- Which among the following can be used as a patch graft?
a. Amniotic membrane
c. Dura mater
f. All of the above.
- Which of the following is not a risk factor for tube exposure?
a. Type of implant
b. Number of anti-glaucoma medication
c. Previous ocular surgery.
d. Young age
- Which is the preferred site for Ahmed glaucoma valve implantation?
d. Both a and b
- Which among the following is not the possible mechanisms behind tube shunt exposure?
a. Excessive tissue tension overlying the tube shunt
b. Immune-mediated inflammatory processes to scleral patch graft
c. Ischemic damage to the conjunctiva
d. Immunological response towards Glaucoma drainage device
- Which among the following signs on follow up is suggestive of impending erosion?
a. Observation of loss of conjunctival capillaries over the tube, usually 1 to 3 mm from the corneoscleral junction
b. Tube corneal touch
c. Posteriorly placed valve
d. Large encysted bleb
Answer 1. f (all of the above)
Various tissues have been advocated as graft material, including human sclera, pericardium, dura mater, fascia lata, cornea, amniotic membrane and porcine intestinal submucosa (KeraSys).
Reference Wolf, A., Hod, Y., Buckman, G., Stein, N., & Geyer, O. (2016). Use of Autologous Scleral Graft in Ahmed Glaucoma Valve Surgery. Journal of Glaucoma, 25(4), 365–370.
Answer 2. a (type of implant)
Some identified risk factors for tube exposure include younger age, presence of inflammation, previous ocular trauma, use of topical corticosteroids, number of preoperative glaucoma medications.
Diabetes, Previous trabeculectomy, and number of pre-shunt glaucoma medications may decrease overall conjunctival health leading to ischemia and conjunctival and graft thinning eventually leading to tube shunt exposure
Reference: Xia, MD and Albert S. Khouri, Scleral Patch Graft Melt Post Tube Shunt Surgery: Grading and Identification of Risk. Journal of Glaucoma.2018,27(8),717-721.https://www.researchgate.net/publication/325587777_Scleral_Patch_Graft_Melt_Post_Tube_Shunt_Surgery_Grading_and_Identification_of_Risk_Factors
Multiple hypotensive medications cause instability of tear film and loss of goblet cells, leading to poor ocular surface status and thereby possibly increasing the likelihood of tube exposure
Reference: V, Zangalli C, Moster MR, et al. Evaluation of risk factors for glaucoma drainage device-related erosions: a retrospective case-control study. J Glaucoma. 2013 https://www.ncbi.nlm.nih.gov/pubmed/24326968
Answer 3. a (supero-temporal)
Supero-temporal quadrant is the preferred surgical site because of easier surgical access, less postoperative exposure, maximal coverage of the tube by patch graft and plate by the eyelid and less diplopia.
Reference: R, Trope GE, Buys YM, et al. Intermediate-term outcome and success of superior versus inferior Ahmed glaucoma valve implantation. J Glaucoma. 2008;17:584–590.
Answer 4. d (Immunological response towards Glaucoma drainage device)
Excessive tissue tension overlying the tube shunt, immune-mediated inflammatory processes, ischaemic damage to the conjunctiva, repeated mechanical trauma by eyelid blinking, abnormal positioning of the tube, and absorption or melting of the graft reinforcing the tube are the causes for tube exposure.
Reference: Tian Xia, MD and Albert S. Khouri, Scleral Patch Graft Melt Post Tube Shunt Surgery: Grading and Identification of Risk. Journal of Glaucoma.2018,27 (8),717-721.
Answer 5. a (Observation of loss of conjunctival capillaries over the tube, usually 1 to 3 mm from the corneoscleral junction)
Erosion of the tube through host sclera or a patch placed at the time of surgery may occur over a period of weeks to months in some cases. During follow-up examinations, observation of loss of conjunctival capillaries over the tube, usually 1 to 3 mm from the corneoscleral junction, is an indication of impending erosion through the surface
Reference: Minckler, D. S., Francis, B. A., Hodapp, E. A., Jampel, H. D., Lin, S. C., Samples, J. R., … Singh, K. (2008). Aqueous Shunts in Glaucoma. Ophthalmology, 115(6), 1089–1098. https://journals.lww.com/glaucomajournal/Abstract/2005/08000/Incidence_and_Management_of_Encapsulated_Cysts.7.aspx