Over
the last few years, a number of companies have introduced their models of Glaucoma
Drainage Devices (GDDs). In a market dominated by the Ahmed Glaucoma Valve
(AGV) and the Baerveldt Glaucoma Implant (BGI), these new players have been
unable to bring any exciting innovations to present a formidable challenge to
the two trusted brands.
The
Paul Glaucoma Implant (PGI) is a valveless GDD designed by Professor Paul Chew
from Singapore. With just a few minor changes to the other valveless implants
already available, the PGI appears to be a case of old wine in a new bottle or
is it old wine in an old bottle?
The PGI is manufactured by Advanced Ophthalmic
Innovations Pte Ltd, Singapore from medical implantable grade silicone.
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| PAUL Glaucoma Implant |
The plate of the PGI has a length of 44.9 mm, width of
23 mm and an extraocular plate surface area of 342.1 mm2.
The PGI has a shorter wingspan compared with
the BGI but extends further posteriorly due to a wider width. The plate surface
area is smaller than the BGI, but larger than the AGV.
The anteroposterior depth of the PGI is larger
compared to that of the BGI which allows the plate to extend further back,
while the shorter breadth (wingspan) reduces the area of the plate tucked under
the recti muscles which may theoretically reduce the postoperative risk of
strabismus and diplopia.
The internal diameter of the PGI is the
smallest amongst the 3 tube shunts at 0.127 m, about one
third the diameter of both the AGV and BGI. The smaller internal tube calibre
increased aqueous flow resistance which theoretically reduces the risk of
postoperative hypotony. The overall slimmer tube caliber allows for a lower
tube profile protruding above the sclera and theoretically reduces risk of
conjunctival erosion while also reducing the risk of tube-endothelium contact
and damage. The lumen can be easily occluded using a 6/0 or 7/0 polypropylene
stent compared to the 3/0 which is usually used to occlude the tube in the BGI.
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| Comparison of the 3 GDDs |
In a 2-year study performed by the innovators,
postoperative
complications included shallow anterior chamber which was self-limiting (n=10
[22.2%]), hypotony requiring intervention (n=4 [8.9%]) and tube occlusion (n=4
[8.9%]).
One eye developed an inferior retinal
detachment requiring vitrectomy a month after implantation but this was not
directly related to the PGI surgery.
Of the 4 eyes with tube occlusion, 2 were
occluded by iris which was treated with argon laser iridoplasty, 1 was blocked
by fibrin and required an anterior chamber washout with tube flushing, and 1
which was blocked by vitreous required an anterior vitrectomy with tube
flushing twice.
All cases of self-limiting shallow anterior
chamber resolved within a month of the surgery.
For the 4 eyes with hypotony that required
anterior chamber injection of viscoelastic, 2 required more than 1 injection of
viscoelastic and 1 had subsequent intervention in the form of anterior
vitrectomy and tube flushing for tube occlusion from vitreous.
One eye had persistent hypotony at 2 years,
which was complicated by blunt trauma to the eye about 1.5 years postsurgery.
One eye underwent tube repositioning as the
tube was felt to be too anterior, with endothelial cell count showing a
downward trend from 2100 to 1400 cells/mm2.
There were no serious complications and no eyes
required explantation of the PGI.
In conclusion, the Paul Glaucoma Implant does
not appear to present any significant practical advantages compared to the presently popular GDDs. However, like any
other device, longer and multi-center studies may throw more light on the
effectiveness of PGI.
There is one very important difference between the PAUL implant and all other plate implants - the tube. Its lumen is much smaller, 0.127 mm vs 0.3 mm which correlates to a 5.5 times difference in lumen crossection area. That greatly reduces the incidence and severity of postoperative hypotony and allows safe use of Mitomycin-C
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