The iStent is one of the new generation medical devices which has recently become available in Australia for use during cataract surgery in patients with glaucoma. It forms part of the new trend towards minimally invasive glaucoma surgery (MIGS).
The stent facilitates intraocular pressure lowering thereby potentially either minimising glaucoma medication, or giving better intraocular pressure control in patients who are poorly controlled.
Whilst the device could theoretically be implanted in any patient with glaucoma, is is currently only TGA approved for use in the course of a cataract operation.
Intraocular pressure lowering in cataract surgery.
It is a well known that cataract surgery lowers intraocular pressure by as much as 30%. As such it is a good way to manage a patient who has concomitant glaucoma and cataract. As an alternative to a trabeculectomy, a cataract operation is a potentially good way to gain further lowering of pressure in people who perhaps are on maximal medical therapy and who have perphaps have also had laser treatment (SLT) but still do not have ideal pressure control.
The iStent, when implanted at the time of cataract surgery gives further lowering of intraocular pressure of around 16%. Whilst this may not seem much, it has to be remembered that every mmHg reduction in intraocular pressure in glaucoma patients results in a 19% reduction in risk of progression(1).
The implantation of the iStent is usually performed at the end of the cataract procedure. This is because the anterior chamber will be maximally deep as the lens has been removed. A gonio lens is required to visualise the trabecular meshwork. It increases surgical time only minimally. The stent comes pre-loaded on an introducer which grips the stent. The stent is then released once in place by depressing a button on the introducer.
There is no difference in the cataract procedure beyond this and no difference in the post operative management.
There is definitely a learning curve associated with its use and is probably best reserved for ophthalmic surgeons who are familiar with the anatomy of the trabecular meshwork.
Following surgery, the intraocular pressure is re-evaluated and it is sometimes possible to reduce the amount of glaucoma medication required.
The iStent works by bypassing the trabecular meshwork which is the site of outflow resistance in patients with glaucoma
The following illustration shows the path of aqueous within the eye and out through the iStent.
The decision to implant the iStent is usually made in the pre-operative period in patients who have glaucoma. The ability to visualise the trabecular meshwork during surgery is crucial and inability to do so would be the main reason not to implant it.
(1) Chauhan A. Acta Ophthalmol 2008;126:1030-1036