What is the ideal conjunctival bleb and how to achieve it? Learning from the Microfistula-XEN procedure
Stephen J. Cringle
William H. Morgan
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Purpose: We ask the following questions: What is the ideal conjunctival bleb following glaucoma filtration surgery (GFS) and how to achieve it? We reviewed our data on cross-linked gelatin microfistula implantations in preclinical and clinical studies of a new type of GFS we developed. This procedure is a minimally invasive glaucoma surgery (MIGS), a modified form of which — the XEN Gel Stent — is now in widespread clinical use.
Methods: The preclinical studies involved the implantation of gelatin microfistula tubes into 168 rabbits and 34 monkeys. The follow-up periods extended out to more than two years in rabbits and six years in monkeys. Drainage from the blebs was monitored following anterior chamber injection of fluorescein. Clinical data from the subsequent clinical trial was also reviewed. Our new form of MIGS, which avoids damage to the conjunctiva, provides a great opportunity to study the mechanisms by which aqueous humor drains from the bleb after GFS.
Results: Long-term drainage was monitored experimentally in both rabbits and monkeys. Essentially, aqueous humor enters the subconjunctival tissue, joins the interstitial fluid, and forms a conjunctival bleb. In the bleb, there are close communications between aqueous humor and the structural molecules of the interstitial or the extracellular matrix, the blood and lymphatic vessels, and parenchymal cells. The presence of conjunctival lymphatic drainage was a key determinant of drainage longevity. We have identified that in long-term aqueous drainage (more than 6 years for monkeys and 2.4 years for rabbits,) the initial lymphatic vessels are very close to the aqueous exit point in the bleb. The placement of the Microfistula-XEN implant in the right position is essential for achieving the best surgical outcomes. The right position for this ab interno approach using a needle-based implanter is that the implant should be placed through the trabecular meshwork, scleral channel, and the distal end of the implant should be located in the superficial region of Tenon’s capsule. The ideal length of the implant tube in the anterior chamber is ~1.5 mm, ~3 mm in the sclera, leaving ~1.5 mm in the conjunctival tissue for a 6 mm implant tube. Good fixation of the eyeball avoiding deformation and a good gonioscopic view are required. The clinical studies suggest that a diffuse, flat bleb produced the optimum outcome. The mechanisms to re-establish balance of interstitial fluid in the conjunctival tissue by which aqueous humor drains from the bleb after GFS have not been fully elucidated.
Conclusions: It is proposed that efficient conjunctival lymphatic drainage of aqueous from the bleb is a key parameter for longevity of bleb drainage. Minimal disruption of the conjunctiva and the formation of a flat diffuse bleb appear to be optimal. Noninvasive methods of examining the conjunctival lymphatics are proposed.
New Concepts in Glaucoma Surgery Series: Volume 1, pp. 275-288 #20
Edited by: John R. Samples and Iqbal Ike K. Ahmed
© Kugler Publications, Amsterdam, The Netherlands
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