A simple surgical solution for the treatment of persistent postoperative hypotony after PRESERFLO MicroShunt implantation

PRESERFLO MicroShunt implantation is a relatively new and increasingly popular treatment for recalcitrant glaucoma. Though relatively uncommon, persistent or severe postoperative hypotony may occur and its treatment presents a significant challenge. Interventional case series of the first 7 consecutive patients who underwent the insertion of a rip-cord suture to treat severe or persistent postoperative hypotony. In 6 patients, the hypotony resolved and vision was restored. The 7th patient had suffered a severe suprachoroidal haemorrhage with a poor visual recovery and limited follow-up, so success was more difficult to assess. Stenting the PRESERFLO MicroShunt with a rip-cord suture is a simple technique to correct persistent or severe postoperative hypotony.


INTRODUCTION
The PRESERFLO TM MicroShunt (PMS, Santen, Osaka, Japan) is an 8.5 mm long synthetic polymer (SIBSpoly(styrene-block-isobutylene-block-styrene) tube with a 70 µm diameter lumen, designed to reduce the intraocular pressure by draining aqueous humour from the anterior chamber to the subconjunctival space. It is implanted via an ab externo approach and functions in a similar manner to a trabeculectomy but with a less invasive surgical technique [1].
In common with all intraocular pressure-lowering external filtration procedures, hypotony may occur after surgery and has been reported in 7.1% of cases after 12 months in one randomised clinical trial, with 2% requiring intervention [2]. Untreated severe hypotony can lead to serious sequelae such as hypotony maculopathy and choroidal haemorrhage.
Reversal of hypotony without losing intraocular pressure control may be challenging. We present a relatively simple surgical technique that has proven successful in addressing a series of cases with clinically significant hypotony after PMS insertion, avoiding explantation of the device.

MATERIALS/SUBJECTS AND METHODS
Seven consecutive patients who presented with hypotony with sequelae after PMS insertion, such as the presence of choroidal effusion, haemorrhage or hypotony maculopathy (with or without anterior chamber shallowing), sufficient to warrant surgical intervention, underwent surgical revision to partially occlude the PMS with a rip-cord suture. Multiple surgeons performed the initial implantation, all revision procedures reported here were performed by one surgeon (KB). The dataset generated or analysed during the current study is available from the corresponding author on reasonable request.
All patients had undergone PMS insertion in a superior quadrant preceded by a 3-minute posterior sub-Tenon's application of mitomycin C (0.5 mg/mL) (MMC) on 3 LASIK shields, followed by copious irrigation of the MMC with balanced salt solution (BSS), as a primary surgical procedure for glaucoma. The youngest patient, case 4, (Table 1) received intraoperative 1.25 µg of intracameral bevacizumab and 40 mg of sub-Tenon's triamcinolone, case 6 received sub-Tenon's triamcinolone and case 7 underwent concomitant phacoemulsification.

PMS occlusion technique
In each case, the conjunctiva and Tenon's was reopened at the limbus to expose the subconjunctival aperture of the PMS. A 7-0 silk corneal traction suture (Ethicon Inc., Johnson & Johnson, Somerville, NJ, USA) was used to aid visualisation. The anterior chamber was then reformed with BSS to pressurise the eye and after removing the needle from the end of a 9-0 polypropylene suture (Ethicon, Inc.) the suture was introduced into the subconjunctival aperture of the PMS and advanced until flow was visibly abolished (Fig. 1A, B). The suture length was then adjusted to re-establish a lower rate of aqueous drainage through the PMS than had been present before stenting (Fig. 1C). The length of the rip-cord suture inside the tube necessary to achieve this was generally around 1-2 mm. In initial cases, because of the short intraluminal length, the external end of the 9-0 Table 1. Baseline and follow-up data for series of cases undergoing Preserflo Microshunt stenting. polypropylene suture was cut short to minimise the risk of it dislodging from the PMS (e.g. if the eye was rubbed). In one later case (case no. 5, Table 1), the external end of the 9-0 polypropylene was buried in a peripheral groove in the cornea, in a manner similar to that often used for releasable trabeculectomy sutures, to facilitate later removal at the slit-lamp (Fig. 1D). This was first performed by one of the co-authors, Dr. Yih-Horng Tham. After flow had been adjusted to the surgeon's satisfaction, Tenon's and conjunctiva were reopposed to limbus with 10-0 nylon (Alcon Inc, Fort Worth, TX, USA) interrupted sutures, taking care not to occlude the PMS nor to dislodge the fine 9-0 polypropylene stent suture from the lumen of the PMS. Table 1 includes baseline, intraoperative and postoperative data from the case series presented.

RESULTS
Most patients (6 out of 7) experienced a full recovery of visual acuity and resolution of the clinical features of hypotony.
One very frail, elderly, 89-year-old patient (case no. 3) with leukaemia and on apixaban treatment, suffered severe vision loss following a large suprachoroidal haemorrhage after the initial implantation, was unable to attend adequate follow-up to judge the outcome in terms of IOP restoration, though his vision remained poor at light perception despite repeated procedures to drain the suprachoroidal haemorrhage.

