Functional assessment of trabecular bypass created by iStent inject using aqueous angiography

Description

A male patient in his late 50s presented with a diagnosis of moderate primary open-angle glaucoma in both eyes with the right eye (RE) having pseudophakia and the left eye (LE) having an immature senile cataract. On examination, the best-corrected visual acuity (BCVA) in the RE was 6/9, and in the LE was 6/60. On gonioscopy, the LE angle of the anterior chamber was open up to the scleral spur. LE fundus examination revealed a cup-to-disc ratio of 0.7:1 with thinning of the inferior neuroretinal rim. Intraocular pressure (IOP) in the LE was 22 mm Hg on brimonidine tartrate 0.2% and timolol maleate 0.5% which was used two times per day. Since the patient had moderate glaucoma with cataract, LE trabecular micro-bypass stent implantation (iStent inject, Glaukos, California, USA) combined with phacoemulsification and intraocular lens implantation was performed under local anaesthesia. After appropriate microscope positioning, two iStent inject were placed at the nasal angle at 8 o’clock (figure 1, red asterisk) and 10 o’clock (figure 1, yellow asterisk) position via the temporal clear corneal incision. Both devices appeared to be anatomically implanted in the correct position as only the flange of the device was visible (with the thorax in trabecular meshwork (TM)). This was followed by aqueous angiography using indocyanine green dye 0.1% to confirm the passage of aqueous and assess the function of the devices.¹ Adjacent to the 10 o’clock device (figure 1, yellow asterisk), collector channels were visible, identified by the continuous flow of dye (figure 1, yellow arrow) indicating optimal trabecular bypass and a functional stent. However, no collector channels were visible adjacent to the 8 o’clock device (figure 1, red asterisk), indicating no aqueous drainage through this device. Postoperatively, timolol maleate 0.5% was stopped, and only brimonidine tartrate 0.2% was continued. LE IOP was 14 mm Hg (on one glaucoma medication) at the first-month postoperative follow-up and 18 mm Hg (on one glaucoma medication) at the third-month postoperative follow-up, with a BCVA of 6/6.

Figure 1.

A: Intraoperative gonioscopic image showing two iStent inject implanted at 8 o’clock (red asterisk) and 10 o’clock (yellow asterisk) position of the nasal angle. B: Intraoperative aqueous angiography image showing the absence of collector channels near the 8 o’clock device (red asterisk) and visible collector channels (yellow arrow) near the 10 o’clock device (yellow asterisk).

Previous studies have reported that phacoemulsification combined with iStent inject can decrease IOP by 2.4 mm Hg to 6.1 mm Hg in the postoperative period.^{2 3} Implantation of iStent inject alone can decrease IOP by approximately 2.3–4.3 mm Hg in the postoperative period.⁴ Huang et al used aqueous angiography to assess the aqueous humour outflow after trabecular bypass surgery in glaucoma patients.¹ Trabecular bypass devices were implanted in high flow region (region of increased angiographic signals) and low flow region (region of decreased angiographic signals) as per aqueous angiography. They found early angiographic signals in high flow region and new angiographic signals in low flow region. In future, performing targeted minimally invasive glaucoma surgery (MIGS) in high or low flow regions based on aqueous angiography can help to improve their outcomes.^{1 5 6}

This case highlights that anatomical implantation of iStent inject within the TM may not ensure its adequate function. This may be the reason for their poor/variable IOP lowering efficacy. Non-drainage of aqueous from the iStent inject may be due to failure in establishing a communication between the aqueous in the anterior chamber and the collector channels due to collapsed TM or Schlemm’s canal, improper placement of the device, and absence of adjacent collector channels.⁷ This clinical finding can encourage further study on intraoperative functional assessment of MIGS procedures by aqueous angiography and enable planning for intraoperative surgical adjustment in the same sitting, if required, to improve the outcomes of MIGS. Aqueous angiography can also aid in improving the design of trabecular bypass devices that optimise functional outcomes.

Learning points.

• Correct anatomical placement of iStent inject within the trabecular meshwork may not ensure its optimal function.

• Aqueous angiography can be used to assess the function of iStent inject and other minimally invasive glaucoma surgery procedures intraoperatively.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).