title: Does the site of filtration influence the medium to long term intraocular pressure control following microtrabeculectomy in low risk eyes?
abstract: OBJECTIVE
  To evaluate the influence of varying surgical site on the medium to long term intraocular pressure (IOP) control in patients undergoing unenhanced small flap trabeculectomy (microtrabeculectomy) in eyes at low risk of failure.
  
  
  METHODS
  A retrospective non-concurrent analysis was performed on two cohorts of patients who underwent unenhanced microtrabeculectomy at different sites by a single surgeon (SAV). The first cohort of eyes was part of a trial to study the astigmatic effect of microtrabeculectomy (results published elsewhere) and all had flaps centred at the 12 o'clock meridian (superior flaps). The second cohort consisted of eyes with flaps created on either side of the 12 o'clock-that is, superonasal in left eyes and superotemporal in right eyes. All case notes were reviewed for the preoperative and presenting IOPs, the number and duration of antiglaucoma medication use preoperatively and, postoperatively, any intraoperative, early, or late postoperative complications. All IOPs measured at 6 months and then yearly intervals were recorded. The baseline characteristics and IOPs at each follow up were compared between the eyes with the superonasal and superotemporal flaps of the non-12 o'clock group against those with superior flaps in the 12 o'clock group up to a maximum of 72 months. Survival was assessed by the site of microtrabeculectomy, with failure considered as any IOP above 22 and 15 mm Hg with or without medications.
  
  
  RESULTS
  All patients had a minimum follow up of 12 months and 12/17 patients in the 12 o'clock group and 17/28 in the non-12 o'clock group completed the full follow up of 72 months. The IOPs at all points in time were lower in the left eyes with superonasal flaps compared to both the superior and the superotemporal groups. This difference was statistically significant between the three groups to the end of 4 years (p = 0.001) and remains clinically significant thereafter with the mean last recorded IOPs of 15.9, 12.4 (p = 0.03), and 14.3 mm Hg in the superior, superonasal and superotemporal groups respectively, with a smaller mean number of drops in the non-12 o'clock group. Kaplan-Meier curves showed a significantly better outcome for the cutoff IOP of 15 mm Hg in the superonasal group (p = 0.003) compared with both the other groups.
  
  
  CONCLUSIONS
  Eyes with superonasal flaps achieve and maintain lower IOPs when compared with both the superior and superotemporal flaps. The results suggest that, when a low target IOP is desired, the site of surgery in an unenhanced filtering procedure should be superonasally sited.
authors.affiliation: Department of Ophthalmology, Eye and ENT Centre, University Hospital, Nottingham NG7 2UH, UK.
authors.forename: A K
authors.initials: AK
authors.lastname: Negi
authors.forename: A W
authors.initials: AW
authors.lastname: Kiel
authors.forename: S A
authors.initials: SA
authors.lastname: Vernon
mesh_terms: D000368:Aged; D002387:Cataract Extraction; D005260:Female; D018463:Filtering Surgery / Q000379:methods; D005901:Glaucoma / Q000188:drug therapy / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D011182:Postoperative Care / Q000379:methods; D011183:Postoperative Complications / Q000209:etiology; D012189:Retrospective Studies; D012307:Risk Factors; D013524:Surgical Flaps; D014130:Trabeculectomy / Q000379:methods*; D016896:Treatment Outcome