i) Why reduce IOP?
Fluid circulates inside the eye in order to nourish the internal structures such as the cornea and crystalline lens. This fluid escapes into a drain located between the edge of the iris and cornea known as the drainage angle. In patients with raised pressure, this angle gets blocked with debris, meaning that the pressure in the eye has to increase to restore the flow of fluid. Unfortunately, high pressures damage the delicate structures within the eye, particularly the optic nerve. Damage to the optic nerve results in loss of peripheral and, occasionally, central vision. There is overwhelming evidence (8) that that reducing IOP lowers the risk of peripheral vision and eventually central vision loss in patients with glaucoma.
ii) Laser trabeculoplasty vs drops
The Glaucoma Laser Trial Research group (6) showed that eyes treated with ALT had lower IOP and better visual fields than fellow eyes receiving drops. Laser trabeculoplasty is particularly useful for the initial treatment of glaucoma and ocular hypertension, but has a place in the treatment of patients already taking some glaucoma drops (9).
iii) SLT vs ALT
SLT is a gentler form of laser trabeculoplasty than ALT, a technique which has been practised since the early 1980’s for the treatment of raised intraocular pressure (glaucoma and ocular hypertension). ALT works by causing scarring of the trabecular meshwork, cannot be repeated and reduces the success of subsequent glaucoma surgery if this is needed. SLT causes minimal damage to the trabecular meshwork since the energy used for SLT is about 80 times less than that used for ALT. Unlike ALT, which generally fails to work after 18 to 24 months, the IOP lowering effect of SLT appears to persist for longer periods (10). Unlike ALT, the success rate of repeat SLT is no different from that of the initial SLT treatment. This means that if the IOP were to rise at a later date, patients have the option of further SLT rather than just drops or surgery (11).