By Gail F. Schwartz, MD
Ever since Albrecht von Graefe first reported that central vision may be compromised after surgery in eyes with chronic glaucoma, physicians have debated the benefit of filtration surgery in advanced glaucoma patients for fear of causing permanent vision loss. Evidence for this so-called "snuff-out" phenomenon varies greatly, ranging from zero to 7.69 percent. This large, retrospective review puts the rate at 2 percent. It also shows that transient vision loss with recovery may be significantly more common than previously reported.
This retrospective study tracked both the occurrence of decreased vision after trabeculectomy and the course of recovery, differentiating between transient and permanent vision loss. The review included 301 trabeculectomy procedures performed with mitomycin C at a tertiary academic glaucoma referral center from January 1999 to September 2003.
The researchers defined postoperative vision loss as mild or moderate if Snellen visual acuity tests demonstrated a decrease in visual acuity of 3 to 5 lines. Severe vision loss was defined as a decrease of more than 5 lines. Vision loss was considered to be permanent if visual acuity did not demonstrate a return of 3 lines within a six-month follow-up period.
Permanent vision loss occurred in 24 of 301 eyes (8 percent). Among them, 13 cases (4.3 percent) had mild or moderate vision loss, and 11 (3.7 percent) had severe vision loss.
In 10 eyes (3.3 percent) with permanent mild or moderate vision loss and 6 eyes (2.0 percent) with permanent severe vision loss, no cause was positively identified.
The most significant risk factors associated with permanent, severe, unexplained vision loss were preoperative split fixation on visual fields, preoperative number of quadrants with split fixation, and postoperative choroidal effusions with eventual resolution.
Transient vision loss was much more common, occurring in 170 of 301 eyes (56.5 percent). Among them, 79 patients (26.2 percent) experienced mild or moderate loss. Mean recovery time for those patients was 88 days (range, six to 720 days).
Ninety-one patients (30.2 percent) experienced severe, though transient, vision loss, with a mean time to recovery of 78 days (range, six to720 days).
Even though this study is limited by its retrospective design and by the fact that no regular postoperative Humphrey visual field examinations were performed, it does provide us with a bigger picture of vision loss and recovery after trabeculectomy.
The authors note another possible limitation: there may have been a bias toward underestimating the incidence of snuff-out if surgeons chose not to operate as frequently on patients with severe disease.
It would be interesting to have a more complete view of the role cataract played in vision loss, as this was the predominant cause of vision decline post trabeculectomy in the Collaborative Initial Glaucoma Treatment Study. Still, this study will help us counsel patients about vision loss and recovery.