Glaucoma is a neuro-degenerative disease of the optic nerve and is the second
leading cause of blindness world-wide. Glaucoma prevalence is estimated to be
70 million world-wide with 7-million blinded from this disease. (1) Glaucoma
affects 300,000 Australians with 25,000 visually impaired. Prevalence increases
with increasing age and population studies have shown that 10% of Australians
over the age of 80 have glaucoma. This prevalence is set to double by the year
2025 due to the aging population. The total annual health cost to Australia in
2005 (direct and indirect) was $1.9 billion per annum. This is set to increase
to $4.3 billion by 2025. The need for a robust and cost effective means for
improving surgical outcomes is clear. This is particularly true in many developing
countries where, for logistic reasons, glaucoma surgery is the only viable
Elevated intraocular pressure is a major risk factor for glaucoma development
and progression. Treatments which reduce intra-ocular pressure (IOP) are
effective in delaying vision loss. Successful glaucoma surgery is more
effective than laser or medical treatment at lowering intraocular pressure,
although the risk of complications largely due to unpredictable wound healing
responses have generally led to surgery being the final option.
Glaucoma surgery lowers intraocular pressure by providing a conduit for aqueous
humor to exit the anterior chamber of the eye into the subconjunctival space,
thus bypassing outflow obstruction in the trabecular meshwork, which is the
principle cause of IOP elevation in glaucoma. Aqueous humor collects in the
subconjunctival space to form a bleb. From here, fluid is absorbed through
the bleb wall or taken up by surrounding blood vessels to leave the eye.
Surgery is usually performed by creating a guarded fistula (trabeculectomy)
or with a glaucoma drainage device where a plate acting as a spacer is placed
in the subconjunctival tissues and connected to the anterior chamber by a tube.
A number of alternative drainage devices have been developed that attempt to drain
aqueous humor through routes other than the subconjunctival space but none of
these to date have been subject to rigorous investigation or enjoy widespread
use. The wound healing response in the subconjunctival tissues after
trabeculectomy and glaucoma drainage device surgery is similar.
Excess scar formation in the subconjunctival tissues is the principal cause of
surgical failure. Post-operative wound contraction and scar formation increase
resistance to aqueous outflow with IOP elevation consequent to a:
- Reduction in the bleb surface area available for aqueous absorption.
- Reducing the porosity of the bleb wall as determined by the thickness
of the bleb wall and the maturation of the scar tissue.
- Reducing in the volumetric absorptive capacity of the surrounding tissues.
To avoid future surgical failure we have constructed a computational model of the
eye-tissue system. This model consists of several sub-models linked together by the
underlying requirement of fluid conservation. These sub-models include fluid production
from the ciliary body within the eye itself, the volume/pressure response of the eye,
the fluid removal from the eye via the trabecular meshwork and uveo-sclearal routes
(in both normal and diseased states), fluid removal from the eye via the tube-shunt,
any prosthesis and a poro-elastic model of the fluid movement into, and sorption by,
the peri-orbital tissues. The model has identified several key processes which need
to be further understood and so model development is currently being coordinated
with experiments performed by our collaborators Professor Jonathan Crowston and
Associate Professor Michael Coote.
- 2009-2011, NHMRC, Regulating fluid mechanics to improve the outcome of
glaucoma surgery, Jonathan Crowston, Michael Cootes, Bruce Gardiner,
David Smith, $269,250.