Preoperative Evaluation for Glaucoma Drainage Device Surgery

From Kahook's Essentials Of Glaucoma Therapy
Primary authors
  • Joshua D. Stein, MD, MS

The success of glaucoma drainage device (GDD) surgery depends not only on the technical skills necessary to perform the surgery but also on careful preoperative planning. During the preoperative evaluation, the surgeon must determine whether the patient is a good candidate for the surgery. Moreover, the surgeon must consider whether GDD surgery is the most appropriate option for lowering intraocular pressure (IOP) or whether other medical or surgical procedures would be better choices. The preoperative evaluation can affect the decisions regarding which type of GDD to use during the surgery, the best location to implant the GDD, and it may identify patients at increased risk for intraoperative or postoperative complications. This chapter discusses information that should be gathered and reviewed by the surgeon before performing GDD surgery.

Patient History

Ocular History

It is important to take a thorough ocular history to identify prior conditions and previously performed surgical procedures that may complicate GDD surgery. Specific conditions that one should inquire about include conditions that can result in scarring of the bulbar conjunctiva, including prior eye trauma and incisional surgical procedures, such as trabeculectomy surgery, intracapsular or extracapsular cataract surgery, or strabismus surgery. Previously placed implants, such as scleral buckles, need to be noted preoperatively, as their presence may affect the location of GDD implantation, or there may be extensive scar tissue around such devices, potentially preventing successful GDD implantation. If silicone oil or other substances that can obstruct the lumen of the GDD have previously been placed in the eye, these substances may need to be removed before a GDD can be successfully implanted. Other conditions, such as scleritis, ocular cicatricial pemphigoid, Stevens-Johnson syndrome, or chemical injuries can also cause thinning and scarring of the conjunctiva, which would make GDD surgery challenging, as would conditions that affect eyelid anatomy or the blink reflex. Whether a patient is phakic or pseudophakic and has undergone previous pars plana vitrectomy can also affect where the GDD can be inserted in the eye.

Medical History and Patient-Related Factors

The surgeon should inquire about acute and chronic medical conditions affecting the patient. For example, if a patient has an acute illness causing coughing, sneezing, nausea, or vomiting, it may be wise (when possible) to delay the GDD surgery, as these symptoms can cause sutures to break, wounds to dehisce, and can increase the risk for intraocular bleeding. Inquiring about chronic medical conditions such as obstructive pulmonary disease or back pain, which limit patients from lying flat on their back for the surgery, may affect the decision regarding the type of anesthesia to use for the surgery. Likewise, if a patient cannot lie still for prolonged periods of time due to Parkinson’s or Alzheimer disease, this too can affect the type of anesthesia required. Patients who have a known history of poor adherence to glaucoma medication regimens need to be counseled about the importance of adhering to postoperative medication regimens to help reduce the risk for postoperative infection or inflammation.

Risk of Infection

Patients who are taking medications, such as corticosteroids or immunosuppressant agents, are at increased risk for infection during and after intraocular surgery. Persons who are immunocompromised are also pre-disposed to infection. Moreover, ocular conditions, such as blepharitis, can also lead to infection. In nonurgent surgeries, it is best to prophylactically treat conditions such as blepharitis to reduce the risk for serious infections (eg, endophthalmitis).

Risk of Hypotony

Patients with a history of chronic intraocular inflammation are at increased risk for ciliary body shutdown and postoperative hypotony after any intraocular surgery, including GDD implantation. Depending on the status of the patient’s glaucoma, one may consider using a flow-restrictive glaucoma drainage implant, such as an Ahmed GDD (New World Medical, Inc), or a smaller-sized non–flow-restrictive device, such as a Baerveldt-250 (Abbott Medical Optics [AMO]), instead of a Baerveldt­-350 (AMO) GDD.[1]

Risk of Bleeding

It is important to inquire about medical conditions and medications that can increase the risk for bleeding during surgery. Medical conditions associated with an increased risk for bleeding include liver disease, uremia, blood dyscrasias, and vitamin K deficiency. Medications that increase bleeding include aspirin, warfarin, and clopidogrel. Vitamins and supplements that can increase the risk for bleeding include vitamin E, ginkgo biloba, and garlic. Whenever possible, the patient should inquire with the medical provider who prescribed the medications about the possibility of discontinuing their use temporarily before surgery. If it is unsafe to discontinue use of these products, the surgery can still be performed, although the surgeon should be prepared to deal with any intra- and postoperative bleeding that may occur.

