Co-managing the Post-Trabeculectomy and PreserFlo Patient

1. Trabeculectomy and PreserFlo: how they differ

Both operations lower IOP the same way: aqueous leaves the anterior chamber and collects under the conjunctiva as a bleb, where it is absorbed. They differ in how the outflow channel is made and how much the surgeon can adjust it after surgery.

Optometrists are seeing more of these patients. Filtration surgery is offered earlier, and patients often return to their optometrist for part of their follow-up. The first three months decide whether a bleb succeeds. Most problems in this window can be fixed if they are caught early.

Trabeculectomy is a guarded sclerostomy. The surgeon lifts a conjunctival flap, makes a partial-thickness scleral flap, removes a block of tissue under it into the anterior chamber, and usually makes a peripheral iridectomy. The scleral flap is closed with sutures that set the resistance to flow. Mitomycin C (MMC) is applied to reduce scarring.

PreserFlo MicroShunt is an 8.5 mm tube made of SIBS (poly(styrene-block-isobutylene-block-styrene)), with a 70 µm lumen.¹ It is passed through a needle track about 3 mm behind the limbus into the anterior chamber, and its outer end sits under the conjunctiva and Tenon's capsule. Small fins stop it migrating. MMC is applied as with trabeculectomy. There is no scleral flap, no iridectomy and no flap sutures.

The best comparison is a randomised trial of 527 patients with primary open-angle glaucoma, both arms using MMC 0.2 mg/mL (Baker et al. 2021).^{2, 3}

Trabeculectomy gets the pressure lower. PreserFlo is more predictable, with less hypotony and fewer visits. Many surgeons now use a higher MMC dose (0.4 mg/mL) with PreserFlo, and real-world series report better success with it.⁴

For the co-managing optometrist, the main practical difference is this: a trabeculectomy can be tuned in the first weeks by cutting or releasing sutures, but a PreserFlo cannot. Early signs of scarring around a PreserFlo bleb are therefore important to report quickly.

2. Hypotony

Hypotony is usually defined as an IOP of 5 mmHg or less. A low number on its own is common in the first week and often harmless. Hypotony matters when it changes the structure or function of the eye. This is clinical hypotony, and it needs the surgeon's attention.

Causes after filtration surgery

• Overfiltration: too much flow through a well-formed bleb. The bleb is usually large and diffuse, and the Seidel test is negative.
• Bleb or wound leak: aqueous escapes through the conjunctiva. The bleb is often low or flat, and the Seidel test is positive.
• Reduced aqueous production: from inflammation, or from a choroidal detachment that lowers ciliary body output and keeps the pressure low.
• Less common: cyclodialysis cleft, retinal detachment.

Clinical signs

Grading the anterior chamber

A shallow anterior chamber is graded by what touches the cornea:

1. Grade I: peripheral iris touches the cornea. Usually watched.
2. Grade II: iris touches the cornea up to the pupil margin. Same-day discussion with the surgeon.
3. Grade III: lens or IOL touches the cornea. Emergency. Risk of endothelial damage and cataract; the surgeon will usually reform the chamber.

Do not miss the shallow chamber with normal or high pressure

Not every shallow chamber after filtration surgery is hypotony. The IOP and bleb together point to the cause:

The last three are emergencies, and the treatment is very different from hypotony. Refer the same day.

Your role

Measure the IOP, grade the chamber, check the bleb and do a Seidel test, and look at the macula and peripheral fundus. Asymptomatic low IOP with a deep chamber and no folds is usually watched, often with atropine from the surgeon. Hypotony with choroidal effusions, maculopathy, a Grade II–III chamber or a positive Seidel test should be discussed with the surgeon that day.

3. How to examine a bleb

A bleb is judged on two things: how it looks and whether it is doing its job, which means the IOP. Record both at every visit, in the same order, so changes are easy to spot.

Step by step

1. History. Ask about pain, redness, discharge, blurred vision, a watery "gush" (which suggests a leak), and whether the patient is using their drops as prescribed.
2. Visual acuity and IOP. Goldmann is fine. Avoid pressing on the bleb with the lids or the tonometer, and note the time of day.
3. Expose the bleb. Ask the patient to look down and gently lift the upper lid by the lashes. Do not push on the globe.
4. Diffuse white light first. Note the bleb's position, extent (in clock hours), height and vascularity, compared with the surrounding conjunctiva.
5. Slit beam. Look at the bleb wall: thick or thin, microcysts present or absent, any sharp edge or demarcation line.
6. Seidel test. Moisten a fluorescein strip and paint it over the bleb and any suture line. Under cobalt blue, a leak shows as a dark stream of aqueous diluting the bright green dye. Do this without pressure first; only press gently if the surgeon has asked for it.
7. Anterior chamber. Depth (graded as in Section 2), cells, flare, fibrin, blood. For trabeculectomy, check the iridectomy is patent. For PreserFlo, find the tube tip and check that it is free in the chamber, not touching the cornea or iris, and not blocked by iris, fibrin or blood.
8. Gonioscopy, when appropriate. It shows whether a sclerostomy is open or blocked by iris, blood or fibrin. Use a small lens with little pressure. In the first one to two weeks, check with the surgeon before doing it.
9. Fundus. Look for choroidal effusions, macular folds and disc swelling.
10. Photograph the bleb. A slit-lamp photo at each visit is the most reliable way to see slow change. Anterior segment OCT, if you have it, shows the bleb wall and fluid spaces.

