A retrobulbar shunt is a silicone glaucoma drainage device that guides aqueous humor from the anterior chamber to the retrobulbar space. While conventional glaucoma drainage devices (such as Baerveldt, Ahmed, Molteno) drain aqueous humor to a plate under the conjunctiva and Tenon’s capsule, the retrobulbar shunt directs aqueous humor into the retrobulbar fat tissue.
| Item | Retrobulbar Shunt | Conventional GDD |
|---|---|---|
| Drainage site | Retrobulbar space (fat tissue) | Sub-Tenon plate |
| Bleb / Plate | Not required | Required |
In conventional glaucoma drainage devices, fibrosis by fibroblasts of the episclera, Tenon’s capsule, and conjunctiva causes encapsulation around the plate, leading to surgical failure 1)2)3). According to pooled data from the ABC and AVB studies, approximately 49% of Ahmed shunts and 37% of Baerveldt shunts fail within 5 years. Since retrobulbar shunts do not require bleb formation, they can avoid this encapsulation problem.
The main indications for retrobulbar shunts are as follows:
Conventional glaucoma drainage devices are indicated for eyes with failed trabeculectomy or those where filtration surgery is unlikely to succeed, such as neovascular glaucoma or uveitic glaucoma 1)2)3). Retrobulbar shunts are positioned as a rescue therapy for cases where these glaucoma drainage devices have also failed.
The biggest difference is the drainage site for aqueous humor. Conventional glaucoma drainage devices (Ahmed, Baerveldt, etc.) drain aqueous humor to a plate under the conjunctiva and Tenon’s capsule, where it is absorbed through a capsule that forms around the plate 2)3). Retrobulbar shunts have no plate and drain aqueous humor directly into the retrobulbar fat tissue. The retrobulbar fat tissue is less prone to fibrosis than the sub-Tenon’s tissue, so the risk of surgical failure due to encapsulation is lower. Additionally, since no bleb is required, the risks of leakage, exposure, and strabismus are minimized.
3-Segment Structure
Anterior portion: Placed in the anterior chamber. Diameter 0.3 mm, same size as Ahmed, Baerveldt, and Molteno implants.
Reinforcement portion: Includes a flange (wing-like part) that fixes the tube at the scleral groove level. Size is 6 × 1.6 mm.
Posterior portion: Introduced into the retrobulbar space (fat tissue). Diameter 1.02 mm, with fenestrations for aqueous humor outflow.
Design Features
No plate: Unlike conventional glaucoma drainage devices, it has no plate, reducing the risk of strabismus and diplopia.
No valve: It does not have a valve mechanism, but postoperative hypotony is prevented by injecting viscoelastic material into the anterior chamber.
Simplified surgery: Surgical time is about half that of standard shunt placement.
Scleral graft: Use of a scleral graft is recommended to protect the tube.
Performed under retrobulbar or sub-Tenon anesthesia. The main steps are as follows.
To prevent postoperative hypotony, inject sufficient viscoelastic material into the anterior chamber after device placement.
Taper steroid eye drops (prednisolone acetate) and antibiotic eye drops (moxifloxacin) over 4 weeks.
In a study of 19 eyes with refractory glaucoma in which all previous glaucoma surgeries had failed, intraocular pressure decreased from a baseline of 35.3±2.3 mmHg to 18.5±1.1 mmHg at 6 months (−16.8 mmHg, −47%, p<0.0001). The number of glaucoma medications also decreased from a baseline of 2.4±0.3 to less than 0.3. 79% of the cases had a history of failed tube shunt surgery.
| Parameter | Preoperative | 6 Months Postoperative |
|---|---|---|
| Intraocular pressure (mmHg) | 35.3±2.3 | 18.5±1.1 |
| Number of medications | 2.4±0.3 | <0.3 |
In a study of 19 eyes with failed conventional glaucoma drainage device plates communicating with the retrobulbar space, intraocular pressure decreased from baseline 33.3±2.1 mmHg to 16.0±1.6 mmHg at 24 months (p<0.00001). In long-term results of 35 eyes, intraocular pressure decreased from 30.94±1.62 mmHg to 13.4±1.23 mmHg at 5 years (p<0.0001).
Compared with the AVB and ABC trials, the retrobulbar shunt showed greater IOP reduction than primary Ahmed shunt and achieved IOP reduction equivalent to primary Baerveldt shunt. The reduction in number of medications was also greater than primary Ahmed shunt and equivalent to primary Baerveldt shunt. With conventional glaucoma drainage devices, 49% of Ahmed and 37% of Baerveldt fail within 5 years1), but the retrobulbar shunt is superior in that it has a lower risk of encapsulation.
In reported studies, no patients experienced complications, vision loss, shunt leakage, infection, or corneal edema. The main event was transient hypotony (<5 mmHg) on postoperative day 1, which resolved with refilling of viscoelastic material into the anterior chamber.
Retrobulbar shunt surgery can be performed in about half the time of standard tube shunt placement. Conventional glaucoma drainage devices require extensive conjunctival dissection, plate placement and suturing, and covering with patch material, but retrobulbar shunt simplifies the procedure because plate placement is unnecessary. However, use of a scleral graft to protect the tube is recommended.
The mechanism of action of retrobulbar shunts is based on diverting aqueous humor into the fatty tissue of the retrobulbar space. The retrobulbar fat tissue has a lower tendency to form scar tissue compared to the sub-Tenon tissue. Aqueous humor reaches the hydrophilic periosteum of the orbit through microchannels between fat cells, achieving continuous aqueous humor circulation.
Animal experiments have shown no evidence of fibrosis around the shunt, indicating that the inherent characteristics of the retrobulbar space limit the risk of fibrous encapsulation.
In conventional glaucoma drainage devices (e.g., Ahmed, Baerveldt), aqueous humor is drained from the anterior chamber through a silicone tube to a plate placed under the Tenon capsule 2)3). The aqueous humor is absorbed through a connective tissue capsule that forms around the plate. However, excessive fibrosis by fibroblasts in the episclera, Tenon capsule, and conjunctiva leads to encapsulation. Encapsulation reduces the aqueous humor absorption capacity around the plate, causing intraocular pressure to rise again and resulting in surgical failure.
Retrobulbar shunts fundamentally avoid this problem of fibrous encapsulation by redirecting aqueous humor drainage from the sub-Tenon space to the retrobulbar space.
Retrobulbar shunts have shown promising results as a rescue therapy for refractory glaucoma. A technique that restores function by connecting a failed conventional glaucoma drainage device plate to the retrobulbar space achieves greater intraocular pressure reduction compared to sequential addition of another glaucoma drainage device.
Future challenges include the following:
It is usable. Retrobulbar shunts were initially studied clinically as a method to restore normal function by connecting a failed, fibrotic, and encapsulated conventional glaucoma drainage device plate to the retrobulbar space. In a study of 19 eyes, intraocular pressure decreased from 33.3 to 16.0 mmHg at 24 months, and in a long-term study of 35 eyes, a pressure reduction from 30.94 to 13.4 mmHg was achieved at 5 years. Greater intraocular pressure reduction has been reported compared to sequential new glaucoma drainage device placement.
- 日本緑内障学会. 緑内障診療ガイドライン（第5版）. 日眼会誌. 2022;126:85-177.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma, 6th Edition. Br J Ophthalmol. 2025.
- American Academy of Ophthalmology. Primary Open-Angle Glaucoma Preferred Practice Pattern®. 2020.
