Copyright of this paper belong to Dr. Shilpi Agarwal and Aravind Eye Hospital
This paper has been republished for the sole purpose of learning and teaching
Aqueous Drainage Devices
Dr. Shilpi Agarwal, DNB Resident, Aravind Eye Hospital, Madurai
Introduction
Glaucoma is derived from “glaucosis” meaning bluish green discoloration of the affected eye1. It is a potentially blinding disease that affects all age groups. As per current definition: glaucoma is a group of eye diseases with mutisectorial etiology, characterized by an acquired loss of retinal ganglion cells, progressive optic neuropathy with morphological abnormalities in the optic nerve head and, visual field defects, in which raised intraocular pressure is a major risk factor. It is very important to diagnose and treat this condition as early as possible since damage caused is irreversible.
Its exact cause is still not known intraocular pressure (IOP) control being the only modifiable source of reigning in glaucoma. Currently pharmacological approach and/or surgical techniques especially trabeculectomy (TRAB) with or without antimetabolites is the main line of management in any kind of glaucoma.
Trabeculectomy was introduced by Cairns in 19682. It aims at creating a fistula between the anterior chamber of the eye and the subconjunctival space thereby bypassing the trabecular meshwork. Eventually most of the filtering bleb fail as subconjuctival fibrosis sets in.
Nowadays with the use of antimetabolites like Mitomycin C (MMC) and 5 fluorouracil (5FU) the survival of the filtering bleb has prolonged to 62.5% at five years after surgery3,4. However, trabeculectomy with antimetabolites is compromised by a number of serious ocular complications like bleb leakage, prolonged hypotony, epitheliopathy, blebitis, endophthalmitis etc.
Eyes with failed filters seldom respond favourably to repeat trabeculectomy. Conjunctival cicatrisation and external bleb failure largely contribute to poor success.
Glaucoma surgeons are often faced with the question of the best procedure to perform. Various anonymous surveys have been done by clinicians which even include surgeons of American & Japanese Glaucoma Society. They showed preference towards TRAB with MMC. However, this trend is slowly changing in favour of tube shunts.
Aqueous drainage devices (ADD) occupy an important place in the surgical armamentarium for the treatment of complicated and refractory glaucoma both as a primary surgical modality and as a secondary procedure where trabeculectomy with or without adjunctive antimetabolite therapy has either failed or is reported to have a very low chance of success.5-11.
However, even shunts are not free of complication. Common complication in AGV are 6,7,8,11 serous choroidal effusion (35%), Flat/shallow anterior chamber (27%), hyphema (17%), endothelial tube apposition (12%), encapsulated bleb (11%), iris tube apposition (10%) tube blocked (4%) hypotony (3.5%), striated keratitis (3%), cataract (3%), corneal decompensation (3%) tube exposure (2%), tube migration (1%), miscellaneous (5%).
The term refractory glaucoma is used for glaucoma which doesn’t respond to the usual medical and surgical treatment and needs subsequent glaucoma procedures. In these patients, the peculiarities involved with deranged anatomical configuration, physiology and dynamics of aqueous circulation is far different from other glaucoma patients.
Current glaucoma drainage devices can be classified into two broad categories. Flow – restrictive, or non – flow restrictive implants. Flow – restrictive, or valved implants provide resistance to aqueous flow, which is intended to reduce the
2 AECS Illumination
risk of hypotony during the early postoperative
period. Non – flow – restrictive, or open tube
drainage, implants provide little resistance to
aqueous flow during the early postoperative period
until a fibrous capsule forms around the plate.
1. non – valved/non-restrictive implants
Molteno, Baerveldt, Schocket.
2. valved /restrictive implants
Ahmed, Krupin, Joseph, Optimed, White.
The valved device (restrictive) prototype
designed by Dr.Mateen Ahmed (Ahmed glaucoma
valve) consists of a receptacle plate & a connector
tube of the plate with the anterior chamber. It
functions as unidirectional valvular system and
under determined pressures of the column of
water that passes through the tube, the valves
can be separated which allows the flow of the
exceeding pressure and it collapses again when
the excess of it has been liberated. This simple
mechanism has proven its efficacy to lower the
incidence of persistent flat anterior chambers in
the immediate postoperative period, a frequent
and risky situation that happens with non-valved
implants. Better results of this valvular implant in
refractory glaucomas have been the motivation to
do this study.
