Filtering blebs fail for a variety of reasons. Only rarely is the blockage at the level of the internal ostium. Other identifiable locations for blockage to aqueous outflow include beneath the scleral flap, at the level of the episclera, and at the episclera-Tenon's fascia-conjunctiva interface. Precise localization of the site of blockage facilitates slit-lamp transconjunctival needle revision and restoration of flow in many eyes.
Normal ultrasound biomicroscopic appearance of a functioning filtering bleb. Clear fluid-filled spaces can be identified in the subconjunctival and sub-Tenon's spaces above the level of the episclera.
Blockage at the level of the internal ostium
Blockage at the level of the intrascleral pathway
Blockage at the level of the episclera
Blockage within Tenon's fascia and the subconjunctival space in this patient with an encapsulated filtering bleb.