Glaucoma Tube Shunt Exposure
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Tube shunt exposure is a rare but potentially vision-threatening complication of glaucoma drainage device surgery. While the exact pathophysiology is not completely understood, several patient and surgery-related risk factors have been associated with increased exposure risk. A wide variety of surgical techniques primarily aimed at fixating and covering the device have been proposed to mitigate the risk of primary exposure and reduce the risk of recurrent exposure following tube shunt revision.
Disease Entity
Glaucoma drainage devices (GDDs) are increasingly utilized as first-line alternatives to trabeculectomy for glaucoma filtering surgery (1). These devices consist of a plate often sutured to the sclera and valved or un-valved tubing that enters the globe into the anterior chamber, sulcus, or vitreous chamber via the pars plana. Tube shunt exposure is a rare but feared postoperative complication of GDDs. Erosion of the overlying tissues can result in hypotony due to fluid leakage, dislodgement of the device, and create an entry point for microbes leading to vision-threatening infections including blebitis and endophthalmitis (2, 3). Exposure can occur anywhere over the tube or plate but is more common over the tube and occurs most frequently close to the limbus (2, 4).Add text here
Disease
The incidence of tube shunt exposure varies by study. The Tube versus Trabeculectomy (TVT) study found shunt exposure in 4.7% (5 out of 107 eyes) after 5 years of follow up. (5). A large meta-analysis from 2010 including 38 studies with 3255 eyes found an incidence of shunt exposure of 2.0+/-2.6% (6).
Etiology
Risk Factors
Several patient and surgical factors have been found to modify the risk of tube exposure. The repeatability across studies varies by risk factor and no single risk factor has been identified in every study.
1. Patient factors
Inflammation and ischemia
Chronic inflammation and ischemia have been postulated to cause microvascular damage to the scleral and conjunctival vasculature resulting in increased risk of erosion and tube exposure. Uveitic glaucoma and inflammation requiring prolonged steroid use prior to GDD surgery have been identified as risk factors for tube exposure among pediatric and adult patients (9, 10). The data on ocular tissue ischemia from microvascular damage secondary to diabetes is more conflicting with Chaku et al. (2016) finding that diabetes was less common among exposure cases compared to controls (10). In contrast, a study by Koval et al. (2013) identified neovascular glaucoma as a risk factor for tube exposure with the majority of neovascular glaucoma cases being secondary to diabetes (11). These conflicting results may reflect small sample sizes and the wide spectrum of diabetic eye disease ranging from minimal ocular involvement to significant ocular ischemia seen in cases of neovascular glaucoma.
Prior ocular surgeries
Prior ocular surgeries and procedures have been proposed to increase the risk of tube exposure. Prior trabeculectomy with antifibrotic agents was associated with increased risk of tube exposure in one study with the use of antifibrotic agents thought to contribute to conjunctival breakdown (12). Koval et al (2013) also identified prior trabeculectomy as a risk factor while Byun et al. found prior incisional surgeries to be a risk factor for exposure (8, 11). In contrast, Levinson et al. (2015) did not find prior incisional surgery to be a risk factor for exposure however it was noted that higher rates of exposure were found for sequential GDDs compared to primary GDDs. Sequential GDDs implanted in both superior and inferior quadrants were found to have higher rates of exposure compared to primary implants (2). Intravitreal injections may also pose a risk for exposure as a study from 2020 by Lui et al. found that recurrent intravitreal injections among patients with neovascular AMD was associated with an increased risk of recurrent tube exposure events (13).
Pediatric patients
While age has not been consistently documented as a risk factor for exposure among adult populations (12), higher risk among pediatric patients has been documented (9, 10). The causative factors are thought to be multifactorial with increased risk being attributed to more frequent eye rubbing behavior and mechanical factors relating smaller orbital volume and increased device mobility. One case-control study found that, younger age, prior incisional surgery, and concomitant surgery at the time of GDD implantation were associated with increased risk of exposure among pediatric patients (14).
Additional demographic factors
Other demographic factors, such as sex and race/ethnicity have shown increased risk in some studies, but results have not been widely replicated. Muir et al. (2014) found that female sex was associated with higher risk of exposure, possibly secondary to smaller on-average orbital volume compared to male patients. Koval et al (2013) found that Hispanic patients were at higher risk of exposure (11). After surgical repair of tube exposure, Huddleston et al. (2013) found that black patients were at increased risk of re-exposure while Thompson et al. (2017) found that white patients were at increased risk of recurrence (4, 15). Variability between studies has been attributed to relatively small sample sizes along with variability in patient demographics between study centers. While associations have been found between socioeconomically deprived areas and delayed time to glaucoma surgery, the relationship between socioeconomic background and risk of postoperative complications including tube exposure has not been well characterized and warrants further investigation (7, 16).
