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Commentary: Survival of vision after glaucoma filtration surgery

Douglas H. Johnson

"...and they lived happily ever after". The classic ending of a fairytale is often assumed to apply to the results of glaucoma surgery. Case presentations in a conference often conclude with "the patient had trabeculectomy". In reality, the story continues, leading to the phrase that glaucoma is plagued by "the curse of long-term follow-up". How effective is glaucoma surgery in preventing significant progression or blindness?

Long-term follow-up is required to answer this question, and few studies have followed patients for longer than five years. Three studies have found surprisingly similar results. Molteno and colleagues analyzed 289 eyes of 193 patients followed for up to 22 years, and concluded that "a steady long-term decline in visual acuity and visual field occurred, decreasing the probability of retaining useful vision up to the time of death to approximately 0.6".¹ This 40% probability of losing useful vision occurred at 15 years postoperatively, and included loss from all causes, not just glaucoma.

Watson et al., followed 150 eyes of 94 patients for up to 22 years, and concluded that, although trabeculectomy was successful in controlling IOP, 59% of patients had progression of visual field damage.² They further concluded that there was a "long-term reduction in the visual acuity and visual fields of about one-third of the patients", unrelated to postoperative IOP, the preoperative field, or other definable factors.²

Parc and Johnson also found a progression to legal blindness in some patients despite filtration surgery, with a 46% probability of blindness from glaucoma at ten years.³ The mean IOP was about 15 mmHg. The diagnosis of blindness was based upon field loss in all but two eyes, and not loss of central visual acuity, and it is probable that these eyes could still be considered "functional" because of preserved central acuity, despite the legal definition of blindness because of field loss. Similarly to the Watson study, progression of visual field damage also occurred even in eyes not becoming blind. Despite lowering of IOP to a mean of about 15 mmHg over the 20-year follow-up period, the probability of progression of visual field damage was 59% by 20 years.

Do these studies mean that surgery is ineffective in treating glaucoma? No, for two reasons. First, if we assume that patients having surgery would have gone blind without the surgery, glaucoma surgery was successful in preventing blindness in over 50% of patients in each of the studies. Second, while the IOP in each of the three studies was about 15 mmHg, several studies have found that even lower IOPs are effective in preventing glaucomatous progression.^4,5 The Advanced Glaucoma Intervention Study (AGIS) found that patients maintaining an IOP of 12 mmHg at all time points after surgery had essentially no progression of field loss after six years, while patients with IOPs of 14 mmHg and higher had increasing numbers of patients develop field progression after six years.⁴

Achieving pressures as low as 12 mmHg is difficult, however, as the majority of patients (75%) in the AGIS study had pressures of 14 mmHg or higher. Indeed, few long-term studies would have been performed if uniformly low pressures, and therefore successful results, were obtained in all cases after glaucoma surgery. Although more aggressive surgical therapy with antimetabolites may obtain lower pressures, the subsequent increased risk of bleb leaks and endophthalmitis^6-11 may neutralize the presumed benefit of pressures in the single digit range.

Long-term outcome studies are valuable for demonstrating the correlation between IOP and progression of glaucoma. In the study by Parc³, patients with field loss both above and below the horizontal at the time of surgery were most likely to progress to blindness. Surgery may be most helpful in preserving vision if performed before significant scotomas are present on both sides of the horizontal meridian.

Long-term studies also demonstrate that glaucoma can be a relentless disease, and that more effective therapies are needed. Neuroprotection will have a definite place in the treatment of glaucoma, and the current findings of such outcome studies will serve as the basis of comparison for the effectiveness of future neuroprotective medications.

References

1. Molteno ACB, Bosma NJ, Kittelson JM. Otago glaucoma surgery outcome study. Long-term results of trabeculectomy 1976-1995. Ophthalmology 1999; 106: 1742-1750

2. Watson PG, Jakeman C, Ozturk M, Barnett MF, Barnett F, Khaw KT. The complications of trabeculectomy: a 20-year follow-up. Eye 1990; 4: 425-438

3. Parc CE, Johnson DH, Oliver JE, Hattenhauer MG, Hodge DO. The long-term outcome of glaucoma filtration surgery. Am J Ophthalmol 2001 (in press)

4. AGIS Investigators. The AGIS study 7: The relationship between control of intraocular pressure and visual field deterioration. Am J Ophthalmol 2000; 130: 429-440

5. Grant WM, Burke JF. Why do some people go blind from glaucoma? Ophthalmology 1982; 89: 991-998

6. Ashkenazi I, Melamed S, Avni I, Bartov E, Blumenthal M. Risk factors associated with late infection of filtering blebs and endophthalmitis. Ophthalmic Surg 1991; 22: 570-574

7. Belyea DA, Dan JA, Stamper RL, Lieberman MF, Spences WH. Late onset of sequential multifocal bleb leaks after glaucoma filtration surgery with 5-fluorouracil and mitomycin C. Am J Ophthalmol 1997; 124: 40-45

8. Wolner B, Liebmann JM, Sassani JW, Ritch R, Speaker M, Marmor M. Late bleb-related endophthalmitis after trabeculectomy with adjunctive 5-fluorouracil. Ophthalmology 1991; 98: 1053-1060

9. Higginbotham EJ, Stevens RK, Musch DC, Karp KO, Lichter PR, Bergstrom TJ, Skuta GL. Bleb-related endophthalmitis after trabeculectomy with mitomycin C. Ophthalmology 1996; 103: 650-656

10. Parrish R, Minckler D. Late endophthalmitis-filtering surgery time bomb? Editorial. Ophthalmology 1996; 103: 1167-1168

11. Soltau JB, Rothman RF, Budenz DL, Greenfield DS, Feuer W, Liebmann JM, Ritch R. Risk factors for glaucoma filtering bleb infections. Arch Ophthalmol 2000; 118: 338-342