Ophthalmologists who manage glaucoma are conscious of both the benefits and the limitations of medical therapy for glaucoma. They are also aware of the positive role that the iStent Inject can have in relation to this.
A large part of my practice is cataract and refractive surgery and a significant number of patients in my care live with glaucoma. As such, I am aware of the limitations associated with glaucoma related medical therapy.
These limitations can include problems with adherence, such as difficulties with administration, remembering to use drops, and reluctance to tolerate side effects. They can also include cost, the concern that medical management may not control intraocular pressure (IOP) fluctuations as well as a surgical approach, and of course, the ocular surface toxicity that can result following the use of topical ophthalmic anti-glaucoma preparations.
glaucoma patients who are medically treated and about to undergo routine cataract, enjoy freedom from drop use
Ocular surface side effects can be related to the agent itself or to the preservatives used, and we often notice tear film instability, now classified as dysfunctional tear syndrome (DTS), and conjunctival inflammation in glaucoma patients on medication.1 It is recognised that the duration and number of anti glaucoma drugs prior to surgery significantly reduce success rates in trabeculectomy surgery.2
Fig 1. iStent inject size in comparison with a 50 cent piece
DYSFUNCTIONAL TEAR SYNDROME
Cataract and refractive surgeons are aware of the problematic effect of DTS on measurable surgical outcomes and on patients’ experience of their visual stability, reliability, and acceptability.
LASIK and, possibly to a lesser degree, PRK are known to disturb corneal sensation and thus initiate a disturbance of the tear film, which must be managed in the post-operative period.
Sophisticated intraocular lenses (IOLs), for example trifocal IOLs, are increasingly used in refractive surgery and are particularly susceptible to DTS, which must be avoided or treated in these patients if symptomatic.
INTRAOCULAR PRESSURE
Cataract surgery alone will, on average, lower IOP and the adjunct use of a microincisional glaucoma surgical device, such as the iStent inject, will add a further pressure lowering effect. In a 2010 study with 15 months follow up, the mean IOP in the iStent (original iStent) group improved from 17.9±2.6 to 14.8±1.2mmHg (17.3 per cent reduction, P<0.05). This reduction was greater than that of the control group (phaco alone) that improved from 17.3±3.0 to 15.7±1.1mmHg (9.2 per cent reduction, P<0.05). The mean number of ocular hypotensive medications was 0.4±0.7 and 1.3±1.0 in the iStent and control groups, respectively (P=0.007). In addition, 67 per cent of iStent subjects and 24 per cent of control subjects no longer required ocular hypotensive medications.3
ISTENT INJECT
Studies on the iStent inject suggest that with the insertion of two stents, IOP lowering of 7–12mmHg may be achieved.
Insertion of the iStent inject may be considered for a patient due to undergo cataract surgery whose IOP is not adequately controlled. A drop in IOP after a combined procedure of up to 10mmHg may be achieved. If more than this is required, then full thickness filtering surgery is usually considered.
Fig 2. iStent inject in the trabecular meshwork
A more common situation is a patient who is due to have cataract surgery, and has well controlled, or at least acceptably controlled IOP on one or more medications. It is rare, in my experience, that such patients will be tolerating their glaucoma medications with no effect on the conjunctiva or the tear film. In my opinion, these patients should have a discussion about the use of the iStent inject, and the degree to which they may benefit from a lesser use of IOP lowering medications.
The iStent inject is approved by the Therapeutic Goods Administration for stand alone surgery, although current MBS item numbers and health fund reimbursement processes do not support this in Australia at the time of writing. Nevertheless, it is reasonable to suggest use of the iStent inject for a glaucoma patient who is being medically treated and is planning refractive surgery.
In my opinion, the potential to reduce the number of topical medications a patient is using, simply by implanting the iStent inject, is advantageous. As a lens surgeon, this is particularly important if I am planning a surgical outcome such as a multifocal intraocular lens or a plan to use monovision.
I also note that many glaucoma patients who are medically treated and about to undergo routine cataract surgery, enjoy the freedom from drop use that often accompanies implantation of an iStent inject.
Dr Paul McCartney BMedSci, MBBS, FRANZCO, FRACS, FRCOphth (UK) is Clinical Associate Professor at the University of Tasmania. He established the private practice Hobart Eye Surgeons in 1998 where he specialises in cataract, refractive surgery, oculoplastics and glaucoma.
Dr McCartney was born in the United Kingdom and educated in Tasmania. He specialised at the Prince of Wales Hospital, Sydney, followed by work overseas in Fiji, Canada, and the United Kingdom. Dr McCartney is an active teacher at the registrar and medical student level. He has been involved in the Glaucoma Inheritance Study in Tasmania, which was part of an international effort to identify the genes responsible for glaucoma. Dr McCartney is actively involved with the East Timor Eye Project, teaching and performing surgery in Timor Leste.
References
- Zhang, Xuemin & Vadoothker, Saujanya & M. Munir, Wuqaas & Saeedi, Osamah. (2018). Ocular Surface Disease and Glaucoma Medications: A Clinical Approach. Eye & Contact Lens: Science & Clinical Practice. 1. 10.1097
- Baudoin C. Mechanisms of failure in glaucoma filtering surgery: a consequence of antiglaucomatous drugs? Int J Clin Pharmacol Res. 1996;16(1):29-41.
- Fea AM. Phacoemulsification versus phacoemulsification with micro-bypass stent implantation in primary open-angle glaucoma: randomized double-masked clinical trial. J Cataract Refract Surg. 2010;36(3):407–412.
The Advantages of iStent Inject
DTS will often lower the quality of vision after LASIK or cataract surgery, especially when a refractive IOL such as a trifocal is used.
The iStent inject permits a lower average postop anti glaucoma medication use than phaco surgery alone.
The iStent inject can therefore be considered as having potential benefit in the refractive cataract and LASIK patient.
