Treatment of POAG and other forms of open-angle glaucoma has been addressed by several large clinical trials comparing medical, laser, and surgical intervention. Two of these trials assessed early surgical intervention. In the Scottish Glaucoma Trial,20 99 patients with newly diagnosed glaucoma were randomly assigned to initial trabeculectomy (46 patients) or conventional medical therapy followed by trabeculectomy if medical therapy failed (53 patients). After a 3- to 5-year follow-up, a greater decrease in IOP was noted in those treated with surgery. More than half of those treated with medical therapy required surgical intervention. There was no difference in final visual acuity between the two groups, but there was greater visual field loss in the medically treated group.
The Moorfields Primary Treatment Trial21 randomized 168 newly diagnosed glaucoma patients into three groups: initial medical therapy, initial laser trabeculoplasty, or initial trabeculectomy. With a minimum follow-up of 5 years, the study found that the greatest decrease in IOP occurred in the trabeculectomy group. In the medical arm and the laser arm, there was equal IOP lowering. There was no difference in visual acuity among the three groups. There was a greater loss of visual field, as measured by the number of absolute defects on Friedmann visual fields, in the medically treated group and the laser-treated group compared to the trabecu-lectomy group.
These two trials have influenced some aspects of glaucoma management. Prior to these studies, glaucoma therapy followed a well-defined course. First, medical treatment was instituted and increased until maximum tolerated levels were reached. Laser trabeculoplasty was the second-line treatment. After the failure of both medical therapy and laser trabeculoplasty, trabeculectomy was undertaken. However, some have proposed that filtering surgery may be more successful prior to chronic IOP-lowering therapy.22 It is important to note that these trials preceded the introduction of prostaglandin analogs, which are the most potent currently available medical treatment and are administered once daily, which could improve adherence. The outcomes may have been different if these agents were available for the medical treatment arm of these studies. By highlighting the potential benefits of early surgical intervention, these studies questioned our traditional therapeutic sequence and were an important impetus for the Collaborative Initial Glaucoma Treatment Study (discussed further below).
The Glaucoma Laser Trial23 and the Glaucoma Laser Trial Follow-up Study23 compared initial treatment with laser trabeculoplasty and a stepwise medical regimen in the treatment of newly diagnosed POAG. In the Glaucoma Laser Trial, 271 patients with similar IOP elevation, optic nerve damage, and visual field loss bilaterally had one eye randomized to initial laser trabeculoplasty and the fellow eye randomized to a stepwise medical regimen. Argon laser trabeculoplasty (ALT) was performed in two 180° sessions approximately 4 weeks apart. The medically treated eye was started on timolol 0.5% twice daily (step 1). If an appropriate IOP was not maintained, timolol was replaced with dipivefrin (step 2). This was followed by replacement with low-dose pilocarpine (step 3) and then high-dose pilocarpine
(step 4). Combination timolol and high-dose pilocarpine (step 5) and combination dipivefrin and high-dose pilocarpine (step 6) followed. After this regimen, treatment options were opened to the best judgment of the treating physician. With 2.5 to 5.5 years of follow-up, this study concluded that initial treatment with laser trabecu-loplasty was at least as effective as initial treatment with timolol.
The Glaucoma Laser Trial Follow-up Study continued to observe 203 patients from the Glaucoma Laser Trial. Median follow-up was 7 years, with a range from 6 to 9 years. This study showed that eyes initially treated with laser trabeculoplasty had a greater reduction in IOP of 1.2 mm Hg and a greater improvement in visual field of 0.6 dB. Because the difference in end points was small, the study group maintained its earlier conclusion that initial treatment with ALT was at least as effective as initial treatment with timolol.
These two studies highlight the benefits of laser trabeculoplasty as a safe and effective initial treatment for glaucoma. However, a significant number of patients initially treated with laser trabeculoplasty subsequently required medical therapy. At 2-year follow-up, 56% of laser-treated patients were using one or more glaucoma medications. These studies were also performed prior to the advent of newer glaucoma medications.
