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Setting target IOP not as simple as it may seem
Although some general guidelines are available, treatment strategy should be focused on individual patients.
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ROME – Target IOP is a complex, dynamic concept that requires knowledge, clinical intelligence and constant updating to become an effective therapeutic means, according to a presentation here.
“It is well known that high IOP is strictly connected with visual field deterioration, and that lowering IOP is the only means we have so far to limit glaucoma-related damage. However, when we focus on target IOP, many factors have to be taken into consideration,” Michele Vetrugno, MD, said here at Rome 2002.
Baseline IOP
Prof. Vetrugno said that baseline IOP, which is the first parameter to be evaluated, should be determined in relation to a number of factors.
“There are pressure variations depending on the time of the day and on the state of activity or rest of the patient. Systemic blood pressure is also a correlated factor, and recently we have developed the concept that the evaluation of baseline intraocular pressure can only be made in relation to the individual corneal thickness,” he said.
Tonometry values should always be recalculated according to pachymetry values, because a thicker cornea normally bears a higher pressure than a thinner cornea, he added.
“For example, an IOP of 21 mm Hg in a 530-mm cornea is actually 21 mm Hg, but it becomes 26 mm Hg in a 450-mm cornea and 15 mm Hg in a 600-mm cornea. Consequently, we run the risk of considering a high-IOP patient as normal, and conversely treating healthy individuals for glaucoma,” he said.
Fairly frequently seen cases of pressure in the low 20s with no visual field loss should also be individually evaluated, according to Prof. Vetrugno, a researcher at Bari University, Italy.
“We might decide that this pressure range is well-tolerated by the patient, but the most recent developments of visual field testing, like frequency doubling perimetry, are showing that these cases are perhaps fewer than we thought. Also, the new morphological investigation technologies often reveal anatomical damages we hadn’t predicted,” he said.
Target IOP
The principal aim of glaucoma treatment is to reduce the rate of ganglion cell loss which is ten times greater in a glaucomatous patient than in a normal patient and is responsible for visual field deterioration.
“Target pressure should re-establish the normal age-related rate of ganglion cell loss, and maintain it over time,” Prof. Vetrugno said.
The guidelines he uses to determine target IOP date from 1998. The three main factors to be considered are the IOP at which the initial damage to the optic nerve was produced the individual rate of visual field progression and the patient’s age (and therefore life expectancy).
Adjunctive factors may be the patient’s family history, his or her ethnic group and the costs and risks of treatment. It is, therefore, focused on the individual patient.
“Target IOP is the goal set by the physician who has considered all these factors and decided that a certain level of IOP reduction will prevent further glaucoma-related damage in that particular case. It is also a dynamic concept, as it might need several adjustments in the course of treatment,” Prof. Vetrugno said.
Assessment of IOP (above left and right) must be made in relation to corneal thickness (left). Tonometry tends to underestimate IOP with thinner corneas and overestimate it with thicker corneas. | |
Individual evaluation
It is important to understand that there is no one target IOP that suits every situation, Prof. Vetrugno said.
“Sensitivity to pressure-related damage is extremely variable from case to case, and you may find that identical pressure levels show very different effects on two patients,” he said.
Taking age into account, there are patients who will never cross the threshold of functional damage and who, in theory, do not need to be treated. On the other hand, there are patients whose rate of retinal ganglion cell loss progresses so fast that it leads to blindness long before the end of life expectancy, he explained.
Most studies, though, indicate that at an advanced stage of the glaucoma, target pressure often needs to be around 15 mm Hg.
“Studies with follow-ups as long as 10 years show that visual field stability and retinal ganglion cell survival are achieved at pressure levels between 16 and 17 mm Hg, and in some cases only at 15 mm Hg. Also, in normal-pressure glaucoma a 30% reduction in IOP has been shown to reduce progression of the disease,” he said.
Data from the Advanced Glaucoma Intervention Study (AGIS) also confirm that a constant IOP below 18 mm Hg prevents further glaucoma progression and preserves visual function in the long term.
“The AGIS study, which is based on long follow-ups dating back to 1988 and on as many as 789 cases, has demonstrated that 100% of eyes with IOP lower than 18 mm Hg had no visual field variation compared to baseline over a period of 6 years,” Prof. Vetrugno said.
He concluded by mentioning the results of a questionnaire completed by a number of American ophthalmologists, asking in two separate questions what IOP level would make them feel tranquil with their glaucoma patients and what IOP levels would make them feel tranquil if they themselves had glaucoma. The average answer to the first question was 16 mm Hg, but to the second question it was 12 mm Hg, he said.
For Your Information:
- Michele Vetrugno, MD, can be reached at Università di Bari, Dipartimento di Oftalmologia y Otolaringoiatria, P.zza G.Cesare 11, Bari, Italy; (39) 08-0547-8970; fax: (39) 08-0547-8918; e-mail: m.vetrugno@oftalmo.uniba.it.
- Rome 2002, the 7th Annual Rome Symposium on Cataract, Refractive and Glaucoma Surgery, was sponsored jointly by Ocular Surgery News, the International Society of Refractive Surgery and the Italian Association of Cataract and Refractive Surgery.