Preoperative Evaluation of Patients Undergoing Drainage Implant Surgery

Summary

The ultimate goal to be wished for with the use of a glaucoma implant is pressure lowering. Most studies reported in the literature, with the use of glaucoma implants, define success as pressures of 21 mmHg or lower. Most severe glaucoma cases, and this usually includes most cases in which a glaucoma implant will be used, require pressures in the low teens, a level often difficult to obtain with glaucoma implants. The pressure-lowering effect of the implant depends on the thinness and permeability of the bleb over the plate. This can be achieved with the use of antifibrosis medication, e.g. systemic steroids, or by modification of the surgical technique, e.g. supra-Tenon's plate placement. Therefore, preoperatively, if a very low pressure needs to be achieved, the feasibility of the use of systemic medications such as steroids and non-steroidal anti-inflammatories needs to be considered. Should the patient have any medical condition or conditions precluding systemic antifibrosis medication use, such as diabetes or hypertension, then modification of the surgical procedure needs to be considered, and this will always include the choice of implant to be used. Supra-Tenon's placement always needs a single-plate implant, the ideal size being the single-plate Molteno implant.

A prospective trial is being planned in which trabeculectomy is being compared to the use of glaucoma implants as a primary procedure for the management of medically uncontrolled glaucoma. The preoperative evaluation in patients undergoing primary implant use would be the same as for patients in whom implants were being used not as a primary procedure, the one exception being that previous glaucoma surgery need not be considered, usually indicating that all quadrants of the eye are available for implant surgery.

There are many factors that have to be evaluated preoperatively in relation to the use of glaucoma implants. Careful preoperative evaluation can assist in achieving ease of the surgical procedure, fewer postoperative complications, and better results in the lowering of the intraocular pressure.

Introduction

In deciding on the use of glaucoma implants for the management of uncontrolled intraocular pressure, a number of important factors need to be taken into consideration. These factors may be summarized as follows:

1.
The underlying cause of the glaucoma.
2.
The anatomy of the eye and orbit under consideration.
3.
Age and ethnicity of the patient.
4.
Previous surgical procedures done in the eye, as well as any additional pathology that may be present, such as cataracts or corneal disease.
5.
Choice of implant, which incorporates the final intraocular pressure desired to be achieved.

Each of these preoperative factors will be discussed.

Etiology of the Glaucoma

Drainage implants were originally introduced to treat refractory glaucomas. These included aphakic and pseudophakic glaucomas, uveitic glaucoma, neovascular glaucoma, glaucoma associated with corneal transplants, congenital glaucomas due to iridocorneal dystrophies such as iridocorneal endothelial dystrophy, and in eyes where previous filters had failed. With the introduction of the use of antimetabolites, such as 5-fluorouracil and mitomycin C, many of the conditions mentioned in this group are now treated with conventional filtering surgery first. Exceptions include neovascular glaucoma, extensive scarring of the conjunctiva, and congenital glaucoma with iridocorneal dysgenesis or with previously failed goniotomy or trabeculotomy. Aphakic glaucoma and glaucoma associated with corneal transplants also do better with glaucoma implants. The 3-year follow-up comparing tube shunt surgery to trabeculectomy in pseudophakic patients showed a higher success rate with tube shunts. Should a new study comparing tube shunts to trabeculectomy in non-pseudophakic patients show similar results, this could result in the choosing of tube shunt procedures more frequently as the glaucoma procedure of choice.

Anatomy of the Eye and Orbit

The anatomy of the eye and orbit will determine what size implant can be placed into the eye. The optimal positioning of five different glaucoma implants was evaluated using cadaver eyes of varying axial lengths. The purpose of the study was to determine the maximum distance that different implants could be placed from the limbus in different quadrants of the eye so that the posterior edge of the implant was at least 2 mm from the optic nerve. The average maximum distance posterior to the limbus of the anterior plate edge ranged between 9.0 mm and 15 mm in the superotemporal quadrant for the glaucoma implants tested. The distances for superonasal, inferonasal, and inferotemporal quadrants ranged between 8.0–14 mm, 9.0–14 mm, and 11.0–17 mm, respectively. The Molteno implant could be placed most posteriorly in relation to the limbus without impinging on the optic nerve, whereas the Ahmed implant was least amenable to posterior placement. The study shows that preoperative evaluation of both orbit size and eye length is important in deciding on the type of implant to be used as well as quadrant placement of the device. The evaluation of orbit size is usually done intraoperatively, whereas nanophthalmos can be evaluated with a preoperative A-scan. Nanophthalmic eyes will be unable to have large implants, as impingement of the optic nerve can occur. The possibility of postoperative motility problems in an eye with a small or sunken orbit, due to bleb size, needs to be evaluated prior to choosing the draining implant. Implants giving a lower-profile bleb, such as the Baerveldt implant, may have to be used.

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