History
The first attempt to implant a drainage device
was made by Roller and Moreau in 1907 when
they performed a double paracentesis and used
horse hair through the corneal punctures to treat
patients with painful absolute glaucoma.
Molteno in 1969 scientifically explained the
pathophysiology of bleb resistance and designed
a functioning implant with an episcleral plate
and tube. His pioneering work on modulation
of bleb inflammation, by an anti – inflammatory
regimen of corticosteroids, fluphenamic acid
and colchicines, to ensure long term implant
patency coupled with the outstanding results of
his implant revolutionized this so far experimental
and anecdotal surgery. His implant has stood the
test of time and became a prototype for all future
implants. He can rightly be called the “father of
glaucoma drainage implants”.
Since then, many authors have thought and
developed different devices with great imagination
and diverse resources, from metal to crystal and
gelatin to cellulose, not leaving aside suture
materials such as silk, catgut etc.
Dr. Molteno developed a prosthetic plastic
device made of polymethyl metha acrylate that
communicates the anterior chamber with a
receptacle plate placed in the subconjunctival
space, through a fine silicone tube, that allows the
aqueous humour to flow into the subconjunctival
space and forms a filtration bleb at the site where
the receptacle plate is located. The molteno
implant is considered the prototype device of
this generation of filtration devices to control
intraocular pressure. Other types of non-valved
devices developed after Molteno’s design, are the
ones described by Krupin and Baerveldt (Table
1). Hypotony and its attendant complications
including corneal damage, choroidal effusion and
suprachoroidal haemorrhage postoperatively 11 are
the most common and troublesome complication
with the free pass devices (non-restrictive).
To avoid these complications, these devices
need to be modified and transformed in a valvular
element. The valved device (restrictive) prototype,
the one designed by Dr.Mateen Ahmed, was
approved by USFDA in November 1993.7,11 It
consists of a receptacle plate and a connector tube
of the plate with the anterior chamber (Table 2).
In the receptacle plate a bivalved valvular system is
found that functions as a ventury system and that
has been denominated “ Ventury – flow”.
What is Ahmed Glaucoma Valve?
The Ahmed Valvular device consists, as shown in
figure 1& figure 2, of four assemblable elements:
1. The receptacle plate that measures 13*16 mm
and has a surface area of 184 mm2, made of
polypropylene and has two lineal supports in
the form of a bar, and four posts that are sealed
through the tension cover and are responsible
for the quality of the tension of the membrane
Vol. XV, No.4, October December 2015 3
Implant type Size Material Valved/non valved
Ahmed glaucoma valve 96 mm2 (S3) Polyprophylene Valved
184 mm2 (S2)
364 mm2 (BI)
96 mm2 (FP8) Silicone
184 mm2 (FP7)
364 mm2 (FX1)
Baerveldt glaucoma
implant
250 mm2 Silicone
350 mm2
Krupin slit valve 183 mm2 Silastic valved
Molteno implant 134 mm2 Polypropylene
Nonvalved
(Signal Plate)
268 mm2
(Double Plate)
Table 1. Design features of current glaucoma drainage implants
FP7 FP8
Width 13.00 mm 9.60 mm
Length 16.00 mm 10.00 mm
Surface area 184 mm square 96 mm square
Tube length 25.00 mm 25.00 mm
Tube inner diameter 0.305 mm 0.305 mm
Tube outer diameter 0.635 mm 0.635 mm
Table 2. Comparision between FP7 and FP8 Ahmed valve
and the valves, besides, such plate has on its
edge the tube in the proximal portion and in
each side of it, two small out bounds perforated
in the center that constitute the fixation
element of the device to the sclera, since it is
through there where the material with which
the valve will be sutured to the sclera passes.
2. The communicating tube made out of silicone
of 0.635 mm of external diameter and an inner
diameter of 0.317 mm, which is inserted and
sticked in the perforation that the silicone
membrane has in the middle and is the element
that will receive the flow of aqueous coming
from the anterior chamber when the free end
is introduced in it.