There is a resurgence of interest in laser trabeculoplasty with the advent of the frequency-doubled Q-switched 532-nm Nd:YAG (neodymium-doped yttrium aluminum garnet) selective laser. Selective laser trabeculoplasty (SLT) lowers IOP without coagulative damage to the trabecular meshwork. Several studies have shown efficacy similar to ALT in controlling IOP.24,25 Since there is less structural damage to the trabecular meshwork, SLT may allow for more retreatment than ALT.26 SLT has been shown to reduce the number of medications needed to control IOP over a 1-year period.27 However, one recent study showed a 68% to 75% SLT failure rate at 6-month follow-up.28
The Fluorouracil Filtering Surgery Study29 examined the role of postoperative subconjunctival 5-fluorouracil (5-FU) in patients who were at high risk for filtration surgery failure. A total of 213 patients who had undergone either filtration surgery and failed or previous cataract surgery were randomized to traditional tra-beculectomy or trabeculectomy augmented with postoperative 5-FU. The 5-FU was administered twice daily for 1 week and then once daily for the second week. After 5 years of follow-up, 51% of eyes treated with 5-FU and 74% of eyes not treated with 5-FU were considered surgical failures. Risk factors for surgical failure included high preoperative IOP, short time interval since last surgery involving conjunctival manipulation, number of previous surgeries with conjunctival manipulation, and Hispanic ancestry. Patients in the 5-FU group had a higher incidence of late bleb leaks (9% vs. 2%). The study members recommended the use of 5-FU after trabecu-lectomy in eyes that had previous cataract surgery or unsuccessful filtering surgery.
The Fluorouracil Filtering Surgery Study clearly showed the benefits of phar-macologic manipulation of wound healing in filtering surgery. Currently, 5-FU is also used intraoperatively as a one-time application.30 Postoperative 5-FU is generally titrated to bleb appearance. With the advent of mitomycin C, a stronger anti-metabolite that can be applied intraoperatively, the surgical options continue to broaden. However, there is an increased risk of bleb leak, hypotony, and late-onset endophthalmitis with antimetabolite use.31,32
The Advanced Glaucoma Intervention Study (AGIS)33 is a long-term study evaluating the outcome of glaucoma uncontrolled with medical therapy. A total of 789 eyes of 591 subjects were randomized between (1) ALT followed by trabeculectomy followed by a second trabeculectomy and (2) trabeculectomy followed by ALT followed by a second trabeculectomy. With 10 years of follow-up, this study highlighted a racial difference in outcomes. Although the IOP lowering was greater in the trabeculectomy-first group for both Caucasians and African Americans, visual function (as measured by visual field, visual acuity, and vision parameters) was better preserved in African Americans in the ALT-first group and in Caucasians in the trabeculectomy-first group.
Initial trabeculectomy increased the risk of cataract formation by 78% and a second trabeculectomy increased risk by almost 300% compared to the first trabeculectomy.34 It is important to remember that when this study began in 1992, antimetabolites were primarily used in higher risk eyes; therefore, fewer than 1% of initial trabeculectomies utilized antimetabolites. Also, prostaglandin agents, topical carbonic anhydrase inhibitors (CAIs), and topical selective alpha-2-adrenergic agonists were not available early in the study.