2. Surgical factors
Tube shunt location
Inferior quadrant tube shunt placement has been associated with higher risk of exposure compared to superior quadrant placement (2, 17). Levinson et al (2015) found exposure rates of 12.8% and 5.4% for inferior quadrant and superior quadrant placement, respectively. The highest risk of exposure was found in the inferior-nasal quadrant (2).
Types of drainage devices
Neither the type of shunt material (silicone or polypropylene), the type of drainage device (Ahmed, Baerveldt, Molteno), nor the size of the drainage device (plate size or number of plates) have been associated with increased risk of exposure (6, 18, 19).
Tube shunt covering and patch grafts
Early studies where GDDs were placed without overlying patch graft material showed exposure rates as high as 30% (20). Over subsequent years, a wide variety of patch graft materials have been introduced ranging from autologous tissues including conjunctiva, tenons, sclera and tragal perichondrium to processed/preserved tissues including cornea, sclera, pericardium, fascia lata, dura mater, amniotic membranes, buccal mucous membranes, and bioengineered collagen matrices. Data on the risk of exposure across patch graft materials is variable with some studies showing no association, others providing evidence for reduced risk with double layered pericardium, and some providing evidence for increased risk with bovine pericardium (7, 21). Many studies cite small sample sizes and retrospective designs as limitations with very few prospective studies addressing the subject. A prospective randomized clinical trial from 2019 comparing amniotic membrane-umbilical cord (AM-UC) and Tutoplast® pericardium patch grafts used anterior-segment OCT to measure graft tissue thinning and found less graft thinning with AM-UC compared to pericardium. No statistically significant difference was found in terms of exposure rate during follow up with a single case of tube exposure in the AM-UC group and two cases in the pericardium group (22).
Surgical techniques utilizing the patient’s own scleral tissue have also been described to cover and fixate the tube and have been used as effective alternatives to patch grafts especially in areas where access to patch graft materials is limited. A variety of techniques including scleral flaps, long scleral tunnels, and interrupted “double scleral tunnels” placed in tandem have been described (7). A retrospective study from 2024 found a rate of tube exposure of 6.9% after 5 years of follow up in 204 eyes that underwent GDD surgery utilizing a long scleral tunnel technique without placement of an overlying patch graft (23).
3. Concomitant ocular surgeries
The impact of combined surgeries on the risk of tube exposure is controversial with conflicting evidence across studies. In two prospective case series, Hoffman et al. (2002) and Scott et al. (2000) found no increased risk of exposure when GDD surgery was combined with cataract extraction or vitrectomy, respectively (24, 25). In retrospective studies, including those examining adult and pediatric populations, combined surgeries have been identified as risk factors for subsequent exposure (4, 14). These conflicting findings may reflect differences in study design (prospective versus retrospective), variable length of follow-up post-operatively, and the relative infrequency of tube exposure with modern surgical techniques.
General Pathology
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Pathophysiology
Although the underlying mechanisms of tube exposure are not fully understood, timing appears to play a role. Early tube exposure, occurring within the first few months after surgery, has been attributed to surgical wound dehiscence and high-grade immune responses leading to rapid melting of the overlying ocular tissues. In contrast, late tube exposure, developing after many months-to-years post-operatively, is thought to result from a low-grade immune response leading to gradual thinning of overlying tissues. Mechanical factors including increased conjunctival tension over the tube, rubbing of the eyelid, and tube mobility leading to friction between the device and overlying tissues may also play a role (7). This process may be more pronounced in populations with compromised conjunctival tissue resulting from pre-existing inflammation, ischemia, exposure to ocular surface irritants, or scarring from prior trauma or ocular surgery including prior exposure to antifibrotic agents such as mitomycin C (7, 8).
