A Clinician’s Guide to Peripheral Iridotomy

Several states have recently enacted legislation which expands optometry’s scope of practice — many of them involving laser procedures. Jeff M. Miller, OD, of the Northeastern State University Oklahoma School of Optometry details the process of performing the laser peripheral iridotomy procedure and discusses pre, peri, and postoperative considerations. 

Identifying Patients Who May Benefit From Laser Peripheral Iridotomy

The most common reason for performing a laser peripheral iridotomy (LPI) is prophylaxis for angle closure in patients who are primary angle closure (PAC) suspects. The procedure relieves aqueous flow obstruction from the posterior chamber to the anterior chamber by establishing an alternate route for the aqueous to flow forward. Once the pathway is created, the bowed iris is allowed to flatten and the angle is opened to a degree.¹

Performing a prophylactic LPI in patients who are angle closure suspects is a clinical judgment. Findings that may indicate LPI include an inability to see structures in at least 180 degrees of gonioscopic evaluation, angles of  10 degrees or less on anterior segment optical coherence tomography (OCT), or an anterior chamber depth less than 2.5 mm.

In addition to glaucoma suspects, patients with PAC – narrow angles with increased intraocular pressure (IOP) or peripheral anterior synechiae visible on gonioscopy – and PAC glaucoma (PACG) – PAC with optic nerve head damage seen on visual fields and OCT – may be candidates for LPI. 

Contraindications to performing LPI include poor views secondary to other pathology (e.g. corneal scars and edema), poor patient cooperation, significant preexisting anterior chamber inflammation, or a compromised angle due to neovascular or other secondary glaucomas. 

Preoperative Considerations

Doctors may vary in their judgment as to where to initiate the LPI. Some doctors prefer the traditional superior 11:00 or 1:00 placement, while others prefer a 3:00 or 9:00 temporal placement. Research has validated both approaches — some evidence suggests that temporal placement reduces postoperative linear streaks of light while other studies indicate a negligible difference between the 2 approaches.^2-4 However,  temporal placement may reduce the risk of visual complaints in patients whose upper eyelid margin could potentially intersect the new LPI.⁴ 

The LPI should be placed at least two-thirds of the distance from the pupil center to the limbus. Ideally, a crypt or other naturally thin area of the iris should be selected as the target to reduce the amount of laser energy needed to penetrate the iris. 

Pretreatment should include brimonidine or apraclonidine to guard against a post-laser IOP spike, and 1% pilocarpine to increase the iris’ tonicity.  

An Abraham laser lens is recommended for the procedure since it prevents the eyelid from interfering and provides a magnified view of the iris. Additionally, pressure from the lens will help to stop any bleeding. Small iris bleeds are common, especially in thicker, brown irides.^5-6 

After bilaterally instilling proparacaine, a small amount of a gel formulation such as hypromellose 0.3% should be instilled into the lens well. Next, the magnified button should be centered over the area where the LPI will be performed. Slit lamp magnification may vary from provider to provider, but a medium (10-16x) setting may provide a good starting point. 

Initiating the Procedure

A yttrium aluminum garnet (YAG) laser is often used to perform the LPI. If available, a green laser (532 nm) can pretreat and thin out a thick, brown iris. In these cases, the energy is set to approximately 600 to 800 mW across a 50 µm spot size and delivered at a duration of 0.1 seconds per application via a foot pedal. A total of 10 to 20 shots are used in a circular or petaloid pattern to thin the iris. Large air bubbles are common with the green laser, but can be dispersed simply by firing again in the same or adjacent location. After pre-treating, the LPI can be initiated with a 1064 nm YAG laser (Figure 1).

YAG energy settings can vary between patients undergoing LPI compared with other laser procedures. Oftentimes, the energy of the YAG laser is set between 2.0 and 6.0 mJ on single, double, or triple burst. Spot size and duration are fixed with the 1064 nm YAG laser. Irides that are lightly colored typically need much less energy, while darker irides require more energy for penetration. 

A typical energy setting to start with is 4.0 mJ with a single or double burst according to doctor preference. Multiple burst mode offers the advantage of faster penetration, but it may increase patient awareness and discomfort. 

The clinician aims the 2 YAG laser focusing beams at the iris plane with no offset in the center of the button of the Abraham lens. The initial shots should be placed in the same area, as poor overlap of laser applications complicates the procedure by liberating pigment and diminishing the view without excavating the tissue. 

Penetration of the iris occurs when a plume of pigment can be seen coming forward into the anterior chamber. This results from the aqueous pushing forward into the anterior chamber. Once the plume is noted, the clinician should continue with the YAG application until the LPI is an acceptable size. 

