Glaucoma · Patient & Clinician Series
When drops are
not enough, surgery
opens a new drain.
Glaucoma surgery does one thing: it lowers eye pressure by giving fluid a better way out. The options range from tiny stents placed during cataract surgery to a full filtering operation or a drainage tube. Each trades pressure lowering against risk and recovery. Here is how MIGS, trabeculectomy and tube shunts compare, and how surgeons choose.
for outflow
first described
restored by surgery
Section 01 — Why Surgery
Pressure is the one
risk factor we can
actually change.
The eye makes a clear fluid, aqueous humour, all the time. It flows from the ciliary body behind the iris, through the pupil, and drains out mainly through the trabecular meshwork and Schlemm's canal in the angle of the eye. In open-angle glaucoma that drain becomes more resistant, pressure rises, and over years the optic nerve is damaged.
Large trials have shown that lowering pressure slows the loss of visual field. Drops and laser are usually first. Surgery is considered when the pressure stays above the target the doctor has set for that eye, when the disease keeps progressing, or when drops cause side effects, cost too much or are not being used reliably.
Glaucoma surgery lowers eye pressure by improving how fluid leaves the eye. MIGS (minimally invasive glaucoma surgery) uses tiny devices or tissue-sparing cuts, usually from inside the eye and often with cataract surgery; it is safer and recovers faster but lowers pressure modestly. Trabeculectomy creates a new drain under a flap in the eye wall, collecting fluid in a blister called a bleb; it can achieve low pressures but has more complications. Tube shunts carry fluid to a plate on the outside of the eye and are favoured after failed surgery or in complex glaucoma. None of them restore lost vision. The choice depends on disease stage, target pressure, previous surgery and the patient's eye.
Section 02 — Interactive
Pick a procedure,
and watch where the fluid goes.
Move the slider to switch between an untreated glaucomatous eye and the main surgical options. Particles of aqueous flow from behind the iris, through the pupil, and leave by the route each operation creates.
| Procedure | Route | Pressure lowering | Recovery & risk |
|---|---|---|---|
| Trabecular MIGS (iStent, Hydrus, goniotomy) | Through the meshwork into Schlemm's canal | Modest; often reduces drop burden | Fast; low risk; no bleb |
| Subconjunctival MIGS (XEN, PreserFlo) | Chamber to a bleb under the conjunctiva | Moderate to substantial | Bleb care needed; antimetabolite usually used |
| Trabeculectomy | Under a scleral flap to a bleb | Often the lowest pressures | Longest follow-up; hypotony, leaks, bleb infection |
| Tube shunt (Ahmed, Baerveldt) | Tube to a plate behind the limbus | Substantial | Used for complex eyes; tube exposure, corneal effects, diplopia |
Section 03 — MIGS
Small devices,
small incisions,
smaller trade-offs.
MIGS was defined in 2012 as procedures that use an ab interno approach (working from inside the eye), cause minimal trauma, have at least modest efficacy, a high safety profile and rapid recovery. Not every device sold as MIGS fits every part of that definition, but it describes the idea.
Trabecular bypass stents. The iStent family places one or more tiny titanium stents through the meshwork into Schlemm's canal; the Hydrus is a longer nitinol scaffold that sits inside the canal. Both are typically implanted at the end of cataract surgery, viewed through a gonioscopy lens. In their pivotal trials, more eyes reached target pressure without drops when the device was added to cataract surgery than with cataract surgery alone. The effect is real but moderate, and these devices work best in mild to moderate open-angle glaucoma where the downstream collector channels still work.
Tissue-removing procedures. Goniotomy with dual-blade instruments, gonioscopy-assisted transluminal trabeculotomy (GATT) and canal-based procedures such as canaloplasty open or remove the meshwork without leaving an implant.
Subconjunctival stents. The XEN gel stent and the PreserFlo MicroShunt drain into a bleb like a trabeculectomy, but with a standardised tube and less tissue dissection. They can lower pressure further than trabecular MIGS, and they inherit some bleb-related risks and the need for an antimetabolite such as mitomycin C.
Section 04 — Trabeculectomy
The operation every
newer procedure is
still measured against.
Trabeculectomy was described by Cairns in 1968 and remains a reference procedure for lowering pressure. The surgeon makes a partial-thickness flap in the sclera, opens a small window into the anterior chamber under it, and usually removes a piece of peripheral iris. Fluid seeps out under the flap and collects under the conjunctiva in a bleb, from which it is absorbed.
Scarring is the main reason a trabeculectomy fails, so antimetabolites, most often mitomycin C, are applied during surgery to slow healing. Early on, the surgeon tunes the flow by adjusting or releasing stitches. The trade-off for lower pressure is a list of possible complications: pressure that is too low (hypotony), a shallow chamber, bleb leaks, choroidal effusion, faster cataract development and, at any time in later years, bleb infection, which can progress to endophthalmitis. People with a bleb are taught to report a red, painful or sticky eye the same day.
Section 05 — Tube Shunts
Tube versus trab:
what the trials
actually showed.