Describing what you see

The Moorfields Bleb Grading System and the Indiana Bleb Appearance Grading Scale both grade area, height and vascularity.^{5, 6} Either is fine; what matters is using the same one each time so the surgeon can compare.

PreserFlo-specific checks

• The tube under the conjunctiva should be covered. Any exposed tube, or conjunctiva wearing thin over it, needs prompt review.
• The tube tip should be visible in the anterior chamber and not touching the endothelium. A tip that has retracted out of view or is in contact with the cornea should be reported.

4. Signs of a failing bleb

Most blebs that fail do so from scarring in the first three to six months. Scarring can be slowed or reversed while it is active, so the aim is to spot it early, not to confirm it once the bleb is flat.

Warning signs

• IOP creeping up across visits, even if still within target. A rise from 8 to 14 mmHg over two weeks is more worrying than a stable 14.
• Increasing vascularity: a redder bleb, engorged or corkscrew vessels, especially at the edges.
• Shrinking bleb: smaller in extent, lower in height, or a sharp edge forming where it meets normal conjunctiva.
• Loss of microcysts and a thicker, opaque bleb wall.
• Encapsulation: a dome-shaped, tense, smooth bleb with a thick wall, usually at two to eight weeks, with rising IOP.
• A deep chamber with high IOP and a flat bleb, in a trabeculectomy, suggests the flap or sclerostomy is scarring or blocked.
• For PreserFlo: a tube tip blocked by iris, fibrin or blood, or a bleb that never forms despite a tube in good position.

Be careful of steroid response

The intensive steroid drops given after surgery can raise IOP in steroid responders. This can look like bleb failure. Report the rise either way; the surgeon may change the steroid rather than treat the bleb.

What to do

Contact the surgeon within 24 to 48 hours if you see any of these signs in the first three months. Include the IOP trend, a description or photo of the bleb, and the patient's current drops. Do not restart glaucoma drops yourself unless the surgeon has asked you to (see Section 7).

5. What the surgeon may do for a failing bleb

When you report early signs of failure, the surgeon has several options. Most are done in the clinic, and your patient may have one or more of them in the first three months. Patients should know these are a normal part of care, not a sign the operation has gone wrong.

Timing matters. Laser suture lysis works best in the first few weeks; with MMC the window is longer. Needling and injections are most effective while the bleb is still red and active, before it has scarred down. This is why the 24 to 48 hour referral in Section 4 matters.

6. Saving a failed bleb

A bleb that has failed is flat and scarred, with IOP above target. It can often still be rescued, so a failed bleb does not mean the patient has to start over with a new operation.

Options, from least to most invasive

1. Needling revision. Done at the slit lamp or in theatre. A fine needle is passed under the conjunctiva to cut the scar tissue around the bleb or over the scleral flap or tube, and 5-FU or MMC is given. It is the usual first step for an encapsulated or scarred bleb. It may need repeating, and success is lower once the bleb has been scarred for a long time.
2. Clearing the internal opening. If the problem is inside the eye (iris, membrane or fibrin over the sclerostomy or tube tip), laser or intracameral tPA can reopen it without touching the bleb.
3. Open bleb revision. In theatre, the conjunctiva is opened, the scar tissue is removed, the flap or tube is freed or reset, and MMC is reapplied. For PreserFlo, the tube can be flushed, repositioned or replaced.
4. New surgery. A new trabeculectomy or PreserFlo at a different site, or a larger glaucoma drainage device such as a Baerveldt, Ahmed or Paul implant. These devices drain to a plate placed further back and are less affected by scarring at the front of the eye.
5. Cyclodestructive laser. Reduces aqueous production. Usually kept for eyes with limited vision or where further surgery is unlikely to work.

The surgeon's choice depends on how long the bleb has failed, the target IOP, the state of the conjunctiva, and the patient's vision and health. The optometrist's most useful contribution is earlier detection: a bleb treated while it is failing has a better chance than one treated after it has failed.

