To reduce the risk of subconjunctival device exposure, MIGS design has evolved by reducing the outer diameter size of the devices, using more flexible material and adapting the surgical protocol. Collagen-derived gelatin XS material cross-linked with glutaraldehyde becomes flexible when hydrated – over 100 times more flexible than the silicone tube used in tube shunts [8]. Ab-interno XS implantation offers the advantage of avoiding conjunctival incision or manipulation. On the other hand, PM implantation requires a conjunctival incision and creation of a scleral pocket. However, in comparison to traditional filtering surgery, the surgical procedure is minimally invasive. The PM material “SIBS” is a recently developed thermoplastic biomaterial resisting biodegradation [9]. Several iterations of micro-shunt design were required to achieve the final design of the PM. Because the first generations of micro-shunt showed a high rate of erosion, the surgical protocol was modified with the creation of a scleral pocket to prevent the fins from eroding the conjunctiva [9]. XS exposure is estimated to occur in 2.0–4.3% of eyes in a recent literature review, and thirty-six cases of XS extrusion have been reported in the literature [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Various risks factors have been suggested by the authors. Most of the cases exhibited conjunctival fragility related to previous glaucoma surgery [23, 24], needling procedures or stent revisions [18, 27, 32,33,34] or even conjunctival perforation during stent implantation [16]. Lanzhofer et al. analyzed the outer stent position using optical coherence tomography (OCT) and showed that the final position of the XS can be either intra-Tenon’s, sub-Tenon’s or subconjunctival [34]. In their study, the only case of XS exposure was observed in the subconjunctival XS group. In addition, a superficial positioning of the stent was noted prior to XS exposure in four cases, highlighting the potential role of Tenon’s capsule in preventing XS exposure [25, 29, 31].
Management of XS exposure varied among the reported cases. In most of the cases, conjunctival suturing was performed, often in combination with stent repositioning or shortening [14, 17, 24, 27, 29, 30, 32, 33]. Recurrence of stent exposure after conjunctival suturing was observed in some cases, requiring additional surgical interventions [17, 29, 32, 33]. Two cases of autologous conjunctival graft along with amniotic membrane graft have been described [23, 33]. Tube removal was performed in 5 cases of XS exposure [25, 26, 28, 31, 34] and in cases of stent exposure complicated by endophthalmitis [13, 15, 16]. Transconjunctival repositioning at the slit lamp, vitamin A ointment, and a soft contact lens were used unsuccessfully in the case reported by Salinas et al. [26]. However, we have found no codified management of XS exposure in the literature.
Few cases of PM exposure have been reported in the literature. Stangos et al. reported two cases of PM exposure in 50 eyes of 40 consecutive patients with PM [35]. Durr et al. reported one patient who developed an exposed PM 6 months after the surgery in a cohort of 85 eyes [3]. However, there is a lack of information concerning the potential risk factors and surgical management of PM exposure. We describe and analyze herein the clinical presentation and management of two cases of exposed PM. These cases share common features. First, both patients presented with severe blepharitis associated with ocular surface inflammation prior to surgery. Associated eye rubbing should be ruled out, since an association between subconjunctival stent displacement and eye rubbing has been reported [36, 37]. However, none of the patients reported eye rubbing behavior. Longstanding use of IOP lowering drops, especially if they contain benzalkonium chloride, can also represent a risk factor for chronic ocular surface inflammation resulting in alteration of the conjunctival tissue [38,39,40]. Indeed, prior ocular inflammation has been identified as a risk factor for tube exposure after glaucoma drainage implant surgery [41]. Second, both cases were characterized by a deficiency in Tenon’s capsule. Our second patient had a history of prior NPDS, which may have contributed to changes in the conjunctiva and Tenon’s capsule. The role of previous ocular surgery in aqueous drainage device erosion has been suggested several times in the literature [42, 43]. Because of the impossibility of pulling the Tenon’s capsule over the PM tube, a simple conjunctival flap was used to cover the PM instead of a Tenon’s flap as recommended [9]. However, because of the limited number of cases, further studies are required to confirm and identify risk factors for exposure.
Management of both patients consisted of surgical repair as first-line treatment. Amniotic membrane graft, tube repositioning and conjunctival suturing were performed. However, early recurrences of PM exposure were observed in both cases. Because of the impossibility of rebuilding a Tenon’s flap and the high risk of bleb fibrosis after repair techniques, the PM was removed in both patients. The procedure was easily performed by pulling the outer portion of the PM tube and suturing the conjunctiva. An uncomplicated XS implantation was performed simultaneously in one patient. For the other patient, a trabeculectomy was performed 1 month after the PM explantation with a good outcome and a functional filtering bleb.
In light of these cases, we would recommend careful screening for and treatment of ocular surface inflammation prior to PM implantation, even if complete control of blepharitis prior to surgery is not always possible in clinical practice. If a deficiency in Tenon’s capsule is noted intraoperatively, scleral fixation of the outer portion of the tube may potentially reduce the risk of further conjunctival perforation. For this purpose, after PM insertion and verification of aqueous humor flow, the external distal part of the tube can be fixed with a 10–0 nylon suture buried within the sclera, preventing the distal end of the tube from pointing towards the conjunctiva. Close monitoring should be performed in such cases because of the higher risk of PM exposure. However, when possible, the distal end of the device should be tucked under a Tenon’s flap, and the Tenon’s layer should be sutured prior to conjunctival closure. As surgical repair is associated with a high rate of recurrence of the PM exposure, as well as a risk of endophthalmitis and bleb fibrosis, we would recommend removal of the device immediately in the case of PM exposure. Another MIGS procedure or filtering surgery can be performed concurrently or as a secondary surgery. However, due to the few cases reported, more studies are required to determine a codified management strategy for PM extrusion.