Risk of Suprachoroidal Hemorrhage

A serious, potentially sight-threatening complication of any intraocular procedure, including GDD surgery, is suprachoroidal hemorrhage. During the preoperative period, the surgeon should identify those patients who have multiple risk factors for this complication. Such risk factors include older age, arteriosclerosis, obesity, short axial length, high myopia, prior intraocular surgery (especially pars plana vitrectomy), use of blood-thin-ning agents, and a history of suprachoroidal hemorrhage.[2] In patients who have several of these risk factors, the surgeon should, whenever possible, try to prevent postoperative hypotony. Using flow-restrictive rather than non–flow-restrictive GDDs may be safer in these patients. Other techniques, such as injecting some viscoelastic into the anterior chamber at the end of the procedure or in clinic if profound hypotony occurs, can help reduce the risk for hypotony-related complications in these patients.

Patient Expectations

As part of the informed consent process before surgery, the patient needs to be made aware of the risks, benefits, and alternatives of performing the surgery. It is important to explain that it is not uncommon for patients to require one or more IOP-lowering medications, even if the surgery is successful and the GDD is functioning properly. Patients should also be informed that additional surgical interventions are necessary if the GDD becomes obstructed or does not sufficiently reduce the IOP. Discussing these points with the patient ahead of time and taking the time to answer questions about the surgery during the preoperative period can help prevent the patient from entering into the surgery with unrealistic expectations.

Preoperative Ocular Examination

Best Corrected Visual Acuity

When evaluating a patient for GDD surgery, the surgeon must gauge the patient’s visual potential by assessing his or her best corrected visual acuity (BCVA). In general, GDD surgery is offered to patients who have reasonable visual potential. In eyes known to have poor visual potential, a less invasive surgical procedure, such as cyclophotocoagulation, may be a more appropriate strategy to lower the IOP. Obtaining an accurate preoperative visual acuity is also useful for determining whether the vision has returned to baseline during the postoperative period after GDD surgery. If the GDD surgery is uncomplicated, the BCVA should quickly return to the preoperative level or better (if the patient had decreased vision as a result of microcystic corneal edema associated with elevated IOP), often within the first week. A decline in BCVA after GDD surgery may alert the clinician to a postoperative complication, such as cystoid macular edema or corneal edema.

Refractive Error

With any intraocular surgical procedure, including GDD surgery, obtaining a manifest refraction during the preoperative period can alert the surgeon to a patient’s increased risk for intra- or postoperative complications. For example, patients who have high levels of myopia are at increased risk for a retinal tear or detachment in the postoperative period. These patients would benefit from a careful evaluation of the peripheral retina before proceeding with GDD surgery. Likewise, patients who have high levels of hypermetropia may have nanophthalmos. To reduce the risk for postoperative choroidal effusions in eyes with nanophthalmos, the surgeon may plan to perform prophylactic sclerostomies immediately before implanting the GDD.

Ocular Motility

One of the complications of GDD surgery is extraocular muscle restriction, which can result in binocular diplopia. Determining whether the ocular motility is normal in the preoperative examination can help the clinician identify whether self-report of diplopia after the surgery can be attributed to the surgery or to a pre-existing ocular or systemic condition.


When evaluating whether a patient is an appropriate candidate for GDD surgery, his eyelid function must be considered. Implanting a GDD in eyes with lagophthalmos or exophthalmos can be problematic because the ocular surface in these patients is often inadequately lubricated. Such patients have an increased likelihood of experiencing breakdown of the conjunctiva overlying the GDD. Before proceeding with GDD surgery, these patients may first require surgery to address their eyelid abnormalities. Alternatively, if a patient has significant eyelid abnormalities that are not easily correctable, endoscopic or transscleral diode cyclophotocoagulation may be more appropriate than GDD surgery. Because patients can develop ptosis as a result of GDD surgery, the clinician should document the presence of any pre-existing ptosis found in the preoperative evaluation.


