Glaucoma Drainage Devices

By Dr.Aditya Mahadevan (PG)

ModeratorsProf Dr.K.Srikanth

Dr.N.Swathi

Introduction Long history of various devices designed to facilitate aqueous flow

out of the anterior chamber to control IOP.

Setons: solid threads, wires or hairs placed to drain aqueous, which was allowed to run alongside these structures.

In 1906, Rollet used a horse hair; and in 1912, Zorab used a silk thread.

In 1969, Molteno devised the first proper glaucoma drainage device (GDD).

Physiology Consist of a silicone tube extending from the anterior chamber(AC),

to a plate, disc or ring beneath the conjunctiva and Tenons capsule Mechanism:

After insertion, collagenous

capsule forms around it

Surrounded by a granulomatous

reaction

Reaction

resolves in 4 month

s

Capsule

remains

stable

Matures over time,

becomes thinner after 6 months

Filtering bleb

Ouflow α Surface area of the implant.

Outflow α 1/Bleb Diameter. This is because the surface tension increases as diameter increases, which in turn will decrease the outflow. Same goes for the bleb thickness.

The edge of the external plate usually has a ridge, through which the tube is inserted. This decreases the chances of obstruction of the posterior opening of the tube.

Indications Generally reserved for cases wherein filtering surgery has failed or is

likely to fail. Recent studies have revealed its use as a primary procedure in

specific conditions. Classically considered in the following scenarios:1. Failed trabeculectomies with antifibrotics2. Active uveitis3. Neovascular glaucoma4. Inadequate conjunctiva5. Aphakia

Implant designs

Subdivided into open tube (non-valved) and flow restricted (valved)

designs

All types have a tube invariably made of silicone. The external

material is usually proplylene or silicone.

Propylene, being more rigid, has a higher tendency to cause

inflammation than silicone.

Implant designs Open tube designs1. Molteno2. Baerveldt3. Schocket

Flow restricted designs1. Ahmed2. Krupin

Other designs: Ex-PRESS device, iStent

Open tube devicesMolteno

Prototype implant.

Original design had single acrylic plate with silicone tube. Further modifications lead to using a propylene body.

Single plate (surface area 135 sq.mm) designs were followed by double plates thereby increasing the surface area (270 sq.mm). Second plate is connected to the first by means of another tube.

Single vs double plates: latter provides better IOP control, but more complications, especially hypotony.

Another modification involved the addition of a ‘V’ shaped pressure ridge on top of the plate enclosing an area of 10.55 sq.mm.

This helps regulate aqueous flow and is supposed to help with the problem of hypotony.

Baerveldt (Abbott):

Unique feature is the large surface area which can still be inserted via a one quadrant incision.

Uses a silicone tube and silicone plate which is barium impregnated for easy radiographic visualization.

Commonly used models are 250 and 350 sq.mm designs. A 350 sq.mm pars plana model is also available.

The plate has fenestrations into which fibrous growth occurs. This serves to reduce the bleb height and prevent its complications including diplopia.

Schocket:

A 360 degree silicone band, as used in detachment surgery, which acts as the reservoir into which the tube is inserted.

Needs a large conjunctival incision.

Modifications include a 90 degree band between 2 recti and insertion into a pre-existing band implanted for buckling purposes

Didn’t provide any added advantage over the existing open tube models.

Flow restricted devicesAhmed (New World Medical):

Most popular device at present.

The silicone tube is connected to 2 sheet valves made of silicon elastomer membranes which are held in the body plate.

These valves are designed to prevent overfiltration leading to hypotony especially in the early post-op period (IOP between 8 and 10mm Hg).

There is an initial hypertensive phase which is more pronounced than open tube devices.

Two models are in use:

1. FP7 which has a silicone body (pediatric counterpart FP8 -102 sq.mm)

2. S2 which uses a propylene body (pediatric counterpart S3- 85 sq.mm)

FP7 is found to have slightly better IOP lower effects than S2

Ahmed vs Molteno:

The IOP lowering effect of Molteno implants were higher.

Success rates of double plated Molteno devices were around 73% after 1 year and 56% after 5 years, compared to around 60% at 1 year and 50% at 5 years for Ahmed devices.

However, Ahmed devices were less likely to cause complications requiring another surgery.

Krupin Implants:

The Krupin-Denver device first developed, was attached to a 180 degree Schocket type explant.

Subsequently the Krupin eye valve with disc was developed.

The valves at the distal end of the tube opens only at IOPs above 10 or 12 mm Hg .

But these devices exposed the valves directly to sub-conjunctival tissue, leading higher rates of failure.

Other devicesEx-PRESS shunt:

Made of a stainless steel tube with a barbed end, to anchor it into the trabecular meshwork, through which a stent is placed.

This is drained underneath a traditional trabeculectomy flap

Theoretical advantages over a trabeculectomy of no tissue excision with less chance of bleeding, quieter eyes postoperatively and simpler operative technique.

iStent:

It is a stainless steel stent that is implanted from an ab interno approach.

The idea is to bypass the trabecular meshwork and drain aqueous from the AC straight into the Schelmm canal.

