers all work by producing a very focused, 'collimated' and 'coherent' beam of light of a single wavelength. The Yag laser uses a Neodymium: Yttrium-Aluminium-Garnet crystal as the lasing medium, hence the abbreviation 'Yag'. The versatility of this laser allows either very short pulses (less than a thousandth of a second when Q switched) or longer pulses to be used. When Q switched, the laser light causes optical breakdown and the formation of a small plasma bubble where it is focused. This enables it to cut tissues which are positioned deeply but where the overlying tissues are transparent, such as the inner structures of the eye.
In ophthalmology, the Q switched Yag laser is typically used to treat posterior capsule opacification and to create peripheral iridotomies.
Posterior capsule opacification
During cataract surgery, the hazy natural crystalline lens of the eye is removed, leaving behind a very thin membrane called the 'capsular bag'. The new lens implant is placed within this thin membranous bag, which originally enveloped the crystalline lens.
Although most of the crystalline lens is removed at surgery, inevitably a few microscopic lens cells remain. Sometimes many months or years after surgery, these cells divide and spread over the back surface of the capsule and lens implant.
This scatters light and causes blurred vision.
In these cases, the Yag laser is used to make a small opening in the opacified posterior (back) surface of the lens capsule. Following this treatment, the capsule is permanently cleared and the lens cells can no longer grow across the new lens implant. The procedure is painless and is performed in the office, taking between 5-10 minutes.
In patients where the front of the eye is narrow, there is a risk that the drainage angle of the eye may become blocked, causing the narrow angle form of glaucoma. In some patients, they only become aware of this when they develop an attack of angle closure glaucoma with a sudden spike in eye pressure and severe eye pain. Although the main factor causing this is the original shape of the eye, the pressure of the iris on the lens causes a phenomenon called 'pupil block' which exacerbates the problem. Aqueous fluid that is unable to flow past the pupil pushes the iris forward and makes a narrow angle even narrower. Another situation where aqueous may not be able to flow past the pupil is where an Implantable Contact Lens (ICL) is implanted into the eye.
In such cases, the Yag laser is used to make an opening in the periphery of the iris, so that the aqueous fluid can flow through this new opening at times when it is unable to flow past the pupil.
Laser peripheral iridotomies in Asian patients are often performed with both the argon and Yag lasers, since their irises are very thick and the opening is very difficult to make with the Yag laser alone. Mild discomfort may be felt at times during the procedure, which is also performed in the office and takes 10-20 minutes.
The non Q switched, frequency doubled Yag laser
The versatility of the Yag laser allows it to perform like an argon laser. When it is not Q switched (in the free running mode), and a separate crystal is used to shorten the wavelength to 532nm, the Yag laser emits a green light which is used in photocoagulation for conditions such as diabetic retinopathy, retinal tears, and wet macular degeneration.