Primary Prevention
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Diagnosis
History
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Physical Examination
Tube exposure can be asymptomatic at time of diagnosis but when symptoms are present, they
can range from minor eyelid irritation and epiphora to severe photophobia, blurred vision, and
eye pain. Since cases of late exposure have been documented years post-operatively and patients
can present without symptoms, complete examination of the device and surrounding tissues is
necessary during routine follow up. Instructing patients to look down (or up in cases of inferiorly
placed devices) with the examiner retracting the eyelid enables examination of both the tubing
and plate along with their overlying tissues. Application of fluorescein stain over the device
allows for inspection of fluid leaks and can help distinguish cases of severe tissue thinning from
device exposure. In cases of exposure, presence of concomitant infection determines the timing
of intervention and in severe cases may necessitate complete removal of the device (7). Signs of
infection at presentation may include eyelid erythema, whitening of the bleb with surrounding
conjunctival injection, purulent discharge, anterior chamber inflammation with or without
hypopyon, and vitritis.
Signs
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Symptoms
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Clinical diagnosis
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Diagnostic procedures
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Laboratory test
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Differential diagnosis
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Management
General treatment
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Medical therapy
In rare cases, spontaneous closure of small conjunctival defects after starting topical antibiotic ointment for infection prophylaxis has been reported. Nevertheless, the treatment of tube shunt exposure is primarily surgical.
Medical follow up
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Surgery
Several surgical techniques and principles have been recommended to repair the site of exposure and mitigate the risk of recurrence. In some instances, particularly cases complicated by severe ocular infections and those with large plate exposures, brisk wound leaks, non-functioning GDDs, and poor health or mobility of the remaining conjunctiva, complete removal of the device may be warranted. In cases involving exposure of an inferior tube, complete removal of the device from its original location and movement to a superior quadrant, if possible, can be considered (7).
Successful repair of tube exposure has been demonstrated using a combination of tube repositioning, tube rerouting, scleral tunneling, suturing the tube to underlying sclera, and secondary patch grafting.
Scleral tunneling and tube suturing
Both scleral tunneling and suturing the tube to the underlying sclera have been used in the repair of tube exposure with the aim of fixating the tube to decrease tube mobility and friction between the tube and overlying tissues. Care should be taken in cases of scleral thinning and high myopia as these may preclude creation of an effective scleral tunnel (7). Huddleston et al (2013) advocate for use of nylon sutures during tube revision after exposure as a higher rate of recurrent exposure was seen with polyester (e.g. Mersiline) possibly due to increased friction or an immune response to the suture material (4).
Tube repositioning and rerouting
Tube repositioning refers to changing the entry point of the tube into the globe and posterior repositioning (e.g. moving an anterior chamber tube into the sulcus) has been promoted to reduce the risk of re-exposure by decreasing the length of extraocular tubing. Tube repositioning techniques and outcomes have been described by Kang et al. (2020) and Joos et al. (2001) (26, 27). Tube rerouting refers to altering the path taken by the tube from the plate into the globe and has been described by Nardi et al. (2021) (28). Rerouting is thought to alter the mechanical forces at play between the tube and overlying tissues to mitigate the risk of re-exposure. Both tube repositioning and rerouting may necessitate the use of tube extensions so the availability of materials for tube extension should be considered preoperatively.
Placement of a secondary patch graft is often performed during tube exposure repair and was shown in a retrospective case series to decrease the risk of recurrent exposure by two-fold compared to direct conjunctival closure without the use of a secondary patch graft (28). Some surgeons have also advocated choosing a different patch material from that chosen for the primary surgery, particularly in cases associated with rapid melting of the initial patch graft material which may be associated with a high-grade immune response (20).
Surgical follow up
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Complications
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Prognosis
Incidence of re-exposure
Re-exposure following exposure repair is common. Huddleston et al (2013) found that 43% (17 out of 40 eyes) required additional surgical intervention after their initial exposure repair with a risk of re-exposure in the tube exposure group of 45% (15 out of 33 eyes) compared to 29% in the plate exposure group (2 out of 7 eyes) (4). Similarly, rates of 41.8% (18 out of 43 eyes) and 44% (41 out of 94 eyes) were found in retrospective case series by Alawi et al. (2020) and Thompson et al. (2017), respectively (29, 15). The success rate of tube revision surgery also varies by indication for tube revision. Weinreb et al. (2019) examined the success of tube revision surgery across multiple indications including exposure, hypotony, uncontrolled IOP, and tube repositioning. Among the tube exposure cases (74 out of 179 eyes), need for additional surgery was found in 50% of cases. Surgical failure defined as the need for further tube shunt revisions, need for an additional types of glaucoma surgery, or uncontrolled IOP at follow up varied by indication for repair with higher rates of failure and post-operative infection seen in the hypotony group compared to the exposure and repositioning groups (30).
Additional Resources
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References
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