|Year : 2007 | Volume
| Issue : 2 | Page : 46-50
Viscocanalostomy without deep scleral excision
Ahmed M Abdelrahman
Department of Ophthalmology, Cairo University, Cairo, Egypt
|Date of Web Publication||11-Nov-2009|
Ahmed M Abdelrahman
8 Morad Street, Morad Tower, Apt 409, Giza
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Purpose. To describe and evaluate a modified viscocanalostomy.
Patients and Methods. Viscocanalostomy without deep scleral excision was performed on 11 eyes of 11 patients.
Results. The mean age of the patients was 51.6 ± 10.6 years. The mean preoperative intraocular pressure (IOP) was 29.5 ± 8.3 mmHg, with a mean number of 2.6 ± 0.7 medications. The mean follow-up period was 7.4 months. The mean IOP at the conclusion of the follow-up period was 10.6 ± 4.5 mmHg, with a mean number of 0.2 0.4 medications. Of 11 patients, 1 patient, who originally had anterior uveitis, developed posterior cortical cataract 5 months postoperatively; 1 patient required Neodymium:YAG (Nd:YAG) laser goniopuncture 4 months postoperatively.
Conclusion. Viscocanalostomy without deep scleral excision is a promising surgical technique for treating glaucoma.
Keywords: deep sclerectomy, nonpenetrating glaucoma surgery, Schlemm′s canal, trabeculotome, viscocanalostomy
|How to cite this article:|
Abdelrahman AM. Viscocanalostomy without deep scleral excision. Middle East Afr J Ophthalmol 2007;14:46-50
The two basic nonpenetrating glaucoma surgical (NPGS) procedures are deep sclerectomy and viscocanalostomy. ,, In both techniques, a superficial scleral flap is dissected, followed by dissection of a deep scleral flap to serve the following functions: (1) opening and unroofing of the Schlemm's canal (SC) and (2) exposing the trabeculo-Descemet's membrane (TDM), through which aqueous humor percolates.  , The excised deep flap should include deep sclera, peripheral cornea, and the SC roof. Therefore, this procedure is actually deep sclerokeratectomy rather than deep sclerectomy only. 
Recently, identification and unroofing of the SC have been facilitated by inserting a trabeculotome through the canal prior to deep flap dissection.  , This approach can also be used as follows: After placing the trabeculotome inside the SC, a deep scleral flap is dissected, starting almost at the level of the SC roof. A central dissection is carried out to expose the TDM, and the deep flap is then excised. Such a procedure does not involve excision of deep scleral tissue, hence, the name viscocanalostomy without deep scleral excision. A computerized MEDLINE search of the literature did not identify any reports similar to this study.
| Patients and Methods|| |
Approval to perform this technique was obtained from the hospital ethics committee. Informed written consent was obtained from all patients. This initial case series included 11 eyes of 11 patients.
A fornix-based conjunctival flap, including Tenon's layer, was raised, and bleeders were cauterized. A 4 x 5-mm, half-thickness superficial scleral flap was dissected to expose the peripheral cornea adequately. A cellulose sponge soaked in a 0.04% mitomycin-C (MMC) solution was placed underneath the superficial scleral flap for 2 minutes. A vertical limbal incision was made deep to the superficial scleral flap, slightly inside its right corner, and fashioned to be perpendicular to the SC course. The incision was gradually deepened under high magnification, with frequent drying of the field using the tip of a dry cellulose sponge. Ultimately, the SC was opened. One of the essential landmarks for its identification is the detection of the horizontal fibers of the scleral spur.
Once the SC had been opened, one of the probes of the Harms trabeculotome (Katena Eye Instruments, Denville, NJ, USA) was introduced into the canal. The trabeculotome was passed horizontally to the left, stopping just before the far edge of the superficial scleral flap. Another vertical limbal incision was performed above the instrument to remove it from the canal. ,, In this way, the instrument remained stable inside a portion of the SC, having an entry point and an exit site [Figure 1]. A horizontal incision was made directly over the trabeculotome visible in the canal to open the canal roof. This incision cut through the limbal tissue and the SC roof [Figure 2]. The instrument was then released from the canal, and the canal floor could be seen. Alternatively, the roof of the SC could have been incised by introducing a blade into the trabeculotome entry incision and cutting upward.
The next step involved creating a TDM window by central dissection through the peripheral cornea to expose the anterior trabeculum and peripheral cornea. The deep flap was then excised after ensuring adequate aqueous percolation [Figure 3].
