• Glaucoma

    Presently the most commonly used glaucoma implants are an offshoot of the original paralimbal acrylic implant developed by Molteno in 1967.1 The long tube Molteno implant developed in 1973 is the prototype for the present day long tube implants such as the Baerveldt implant, Ahmed implant, and Krupin disc.

    Many studies and reviews involving glaucoma implant use have appeared in the literature over the past 40 years. Nonetheless there remain many aspects of glaucoma implant use that need to be considered if the success of implant use is to be improved. This review will try to answer some questions that the users of glaucoma implants may themselves have pondered:

    • Whether the indications for implant use have changed
    • Which implants to choose
    • Whether plate size or biomaterial matter
    • Whether the common complications seen over the past 40 years have changed

    The glaucoma implants in use today come in different sizes, are valved or nonvalved, and consist of a variety of biomaterials. Whether these features influence the efficacy of the implants, and therefore the choice of implants, needs to be considered.
    Initially implants were used in patients with previously failed glaucoma surgery and in patients known to do poorly with conventional glaucoma surgery. Included in the latter group were such conditions as neovascular glaucoma, uveitis, aphakic and pseudophakic glaucoma, paralimbal conjunctival scarring, and congenital glaucoma with iridocorneal dysgenesis.

    While glaucoma implants may still be the procedure of choice in some of these conditions, ie, neovascular glaucoma, others may now be treated with conventional glaucoma surgery plus antimetabolite therapy or with the newer devices such as the Express shunt under a scleral flap.2 Furthermore the glaucoma implant may find a place as a primary surgical procedure.


    A recent study comparing the implants to trabeculectomy in patients who had previous failed trabeculectomy or were pseudophakic with uncontrolled glaucoma concluded that results obtained with the implants were comparable to those seen with trabeculectomy.3 A study comparing trabeculectomy to implant use in patients who have not had previous glaucoma surgery is planned. Thus indications for implant use have certainly changed and will probably change more in the future.

    Which implant to choose and does size or biomaterial matter?

    Studies comparing different implants, although mainly retrospective, and containing different variables, still seem to indicate that by and large, the pressure lowering effect of different implants, including plate size, is similar if followed for periods of greater than 12 months.4 Certain factors may influence the choice of implant:

    • Hypotony is less likely to occur with valved implants, but in nonvalved implants, this can be equally prevented with the use of stents and by tying off the tubes.
    • Valved implants are more likely to develop a more aggressive hypertensive phase and possibly a thicker bleb capsule due to the influx of initial glaucoma aqueous, which has been shown to contain proinflammatory substances.

    Studies comparing plate size seem to indicate that although initially the larger plates (double plates or large Baerveldt plates) may have a better pressure lowering effect, in the long term, this advantage is lost. This would seem to indicate that double plate implants do not offer any advantage over the smaller single plates, which are more easily inserted.5 The ideal plate size may be similar to the single plate varieties available, namely the Ahmed 184 mm2, Baerveldt 250 mm2, and Molteno3 175 mm2.

    At present the biomaterials composing the different implants may or may not influence efficacy, as studies comparing silicone to polypropylene Ahmed implants have shown.6 The concurrent use of antimetabolites with implants has not been shown to be of any significant use. Analyzing the literature on the efficacy of the presently available implants, the single plate implants seem to be equivalent in efficacy, with the valved implants perhaps having a more intense hypertensive phase and a slightly less efficacious pressure lowering effect due to a thicker capsule over the plate.7

    A recent study has shown that the distance that an implant is placed from the limbus is of some importance. The maximum distance that an implant can be placed from the limbus, before encroaching on the optic nerve, varies between different implants; the single plate Molteno can be placed most posterior, whereas the Ahmed FP7 and S2 are the least amenable to posterior placement.8

    Complications seen with implants

    Hypotony, one of the early complications, may still occur, but as mentioned, may be addressed by valved implants or obstructing the tube by various methods.

    The most difficult complication still remains bleb encapsulation. Management of bleb encapsulation still remains elusive, but can be somewhat alleviated by the use of the original Molteno antfibrosis regimen of steroid, nonsteroidal anti-inflammatories, and colchicine. The placement of the implant in a supratenons pocket has also been described.9 The use of antimetabolites such as mitomycin C has not proven to be useful.

    The problem of encapsulation remains an area where further research would be useful. Preventing aqueous from the plate surface in the immediate post operative period, as well as for a few weeks thereafter, may result in less fibrosis of the bleb by preventing the effects of the initial glaucomatous aqueous and its contents, such as prostaglandins and transforming growth factor beta (TGFb), from inciting early inflammation.

    Summary

    • The implant of choice in terms of efficacy may be any of the larger single plate varieties.
    • Double plate implant use may have become obsolete.
    • The indications for implant use have changed and may soon include its use as a primary procedure for failed medical therapy.
    • The need to decrease bleb encapsulation still remains the biggest challenge, associated with glaucoma implant use.

    References

    1. Molteno AC. New implant for draining in glaucoma.Br J Ophthalmol. 1969;53(3):161-168.
    2. Dahan E, Carmichael TR. Implantation of a miniature glaucoma device under a scleral flap.J Glaucoma. 2005;14(2):98-102.
    3. Gedde SJ, Schiffman JC, Feuer WJ et al. Treatment outcomes in the tube versus trabeculectomy study after one year of follow-up.Am J Ophthalmol. 2007;143(1):9-22.
    4. Tsai J, Johnson CC, Dietrich MS. The Ahmed shunt versus the Baerveldt shunt for refractory glaucoma: a single-surgeon comparison of outcome.Ophthalmology. 2003;110(9):1814-1821.
    5. Molteno AC, Bevin TH, Herbison P, Houliston MJ. Otago glaucoma surgery outcome study: long-term follow-up of cases of primary glaucoma with additional risk factors drained by Molteno implants.Ophthalmology. 2001;108(12):2193-2200.
    6. Law SK, Nguyen A Coleman AL, Caprioli J. Comparison of safety and efficacy between silicone and polypropylene Ahmed glaucoma valves in refractory glaucoma.Ophthalmology. 2005;112(9):1514-1520.
    7. Nouri-Mahdavi K, Caprioli J. Evaluation of the hypertensive phase after insertion of the Ahmed glaucoma valve.Am J Ophthalmol. 2003;136(6):1001-1008.
    8. Kahook MY, Noecker RJ, Pantcheva MB, Schuman JS. Location of glaucoma drainage devices relative to the optic nerve.Br J Ophthalmol. 2006;90(8):1010-1013.
    9. Freedman J. Update on Tube-Shunt Procedures for Glaucoma. In: Grehn F, Stamper R, eds. Essentials in Ophthalmology.Glaucoma. New York: Springer-Verlag; 2006:173-185.

    Author Disclosure

    The author states that he has no proprietary interest in any of the devices discussed in this article.