Early studies are showing promising results.

Basic surgical principles Although different devices have variations in the surgical procedures,

the basic techniques apply in general.

All procedures start with adequate surgical exposure with the help of a traction suture.

Superotemporal quadrant is preferred, as it offers better exposure and is less likely to cause diploplia (protects the superior oblique muscle).

Fornix based flap is commonly fashioned, so that the foreign body plate does not come in contact with the suture line as in a limbus based flap.

After exposing the scleral bed, a muscle hook is used to isolate 2

adjacent recti muscles (usually superior and lateral recti).

Valved tubes need to be irrigated, before placing them, to open them

up (Priming).

The external plate is tucked posteriorly into the sub-Tenon space,

underneath the recti and sutured with non-absorbable 8-0 or 9-0

nylon to the sclera.

The anterior border must be atleast 8 to 10 mm away from the

limbus, to keep them away from the palpebral area and the insertion

of the recti.

Tube is then cut, bevel up, so as to extend 2-3 mm into the anterior

chamber (AC).

Then a 23-gauge needle is used to create a tract, parallel to the iris

plane, to enter the AC, without touching the cornea.

Tube is entered via the needle track into the AC

To prevent erosion of the tube at the limbus, pericardium, donor

sclera or cornea is sutured over the tube at this site.

The conjunctiva is sutured using 6-0 Vicryl.

Special considerations To prevent hypotony in open tube models:

1. Placement of tube 3 to 4 weeks after inserting the plate.

2. Ligature of tube with Vicryl to prevent drainage till the suture dissolves.

3. Usage of stents.

Pars plana insertion:

1. In eyes that have undergone vitrectomy such as aphakics.

2. Tube doesn’t touch cornea. Useful in pre-existing corneal grafts

3. In cases with extensive peripheral anterior synechiae.

Post-op management

As with trabeculectomy, patient is treated with topical steroid, antibiotics ad cycloplegic agents.

Early IOP spikes are best managed medically, with the help of aqueous suppressants and this usually resolves in 1 to 6 months.

Occluding sutures, if applied, may also be removed or left to dissolve on its own.

Complications Hypotony:

Most common in open tube devices. Treatment as discussed

Another cause maybe due to a leakage around the tube. Avoided by creating water tight entries. Management is by forming the AC, repositioning the tube.

Elevated IOP:

As a result of inflammation. This is treated medically and usually resolves in 1-6 months

Early causes include occlusion by blood clot, iris, vitreous membrane.

Treated by irrigating the tube with a paracentesis or opening with Nd-Yag laser.

Migration, extrusion and erosion:

Posterior migration out of the AC may occur if the tube is not secured, or anterior migration due to plate migration may occur.

Migration is common pediatric patients because of the growth of the eyeball.

Treatment involves securing the tube or plate, if necessary, or repositioning the tube.

Erosion of the tube through the overlying conjunctiva is another possibility. Cover with pericardium, patch sclera or cornea minimizes the occurrence.

Diplopia and ocular motility disturbance:

Risk factors: superonasal incision, large plates like Baerveldt, large bleb height/diameter.

Corrective measures include removal of the device, placement of smaller devices and/or shifting to the supero-temporal quadrant.

Corneal decompensation:

Tube-cornea touchis the major cause. May also be due to retrograde flow into the AC especially in non-valved shunts.

Reforming the AC or repositioning the tube maybe necessary.

Endophthalmitis:

As with any invasive procedure, there is a risk of endophthalmitis.

Early diagnosis and treatment as per protocol is essential.

May require immediate removal of the contaminated device.

Outcomes Long term outcome studies revealed success rates ranging from 65 to

85% in achieving IOP control.

Variations between different study models and among different devices exist.

Molteno implants, in general, lower IOP more than Ahmed implants but have higher risk of complications and need for resurgery.

Success is lower in pediatric glaucomas, with failure rates of more than 30%, and most of them requiring other surgeries.

Drainage devices have been particularly successful in neovascular glaucoma (success rates 62% at 1 yr but diminished with time).

Tube versus Trabeculectomy (TVT) study Multicenter RCT comparing tube shunts with trabeculectomy with mitomycin-

C(MMC) in eyes with previous cataract and/failed glaucoma surgery.

A 350 sq.mm Baerveldt implant was used.

During the 1st year of follow-up, the tube patients were more likely to maintain IOP and avoid hypotony or loss of light perception or resurgery as compared to trabeculectomy patients. However tube patients needed more supplemental therapy.

At the end of 5 yrs of follow-up, IOP control was found to be similar in both groups but the trabeculectomy group needed additional surgery more than the tube group.

Primary TVT study Recently concluded RCT, comparing tube and trabeculectomy with

MMC as a primary procedure in patients with uncontrolled glaucoma.

Used the 350 sq.mm Baerveldt implant.

At the end of 1 yr of follow-up, it was found that greater IOP reduction was found in the trabeculectomy group.

Failure rates were also higher in the tube group (20% in tube vs 8% in trabeculectomy).

No significant difference in the rates of complications and interventions.

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