Fig. 2.1
(a) First-generation iStent®. (b) Second-generation iStent inject®. Image Courtesy of Glaukos
The two models GTS100R and GTS100L are identical except the body faces opposite directions to assist with nasal stent placement. The GTS100R is designed for the right eye and GTS100L is designed for the left eye. The inferonasal quadrant is the preferred site for insertion as it is thought to be the area with the highest density of collector channels [15]. There are about 20–30 collector channels surrounding the globe that drain aqueous from Schlemm’s canal to aqueous and episcleral veins [16].
2.2.2 Second-Generation iStent Inject®
Glaukos has recently developed a second-generation trabecular bypass stent, the iStent inject® (GTS400), which is currently CE (Conformité Européenne) marked in Europe and is undergoing clinical trials in the United States (Fig. 2.1b). This device is also a single-piece, heparin-coated, titanium device. This device is smaller than the first-generation device, measuring 360 μm in length and the conical head measuring 230 μm in width. The conical head sits in Schlemm’s canal with four flow outlets, each measuring 50 μm in diameter. The trabecular meshwork holds the thinner thorax region and the flange contains an inlet orifice measuring 80 μm in diameter. Since this device is symmetrically designed, it can be used in either eye. The iStent inject is preloaded on the G2-M-IS injector which houses two stents, enabling insertion of two devices with one injector.
2.3 Indications
The iStent® trabecular micro-bypass stent is currently FDA approved to be used in conjunction with cataract surgery for the reduction of IOP in open angle glaucoma. Approximately 1 in 5 patients with a visually significant cataract is on at least one ocular hypotensive medication for glaucoma or ocular hypertension [17]. However, the appropriate patient should have mild to moderate glaucoma, demonstrating less than −12 dB MD on their visual field. A patient who has uncontrolled glaucoma, who is on three or more ocular hypotensive medications, or who requires very low IOPs (below episcleral venous pressure) would likely benefit from the more traditional combined phacoemulsification/trabeculectomy or tube shunt procedure, which provides better control for IOP in more advanced glaucoma.
FDA approval guidelines require that the iStent be implanted concurrently with phacoemulsification. There is often better visualization of the angle structures after the cataract is removed. Furthermore, cataract surgery alone has been shown in multiple studies to lower the IOP [18–20]. It is thought that the IOP lowers because there is a reversal of the compression of the ciliary body, Schlemm’s canal and trabecular meshwork complex after the enlarged cataractous lens is removed. The conventional wisdom is that the IOP lowers an average about 2–4 mmHg after cataract surgery [21]. Recent studies have shown IOP can lower as much as 8 mmHg especially when starting with a higher preoperative IOP [22]. By creating a bypass through the trabecular meshwork, the iStent can work in conjunction with cataract extraction to enhance the natural physiologic outflow system.
This device has been successfully implanted in eyes with POAG and secondary open angle glaucoma such as pseudoexfoliation and pigmentary glaucoma. Preoperative gonioscopy should rule out peripheral anterior synechiae (PAS), rubeosis, or other angle abnormalities that may make proper placement difficult. A more pigmented trabecular meshwork will also ease proper placement of the device, but is not necessary as identification of Schlemm’s canal can be seen with blood reflux into the episcleral venous system during periods of transient intraoperative hypotony. With experience, one can consider implanting the device in eyes with a slightly narrow angle that is expected subsequently to deepen after phacoemulsification.
2.4 Contraindications
This device is contraindicated in eyes with primary or secondary angle closure glaucoma, as access to the trabecular meshwork is restricted. This device is also contraindicated in patients with pathology that may cause increased episcleral venous pressure such as thyroid eye disease or Sturge–Weber Syndrome. In these cases, the cause of increased IOP is not secondary to resistance in the juxtacanalicular meshwork and so the iStent would not be expected improve IOP. Lastly, one should be cautious while implanting this device in patients with corneal haze or opacities due to limitations in visualization of the angle structures.
2.5 Surgical Procedure
1.
Preoperatively, one should carefully examine the angle gonioscopically to ensure that the angle structures, in particular the trabecular meshwork, are clearly visible. There should be no evidence of angle closure, rubeosis, angle recession, trauma, or other angle abnormalities that may make implantation of the iStent difficult.
2.
It is advisable to perform phacoemulsification in the standard technique through a temporal clear corneal incision or limbal approach. This will allow implantation of the iStent nasally.
3.
After implantation of the intraocular lens (IOL), if the pupil is extremely dilated, an intracameral miotic can be instilled.
4.
After irrigation/aspiration, the anterior chamber should be filled with additional cohesive viscoelastic to maintain the anterior chamber.
5.