3. The silicone membrane that, when tensioned,
will be the unidirectional bivalved valve.
4. The cover that produces the tension in the
silicone membrane, made of polypropylene.
That is the final part of the assemblement
and covers the valves of the membrane, giving
support at the same time to the posts of the
receptacle plate.
How does it work?
The physical principle on which the valvular
“ventury-flow” system is based on and considered
in Bernoulli’s equation. In laboratory studies in
vitro and under atmospheric pressure conditions,
the valves open when the pressure of the column
of water is over 8 to 12 mmHg and close when
the pressure is under 6 mmHg.
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Surgical technique
Anaesthesia
Retrobulbar or peribulbar anaesthesia and a lid
block are routinely given.
The quadrant selection
The implant should preferably be put in the
superior temporal quadrant, since it is the easiest
and safest one because there are no muscular
structures that can be affected in the future due to
the fibrosis. However, if this cannot be done, the
preferred quadrants in order are: inferior temporal,
superior nasal and inferior nasal.
If the implantations have to be done in the
nasal quadrants, we suggest that it be fixed to the
episclera at a maximal distance of 6 to 8 mm from
the limbus, due to the shorter distance in the nasal
side between the ocular globe, and the optic nerve.
Steps of AGV implantation has been summarized
in the figure 3 to figure 10.
Valve preparation
Under sterile conditions, with a 23 gauge cannula
on a 3 ml syringe that contains isotonic saline
solution for ocular irrigation, priming of the
implant and initial and definitive unsealing of
the silicone valves are performed. The tip of the
cannula is introduced 3 to 4 mm into the tube
since the pressure required to prime the tube is
over 80 to 100 mmHg and could (if not held with
strength) push the implant away. It is suggested,
therefore, that the body of the valve be held with
the other hand while the priming is performed.
One of the main causes of no tensional
decrease in the immediate postoperative period
is the failure of priming the valve before its
implantation.
When the initial spurt of water is observed one
can notice an immediate decrease in the resistance
of the implant to the outflow of the liquid.
Preparation of the receptor scleral bed
Once the quadrant to be used has been selected, a
90 degree peritomy should be done dissecting the
conjunctiva along with tenon as close as possible to
the normal insertion of the conjunctiva. On both
sides of the peritomy a 5 mm relaxing incision
should be done in an oblique direction towards
the recti muscles tendons in order to improve the
scleral exposure without extending the peritomy,
allowing a better closure of the conjunctival edges.
Insertion and attachment of the valve’s
body
The next step is the introduction of the valve’s
body in the pocket of conjunctiva. The body of the
implant should be pushed until the anterior edge
of the valve (where the suture is) reaches 8 mm
posterior to the limbus. During this procedure the
hand that is pulling the conjunctiva-tenon “sheet”
should be maintained very well stretched, allowing
an easier insertion of the implant’s body in its place
avoiding the rolling of the tenon.
Once the body has been inserted into the
pocket, the distance at which the valve will be
sutured should be measured with a caliper. The
distance usually required to suture the valve, is 8
mm posterior to the corneoscleral limbus, except
if the implantation should be done in the nasal
quadrants, where we suggest it be fixed 6mm away
from the limbus due to the shorter distance to the
optic nerve in the nasal side of the globe. Sutures
should be put in the episclera, but should be long
in their transcleral trajectory, so that one should
not have to tighten initial knot very hard to obtain
a good confrontation of the valve with the sclera.
The valve should be fixed to the episclera with
a triple – throw surgeon’s knot (7-0 silk). The first
throw should have two turns in one direction.
The second throw is applied by looping the suture
once over the needle in the direction opposite
to the first, and the third throw is completed in
similar way.
Once the sutures are finished the ends are
cut 2mm long in order to avoid an unexpected
untying of the suture, which would produce
instability and migration of the implant from its
original place.
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There are two options for the fixation of the
valve:
1. Tying the sutures on each eyelet as described
above, or
2. Tying both sutures in a knot underneath the
tube.
