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Sunday, August 19, 2001

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Reshaping the future of vision

NOVELTIES have appealed to us since the beginning. So it is not unusual for patients and doctors to share a fascination for the latest procedure or the newest equipment. Lasers are at the focal point of these converging interests.

The era of lasers began in the 1950s. The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. It is a device that produces a powerful beam of highly "coherent" electromagnetic radiation. The uses of lasers in today's world are too numerous to list - be it the CD player at home or the bar-code reading device at the supermarket - and its uses will continue to grow.

Laser light has the ability to carry an enormous amount of energy that is harnessed by doctors to burn, disrupt or ablate abnormal or unwanted tissues in the eye in a controlled manner without any damage to adjacent tissues. The eye, with its optical accessibility to light, is especially receptive to laser treatment.

This article describes how the destructive power of the laser is harnessed to be a sight-saving device. India does not lag behind the First World countries - either in availability of instruments and equipment or in knowledge and expertise.

Lasers in retinal diseases: The retina is the light sensitive layer of the eye. Its central and most sensitive area is the macula, which helps one see fine details.

Retinal vascular disease: Diabetes mellitus (high blood sugar) and hypertension (high blood pressure) can affect the retina. Small retinal blood vessels are damaged and leak, resulting in swelling of the macula. The ability to read and appreciate fine detail is impaired. Tiny laser burns, a few hundred microns in size, are applied on the retina closing the tiny leaking vessels, thus restoring retinal function. In patients with advanced diabetes, fragile new blood vessels bleed into the cavity of the eye. The destructive power of the laser is used to destroy the stimulus for formation of abnormal new vessels and lower the risk of bleeding.

Ocular tumours: Laser is particularly useful in treating children who develop a cancer called retinoblastoma. This leads to a white reflex at the pupil, much like that of a cat's eye caught in the headlamps of a car. Laser is also used to treat many other vascular tumours and malignant melanoma. Transpupillary Thermotherapy, a new technique using the diode laser, has been found to be effective in treating many eye cancers.

Retinal detachment: Lasers are also widely used to treat retinal detachment - a condition where holes in the retina cause it to separate from the underlying layers leading to loss of vision. As a preventive measure, laser is used to seal the hole. Lasers are also used extensively in advanced stages of retinal detachment along with the vitrectomy surgery.

Age related macular degeneration: Age related macular degeneration (ARMD) is another disease that affects the macula of the retina in the elderly. Reading vision is affected and gainful employment impossible. In one form of ARMD, early laser treatment arrests its progression. Transpupillary Thermotherapy is also effective in a subset of ARMD patients. Photodynamic Therapy (PDT) is the new weapon in the battle against ARMD in situations where simple laser can do more harm than good. This laser- assisted techinque has raised the hopes of many ARMD patients for whom no treatment was available until recently.

Lasers in cataract surgery: Cataract is a condition where the normally clear lens loses transparency with age. Phacoemulsification is currently the most advanced technique of cataract surgery. It involves breaking the hard cataractous lens within the eye using ultrasonic energy.

Laser cataract surgery has received much attention. Although the use of Erbium:YAG laser energy does have some theoretical advantages over ultrasonic energy, it is still to gain widespread acceptance. The laser cannot remove the harder varieties of cataracts that are common in India. In contrast, experienced surgeons can remove 95 or 99 per cent of cataracts using the ultrasonic phacoemulsifier. The few published reports of clinical trials using laser-based cataract systems indicate that phacoemulsification is at present a better technique.

Treatment after-cataract: A common post-operative change after intraocular lens implantation is opacification of its supporting capsule - a condition called after-cataract. This results in blurring of vision. The least invasive and safest method of treating an after-cataract requires the use of a neodymium yttrium-aluminum-garnet (Nd:YAG) laser. This laser is an accepted mode of treatment.

Laser corneal refractive surgery: Light entering the eye passes through the cornea (the front surface of the eye), which bends the light rays to initiate focussing on the retina. Refractive errors like myopia (short sight) and hyper metropia (long sight) are conditions where light does not focus accurately on the retina. Clear vision is not possible without the aid of correcting spectacles or contact lenses. Refractive errors are a significant cause of visual impairment in India.

Excimer Laser: In the 1980's Dr. Rangaswamy Srinivasan, a New York-based scientist of Indian origin, introduced the excimer laser to ophthalmology to reshape the corneal contour to correct refractive errors. It delivers high-energy ultraviolet rays to the corneal surface resulting in sub-micron precision in the corneal sculpting. The introduction of Laser-Assisted In Situ Keratomileusis (LASIK) increases the predictability of the procedure freeing patients from their dependence on glasses and contact lenses.

"Super" Vision: The most exciting and recent development is customised corneal ablation that takes into account the ocular characteristics of individual patients. Aberrometry is a procedure that allows an instrument to map the optical imperfections of the eye's optical system. The ability to correct these aberrations allows the patient to achieve visual acuity better than what he or she had before the procedure - a phenomenon called super-or hypervision.

Lasers in Glaucoma: Glaucoma is the second most common cause of blindness in the world. Aptly named a "silent thief of sight", this disease is largely underdiagnosed because of its silent nature. Glaucoma is a condition that results in an increase of eye pressure. This damages the optic nerve and, in time, results in total loss of vision. Early diagnosis and management is the key. Lasers are used in various stages of glaucoma in a preventive, supportive or active role. New lasers are under evaluation to be used instead of surgery. Endoscopic lasers are also in trials. In a decade, glaucoma will be treated with simple laser techniques, safely and effectively.

* * *

While newer lasers are under evaluation, research is underway to determine the prevalence of glaucoma in India, of which very little is known. In conclusion, laser has generated equal excitement in the scientist as well as the common man. As a form of energy, it has applications in modern warfare. But the idea of destroying to create and preserve is something that resonates with Indian concept of the trimurthi, and the cyclic creation and destruction of the universe. Doctors create new methods of treatment, and ultimately preserve vision by using the inherent destructive nature of lasers. The medical uses of the laser, a result of many decades of research by medical scientists, give us hope that mankind will not apply it to self-destruction.





The writers are with Sankara Nethralaya, Chennai.

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