Treating POAG: How Low Should IOP Go?

Choosing a target intraocular pressure has always been an inexact science. The greater and more consistent the reduction in IOP, the greater the reduction of subsequent nerve damage. The target pressure is different for each patient and there is no clearly, scientifically defined IOP level below which an eye is safe from pressure-related damage. Some clinicians set a target at a percentage reduction from baseline while others use more complicated formulas that have no proven validity.

The results of the Advanced Glaucoma Intervention Study (AGIS) have proven that the popular thinking about low IOP preventing damage is correct.¹ In this study, patients with advanced glaucoma who had failed medically were randomized into surgical groups employing a preset algorithm of argon laser trabeculoplasty and trabeculectomy. A side arm of the study examined the role of IOP-reduction in preventing further damage.

Patients were grouped into categories based upon the percentage of study visits where the IOP was below 18mm Hg. Patients who presented with 100% of study visits below 18mm Hg had, on average, little deterioration of their visual fields over six years. The patients in this group had an average IOP of 12.3mm Hg. Patients who had fewer than 50% of study visits in which IOP was below 18mm Hg had much more significant visual field deterioration. These patients had an average IOP of 20.2mm Hg.

The AGIS study clearly shows that low post-intervention IOP is associated with a positive outcome (visual fields are better preserved). This is one well-conducted scientific study that definitively shows that lowering IOP is beneficial in glaucoma management. However, must the study results be matched in all glaucoma patients? Should we insist on an IOP of 14mm Hg or less on every patient? While this research seems impeccable and the results credible,
clinicians need to understand how to integrate this information into clinical practice.

While patients who had IOP < 14mm Hg (average 12.3mm Hg and all study visits under 18mm Hg) did the best, remember that the patients in this study had advanced glaucoma and already had failed on medical therapy. Not all patients have advanced glaucoma; hence an IOP under 14mm Hg may be excessive. As we force the IOP lower and lower, so we decrease the patient's quality of life in terms of expense, adverse therapeutic effects and other unforeseen consequences. You still must consider many factors when determining a patient-specific goal for IOP range.

While the below-14mm Hg group experienced visual field changes that were nearly zero, this represents an average change which incorporates patients who actually had improvement in their visual fields throughout the study. This means that there were also patients in this low-IOP group who did experience visual field deterioration. Please be aware that an IOP of 12-14mm Hg does not guarantee that the patient will not experience further glaucomatous losses.

The significance of this study is that, for patients with advanced glaucoma, attaining low IOPs (in the 12-14mm Hg range) is an indication that we are likely benefiting the patients. Remember that maintaining a steady diurnal IOP curve is just as important to maintaining optic nerve health as is attaining low IOP. Fortunately the newest class of glaucoma medications, the prostaglandin analogs, affords us the ability to medically reach lower IOP than ever before, as well as stabilize the IOP diurnal curve.

1. The AGIS investigators. The Advanced Glaucoma Intervention Study (AGIS): 7. The relationship between control of intraocular pressure and visual field deterioration. Am J Ophthalmol 2000; 130: 429-40.

Other reports in this section

• Crystalline Lens Subluxation
• Ciliary-Block Glaucoma
• Primary Open-Angle Glaucoma
• Pigment Dispersion Syndrome and Pigmentary Glaucoma
• Pseudoexfoliation Syndrom and Pseudoexfoliative Glaucoma
• Anterior Uveitis
• New Insights Into Treating Ocular Hypertension
• Treating POAG: How Low Should IOP Go?
• Understanding Nerve Fiber Layer Analysis