Selection of the suture
We suggest to use 7-0 silk without a silicone
cover, because it is a non-rigid material, produces
an acceptable fibrous reaction in the place of the
suture that allows a faster stabilization of the
implant, stays in its place for many years, and
seldom produces intolerance or rejection. We have
used nylon 9 -0 or 10-0, but we have noticed it
cuts the sclera more easily, especially if it is thin
and not elastic, and that ends of the suture can
perforate the conjunctiva due to its rigidity. We
do not recommend the use of absorbable sutures
due to the possibility of premature instability of
the implant.
Anterior chamber entry incision
Traditionally introduction of the silicone tube in
the anterior chamber has been done through a
limbal paracentesis with a 22 or 23 gauge needle,
parallel to the plane of the iris, covering the tube
afterwards with a small piece of preserved donor
sclera.
With this technique a tunnel is made in the
sclera with a 23 gauge needle starting 4 or 5
mm away from the limbus upto this site, where
the needle enters the anterior chamber. In order
to do this, 5 or 6 mm of the needle tip should be
bent making a 70 degree angle with the level facing
inwards. Then the needle is bent again near its
base in order to avoid obstruction of the visibility
during the procedure.
The needle is connected to a syringe with
isotonic saline solution and primed to prevent the
flow of aqueous humor through the needle when
it penetrates the anterior chamber because of the
difference in pressures.
The needle is connected to a syringe with
isotonic saline solution and primed to prevent the
flow of aqueous humor through the needle when
it penetrates the anterior chamber because of the
difference in pressures. The form we have given
the needle makes the tip of it trying constantly
to escape the ocular globe when we are doing the
scleral tunnel. This situation allows us to see the
tip of the needle while doing the tunnel. Once we
get to the limbus, which we can notice because of
the depression seen in this area, the direction of
the needle, is changed pressing down the tip of it
in order to penetrate the anterior chamber in the
plane parallel to the iris, allowing the tube to stay
in a middle plane between the iris and the cornea.
Preparation and insertion of the tube in
the anterior chamber
The silicon tube must be trimmed level upto
extend 2 to 3 mm into the anterior chamber,
leaving the opening of the tube towards the corneal
dome. The tube is inserted through the entry
track into the anterior chamber with a forceps
or a special tube introduce. The introduction of
the tube must be done very slowly and carefully
in order to avoid creating a false track into the
suprachoroidal or supraciliary spaces.
When the tube reaches the limbus, a greater
resistance to the introduction of the tube is
frequently felt due to the rigidity of the eye in
this zone and to the change in direction necessary
to penetrate the anterior chamber. Generally it is
enough with a simple “massage” over the track at
the limbus towards the anterior chamber while
the tube is pushed inside to facilitate its entrance
through this course.
Once the tube is seen in the anterior chamber
it should be introduced completely without
leaving it loose in the space between the valve’s
body and the beginning of the intrascleral tunnel,
since this could produce instability of the implant
and the possible migration of it.
The proper position of the tube is within the
anterior chamber, without contacting the iris or
lens posteriorly and the cornea anteriorly. If the
tube is too long after insertion, it is removed,
trimmed and reinserted.
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Reposition and suture of the conjunctiva
At this moment the valve’s body and total course
of the tube should be completely covered upto the
limbus with tenon and conjunctiva, in order to
protect the implant from the friction of the eyelid.
Sometimes this maneuver is difficult due to the
lack of tenon in old patients, making it necessary
to pull the tenon that is found in the inferior
fornix, so it can cover all this space.
Episcleral sutures are tied on each side of the
conjunctival flap trying to leave the central part
of the conjunctiva (between the sutures) tense
and mounted over the limbus. Sutures can be
done using the same material that was used for
the fixation of the valve’s body (silk 7-0) in one
tenon and conjunctival plane fixing them to the
episclera on both sides at the limbus.
Once the procedure has finished, 4 to 8 mg
of perilimbal subconjunctival dexamethasone is
administered. It is advisable to use a mydriatic
cycloplegic, such as 1% atropine twice a day,
or a combination of 10% phenylephrine and
tropicamide or cyclopentolate twice a day, plus
a steroid drop such as prednisolone combined
with an antibiotic and place a rigid occlusion
just to cover the eye without producing any
compression, since this would activate the flow of
aqueous humour through the valved mechanism
and produce a flat anterior chmber. Cycloplegics
and antibiotic steroid drops should be used for
at least four weeks.