IOP as a cornerstone to glaucoma management

The measurement and management of intraocular pressure (IOP) in patients with glaucoma is critical. Even with the onslaught of new technologies to monitor progression and make earlier diagnoses, IOP remains a crucial data point in the optometric examination. The focus of all current means to quell the progression of glaucoma is control over IOP. As such, a careful review of glaucoma facts and key research endeavors is prudent. 

In the United States, glaucoma is still a very prevalent disease state. Glaucoma is one of the leading causes of blindness in the U.S.-more than 4 million Americans have a glaucoma diagnosis, and an additional 2 million may have it but are not yet aware. What’s more, approximately 5-10 million people in the U.S. have elevated IOP. These rates are projected to triple by 2050, with the south and southwest being prime areas.¹

A number of diagnostic tests are used for the detection and progression analyses of glaucoma, including (with corresponding CPT codes):

• Gonioscopy, 92020

• Perimetry, 92083

• Tonometry, (bundled with exam code)

• Serial/diurnal measurement, 92100

• Optic nerve head (ONH) tomography/topography, 92133

• Pachymetry, 76514

• ONH photography, 92250

Of the tests noted, gonioscopy and serial tonometry are occasionally omitted in the care of glaucoma patients. Gonioscopy is a necessary procedure to be performed upon any glaucoma diagnosis because open-angle and narrow-angle glaucomas may require different treatment strategies. Gonioscopy should also be done on a scheduled basis as the patient ages because the angle can narrow over time. Serial tonometry (usually defined as four measurements of IOP over at least a 4-hour period) may also be underperformed and yet still offers useful results. A variable diurnal curve of IOP poses increased risk for the glaucoma patient and should be monitored. 

Advanced Glaucoma Intervention Study

Arguably one of the most influential glaucoma research trials to date has been the Advanced Glaucoma Intervention Study (AGIS).² AGIS enrolled patients with known unstable glaucoma (such as worsening visual field loss and/or advancing atrophy of the nerve fiber layer with optic nerve head atrophy). This trial was large in that it enrolled 591 patients and monitored the progression of 789 eyes. AGIS randomized the treatment of eyes to either argon laser trabeculoplasty (ALT) or surgical trabeculectomy.

There was no difference that was statistically significant in the outcome between treatment groups. However, an analysis of the data did reveal some very important points regarding IOP management in unstable glaucoma patients. Eyes in which 100% of visits over a 6-year period had an IOP <18 mmHg also had a visual field defect score (from baseline) close to 0. This was in stark contrast to those eyes in which only 50% of visits had an IOP <18 mmHg and visual fields continued to worsen. Furthermore, the lower the IOP, the more reduced the progression. Those with 15 mmHg IOP showed half the progression of those with 18 mmHg, and those with 13 mmHg showed half the progression of 15 mmHg. The results indicated that lower IOP resulted in patients who were far better managed.² So, with unstable glaucoma patients, low IOP is an excellent goal.

Collaborative Initial Glaucoma Treatment Study

The Collaborative Initial Glaucoma Treatment Study (CIGTS) answered a question that had been burning at the time.³ Optometrists frequently manage glaucoma patients who struggle with medical therapy compliance. The pertinent question is whether surgery for newly diagnosed patients would positively affect quality of life without the need for regular medication instillation. CIGTS answered this question by examining 607 newly diagnosed glaucoma patients and randomized them to either topical therapy or surgical trabeculectomy. After 5 years, the end result was that new glaucoma patients’ quality of life was unchanged. As a result, glaucoma managers prescribe treatment modalities that are best for the individual and do not leap toward surgery until it is indicated.

Collaborative Normal Tension Glaucoma Study

The Collaborative Normal Tension Glaucoma Study (CNGTS) examined a very challenging group of glaucoma patients.⁴ It enrolled and monitored 145 eyes with normal tension glaucoma (eyes over 10 measurements of IOP that was never measured over 24 mm Hg). The treatment group’s target was a 30% reduction of IOP using any drops, ALT, or trabeculectomy. There was an observation group that progressed, of course, at an 80% rate without therapy. What was staggering was that the treatment group (though better than the observation group) progressed at a rate of 40%. Normal tension glaucoma continues today to be a very difficult disease to manage without progression.

