Gonioscopy may offer a reasonable and cost-effective method to evaluate the function of the trabecular micro-bypass device post-implantation and it could help in adjusting the therapy after the surgery.
Gonioscopy forms an essential part of the glaucoma evaluation, enabling the structure of the iridocorneal angle to be seen, which is important in diagnosing and monitoring eye conditions associated with glaucoma. However, Schlemm’s canal (SC) can be poorly visible in the angle, especially in excessively pigmented eyes.
Blood reflux from episcleral veins to the SC is a well-known sign that helps one to visualise the SC1 and can be caused by the pressure of the goniolens on the episcleral veins, reduced intraocular pressure (IOP) or both.2 Three quarters of outflow resistance of the aqueous humour in normal eyes comes from the trabecular meshwork (TM), while 25% of the resistance comes from SC.3 The juxtacanalicular meshwork is responsible for normal outflow resistance in the TM.2
Many glaucoma surgeries aim to overcome this resistance of outflow, which may reduce the IOP. The trabecular micro-bypass implant iStent inject (Glaukos Corporation), which received CE Mark clearance in Europe in 2010 for the treatment of open-angle glaucoma,4 aims to connect the anterior chamber directly with SC, thus overcoming the resistance of the TM.5
In one study, blood reflux was seen in nearly all (91%) of eyes immediately after implanting iStent,6 but there are few case reports about the late hyphaema that can occur through the implant following surgery.7-8 We evaluated blood reflux during gonioscopy 3 to 6 months following implantation of either the trabecular micro-bypass devices iStent inject or the newer-generation device, iStent Inject W, to evaluate the effectiveness of the implantation and adjust the drugs therapies patients were on if necessary.
In a case series, 22 eyes from 20 patients who had been diagnosed with either early primary open-angle glaucoma (POAG) or ocular hypertension (OHT), and who also had cataracts and prior difficulties with eyedrops, received implants of either iStent inject or iStent inject W combined with their cataract surgeries. All procedures were carried out by the same surgeon at the department of ophthalmology in Staedisches Klinikum Dresden hospital in Dresden, Germany.
Two stents were implanted in each eye nasally. 14 eyes received iStent inject and eight eyes were implanted with iStent inject W. Patients were treated with at least one hypotensive eye drop before the surgery.
The patients were controlled regularly after the surgery and the IOP was measured at each visit. The required drugs therapy was reduced in most cases and adjusted to reach the target pressure according to the guidelines of European Glaucoma Society, with consideration of the IOP before the surgery.
There were no significant complications intraoperatively or after the surgery. Gonioscopies with Goldmann 3 mirror classic contact lens 903 was performed on the eyes 3-6 months after surgery to evaluate the blood reflux through the devices.
The gonioscopy lasted about 1 minute with light pressure applied to the lens and slightly greater pressure applied nasally, before the pressure was eased off and evidence sought of possible blood reflux from the lumina of either of the stents.
Following the surgeries, patients had a mean IOP of 18 mmHg and a mean of 1.25 glaucoma eye drops was used. The patients did not require any further interventions and there were no complications observed intraoperatively or postoperatively.
Three of the patients who had an IOP of less than 15 mmHg had blood reflux through one of the two iStents during gonioscopy, but this was not visible in the other 19 eyes.
An 82-year-old woman with POAG had an IOP of 13 mm Hg. She was prescribed brinzolamide (Azopt, Alcon Laboratories) eyedrops with a dosing schedule of twice a day.
Gonioscopy was performed 3 months following the surgery. The iStent inject w was implanted. The results can be seen in Figure 1.
An 85-year-old male with POAG had an IOP of 12 mmHg. He was provided with derzolamide plus timolol eyedrops (Cosopt, Merck & Co) for twice-daily use.
A gonioscopy was performed 5.5 months after the surgery. iStent inject was implanted (see Figure 2).
A 72-year-old male with POAG had an IOP of 15 mmHg. He was prescribed tafluprost (Taflotan, Santen Pharmaceutical) eyedrops to be taken once a day.
Gonioscopy was performed 3.5 months following his surgery. iStent inject W was implanted. Results can be seen in Figure 3.
From the findings of these three cases, we can see that all the patients who had late blood reflux had an IOP under 15 mmHg, whereas other patients with higher IOPs did not. It is well known that the IOP following iStent implantation should not be lower than the pressure in the episcleral veins, which is usually 6-9 mmHg, but it can be elevated in glaucoma patients.9
Moreover, it is widely accepted that blood reflux in SC is more likely to occur in eyes with low IOP2 since lowering the IOP reportedly facilitates blood movement into SC.10
The blood reflux has been classified into three patterns:
It is important to note that the diameter of SC is between 50.8 μm and 393.5 μm,11 and this diameter is smaller in older patients in comparison with children. Moreover, the diameter of SC is smaller in glaucoma patients compared with those who do not have glaucoma.11
We know that the central outlet in iStent inject and iStent inject W is 80 μm and that the side flow outlet is 50 μm. This may lead to the ineffective implantation of the device in some cases, such as where the diameter of SC is small, which may obstruct the outflow.
This could be the explanation for observing blood reflux just from one stent in our three patients and not in both stents, but it could also be related to the IOP and the amount of blood in SC. These considerations may also explain the minimal blood reflux seen in the third patient, which does not obscure the vision of the angle.
On the other hand, the excessive treatment of IOP in glaucoma patients following implantation of the stent may lead to a spontaneous recurrent microhyphaema in the anterior chamber. In these patients, reducing the therapy would be more reasonable and IOP phasing may offer a better view to optimise it.
In conclusion, gonioscopy may offer a cost-effective method of evaluating the function of iStent after implanting it and could help in adjusting the therapy after the surgery.
Abdelwahhab Azzawi, MD, FEBO, MRCPS (Glasg)
Dr Azzawi is an ophthalmic consultant based at Department of Ophthalmology, Staedtisches Klinikum Dresden, Dresden, Germany. He has no financial disclosures.