Position the microscope for better gonioscopic viewing of the angle. Rotate the patient’s head 35° away from you and rotate the microscope 35° towards you. An intraoperative gonio prism is placed on the cornea to adequately view the trabecular meshwork. To ensure a clear view, a light touch is required to prevent striae induced from the gonio prism.
6.
The iStent can then be inserted through the same temporal incision for cataract surgery (use the GTS100R for the right eye and the GTS100L for the left eye).
7.
Traverse the anterior chamber with the inserter with the implant on the tip of the inserter.
8.
View the angle under high magnification and approach the upper third of the nasal TM at a 15° angle with the tip pointed inferiorly (Fig. 2.2). If it is difficult to visualize the trabecular meshwork, transient hypotony can help cause blood reflux into Schlemm’s canal for better visualization. Schlemm’s canal is located just posterior to the TM.
Fig. 2.2
iStent® aimed 15° into the trabecular meshwork (TM) under high magnification. Blood reflux from Schlemm’s canal with hypotony can improve visualization of TM
9.
Engage the TM and advance the iStent into Schlemm’s canal. The device should be placed close to areas of heavier pigmentation as the collector channels are expected to be nearby. Proper placement into Schlemm’s canal typically demonstrates a small reflux of blood; however, this does not always occur even with proper placement.
10.
The device is released by pushing the button on the inserter. Tap the snorkel on the side to ensure it is placed securely.
11.
If there is resistance on insertion, one can try again a half clock hour more inferiorly.
12.
Irrigate the anterior chamber with irrigation/aspiration to remove all viscoelastic to achieve physiologic pressure at the end of surgery.
13.
Some surgeons prefer to perform phacoemulsification after implantation of the iStent device. The advantage to this is that there may be less corneal edema and one can use a smaller corneal incision, which may help with anterior chamber stability during implantation of the device. Furthermore, the extra viscoelastic needed to implant the iStent if performed after the cataract surgery is not necessary. However, the advantage of placing the iStent after phacoemulsification is that the angle will be deeper after the lens is removed, aiding visualization of angle structures.
14.
Routine postoperative management after phacoemulsification should be used after cataract surgery with iStent placement. If the patient was on only one preoperative ocular hypotensive drop, one can consider stopping this postoperatively and resume it if necessary. Some surgeons use loteprednol instead of prednisolone acetate postoperatively to avoid a steroid response.
2.6 Outcomes
2.6.1 First-Generation iStent Prospective Studies
Initial studies by Spiegel et al. [23,24] first demonstrated the successful implantation of the iStent with phacoemulsification. Their prospective noncomparative studies demonstrated an IOP reduction of 4.4 ± 4.5 mmHg after 12 months of follow-up, with 50 % of subjects achieving adequate IOP control off of medications (Table 2.1). Fea [25], was the first published double-masked randomized controlled trial comparing IOP reduction between phacoemulsification alone (control) and phacoemulsification combined with iStent implantation. After a washout of all postoperative medications at 15 months follow-up, the mean IOP difference was almost 2.5 mmHg between the two groups with the combined group having the lower mean IOP. Furthermore, after a 15-month follow-up, 67 % of the combined group no longer required glaucoma medications compared to only 24 % in the control group, demonstrating that the iStent is an effective way to reduce the medication burden in patients with concomitant cataracts with mild to moderate glaucoma.
Table 2.1
Results after Glaukos iStent® implantation
Study | Number of iStents | Combined with phaco? | Number of eyes | Follow-up (months) | Mean IOP reduction (mmHg) | Mean reduction in medications |
|---|---|---|---|---|---|---|
Spiegel et al. [23] | 1 | Yes | 48 | 6 | 5.7 ± 3.8 | 1.5 ± 0.7 vs. 0.5 ± 0.8a |
Spiegel et al. [24] | 1 | Yes | 42 | 12 | 4.4 ± 4.5 | 1.2 ± 0.7 |
Fea [25] | 1 | Yes | 12 | 15 | 3.2 ± 3.0 | 2.0 ± 0.9 vs. 0.4 ± 0.7 |
Samuelson et al. [26] | 1 | Yes | 109 | 12 | 1.5 ± 3.0 | 1.4 ± 0.8 |
Craven et al. [27] | 1 | Yes | 98 | 24 | 18.6 ± 3.4 vs. 17.1 ± 2.9a | 1.6 ± 0.8 vs. 0.3 ± 0.6 |
Arriola-Villalobos et al. [28] | 1 | Yes | 19 | 54 ± 9.3 | 3.2 ± 3.9 | 0.5 ± 1.0 |
Fernández-Barrientos et al. [29] | 2 | Yes | 17 | 12 | 6.6 ± 3.0 | 1.1 ± 0.5 vs. 0a |
Belovay et al. [30] | 2 to 3 | Yes | 53 | 12 | 18.0 ± 4.0 vs. 14.3 ± 2.9 | 2.0 ± 1.4
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