DISCUSSION
Traditionally, trabeculectomy has been the first line surgical treatment for recalcitrant glaucoma and evidence suggests that it is still the most effective IOP-lowering procedure available in the longer term for patients at a low risk of filtration failure from scarring. However, trabeculectomy is unpredictable, and postoperative care, intensive and laborious for both surgeon and patient, not ideal in elderly patients. Good surgical technique and postoperative management also require extensive training and experience. For that reason, trabeculectomy is best suited to high volume trabeculectomy practices.
Less invasive surgical techniques that have been developed in the last decade offer the hope of approaching the efficacy of trabeculectomy with less invasive surgery, less laborious followup, and a lower risk of adverse events. Subconjunctival minimally invasive surgical devices such as the PMS are tempting, as they offer greater efficacy than Schlemm's canal-based surgical procedures, with a less invasive technique than trabeculectomy, offering the potential of less postoperative manipulation and fewer postoperative visits.
However, in one randomised surgical trial of InnFocus Microshunt (later renamed Preserflo) vs Trabeculectomy [2], 28.9% of patients developed transient hypotony (defined as IOP < 6 mmHg at any time), with 7.1% persistent (defined as IOP < 6 mmHg at two consecutive visits after week 1). In 2%, some form of intervention was required, either surgical or office-based viscoelastic or air injection (7.6% in the trabeculectomy group, p = 0.09). 0.5% of patients were reported to have developed hypotony maculopathy. 1.8% of early hypotony events (on or before month 3 visit) were unresolved in the MicroShunt group (2.5% in the trabeculectomy group). For late hypotony events (after month 3), 2.7% were unresolved in the MicroShunt group (25% in the trabeculectomy group).
In theory, the PMS implant, designed with the Hagen-Poiseuille law in mind, is 8.5 mm in length with a 70 µm lumen and should produce sufficient resistance to avoid hypotony [1] This has been confirmed in animal studies [7] but questioned in one in vitro study [8]. Either way, in clinical practice, hypotony can sometimes still occur and, uncommonly, with devastating effect.
Persistent hypotony after PMS insertion presents a challenge. If a trabeculectomy results in hypotony, viscoelastic can be injected at the slit-lamp as a temporising procedure and, if necessary, the scleral flap can be resutured to reduce flow. With a fixed, small-diameter PMS, in the authors' experience, viscoelastic injection must be approached with great caution as it runs a much higher risk of high IOP spikes. The remaining options are ligation, internal occlusion with a rip-cord suture that can be removed later or removal of the PMS. Intraluminal stent suture occlusion is an attractive option because, in contrast to the others, is titratable to some degree, ie. the suture can be fed up and down the PMS and flow visualised and adjusted according to the degree of drainage. Ligation of such a small device is potentially more difficult, less predictable, and runs the risk of a binary outcome, ie. persistent hypotony or very high IOP, as it is difficult to adjust flow with a ligature.
One problem with such a rip-cord is the possible requirement for later removal, at a time when sufficient bleb encapsulation has occurred. Initially the sub-conjunctival end of the suture was left short, out of a concern that, with only 1-2 mm into the tube, it might easily dislodge. As there were no cases in which this happened and as removing the rip-cord required reopening conjunctiva, albeit at the slit-lamp in most cases, in one later case, the external end was buried in a releasable suture-type loop in the peripheral cornea, so that it could be removed later without opening conjunctiva (Fig. 1D). This has since become our standard technique in these cases. The rip-cord suture was removed later in the course of follow-up in 3 cases and at last follow-up all patients had satisfactory IOP control without ocular hypotensive medication. (Table 1) It would be of great value to identify potential risk factors for persistent hypotony in glaucoma patients. Risk factors for postoperative choroidal detachments after different types of glaucoma surgery are: pseudoexfoliative glaucoma, older age, pseudophakia (lens status), hypertension, greater IOP reduction and thicker cornea [9,10]. And those seem to differ from those of hypotony maculopathy: young age, male gender and myopia [11]. Our cases were mainly female (5 out of 7), above 50 years old (6 out of 7), Caucasian (5 out of 7) and phakic (6 out of 7).
The dose of mitomycin C applied in our routine practice is higher than that used by Baker et al. [2], though this dose is not outside the levels used in other practices, and higher levels have been used in other studies with higher apparent rates of success [4]. It is worth noting that even with the lower dose levels used by Baker et al. [2], a significant early hypotony rate was still observed.
Although suprachoroidal haemorrhage is infrequent, case 3 illustrates the potentially catastrophic consequences of hypotony. A similar case of a 76-year-old patient who was being treated with new oral anticoagulants and suffered a suprachoroidal haemorrhage following PMS insertion has been reported in the literature [12].
One possible method to avoid severe hypotony if those at risk could be identified, might be the primary insertion of a rip-cord suture as described here. As used subsequently, the subconjunctival end can be run through a corneal groove and retrieved at the slit-lamp, in the same manner that is often used for trabeculectomy releasable sutures, at a safe time after surgery (e.g. 3-4 weeks), if the postoperative pressure is above the desired target.
The rip-cord stenting technique described here seems to be a useful surgical technique to treat vision-threatening, severe or persistent postoperative hypotony after PMS implantation.

Summary
What was known before • Preserflo Microshunt can induce hypotony.
• Often, to sort it out, removal of the Preserflo is required and this leads to surgical failure.
• Patients would probably need subsequent surgical intervention.
What this study adds • This alternative solution will allow the Preserflo to remain and continue working avoiding surgical failure and preventing further glaucoma surgery.

DATA AVAILABILITY
The dataset generated or analysed during the current study is available from the corresponding author on reasonable request.