7. Post-operative medications

Regimes vary between surgeons, so always follow the discharge letter. The example below is typical. Note that the steroid course is much longer than after cataract surgery, because its job is to control scarring, not just inflammation.

Glaucoma drops

• In the operated eye, glaucoma drops are usually stopped on the day of surgery. The bleb needs aqueous flowing through it to stay open. Oral acetazolamide is also stopped.
• In the other eye, glaucoma drops continue as normal. Patients often stop all their drops by mistake. Check this at every visit; the unoperated eye can lose control quickly.
• Restarting drops in the operated eye is the surgeon's decision. They may do this if the IOP stays above target after other measures, or to support a bleb being treated. Aqueous suppressants (beta-blockers, carbonic anhydrase inhibitors, alpha-agonists) are often chosen, in preservative-free form where possible. Some surgeons avoid prostaglandins in the early weeks because of inflammation.
• Many patients need no glaucoma drops at all after surgery. In the Baker trial, the average was under one medication at 12 months.² Needing a drop later is not necessarily failure; the target is the IOP, not zero drops.

Before surgery

Many surgeons switch patients to preservative-free drops or add a short course of steroid in the weeks before surgery. Long-term preserved drops inflame the conjunctiva and make scarring more likely. If you are managing a patient listed for surgery, ask the surgeon whether they would like this.

Prescribing

Therapeutically endorsed optometrists can prescribe most of these drops, but changes to the steroid taper or glaucoma drops in the operated eye should be agreed with the surgeon. The surgeon is judging the bleb, and the taper is part of that plan.

8. Counselling the patient: the first three months

Patients who know what to expect use their drops properly, come to their visits, and report problems early. Each of these improves the chance that the bleb survives.

Visits

Expect frequent visits: typically day 1, week 1, week 2, then every two to four weeks until three months. Trabeculectomy patients often need more visits than PreserFlo patients because the flow is being adjusted. A patient who misses visits in this period is at real risk.

What is normal

• Blurred vision for two to six weeks. The focus will change as the eye settles.
• A gritty, scratchy feeling from the stitches and the bleb, and some redness.
• A slightly droopy upper lid, which often improves.
• Procedures in the clinic, such as stitch removal, laser, injections or needling. These are part of the plan, not a sign that something has gone wrong.

Glasses

The refraction is not stable for some weeks, and astigmatism can change. Wait until about two to three months, and until the surgeon is happy that the IOP is stable, before prescribing new glasses. Temporary readers or an old pair are fine in the meantime.

Activities

• Wear the eye shield at night for two to four weeks, or as advised.
• Do not rub or press on the eye.
• Avoid heavy lifting, bending with the head down, and straining for about two weeks.
• Avoid swimming, spas and eye make-up for about four weeks.
• Return to desk work after about one to two weeks; dusty or manual work may need longer.
• No contact lens wear in the operated eye unless the surgeon approves it.
• Driving once the patient meets the vision standard and feels comfortable.
• Flying is fine.

Drops

• Use the steroid exactly as prescribed and do not stop it early. It is what keeps the bleb open.
• Stop the glaucoma drops in the operated eye, but keep using them in the other eye.
• Bring all drops to each visit.

Warning signs: call the same day

Teach patients the RSVP rule. Any of these needs the surgeon or emergency department the same day:

• Redness that is increasing
• Sensitivity to light
• Vision getting worse
• Pain

Also report discharge, a white or milky look to the bleb, or a sudden gush of watery fluid.

For life

The risk of bleb infection (blebitis, which can progress to endophthalmitis) is lifelong, and higher with thin MMC blebs. Patients should treat any red or sticky eye as urgent, never assume it is "just conjunctivitis", and tell any doctor or optometrist that they have had glaucoma surgery.

9. When to contact the surgeon

Blebitis deserves special mention. It can present years after surgery as a red eye that looks like conjunctivitis. The clue is the bleb itself: white, milky and full of inflammatory cells against an injected background. Refer the same day; do not treat it as conjunctivitis.

Summary

• Trabeculectomy gets the pressure lower and can be adjusted after surgery. PreserFlo is more predictable, with less hypotony and fewer interventions.
• Low IOP alone is not an emergency. Hypotony with a shallow chamber, folds or effusions is.
• A shallow chamber with normal or high IOP is not hypotony and needs same-day referral.
• Examine the bleb the same way at every visit, photograph it, and do a Seidel test.
• Rising IOP and a redder, smaller or encapsulated bleb in the first three months are reasons to call the surgeon within 48 hours.
• Glaucoma drops stop in the operated eye and continue in the other eye. Restarting them is the surgeon's call.
• RSVP symptoms need same-day review, for life.