Both of these options avoid a gaping effect on the scleral flap, which can cause aqueous flow from the anterior chamber and distort the anatomy.

If the cornea is perforated full thickness, during placement of the traction suture this should be easily identified by not only sensation but also the egress of aqueous. Simply withdraw the needle and hydrate the wound if need be and place the suture more superficially elsewhere. Should the anterior chamber have gone very shallow, simply wait and it will refill allowing the operation to proceed as usual.

We recommend releasing the traction suture prior to entering the anterior chamber to avoid the risk of anterior chamber shallowing and difficulty closing the scleral flap due to countertraction (if the traction suture is in the 12 o-clock position).

This can lead to difficulty in dissecting the scleral flap, as the Tenon’s tissue can make it difficult to judge the depth of the scleral flap. Releasable sutures can also become embedded in Tenon’s which is adherent to the sclera, making them prone to snapping when trying to remove them.

We find combined closure of the Tenon’s and the conjunctiva at the limbus very helpful. If the Tenon’s tissue is not brought forward at the limbus, then it can stick down posteriorly with scarring and cause the trabeculectomy to fail. Meticulous dissection at the start of the operation facilitates bilayered closure at the end.

Some patients do however have very friable conjunctiva, and no matter how careful the dissection, conjunctival buttonholes may occur. Should a conjunctival button hole arise, it can be cut out if sufficiently anterior and enough tissue is available to be brought down to the limbus after the buttonhole has been excised. If the buttonhole is posterior, then we would ordinarily repair with a purse-string suture, of either 10/0 vicryl or nylon. Both these sutures have their merits. Vicryl is absorbable and due to the “vicrylitis” or tissue inflammation, encourages tissue healing. The healing, however, may be overaggressive and encourage scarring of the trabeculectomy. The nylon suture is only slowly absorbable, so may need removal at some stage. It is, however, less inflammatory for the tissues. Care must be taken to gently close the buttonhole, so as not to cheese-wire the tissue and create an extension of the original buttonhole. Completion of the suture from the undersurface of the conjunctiva ensures very good burying of the knot and fewer problems.

The key is recognition on the operating table. First, if there is an area of thin sclera, try and avoid this and move the site of the flap, laterally or medially to an area of healthier sclera.

If one does inadvertently create a full-thickness hole in the scleral flap, it can be subtle and we always recommend checking for leaks. Use of 2 % fluorescein after the flap has been sutured closed and the anterior chamber refilled to a physiological pressure with BSS via the paracentesis helps considerably in this respect. If a full-thickness hole is detected, then this should be closed with either a patch of Tenon’s (autograft), or tutoplast at the time of surgery.

If the flap is very thin overall, one can cut a flap around the original flap (see Fig. 4.2a), at a deeper depth, thus incorporating the original thin flap. This a good option, as normal anatomy is retained, it facilitates good flap closure and the position of the trabeculectomy remains optimal.

An alternative is to move to a new site.

Finally, if the leak is persistent and material scarce, a rotational scleral flap may be fashioned and folded over the original thin scleral flap (see Fig. 4.2b).

In order to detect valving, it is important to check drainage at the time of surgery. Once the sclerostomy and peripheral iridectomy are created, the scleral flap should be folded down, but not sutured down and the anterior chamber refilled with BSS via the paracentesis. If BSS is freely flowing out of the flap and the anterior chamber is spontaneously shallowing, then valving is not present and the flap can be closed. If on the other hand, no flow is seen from the back end of the flap once the anterior chamber is filled it indicates that aqueous will not drain from the flap, even if all the sutures have been removed. If this occurs two measures can be taken. The first is to cut-down the side arms more anteriorly. This however should not be undertaken, if the side arms are already very anterior, as it can lead to postoperative limbal leaks. The alternative is to enlarge the sclerostomy posteriorly. Once, one or both of these steps have been undertaken, then fold the flap back down and test again for flow. Once, you are confident that flow is present, then the flap can be sutured down.

If adequate dissection is not performed then the subsequent dissection may either cut the suture out or be completely distorted by the suture. The same principal applies if undertaking a revision. Take the whole operation down and explore using 2 % fluorescein. Suture the defect and then restore normal operative anatomy in your usual fashion.

Should the sclerostomy be made too posteriorly, the danger is catching the ciliary body tissue. This can lead to profuse bleeding, or even creation of a cleft. Should this situation arise the bleeding usually stops by closing the flap, allowing blood to drain externally, and simply waiting. In the unlikely event of a cleft being created this may need suturing to close.

There is a shift in paradigm regarding the creation of a surgical peripheral iridectomy. Some surgeons advocate not performing surgical peripheral iridectomy in eyes that have deep anterior chambers and are not predisposed to the iris blocking the ostium (De Barroset al. 2009). However, the risk of iris obstruction will remain should the anterior chamber shallow in the absence of a peripheral iridectomy. While this can be controlled on the operating table, care must be taken when removing releasable sutures and massaging the bleb, in an outpatient setting.