Blebs fail when the conditions necessary for their continued existence cease to exist. They need a steady flow of aqueous humor to maintain a passageway at the outer edge of the scleral fistula and to distend the interstitial space within the bleb or else connective tissue cells in the episclera and Tenon's capsule may bridge across and fill in those spaces. Initial bleb survival is aided when the flow of aqueous maintains sufficient tissue turgor pressure to collapse the conjunctival and Tenon's capsule blood vessels and when the aqueous is free of inflammatory cells or flare. Under such conditions there is little source of serum-derived growth factors to stimulate fibrosis or vascularization but an adequate nutrient supply to the cells in the bleb.<sup>16 Palmberg P. Surgery for complications.  London: Blackwell Scientific (1999).  476-478

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