SEQUENTIAL AB INTERNO GONIECTOMY (SAG)

Minimally invasive glaucoma surgeries (MIGS) targeting Schlemm’s canal (SC) are based on the hypothesis that resistance to aqueous outflow is primarily governed by the trabecular meshwork (TM). These procedures can be divided in incisional and excisional approaches. The rationale behind this approach is an excision of the trabecular meshwork which is described as being a major factor in outflow obstruction.

When the TM is excised over its total width the risk of undesired wound healing in the TM and the formation of peripheral synechiae is greatly reduced. Whereas this technique also creates a greater exposure of the opening of the collector channels, recent studies have highlighted the importance of a complex system of tension and counter-tension of different tissues involving valve-like structures regulating the outflow of aqueous humour. A full-width excision over 120 to 180 degrees might weaken these structures and thus impair these mechanisms.

A sequential goniectomy alternating excisional sections and spared TM may provide the advantages of full-width TM excision while preserving crucial anatomical structures and functions.

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The operation starts like most MIGS procedures with the creation of a main incision and a sideport. This is followed by the instillation of an ophthalmic viscosurgical device or the utilisation of an infusion system either external with an anterior chamber maintainer or integrated in the MIGS-device.  

The MIGS device is introduced via the main incision and then moved towards the opposite anterior chamber angle under gonioscopic observation. 

The excision is carried out in the usual fashion with the excising instrument being introduced into SC but it is then moved forward for one clock hour only. Then the instrument will be taken out of SC, moved on for one clock hour and inserted into SC again. This sequence will be repeated up to five times treating up to 180 degrees of the anterior chamber angle.

Surgery is finished with the removal of the OVD, hydration of the corneal incisions and may be concluded with the instillation of intracameral miotics, steroids or antibiotics.

Intra- and postoperative inspection have demonstrated the feasibility of the surgical technique and the postoperative appearance of multiple sectoral (clock hour) TM excisions.

The excision of the TM as the inner wall of SC poses a feasible and well-established approach in microinvasive glaucoma surgery. Sequential excision may preserve essential anatomical structures that regulate the outflow of aqueous humour.

The SAG-technique, or better the SAG principle, can be performed with all excisional systems available. A larger, multi-centre study will have to show if this technique has the potential to reduce the share of failures in microinvasive glaucoma surgery.