Early Manifest Glaucoma Trial

The Early Manifest Glaucoma Trial (EGMT) was another trial that demonstrated the importance of IOP management.⁴ The patients enrolled had mild to moderate visual field defects and pre-treatment IOP of less than 30 mm Hg. The question was whether to initiate treatment right away, to delay initiation, or to simply manage. This study enrolled subjects with primary glaucoma and some secondary glaucomas, as well. Some 255 newly diagnosed patients were randomized to ALT, beta-blocker treatment, or observation (observe only or delayed treatment). Unsurprisingly, the observation group progressed 12 months earlier than the treatment group(s). The key IOP finding was that every 1 mm of decrease of managed IOP resulted in a 10% decreased risk of progression. In this multifaceted group of new glaucoma patients, lower IOP was better, as was the case in the advancing glaucoma patients via AGIS. 

Ocular Hypertension Treatment Study

The Ocular Hypertension Treatment Study (OHTS) was one of the largest trials ever conducted in the realm of glaucoma.⁵ OHTS was a prospective study of patients aged 40-80 years (1636 patients) with ocular hypertension solely (no glaucomatous damage), in which 50% were treated and 50% were observed. Though lowering IOP in the treatment group reduced risk by 50% over the 5-year investigation period, it indicated that optometrists would have to treat 100 patients with ocular hypertension (OHTN) per year to prevent one from progressing to primary open-angle glaucoma (POAG). This result was somewhat surprising because many predicted that preventative treatment would be far better. Very important risk factors for the progression of OHTN to POAG were also identified, including:

• Age: 22% increased risk per decade

• Race: African-American highest risk

• Higher initial IOP: 10% increased risk per 1 mm Hg

• Thinner corneas: higher risk

• Larger C/D ratio: 32% increased risk with 0.1 increase in cup-to-disc ratio

• Heart disease: higher risk

An additional risk factor was identified that continues today to be a very important in-office test: central corneal thickness (CCT).⁵

OHTS revealed the following data:

• 36% of patients with IOP >25.75 mm Hg and CCT <555 µm progressed from OHTN to POAG

• 6% of patients with IOP >25.75 mm Hg and CCT >588 µm progressed from OHTN to POAG

• 15% of patients with C/D ratios of .3 round and CCT <555 µm progressed from OHTN to POAG

• 4% of patients with C/D ratios of .3 round and CCT >588 µm progressed from OHTN to POAG

As it turned out, CCT is a powerful predictor of progressing from OHTN to glaucoma. The relative risk of progression of OHTN to POAG increased 81% for every 40µm thinner central cornea tested in the OHTS.⁵

This cursory review of some landmark glaucoma research trials underscores the importance of IOP control. Generally, lower IOP is preferred to the alternative. Over the coming years, optometrists will likely see impressive novel technologies enter their practices. It is likely that new medical therapies and superior surgical advances will come to bear. IOP measurement and management will doubtfully go away. Instead, IOP is likely to continue to be the cornerstone of glaucoma management, thereby being a critical cog in the care of our glaucoma patient base.ODT

References

1. Glaucoma Research Foundation. Glaucoma Facts and Stats. http://www.glaucoma.org/glaucoma/glaucoma-facts-and-stats.php. Accessed Dec. 9, 2013.

2. The Advanced Glaucoma Intervention Study (AGIS): 4. Comparison of treatment outcomes within race. Seven-year results. Ophthalmology. 1998 Jul;105(7):1146-64.

3. Comparison of glaucomatous progression between untreated patients with normal-tension glaucoma and patients with therapeutically reduced intraocular pressures. Collaborative Normal-Tension Glaucoma Study Group. Am J Ophthalmol. 1998 Oct;126(4):487-97.

4. Heijl A, Leske MC, Bengtsson B, Hyman L, et al. 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.

5. Kass MA, Heuer DK, Higginbotham EJ, Johnson CA, et al. The Ocular Hypertension Treatment Study: a randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002 Jun;120(6):701-13.