In their excellent article, Broadway and colleagues1 follow an established convention in defining surgical success in trabeculectomy as an intraocular pressure (IOP) of 21 mm Hg or less without medication. The aim of glaucoma surgery is to reduce IOP to a level where there is nonprogression of visual field loss. The IOP likely to achieve that aim is unknown, but a value of less than 15 mm Hg has been suggested by Hitchings.2 Field stability is found when the IOP is below 17 mm Hg,3 with 58% of fields stable in eyes with pressures below 15 mm Hg compared with only 15% of eyes with pressures above 15 mm Hg.4 However, the matter may not be quite this clear-cut, as a 20-year follow-up of trabeculectomy in a stable, white population in England was unable to demonstrate field stability "... whatever cut off point is used for IOP."
Clearkin L. Adverse Effects of Topical Antiglaucoma Medication. Arch Ophthalmol. 1995;113(7):849-850. doi:10.1001/archopht.1995.01100070019006