It doesn’t take a brain surgeon to figure out whether a patient with end stage glaucoma and pressure in the 30’s on maximum tolerated therapy needs glaucoma surgery. But for the large group of glaucoma patients with moderate to advanced field loss, ambling along with pressures in the mid teens, high teens, and even occasionally low 20’s, how do we determine when to take the next step? Many experts set a “target pressure” that each patient needs to stay under to remain safe. But does exceeding this barrier automatically condemn a patient to further intervention, whatever the cost? Should our patients live in constant fear that one day their pressure will cross a threshhold that we set based on an educated guess of their particular risk? Let’s first look at the origin of target pressures and how we calculate them.
We should thank Paul Palmberg M.D. for popularizing the target pressure concept. He originally saw it as a means of quality control, to show that a patients’ glaucoma status had been analyzed and a goal synthesized from the data. Factors considered in the analysis included severity and rapidity of field loss at a given pressure, family history of blindness, fellow eye status, and life expectancy. But there was little data to suggest what numerical pressure was actually safe for most patients.
Thanks to the EMGT1 and CIGTS2 studies, we now have some quantitative data regarding the risk of further progression when a patient’s IOP has been lowered a specific amount. For patients with glaucomatous damage, further lowering of the pressure beyond a 25% reduction is associated with corresponding reduction of risk. For most patients who have had their pressures lowered around 35% or more, there may be no further benefit. A post hoc analysis of the AGIS study gives us an idea how to stratify patient risk by absolute pressure alone3. For patients with moderate field loss on average, the percentage of time the IOP remained under 18 was a significant predictor of the risk of progression. Corneal thickness may help substratify patients within this framework in borderline cases.
General lessons from all of these trials should be taken into account in setting a goal. For example, a patient with chronic open angle glaucoma with a pretreatment pressure of 50 will have developed damage already unless picked up very quickly. However, that does not mean that he can tolerate a pressure of 25 (a 50% reduction). Similarly, a patient who developed progressive damage at a pressure of 18 may have considerable risk of progression at a pressure of 16.
Despite recent breakthroughs in our knowledge of risk of progression, we still are making educated guesses. At all but the highest pressures, not all patients will progress. Some patients may have non-pressure dependent optic neuropathies that are beyond our current understanding and treatment capabilities. Around half of patients with normal tension glaucoma will not progress even without treatment5. All interventions, from adding a prostaglandin to putting in a third Baerveldt implant, have their own risk-benefit ratios.
In a prior symposium (Clinical controversies: target pressures – what are they; 45th Annual Symposium of the New Orleans Academy of Ophthalmology; 1998), Dr. Palmberg states that an epiphany came when he was reviewing 20-year-old charts on glaucoma patients. He noticed that patients with pressures in the high teens often progressed, though this was not picked up at the time. In reality, it was the prior glaucoma doctor’s blind faith in his target pressure (likely 21 for all patients at that time, and clearly inadequately precise) that gave him a false sense of security. The crime was that on careful review of visual fields, this progression was apparent. So not reaching a target pressure is first and foremost a signal that we need to scrutinize the clinical picture. Then we can carefully consider what the next step in the treatment would be and estimate a risk-benefit ratio.
So when do we decide to “take the plunge” and go to the operating room? For most patients who have “borderline” pressures with reasonable reductions on treatment, increased vigilance is the rule. If a patient is going to progress at a somewhat reduced pressure, it will likely be at a slow rate. Motivated, intelligent patients who push us to tell them their target pressure should be made to understand this. Visual fields can be performed as much as twice yearly in a patient who sits in a grey area. Progression can be diagnosed before the patient suffers a functional decline. The risk-benefit profile of further IOP-reducing maneuvers varies for each patient and over time. Thanks to recent clinical trials, an increased awareness of the potential dangers of even “high normal” pressures has come about. Still, in a patient with a chronic, treated illness which may be progressing slowly, if at all, we should clearly establish and confirm progression whenever possible before submitting them to additional risk. In an end-stage patient, prior records can sometimes be obtained to establish the recent course of the disease. If not, a spot judgement can be made guided by patient factors, helped by our knowledge of clinical trials.
References:
1 Heijl A, Leske MC, Bengtsson B, Hyman L, Bengtsson B, Hussein M; Early Manifest Glaucoma Trial Group. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial. Arch Ophthalmol. 2002 Oct;120(10):1268-79.
2 Lichter PR, Musch DC, Gillespie BW, Guire KE, Janz NK, Wren PA, Mills RP; CIGTS Study Group. Interim clinical outcomes in the Collaborative Initial Glaucoma Treatment Study comparing initial treatment randomized to medications or surgery. Ophthalmology. 2001 Nov;108(11):1943-53.
3 The Advanced Glaucoma Intervention Study (AGIS): 7. The relationship between control of intraocular pressure and visual field deterioration. The AGIS Investigators. Am J Ophthalmol. 2000 Oct;130(4):429-40.
4 Gordon MO, Beiser JA, Brandt JD, Heuer DK, Higginbotham EJ, Johnson CA,Keltner JL, Miller JP, Parrish RK 2nd, Wilson MR, Kass MA. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002 Jun;120(6):714-20.
5 Drance S, Anderson DR, Schulzer M; Collaborative Normal-Tension Glaucoma Study Group. Risk factors for progression of visual field abnormalities in normal-tension glaucoma. Am J Ophthalmol. 2001 Jun;131(6):699-708.
Have a question or comment on this article? Use the “Comment” link above to leave your thoughts, and the author will respond.