A high-viscosity hyaluronic acid (Healon GV) was injected repeatedly through the opened ends of the SC. The superficial scleral flap was then sutured loosely with two 10-0 nylon sutures, one at each corner. Healon was injected under the superficial scleral flap to maintain the space underneath. The conjunctiva was sutured to the corneal periphery. A combined antibiotic/steroid ointment (Maxitrol) was applied at the conclusion of the surgery. In cases scheduled for combined surgery [Figure 4], phacoemulsification was performed through a separate clear corneal temporal incision.
| Results|| |
The mean age of the patients was 51.6 ± 10.6 years. The mean preoperative IOP was 29.5 ± 8.3 mmHg, with a mean number of 2.6 ± 0.7 medications. Of 11 patients, 4 patients had primary open-angle glaucoma; 2 patients had chronic angle-closure glaucoma; 4 patients underwent combined phacoemulsification and deep sclerectomy; and 1 patient had open angle-glaucoma associated with recurrent anterior uveitis.
Intraoperatively, microperforation of the TDM occurred in 1 eye, and peeling of the inner wall of the SC was necessary to enhance percolation in another eye. The mean follow-up period was 7.4 months. The mean IOP at the conclusion of the follow-up period was 10.6 ± 4.5 mmHg, with a mean of 0.2 ± 0.4 medications. One patient, who originally had anterior uveitis, developed posterior cortical cataract 5 months postoperatively; a second patient required Neodymium:YAG (Nd-YAG) goniopuncture 4 months postoperatively.
| Discussion|| |
Schlemm's canal, which is located deep at the corneo-scleral junction,  is an important landmark during NPGS. Deep flap dissection during NPGS is carried out in 3 steps. The first step is the dissection of the scleral portion, starting 4 mm posterior to the limbus, passing toward the cornea, and leaving approximately 10% of scleral bed thickness covering the uvea. The initial incision may cut down to the uveal tissue, exposing it. This step helps to establish a plane of dissection that leads to the SC and also creates a space for aqueous humor to accumulate and form an intrascleral bleb. The second step is opening the SC along its posterior border and deroofing it. This step opens the ends of the SC to allow for aqueous outflow. This mechanism provides the main route for aqueous outflow in viscocanalostomy. The third step is central dissection to expose the TDM, through which aqueous humor will flow out of the eye. Finally, the deep flap, composed of sclera, the roof of the SC, and the cornea, is removed. ,
This technique constitutes a modified viscocanalostomy. The step involving deep scleral flap dissection has been omitted, as the SC is identified through a different surgical step. The mechanism of action here is similar to that in viscocanalostomy insofar as the principal route of aqueous outflow is through the dilated ostia of SC, with the exception that deep excision is avoided; hence, the procedure is a modified viscocanalostomy.
Sinusotomy is the first type of NPGS to have been described.  This procedure involves deroofing the SC over 120μ of the superior limbus, with the assumption that the site of resistance for aqueous outflow in primary open-angle glaucoma is located at the level of the intrascleral aqueous veins. However, the main sites of aqueous obstruction have been found to be located at the outer wall of the SC and the juxtacanalicular trabeculum.  The modified viscocanalostomy procedure described in this study can be considered as a combination of sinusotomy and exposure of the TDM, which is the principal aqueous percolation site.
In viscocanalostomy without deep scleral excision, aqueous humor leaves the anterior chamber through the TDM; it accumulates in the pre-Descemet's space, deep enough to form a lake under the loosely sutured scleral flap. The formation of such a lake is also promoted by the injection of Healon GV under the scleral flap.  In addition, the application of MMC underneath the scleral flap retards healing. The accumulated aqueous humor leaves via many routes, but primarily through the ostia of SC.  External filtration into the subconjunctival space may also occur.
The technique described avoids the surgically induced scleral thinning that can follow removal of a layer of sclera. Such thinning might lead to staphyloma formation in certain high-risk patients, especially if IOP elevation recurs. Patients at high risk are those with thin sclera, such as in cases of high myopia or chronic uveitis, especially when associated with rheumatoid or juvenile arthritis.  Therefore, this modified technique expands the indications for NPGS and increases its safety. The use of antimetabolites during classic NPGS may increase the risk of scleral ectasia,  whereas the procedure described in this report probably reduces this risk.
In this modified viscocanalostomy, there is no need to expose any uveal tissue, a step occasionally required during deep scleral flap dissection to establish the proper plane leading to the SC.  Serious bleeding from deeper blood vessels during deep scleral flap dissection should not occur with this procedure.
It has been found that the exact plane leading to the SC might be missed during dissection of the second flap, even by experienced surgeons. According to Dietlein et al,  the outer wall of the SC was not included in the deep flap in 48% of the specimens examined, indicating a lack of unroofing in these excised deep flaps. With the modified technique used in the present study, dissection starts with the identification and opening of the SC. Whereas the key step of unroofing the SC might be missed when performing the technique of deep flap dissection in classic NPGS, this will not happen when performing the modified technique used in this study because the trabeculotome is inserted inside the SC from the start. 
To summarize, viscocanalostomy without scleral excision is a NPGS that preserves the normal thickness of the scleral bed and facilitates identification and unroofing of the SC. This modified technique also makes the use of antimetabolites relatively safer because it avoids uveal tissue exposure.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]