AGIS data highlight the importance of IOP control in reducing the risk of glaucoma progression. Retrospective analysis showed that the group of patients who maintained IOP less than 18 mm Hg (mean IOP, 12.3 mmHg) at all visits over a 6-year period showed no change in their mean AGIS visual field score.35 AGIS also highlights the importance of IOP fluctuation as a risk factor for visual field progression. Patients with IOP fluctuations as measured by standard deviation of office-measured IOPs of 3mmHg or more had a statistically greater risk of progression compared to patients with IOP standard deviations less than 3 mm Hg. Each 1 mm Hg increase in IOP fluctuation increased the risk of visual field progression by 30%.36
The Collaborative Normal-Tension Glaucoma Study (CNTGS)14,37 enrolled 230 patients with unilateral or bilateral normal tension glaucoma (NTG). The diagnosis of NTG was based upon characteristic optic disk and visual field findings in the absence of any documented IOP greater than 24mmHg. Study patients had a median IOP of 20mmHg or less from 10 baseline measurements. One eye of each patient was randomized to either observation or aggressive treatment to lower IOP by 30%. Randomization occurred immediately if there was evidence of recent visual field progression or if fixation was threatened. Eyes not meeting these criteria were observed without treatment until visual field change, optic nerve head change, or a disk hemorrhage was documented, at which time they were randomized. Sixty-two percent of eyes were randomized. Study end points were optic disk progression or visual field loss. When visual field progression was evaluated, baseline data in the control group were obtained at the time of randomization, whereas in the treatment group, a new baseline was established when the 30% IOP reduction was realized. Stabilization occurred an average of 219 days after randomization. End points were significantly more common in the control group (35% vs. 12%) when the effect of cataract was removed. The rate of cataract formation was significantly less in the control group (14% vs. 38%). There was no statistically significant difference in the rate of cataract formation between the control group and the medically treated subgroup.
If visual fields at the time of randomization were used as the initial baseline, the rate of progression of visual fields between the treatment group and the control group were statistically indistinguishable.37 A significant decline in the mean deviation values occurred in the treatment group from the time of randomization to the time of IOP stabilization. The decline in mean deviation disappeared after adjustment for reduction in foveal threshold. Because glaucomatous progression usually does not affect only the foveal threshold, the study authors postulated that the progression of visual field loss in the treated group between randomization and stabilization of IOP reduction might be related to cataract formation. When the effects of cataract were addressed in the analysis, visual field progression was greater in the control group at both 3-year and 5-year follow-up.
The CNTGS confirmed the role of IOP in the progression of visual field loss in this patient population. It also showed the potential complications induced by treatments aimed at reducing IOP. Since this study was done prior to the introduction of topical CAIs and prostaglandin analogs, and since beta blockers and alpha-adrenergic agents were not allowed by the study protocol, in actual clinical practice, surgical intervention to reach the 30% IOP reduction may be needed less often. The CNTGS identified variable rates of visual field progression with some patients showing progression within several months; however, 50% of patients who received no treatment showed no visual field progression in 5 years.38 Individual factors that increased the risk of progression included migraine, female sex, and disk hemorrhage at the time of diagnosis. Asian patients had a longer mean time to progression than did Caucasian patients. Therefore, it may be prudent to follow lower risk patients without treatment to obtain adequate baseline studies to identify individual stability.
The Ocular Hypertension Treatment Study (OHTS)9,15 was designed to determine whether medical treatment of elevated IOP in the absence of optic nerve or visual field abnormalities is beneficial. Sixteen hundred thirty-six subjects whose IOP was between 24 and 32 mm Hg in one eye and at least 21 mm Hg in the fellow eye, with normal optic nerves and visual fields, were randomized to observation or medical treatment designed to lower IOP by at least 20%. With 5 years of follow-up, the risk of progressing to glaucoma was 4.4% in the treated group and 9.5% in the untreated group. Of these end points, slightly more than 50% were only optic disk changes, thus highlighting the importance of careful, continued optic disk exami-nation.15 Individual risk factors for progression included older age, higher IOP, larger cup-to-disk ratio, greater pattern standard deviation, and thinner central corneal thickness. Interestingly, the presence of diabetes was found to be protective. However, the diagnosis of diabetes was based only upon patient self-reporting, and no independent testing or confirmation was obtained. Also, the presence of any diabetic retinopathy was an exclusion criterion at baseline. The reliability of these predictive factors in creating a risk calculator has been recently supported.39
The OHTS data clearly shows the benefits of reducing IOP in ocular hypertensive patients. However, it is important to consider that 90.5% of untreated patients showed no evidence of progression over 5 years. Therefore, not all ocular hypertensive patients require treatment, and individual patient risk assessment is needed prior to initiating therapy.
