Mixed Mechanism Glaucomas
Combined mechanism glaucomas refers to an eye with elevated pressure in which both open-angle and angle closure glaucoma mechanisms are present.
Usually the structure of the eye is similar to that found in eyes with angle closure glaucoma – that is; the globe is somewhat smaller than ideal, with the result that the internal ocular components are crowded together. This means the coloured part of the eye (the iris) is very close to the trabecular drainage tissues, making it difficult for the aqueous fluid to gain access to the drain, and raising the possibility that the iris tissues can completely block the drain.
A peripheral iridotomy procedure, usually performed with a laser, permits the iris to fall back, away from the drain, allowing the aqueous to access the drain and thereby permitting the pressure to fall to normal levels.
However, if iris tissue (because of its previous proximity) has blocked the drain intermittently or repeatedly, the drainage system may be functionally impaired. Consequently, this will abnormally resist the drainage of aqueous even when opened by laser peripheral iridotomy.
Occasionally the affected eye also has an open-angle glaucoma mechanism causing raised pressure, so that once the angle closure component has been eliminated, the drain still does not operate efficiently and the pressure remains too high for the safety of the optic nerve fibres. Long-term drops, laser treatment to the drain itself, or even drainage surgery may become necessary to preserve sight, just as in the open-angle glaucomas.
These situations are known as "combined mechanism glaucomas". Often the diagnosis can only be made once the angle closure element has been removed, and the pressure subsequently monitored. It means that when angle closure has been treated successfully long-term care and observation remain vital to ensure that should open angle glaucoma occur it is promptly detected thus minimising the risk of unnecessary blindness.