What causes thinning and melting of a pericardium graft in a patient with a history of glaucoma and recent glaucoma drainage device surgery?

Pericardial Graft Thinning and Melting in Glaucoma Drainage Device Surgery

Pericardial graft thinning after glaucoma drainage device surgery is a recognized complication that occurs due to gradual tissue resorption and biological degradation of the donor material, though the exact mechanism remains incompletely understood. 1, 2

Mechanism of Graft Thinning

The thinning process appears to be a biological phenomenon where the preserved cadaveric pericardial tissue undergoes gradual degradation over time. 1 This is not an inflammatory rejection process, but rather represents:

• Tissue resorption where the graft material is slowly absorbed by the body without triggering an immune response 1
• Mechanical stress from overlying conjunctival movement and blinking forces that may contribute to gradual tissue breakdown 2
• Avascular environment where the graft lacks its own blood supply and depends on host tissue integration, which may be incomplete 2

Clinical Evidence and Incidence

Research demonstrates that graft thinning occurs across all patch graft materials used in glaucoma surgery:

• Pericardium grafts show asymptomatic thinning in approximately 26% of cases (6 out of 23 eyes), where the tube becomes visible beneath intact conjunctiva 2
• Donor sclera exhibits similar thinning rates at 26% (6 out of 23 eyes) 2
• Donor dura demonstrates thinning in 22% of cases (4 out of 18 eyes) 2
• No single material proves more resistant to melting than others 2

Important Clinical Distinctions

Graft thinning does not equal tube erosion. 1, 2 The critical distinction is:

• Asymptomatic thinning: The graft becomes translucent and the tube is visible, but the conjunctiva remains intact and the tube is not exposed 1, 2
• True erosion: Full-thickness breakdown where the tube penetrates through both graft and conjunctiva, requiring surgical intervention 2

In the largest comparative study, only 3 cases out of 64 eyes (4.7%) required reoperation for actual tube erosion over 24 months, while asymptomatic thinning was far more common 2

Risk Factors Contributing to Thinning

While the evidence doesn't definitively establish causation, several factors are associated with increased risk:

• Inadequate graft coverage of the tube, particularly at the limbus where mechanical stress is highest 3
• Suture-related irritation from braided materials like Mersilene polyester, which may contribute to tissue breakdown 3
• Elevated intraocular pressure and inflammation, which can accelerate tissue degradation 3
• Poor wound closure technique leading to chronic low-grade inflammation 3

Clinical Management Approach

For asymptomatic graft thinning without tube exposure, observation is appropriate with close monitoring. 1 However, vigilance is essential because:

• Thinning can progress to frank erosion requiring surgical revision 2
• The time course is unpredictable, ranging from 6 to 21 months postoperatively 2
• Early detection allows for elective rather than urgent intervention 4

If tube erosion occurs, immediate surgical revision is mandatory with re-coverage using either double-layered pericardial graft, amniotic membrane over pericardium, or alternative donor material 5, 4

Prevention Strategies

Based on consensus recommendations from experienced surgeons:

• Use double-layered graft coverage over the tube to provide redundancy if one layer thins 4
• Ensure adequate Tenon's membrane closure before conjunctival closure to provide additional tissue barrier 3
• Select nylon over braided sutures to minimize chronic irritation 3
• Rotate suture knots posteriorly beneath the graft to create a lower profile 3
• Consider amniotic membrane as an alternative which shows stable thickness on optical coherence tomography over 22 months 5

Critical Caveat

The evidence base consists primarily of retrospective case series with relatively short follow-up (10-24 months). 1, 2, 5 Long-term data beyond 2 years is limited, and the true incidence of late graft failure remains unknown. 1 This underscores the importance of indefinite long-term monitoring in all patients with glaucoma drainage devices and patch grafts. 4