Glaucoma drainage devices carry fluid through a tube to a plate placed around the equator of the eye. Valved designs such as the Ahmed limit early flow; non-valved designs such as the Baerveldt are tied off temporarily until a capsule forms around the plate.
The Tube Versus Trabeculectomy (TVT) study compared the two in eyes that had already had cataract or glaucoma surgery. At five years, the tube group had a lower rate of failure than the trabeculectomy group, with similar pressures in both. The later Primary Tube Versus Trabeculectomy (PTVT) study looked at eyes without previous incisional surgery and found the opposite balance: trabeculectomy achieved somewhat lower pressures with fewer medications, while serious complications were not less frequent with tubes. Comparisons between Ahmed and Baerveldt implants (the ABC and AVB studies) found slightly lower pressures with the Baerveldt and more hypotony-related problems, against a higher failure rate for the Ahmed.
The practical reading: in an eye with no previous surgery, many surgeons still favour trabeculectomy or a bleb-forming stent when low pressure is needed; after failed surgery, scarring, or in neovascular and uveitic glaucoma, a tube is often the more dependable choice.
Section 06 — Choosing
The right operation
depends on the eye,
not the trend.
There is no single best glaucoma operation. Surgeons weigh how far the pressure must fall, how advanced the damage is, and what the eye has already been through.
| Situation | Often considered |
|---|---|
| Mild–moderate open-angle glaucoma with cataract | Cataract surgery combined with trabecular MIGS |
| Drops working but poorly tolerated | SLT laser, or MIGS to reduce the number of medications |
| Advanced damage, low target pressure | Trabeculectomy or a subconjunctival stent with mitomycin C |
| Failed previous surgery or heavy scarring | Tube shunt |
| Neovascular or uveitic glaucoma | Tube shunt, often with treatment of the underlying cause |
| Primary angle-closure | Laser iridotomy or lens extraction first; see the angle-closure guide |
Laser first is reasonable. In the LiGHT trial, selective laser trabeculoplasty used as the first treatment controlled pressure without drops in roughly three-quarters of eyes at three years, and was cost-effective compared with starting on drops. Laser does not prevent later surgery, but it can delay it.
Section 07 — Afterwards
Surgery is a step
in glaucoma care,
not the end of it.
Whatever the operation, glaucoma remains a lifelong condition. Pressure, the optic nerve and the visual field are monitored for life, because drains can scar and pressure can creep back up years later.
After filtering surgery, follow-up in the first weeks is frequent and steroid drops are used to control healing; some surgeons inject more antimetabolite or needle the bleb if it starts to scar. After trabecular MIGS with cataract surgery, recovery is usually close to that of cataract surgery alone. In every case, the question to ask before surgery is not only “how low will my pressure go?” but “what are the risks for my eye, and how will we know if it is working?”
Section 08 — FAQ
Frequently asked questions
about glaucoma surgery.
MIGS stands for minimally invasive glaucoma surgery. It is a group of procedures, usually done from inside the eye through a small corneal incision, that improve drainage of fluid with less tissue trauma and a faster recovery than traditional filtering surgery. The pressure lowering is generally more modest, so MIGS is mostly used for mild to moderate open-angle glaucoma, often at the same time as cataract surgery.
For lowering pressure as far as possible, trabeculectomy usually achieves lower pressures than trabecular MIGS devices. It also carries more risk: low pressure, bleb leaks, infection of the bleb and faster cataract formation. Which is better depends on how advanced the glaucoma is, the target pressure, previous surgery and the patient's ability to attend follow-up.
No. Glaucoma surgery lowers eye pressure to slow or stop further damage to the optic nerve. Vision or visual field already lost to glaucoma does not come back. This is why surgery is considered when pressure cannot be controlled well enough with drops or laser, before more damage occurs.
Many people need fewer drops after surgery, and some need none, but this is not guaranteed. After MIGS in particular, reducing the number of medications is often the main goal. After any glaucoma procedure the pressure is still checked for life, and drops may be restarted if it rises again.
Yes. Several trabecular bypass devices were first approved for use combined with cataract surgery, and combining them avoids a second operation. Removing the cataract itself can also lower pressure somewhat, especially in eyes with narrow angles.
A glaucoma drainage device, also called a tube shunt, is a small silicone tube placed into the eye that carries fluid to a plate stitched onto the outer wall of the eye under the conjunctiva. Common designs include the Ahmed and Baerveldt implants. Tubes are often used after a failed trabeculectomy, in eyes with heavy scarring, or in complex glaucoma such as neovascular or uveitic glaucoma.
Recovery after trabecular MIGS combined with cataract surgery is usually similar to cataract surgery alone. After trabeculectomy or tube surgery, follow-up is more intensive in the first weeks, with frequent visits, possible stitch adjustment and restrictions on heavy lifting, bending and eye rubbing. Your surgeon will give a plan specific to the procedure.
Selective laser trabeculoplasty (SLT) is a quick clinic laser that improves drainage through the trabecular meshwork. In the LiGHT trial it was an effective first treatment for open-angle and ocular hypertension, with many patients controlled without drops for years. Its effect can wear off, and it can be repeated. It is often tried before incisional surgery.
References & Evidence Base
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citations.
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