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Set A Target IOP - David can you explain the formula?

Submitted by Agnes on Thu, 10/01/2009 - 6:48pm
The Early-Manifest Glaucoma Treatment Study showed that IOP reduction by at least 25% reduced progression from 62 to 45% in the treated group compared to an untreated group.19 The Collaborative Initial Glaucoma Treatment Study (CIGTS) lowered the IOP by 35%, demonstrated equivalence of medical and surgical treatment, and decreased disease progression to less than 15%.20
IOP lowering needs to be individualized with the goal of preventing any decrease in the QOL during the patient′s lifetime. That in essence is the target IOP. There is, however, no hard evidence for the concept or the methods used to determine the target. The following factors should be considered at the time of presentation to customize the target IOP:21
  • Structural damage: optic disc and RNFL
  • Functional damage on WWP
  • Baseline IOP at which the damage occurred (correlate the above two with baseline IOP).
  • Age
  • Presence of additional risk factors
Target IOP has to be individualized based on patient′s clinical profile. This can be calculated using tables, graphs or formulae. The formula used by CIGTS is shown below. This is similar to that published by Jample et al.22
Formula for target IOP =
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While we can formally calculate the target IOP in this manner or using graphs or tables, the rule of thumb is to reduce the IOP by at least 20% in mild, 30% in moderate and more than 40% in severe glaucoma. Generally, the formulae and other methods will provide similar values.
The higher the IOP, the larger the reduction required. If a patient has a starting IOP of 40 mmHg, we would opt for a larger percentage reduction. A 20% reduction from 40 mmHg would bring the IOP into the 30s; which is not good enough even for pre-perimetric glaucoma. In an advanced glaucoma (evident by structural and functional damage), in a young or middle-aged patient, one may choose to reduce IOP by 50% from the baseline. However, for the same clinical findings in a very old patient, the target may be set to a higher level so as to minimally hamper the QOL for that individual.
There are limitations to the target IOP approach. There is no sure-fire method of estimating it and no hard evidence that it works. Also, we do not know what aspect of the IOP actually causes the damage (peak IOP, fluctuations, short spikes, etc). Currently, we monitor the patient using visual fields; we need a more sensitive outcome measure to monitor the patient to reset the target if necessary. At the moment, however, it is a good concept to manage the patient.
A word of caution, there is a real danger of using the target IOP approach. Despite popular belief that ″lower is better″, not every patient requires the IOP to be lowered to a mean of 12 mmHg. Also, the target IOP is just a guideline, not a number to be strictly adhered to; it is better to use a range rather than a single number. Using a range of IOP provides safety from unnecessary aggressive therapy.
The target IOP is not a fixed magic number. Neither is it a static number, but changes depending on the results on long- term monitoring. If a patient is progressing on the target IOP we have set, we may need to lower it further. If a patient is stable on our target IOP, it may well be that it could be readjusted higher; we may try to withdraw some treatment.

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