Glaucoma surgery is indicated when target pressures are not achieved, or when neural tissue or visual function is progressively lost despite maximally tolerated medical and laser therapies.
Target pressure is generally accepted to be the pressure at which progression of glaucomatous
optic neuropathy is unlikely to continue. It is an attempt to prevent progression in a prospective manner. Target pressures need to be re-evaluated periodically and re-set at a
lower level if progression continues.
At the present time, the success of target pressure estimates can only be determined in a retrospective manner after many years of treatment.Recent multi-center, randomized controlled trials have demonstrated the efficacy of lowering intraocular pressure (IOP) in reducing both the risk of developing glaucoma and progression of the disease, and provide some help in choosing the initial target pressure.
Studies clearly show the benefit of IOP reduction in the management of glaucoma and selected patients with ocular hypertension, and help us to set initial target pressures. Lower pressures 12–15 mmHg clearly result in a lower risk of progression,but even reducing IOP by 20% has a protective effect. Advanced disease requires lower pressure when compared with early disease in order to halt or minimize the risk of of progression. It is for this latter group that surgery should be considered sooner than later.
The risk of progression posed by IOP must always be balanced with the risks of treatment. This is especially true when surgery is being considered. There is even some 4 Sit and Trope discussion as to whether patients are being over-treated in the zeal to reach the target pressure, particularly with early glaucoma. It is instructive to consider that the OHTS
found that 90% of untreated ocular hypertensives did not progress over 5 years.
Clearly, however, patients with advanced disease require aggressive therapy. However,not all glaucoma patients require an IOP of 12–14 mmHg. For example, an 85-year-old with a 0.75 cup-to-disc ratio and an IOP of 18 mmHg will likely not go blind from progressive optic neuropathy despite this IOP level. However a 55-year-old with a 0.9 cup-to-disc ratio and the same IOP level with a life expectancy of at least another 20 years is at greater risk of blindness if IOP is not dropped into the low teens. Spaeth has suggested that the goal of treatment is not to prevent disease progression, but to prevent patients from becoming symptomatic or from becoming more symptomatic
