Fig. 7.1
Creation of the radial incision of the limbus for the identification of the Schlemm Canal.
(a) Surgeon’s view. On the bed of the scleral pouch, the surgeon creates a radial incision approximately 1 mm long with 15° blades, in correspondence with the surgical flap. In this phase, it is advisable to use strong magnification to view the details. The anterior edge of the area to be cut must correspond to the anterior projection of the Schwalbe line; the posterior margin must correspond to the scleral spur. The trabeculate and the attached Schlemm canal can be identified between these two lines.
(b) Side view. The initial incision is gradually and progressively deepened until the Schlemm Canal can be observed in a slightly anterior position to circumferential fibers of the scleral spur (barely posterior to the gray limbal zone). The incision is deepened delicately until the Schlemm Canal can be identified: opening the roof of the canal is generally accompanied by the escape of a small quantity of blood or a drop of aqueous humor (due to the pre-set pressure gradient), often a pink color thanks to the blood present.
The goal is to open the external wall of the canal without perforating the internal wall, thus avoiding penetration of the AC: the surgeon visualizes the internal wall of the canal that will appear to be slightly pigmented. If a large amount of liquid escapes after the surgeon has created the incision, he may have inadvertently penetrated the AC. If this is the case, the incision must be sutured quickly and another incision must be created just posterior to the first.
(c) Surgeon’s view and gonioscopic view during the identification of the Schlemm Canal. In the lower part of the figure (viewed from above), the conjunctival peritomy, the small radial incision of the sclera that extends posteriorly from the limbus and the Schlemm Canal are observed. The scleral flap is not visible in the drawing.
In the upper portion of the figure (gonioscopic vision), the pigmented layer (Schlemm Canal) and the scleral spur (a whitish color) can be observed
(d) Control of the correct identification of the Schlemm Canal. The correct position and the patency of the Schlemm Canal can be controlled using a small length of prolene or 6.0 nylon suture thread (diagnostic incannulation of the canal): the thread should easily slide into the canal or present only minimal resistance, through the left and through the right sides of the radial incision. If there is considerable resistance to the pass of the thread, the surgeon must suspect that he has penetrated the wrong area, such as the suprachoroidal space; in these cases, the incision can be deepened or a second radial incision can be created parallel to the first to allow correct identification of the Schlemm Canal.
At this point, many authors suggest creating a paracentesis to allow reformation of the AC using VES if necessary. Alternately, the paracentesis should be created immediately after the scleral flap. The surgeon then proceeds with the trabeculotomy
Fig. 7.2
Trabeculotomy procedure .
(a) Lateral vision. To facilitate the successive introduction of the trabeculotome once the Schlemm Canal has been identified, some authors suggest the creation of an incision parallel to the limbus, introducing the inferior arm of Vannas scissors into the canal. A special probe, such as Harms or a Mc Pherson trabeculotome, is delicately inserted into the end of the Schlemm Canal, and pushed as far as possible. Two curved arms, parallel to each other, extend from the handle; the lower arm is inserted into the canal, the upper arm serves as a guide and allows indirect visualization of the position of the arm inside the Canal; the wall of the trabeculate is cut (a). In this phase, the surgeon holds the trabeculotome in one hand and delicate toothed forceps in the other to lift the scleral flap. Right and left trabeculotomes are available.
(b) View from above. At this point, the surgeon rotates the two arms of the trabeculotome towards the middle of the AC (b, see red arrows), until ¾ of the two arms of the trabeculotome can be seen inside the AC: this maneuver opens the trabeculate. In this phase, the surgeon must take care not to damage the iris or the cornea. Leaving the distal tip of the trabeculotome facing the middle of the AC, the surgeon withdraws the instrument from the Schlemm Canal, opening the trabeculate as far as the point of insertion.
The trabeculectomy must extend left and right for a total angle of approximately 120°: consequently, after having extended the trabeculate incision for 60° in one direction, the surgeon then extends the cut for 60° in the opposite direction, introducing the trabeculotome on the side opposite the incision of the canal.