The forming hole will often disappear after a shot as the laser energy moves the iris’ elastic tissue around. Firing again in the same spot will allow the hole to reappear. If a bleed occurs, evidenced by a thin column of blood that streams down from the site, the clinician must pause firing and apply gentle pressure with the Abraham lens. The column of blood may remain suspended, but active flow should abate within 30 seconds. 

The procedure can be time-consuming in patients with a thicker iris, with much of the time spent waiting for dispersed pigment and debris to settle and the doctor’s view to clear. Small cavitation bubbles are possible, but they rarely impede the view for more than a few seconds (Figures 2a, 2b and 2c).

Although there is no set size for a LPI opening, researchers suggest openings between 0.25 and 0.5 mm. Openings narrower than 0.2 mm increase the likelihood of LPI closure, while openings wider than 0.5 mm increase the risk of dysphotopsia.^4,7 Since the Abraham lens button magnifies the doctor’s view, the LPI will appear larger while performing the procedure. After the procedure is complete the clinician should inspect the area without the Abraham lens in place to better gauge the size.⁸ 

Postoperative Care

Immediately following the procedure, an additional 1 to 2 drops of brimonidine or apraclonidine should be instilled and IOP should be measured. A significant IOP spike will warrant further treatment with topical brimonidine or oral acetazolamide (2-250 mg oral tablets) to bring the IOP to an acceptable level.  

All patients should receive prednisolone acetate 4 times daily, or difluprednate twice daily. Additional topical or oral glaucoma medications can also be prescribed if necessary. A follow-up should be conducted 1 to 2 weeks postoperatively, with instructions to return in the case of any increased redness or pain, or decreased vision.

Chart recordings should include the number of shots required, the energy per shot, and the total amount of energy used. There is no optimum energy as every iris will require different amounts. A thin iris LPI might be completed with as few as 5 shots using low (3 mJ) energy per shot while a thick, brown iris may take many times that and, occasionally, require more than 1 treatment session. 

The 1 to 2 week follow-up visit should include an inspection of the LPI patency, IOP measurement, gonioscopy and anterior segment OCT. The latter 2 methods will ensure that the LPI achieved an adequate opening compared with preoperative measurements.

The peripheral iridotomy occupies a unique space within optometry’s laser procedures. While it will likely be performed less often than YAG capsulotomy or select laser trabeculoplasty, it can be the most technically challenging of these procedures — particularly in patients with a thick iris. However, a precise focus and consistent aim of the YAG laser in the target area on the iris will help to optimize outcomes in these patients.   

References

1. European Glaucoma Society terminology and guidelines for glaucoma, 4th edition – part 1. Br J Ophthalmol. 2017;101(4):1-72. doi:10.1136/bjophthalmol-2016-EGSguideline.001

2. Vera V, Naqi A, Belovay, GW, Varma DK, Ahmed IIK. Dysphotopsia after temporal versus superior laser peripheral iridotomy: a prospective randomized paired eye trial. Am J Ophthalmol. 2014;157(5):929-35. doi:10.1016/j.ajo.2014.02.010

3. Srinivasan K, Zebardast N, Krishnamurthy P, et al. Comparison of new visual disturbances after superior versus nasal/temporal laser peripheral iridotomy: a prospective randomized trial. Ophthalmology. 2018;125(3):345-51.  doi:10.1016/j.ophtha.2017.09.015

4. Spaeth GL, Idowu O, Seligsohn A, et al. The effects of iridotomy size and position on symptoms following laser peripheral iridotomy. J Glaucoma. 2005;14(5):364-7. doi:10.1097/01.ijg.0000177213.31620.02

5. Radhakrishnan S, Chen PP, Junk AK, Nouri-Mahdavi K, Chen TC. Laser peripheral iridotomy in primary angle closure: a report by the American Academy of Ophthalmology. Ophthalmology. 2018;125(7):1110-20.  doi:10.1016/j.ophtha.2018.01.015

6. Golan S, Levkovitch-Verbin H, Shemesh G, Kurtz S. Anterior chamber bleeding after laser peripheral iridotomy. JAMA Ophthalmol. 2013;131(5):626-9. doi:10.1001/jamaophthalmol.2013.1642

7. Fleck, BW. How large must an iridotomy be? Br J Ophthalmol. 1990;74(10):583-8. doi:10.1136/bjo.74.10.583

8. Miller, J. Peripheral Iridotomy and Iridoplasty. In Lighthizer, et al (Ed) The Ophthalmic Laser Handbook. Wolters Kluwer;2022:226-241.