One of the most important factors to consider when evaluating patients for GDD surgery is the health of the bulbar conjunctiva. Careful inspection is required to evaluate the viability of the bulbar conjunctiva in the quadrant where the surgeon is considering implanting the GDD. This is particularly critical in patients who have risk factors for scarring, such as those with a history of intraocular surgery, ocular trauma, or an acid or alkali burn. Extensive conjunctival scarring in the quadrant under consideration may make it technically challenging for the surgeon to adequately mobilize enough healthy conjunctival tissue to completely cover the implant and overlying patch graft. If conjunctival scarring is identified preoperatively, the surgeon may elect to implant the GDD in a different quadrant that has healthier bulbar conjunctival tissue. If considerable conjunctival scarring is present, the surgeon may need to perform a conjunctival autograft with tissue from an adjacent quadrant or from the contralateral eye. Alternatively, it may be preferable to consider performing a cyclodestructive procedure instead of attempting GDD surgery in these patients. In addition to visual inspection of the bulbar conjunctiva to identify scar tissue, one can instill a topical anesthetic in the eye and then gently manipulate the conjunctiva with a cotton tip applicator to get a sense of how adherent it is to the underlying sclera.


Evaluating the quadrant where the GDD will be implanted for scleral thinning is necessary to avoid scleral perforation during the surgery. If the sclera is too thin, as a result of scleromalacia or other causes, it may be unsafe to attempt to implant a GDD in the affected quadrant. It is also useful­ during the preoperative period to check for evidence of previously created sclerostomies from prior glaucoma or retinal procedures. The surgeon would want to avoid prior trabeculectomy flaps and other sclerostomies during implantation of the GDD by rerouting the tube away from these locations. Inserting the tube through a previously created sclerostomy may lead to leakage of aqueous around the lumen of the tube, resulting in postoperative hypotony.


During preoperative examination of the cornea, the surgeon should pay careful attention to the corneal endothelium. Corneal guttae on the surface of the endothelium may be a sign of Fuchs’ endothelial dystrophy. Patients with Fuchs’ endothelial corneal dystrophy are at increased risk for corneal decompensation after GDD surgery. In these patients, and with other patients who will likely undergo endothelial or penetrating keratoplasty in the future, the surgeon should plan to position the GDD as far away from the corneal endothelium as possible. If the patient is pseudophakic or aphakic, the surgeon should make every attempt to implant the GDD in the posterior chamber, and, if the patient had previous pars plana vitrectomy surgery, positioning the GDD in the vitreous cavity can reduce the risk for corneal decompensation after GDD surgery. It is also useful for the clinician to examine the cornea for scarring, as this may limit visualization of the tip of the tube both during the surgery and in the postoperative period. Often, patients with markedly elevated IOP before GDD surgery have microcystic corneal edema present. In these patients, temporarily lowering the IOP preoperatively with the use of aqueous suppressants and oral or intravenous carbonic anhydrase inhibitors may help clear up the edema and improve visualization of the tip of the tube during the surgery.

Anterior Chamber

When assessing the anterior chamber, one should take note of the anterior chamber depth and the presence of material in the anterior chamber that can get into and occlude the tube once it is inserted in the eye. There are several conditions that can result in the presence of a shallow anterior chamber. Individuals with high levels of hypermetropia often have short axial lengths and shallow anterior chambers. Persons who are phakic and who have large lenses that cause bulging forward of the iris can also exhibit shallowing of the anterior chamber. Scarring from previous intraocular surgery or trauma can result in iridocorneal touch, which can limit the depth of the anterior chamber. In all of these scenarios, it may be technically challenging to position the tip of the tube in the anterior chamber far enough away from the corneal endothelium. When possible, addressing the underlying condition (eg, by performing laser peripheral iridotomy or cataract extraction) may be necessary prior to or concurrent with GDD implantation. Materials that can occlude the lumen of the GDD include vitreous, fibrin, or blood. If these substances are known to be present during the preoperative examination, the surgeon may need to address them before implanting the GDD into the eye, by performing a vitrectomy to remove vitreous or an anterior chamber washout to remove extensive hemorrhage or fibrin. In patients with active uveitis, use of topical, subconjunctival, or sub-Tenon’s corticosteroids during the preoperative period and at the time of surgery can limit postoperative inflammation.


If neovascularization of the iris is present in the preoperative evaluation, an injection of an anti-vascular endothelial growth factor (VEGF) agent should be considered. These agents can reduce the risk for intra- and postoperative bleeding during GDD surgery, and, occasionally, anti-VEGF agents can cause regression of neovascularization of the angle and resolution of the IOP elevation, thus negating the need for GDD surgery.[3] If anterior segment neovascularization is noted, the surgeon may consider indirect panretinal photocoagulation concurrent with GDD surgery. If the surgeon notes signs of iridocorneal endothelial (ICE) syndrome during the preoperative examination, he or she should try, whenever possible, to implant the GDD into the posterior chamber to reduce the chance that ICE material can proliferate into the lumen and obstruct the GDD.