The Early Manifest Glaucoma Trial (EMGT)16 randomized 255 patients with POAG, NTG, or exfoliation glaucoma to topical beta blocker therapy followed by laser trabeculoplasty as needed or observation with the option of delayed treatment if there was sustained IOP elevation greater than 34 mm Hg. Outcome parameters were progression of visual field and optic nerve changes, as documented by stereoscopic disk photography. The median entry IOP was 20 mm Hg. The treated group achieved an average IOP reduction of 25%, while the observation group had no significant IOP reduction. The risk of glaucoma progression was 45% in the treated group and 62% in the control group after average follow-up of 6 years. Individual risk factors that increased the risk of glaucoma progression included increased baseline IOP, exfoliation syndrome, bilateral disease, worse mean deviation on automated visual fields, older age, and frequent disk hemorrhage. Each 1 mmHg reduction in IOP was associated with approximately a 10% reduction in risk of glaucoma progression.40
The EMGT prospectively shows the benefits of IOP reduction in lowering the risk of glaucoma progression in patients with preexisting glaucoma. Since there remained a 45% risk of glaucoma progression in those patients who achieved on average a 25% IOP reduction, this study tends to indicate that a lower initial target pressure may be needed when managing this patient population.
The Collaborative Initial Glaucoma Treatment Study (CIGTS)41,42 compared filtering surgery with medical treatment in newly diagnosed POAG, pigmentary glaucoma, and exfoliation glaucoma in 607 patients. Each arm is followed by laser trabeculoplasty if there is treatment failure. The surgically treated group had a mean IOP at follow-up of 14 to 15 mm Hg (48% reduction), and the medically treated group had a mean IOP at follow-up of 17 to 18 mm Hg (35% reduction). After 5 years of follow-up, the mean visual field scores of each group were similar. The risk of significant loss of visual field as defined by an increase of at least 3 units of the visual field score was 10.7% in the medically treated arm and 13.5% in the surgically treated arm. The likelihood of cataract surgery was about three times greater in the surgically treated group (17.3% vs. 6.2%). Quality of life analyses were generally similar between the two groups, with the exception of local eye symptoms, which were statistically greater in the surgically treated group.
The CIGTS data are relevant to current ophthalmic practice. It shows that despite obtaining lower IOPs in this group of early glaucoma patients, initial trabeculec-tomy (with the use of antimetabolites—primarily 5-FU—at the surgeon's discretion) has similar glaucoma stabilization rates over 5 years compared to initial medical treatment. Given that 45% of treated patients in EMGT progressed despite achieving a 25% target IOP reduction, the relative stability in the visual field scores with medically obtained IOP reductions of 35% in the CIGTS study tend to indicate that a lower initial target pressure may be more efficacious.
The European Glaucoma Prevention Study (EGPS)43 is a randomized double-masked controlled clinical trial evaluating the effect of dorzolamide versus placebo on the development of glaucoma in 1,081 patients. Interestingly, IOP reduction at
5 years was 22% in the treated group and 19% in the placebo group. End points were defined as visual field worsening, optic disk change, or an IOP of 35 mm Hg or greater in the same eye on two visits within 1 week. The rates of end point occurrences (13.7% dorzolamide vs. 16.4% placebo) were not statistically different.
The EGPS results must be considered in context with evidence from OHTS, CNTGS, and EMGT and the retrospective but consistent evidence obtained from AGIS that strongly support the benefit of IOP reduction. Several issues are concerning when evaluating the EGPS data.44 The ''placebo effect'' was much higher than one would normally anticipate. A significant number of patients were lost to follow-up studying EGPS, which could skew the results. Finally, in the absence of a clinically significant IOP reduction between the two arms, the study will not be able to identify the benefit of lowering IOP in reducing the risk of glaucoma development. It will be interesting to see follow-up results from this study and the outcomes of an expected combined evaluation with the OHTS data set.
These clinical trials provide an excellent framework of rigorous evidence-based medicine to direct our management of glaucoma (table 10.1). IOP appears to have a dose-response relationship with glaucoma development and progression. Lowering IOP with medications, trabeculoplasty, and surgery are all effective in reducing the risk of glaucoma progression.
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