The trabeculotomy must not involve the point at which the scleral floor has been cut: in this position the internal wall of the Schlemm Canal must be preserved (b, blue arrow), otherwise the procedure is transformed into a perforating procedure and a basal iridectomy will be necessary to prevent the prolapse of the iris.
Following the trabeculotomy , there may be a reduction in the AC depth or hypoema (blood reflux) may be observed. Occasionally, between the first and the second trabeculotomy, it may be necessary to introduce cohesive VES through a paracentesis to reposition the iris in a posterior position and deepen the AC that may collapse when perforated due to low scleral rigidity
Suturing the Flap and the Conjunctiva
Trabeculotomy is not associated with outflow of the aqueous into the sub-Tenon space and consequently a suture is required to close the scleral flap . Five-six single sutures in 10.0 nylon are normally sufficient. The conjunctiva and the Tenon capsule are closed with the procedure used for viscocanalostomy (see Chap. 4).
Phacoemulsification
The trabeculotomy procedure con be successfully combinated with phacoemulsification . There are different options for the construction of the access tunnel to the AC: with one or two scleral flaps.
There are no significant differences with respect to viscocanalostomy, deep sclerectomy and canaloplasty (see Chap. 4) regarding the successive phases of phacoemulsification.
Results
Numerous studies have demonstrated that, by reducing resistance to aqueous outflow, the trabeculotomy leads to a decrease in the IOP with good results in cases of congenital glaucoma, open- or closed-angle primary glaucoma and particularly in pseudo-exfoliative glaucoma.
In congenital glaucoma , the results of the trabeculotomy procedure are linked to the eziology of the glaucoma. As for goniotomy, trabeculotomy will also lead to excellent results (success in 80–90% of cases), in cases of congenital glaucoma with good prognosis, for example, glaucomas that appear in the period immediately after birth and within the first year of life.
The advantage of this procedure is that it does not require the formation of a filtering bleb and consequently, it is not associated with long-term risks associated with the presence of this bleb.
In terms of disadvantages, it should be pointed out that goniotomy does not lead to the formation of a conjunctival scar typical of trabeculotomy. This would have serious consequences for a child affected by glaucoma: during the patient’s life if filter surgery is necessary, a scar created by the previous trabeculotomy will reduce the percentage success of the procedure (even when antimetabolites are used).
The general opinion is that the tonometric reduction achievable with the trabeculotomy is lower than that achievable with the trabeculectomy procedure; consequently, even in the event of combined surgery, it is advisable to opt for the trabeculotomy procedure only in those eyes affected by mild glaucoma or simply with raised IOP, without advanced peripheral damage, in which case a phaco-trabeculectomy is advisable.
Complications
These are divided into intra-operative and post-operative .
Intraoperative
Errors may occur during the preparation of the conjunctival-capsular and the scleral flaps. They include lacerations of the conjunctiva or a flap that is excessively superficial (or deep), a common occurrence with other anti-glaucoma techniques. In these cases, the tissues should be carefully sutured and the position of the incision should be changed.
It is also possible that false pathways have been created in the AC or in the supra-choroidal space. There are also some complications typical of the trabeculotomy, such as the detachment of the Descemet, caused by the movement of the trabeculotome when it is rotated towards the AC with the tip pointing towards the corneal endothelium.
Another possible error caused by the trabeculotome is the creation of cyclo-dialysis . The resulting drop in IOP may necessitate further surgical treatment.
Frequently (in up to 84% of cases according to Tanihara), the laceration of the trabeculate is accompanied by modest hypoema: during the procedure, this can be controlled by injecting low resting molecular weight cohesive viscoelastic into the AC; the clot will normally reabsorb within a few days.
Damage to the crystalline or the iris is possible but rare.
Postoperative
The most common postoperative complications are:
- 1.
Residual hypotonia
- 2.
Anterior peripheral synechias that can reduce or eliminate the success of the procedure
- 3.
Formation of a conjunctival bleb, that can lead to hypotonia. This necessitates a revision of the wound and a new suture of the scleral flap.
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