Status of the Lens

When planning for GDD surgery, the surgeon should take note of whether the patient’s eye is phakic, pseudophakic, or aphakic. Patients who are phakic should be informed that the surgery can hasten the development of cataracts. In addition, the surgeon should try to leave the tip of the tube short so that it will not get in the way during future cataract surgery. In patients who are phakic, it is necessary to implant the GDD in the anterior chamber. In patients with pseudophakia or aphakia, the surgeon may want to consider implanting the GDD into the posterior chamber, to reduce the risk for postoperative corneal decompensation in high-risk patients, such as those with Fuchs’ endothelial dystrophy.

Intraocular Pressure

All patients who are being considered for GDD surgery have levels of IOP that are higher than desirable, which increases their risk for irreversible glaucomatous damage. During the preoperative evaluation, the surgeon should think about whether there is a need for short-term IOP control, long-term IOP control, or both. All patients who present with extensive glaucomatous damage will require long-term IOP control. However, there will be a subset of these patients who have such extensive damage to their optic nerve already or a high enough IOP that they are also at risk for vision loss in the short term if the surgeon implants a nonvalved GDD; it takes weeks before the GDD becomes functional. Other patients may present with a markedly elevated IOP, as a result of anterior segment neovascularization, uveitis, or exposure to corticosteroids, but the eye care provider has identified the problem before any glaucomatous damage has occurred. In this subset of patients, the goal of GDD surgery is to immediately lower the IOP to a safer level; yet, achieving a very low IOP over the long term may not be necessary because these patients have healthy nerves with little or no damage. In patients who present with markedly elevated IOP and require short-term IOP control, implanting a flow-restrictive GDD, such as an Ahmed FP7 (New World Medical, Inc), can be effective at immediately lowering the IOP. If the surgeon prefers to implant a nonvalved GDD, such as a Baerveldt GDD, because these devices are tied off for the first few weeks after implantation, venting slits must be created in the lumen of the GDD to help regulate the IOP in the short term until it is safe to open the tube. There is some evidence that GDDs with larger plates can achieve a lower IOP in the long term than can other devices with smaller plates. An ongoing randomized clinical trial is testing this theory.[4] Assuming this theory is correct, many surgeons prefer to implant GDDs that have larger-sized plates, such as the Baerveldt-350, in patients with advanced glaucoma who require a long-term low target IOP (in the low teens or lower), rather than devices with smaller-sized plates, such as an Ahmed GDD. Many of these patients can tolerate an IOP that is slightly above target for a few weeks after GDD implantation as they wait until it is safe for the tube to open and aqueous to flow through the GDD.

A small subset of patients have advanced glaucoma requiring long-term IOP control from a GDD with a larger plate size (eg, a Baerveldt-350), yet the preoperative IOP is also so high, or the damage to the optic nerve so advanced, that it would be too risky to implant a nonvalved GDD and simply wait a few weeks before the IOP can be lowered by opening the GDD. The surgeon has several options for managing these patients. One option is to implant a nonvalved Baerveldt-350 GDD and create venting slits in the lumen of the tube, with the hope that the venting slits will effectively lower the IOP in the short term until it is safe for fluid to flow through the GDD and collect around the plate. A second option is to combine the implantation of the Baerveldt-350 with an orphan trabeculectomy; one that is performed for the sole purpose of achieving an immediate lowering of IOP during the postoperative period until the GDD becomes functional. A third option is to perform a double-tube surgery, simultaneously implanting a Baerveldt-350 in the superotemporal quadrant to achieve long-term IOP control and an Ahmed S3 in the superonasal quadrant that, along with the use of aqueous suppressants, can achieve short-term IOP control until the Baerveldt-350 becomes functional. A fourth option is to implant a biplate Ahmed GDD. These devices contain a flow-restrictive element so that the IOP can be lowered immediately, but they also have 2 plates and thus a larger surface area for aqueous to accumulate, providing long-term IOP control.

