Topographic and keratometric astigmatism up to 1 year following small flap trabeculectomy (microtrabeculectomy)

Abstract

In the 1990s a number of authors have studied the refractive changes induced in the cornea as a result of the procedure.2-6 Several surgical modifications have been described, [7][8][9][10][11] each of which may theoretically influence the changes noted in the cornea following surgery. In 1995 we reported the intraocular pressure results following microtrabeculectomy, a small flap modification of the original design which we have utilised routinely since 1991.11 The scleral trapdoor at 2 × 2 mm is approximately one sixth of the area of the Cairns' procedure, and the 0.75 × 0.75 mm internal opening is just one seventh of the area of the original osteum. As a smaller sized operation theoretically should result in less surgical trauma, we performed a prospective study to investigate the changes in corneal curvature following microtrabeculectomy. We utilised vector analysis and vector decomposition techniques to determine the "with the rule" and "against the rule" changes induced in the cornea. Materials and methodsFollowing ethics committee approval and informed consent, a cohort of eyes requiring filtering surgery underwent microtrabeculectomy. None of the eyes had had previous ocular surgery. Full details of the surgical technique have been described elsewhere.11 Briefly, a limbus based conjunctival flap was fashioned commencing 4 mm from and exposing the limbus. SuYcient scleral cautery was then performed using a battery powered bipolar cautery (Mentor) over an area approximately 3 mm × 3 mm, suYcient to avoid haemorrhage during the procedure. A 2 mm × 2 mm scleral trap door was constructed, for this study centred at the 90 degree meridian, and an anteriorly sited 0.75 mm diameter internal sclerostomy was achieved with a Kelly punch (Storz). A small basal peripheral iridectomy was followed by two 10/0 nylon scleral trapdoor sutures and a running 10/0 gauge monofilament polyglycolic acid suture to the conjunctiva. Balanced salt solution (Alcon) and air were then injected into the anterior chamber via a paracentesis performed at the start of surgery. Postoperative medications routinely employed were atropine 1% drops three times a day and betamethasone 0.1% with neomycin sulphate 0.5% four times a day for the first week. At the 1 week review the atropine was stopped and the steroid/antibiotic reduced at the discretion of the clinician. All patients continued topical steroids for at least 1 month but for no longer than 3 months postoperatively. All surgery was performed by the same surgeon (SAV) who was experienced in the technique and no suture manipulations by laser or otherwise were used postoperatively.In order to provide the raw data for analysis, manual keratometry with a Javal/Schiotz keratometer and computerised corneal videokeratoscopy with the Eyesys corneal analysis system

A review of astigmatism and its possible genesis

“…82 The size of the incision appears to be correlated to the amount of induced astigmatism with smaller incisions (as used in 'microtrabeculectomy' procedures) leading to a smaller astigmatic change. 80 The exact cause of the corneal change following trabeculectomy is not known, although it has been postulated that it relates to tension from sutures used in the surgery, 82 cauterisation of the wound 80,82 or wound healing factors 79 leading to steepening of the cornea in the superior meridian.…”

“…76 Most studies have noted these corneal changes to be transient 74,77,78 but significant change can persist up to six months following surgery. 75,76 Trabeculectomy for glaucoma can cause significant corneal change, which is typically a steepening in the vertical meridian, leading to an increase in regular WTR [79][80][81] and irregular astigmatism. 82 The size of the incision appears to be correlated to the amount of induced astigmatism with smaller incisions (as used in 'microtrabeculectomy' procedures) leading to a smaller astigmatic change.…”

Keratometric astigmatism evaluation after trabeculectomy

“…These results were on eyes which had a 4x3 mm scleral trap door with two 10/0 nylon sutures. Vernon et al (1999) reported in a study of small flap (2mm x 2mm sclera trap door and o.75 mm internal sclerotomy) trabeculectomy procedure performed at the 90 degrees meridian on 16 eyes with the use of 10-0 nylone suture. By vector analysis, the mean surgically induced refractive changes (SIRC) in cylinder power vectors induced at 1, 3, 6, and 12 months as measured by manual keratometry were 0.68, 0.38, 0.52, and 0.55 dioptres, and by keratography 0.75, 0.66, 0.59, and 0.64 dioptres.…”

Do scleral flap dimensions influence reliability of intraocular pressure control in experimental trabeculectomy?

“…18 Trabeculectomy using a small scleral flap appears to provide medium to long-term IOP control comparable to large flap techniques 12,15,19 and may offer potential advantages: reduced surgical tissue trauma, a larger area of undisturbed sclera and conjunctiva should repeat surgery be required, and reduced astigmatism induction. 20 Potential disadvantages include possible decreased control over aqueous flow, technical challenges including difficulty in dissecting a large treatment area for antimetabolite application and difficulty in controlling subconjunctival bleeding if the conjunctival incision is small. Larger scleral flaps might give better tissue apposition over the scleral bed, and may give better IOP control.…”