Extent of Glaucomatous Damage

When evaluating a patient for GDD surgery, the surgeon must carefully consider the extent of glaucomatous damage present. Patients who have elevated IOPs with minimal or no damage to the optic nerve and retinal nerve fiber layer may not require a very low IOP during the post-operative period. In contrast, those who have extensive visual field loss and cupping of the optic nerve may require very low target pressures after the surgery. According to preliminary results of the Ahmed Baerveldt Comparison Trial, patients with extensive damage who require a very low IOP after GDD surgery may do better with a Baerveldt-350 than an Ahmed GDD.[5]


In patients who are undergoing GDD surgery for neovascular glaucoma, it is useful to inspect the retina to determine whether the patient would benefit from concomitant panretinal photocoagulation along with the GDD implantation.


Before proceeding with GDD surgery, it can be helpful to perform gonioscopy to identify the presence of peripheral anterior synechiae in the region of the drainage angle where the GDD will be implanted. In patients with pseudophakia, who have extensive peripheral anterior synechiae, the surgeon can avoid the synechiae and position the tube tip away from the corneal endothelium by implanting the GDD in the posterior chamber rather than the anterior chamber. Sulcus placement of the GDD can be ideal in this situation. In patients with neovascular glaucoma, preoperative gonioscopy can identify areas of active angle neovascularization, so these locations can be avoided during tube insertion. Patients with Fuchs’ heterochromic iridocyclitis are also at increased risk for bleeding with GDD implantation, and gonioscopy can alert the surgeon to be prepared for this postoperative complication.


During the preoperative assessment, the surgeon should check that there are no pieces of hardware, such as scleral buckle elements, present in the location where the plate of the GDD will be attached to the globe. If the patient has a scleral buckle present, the surgeon should consult with a retina specialist to determine whether it is safe, if necessary, to remove the element to free up space for the GDD. Some of the more modern scleral buckles may not require removal, as often the plate of the GDD can be positioned posterior to the buckle or sewn to the buckle element itself. If silicone oil is present in the eye, it will likely need to be removed before GDD implantation because it can obstruct the GDD or exit the eye via the GDD.

Key Points

  1. Decisions made during the preoperative evaluation can play an important role in the ultimate success of GDD surgery.
  2. Factors affecting the decision about what type of GDD implant to use include the extent of glaucomatous damage present, the target IOP desired, the absolute level of IOP at the time of surgery and whether the eye can likely sustain this level of pressure for several weeks after implantation of a non–flow-restrictive device, and the type of glaucoma.
  3. Important factors that affect the location of GDD placement in the eye include the presence of conjunctival scarring from prior surgery, whether the patient is phakic or pseudophakic, the depth of the anterior chamber, whether the eye has previously undergone pars plana vitrectomy, and the presence of previously placed hardware, such as scleral buckle elements.
  4. Surgeons should plan ahead and, on occasion, modify their technique in patients at increased risk for infection, hypotony, bleeding, or suprachoroidal hemorrhage. Whenever possible, discontinuation of the use of medications that may predispose patients to intraoperative or postoperative bleeding should be considered.
  5. Prior to proceeding with GDD surgery, it is essential to take the time to have a discussion of the risks, benefits, and alternatives of the surgery with the patient, to answer questions she may have, and to help set realistic expectations.


  1. Schwartz KS, Lee RK, Gedde SJ. Glaucoma drainage implants: a critical comparison of types. Curr Opin Ophthalmol. 2006;17(2):181-189.
  2. Jeganathan VS, Ghosh S, Ruddle JB, et al. Risk factors for delayed suprachoroidal haemorrhage following glaucoma surgery. Br J Ophthalmol. 2008;92(10):1393-1396.
  3. Duch S, Buchacra O, Milla E, et al. Intracameral bevacizumab (Avastin) for neovascular glaucoma: a pilot study in 6 patients. J Glaucoma. 2009;18(2):140-143.
  4. Barton K, Gedde SJ, Budenz DL, Feuer WJ, Schiffman J; Ahmed Baerveldt Comparison Study Group. The Ahmed Baerveldt Comparison Study: methodology, baseline patient characteristics, and intraoperative complications. Ophthalmology. 2011;118(3):435-442.
  5. Budenz DL, Barton K, Feuer WJ, et al. Treatment outcomes in the Ahmed Baerveldt Comparison Study after 1 year of follow-up. Ophthalmology. 2011;118(3):443-452.