Article

Dogma vs Data: Placement of Laser Peripheral Iridotomy

Does improved understanding of dysphotopsia support traditional superior placement?

Angle-closure glaucoma, while less common than open-angle glaucoma, contributes to a high rate of glaucoma-associated blindness and visual dysfunction. It is now well established that achieving an anatomical opening of the anterior chamber angle can mitigate these adverse events. The results of the EAGLE study1 demonstrated the value of early lensectomy in certain eyes with angle-closure spectrum disease, but laser peripheral iridotomy (LPI) remains a safe alternative for many patients. Even though the results of the recently published ZAP trial2 suggests observation may be appropriate in many eyes with primary angle-closure suspect status, LPI remains an important part of the treatment armamentarium for angle-closure spectrum disease.

Historically, LPI and surgical iridectomies have been performed superiorly, with the hope that the upper lid would prevent stray light from passing to the retina and causing dysphotopsia. However, numerous studies have demonstrated the possibility of dysphotopsia development when LPIs are placed superiorly.3 Classically, these dysphotopsias are described by patients as a horizontal line that is present in the lower visual field. Lid position relative to the LPI does appear to have some effect in some studies; complete lid coverage of the LPI eliminates dysphotopsias whereas partial coverage allows for dysphotopsias to develop. Interestingly, complete lack of lid coverage can also result in dysphotopsias disappearing.4

The mechanism for this unique dysphotopsia has become increasingly well understood. The upper lid position, in combination with the tear film, causes the development of a base-up prism along the upper lid margin.4,5 This can allow light to be bent upward into the iridotomy. Because of this process, dysphotopsias can occur even in the presence of some degree of lid coverage. Diffraction of light results in the horizontally oriented linear shape to the positive dysphotopsia, and this image being projected to the superior peripheral retina causes the image to manifest in the inferior visual field.

In the office, dysphotopsia of this nature can be evaluated with a careful history and exam, as well as an assessment of the change in dysphotopsia with changes in lid position. With increased lid closure, the dysphotopsia should disappear. It may also disappear with lid elevation in which the prismatic effect of the tear film is eliminated from the equation. It is this latter observation that guides LPI placement for the majority of patients.

By allowing complete exposure of the LPI, light that enters the opening simply scatters without coalescing into a discrete linear phenomenon. Therefore, placement of the LPI along the horizontal meridian should theoretically reduce the risk of positive linear dysphotopsia development.

The literature has been quite mixed in its conclusions regarding LPI placement. Two large, prospective trials have been conducted in recent years to help. In the first of these trials, 1 eye of each patient was randomized to a superior versus temporal LPI.5 The rates of dysphotopsia were markedly reduced in horizontal LPI eyes, from 8% to 2%. Interestingly, in an alternative randomized clinical trial in which patients, not eyes, were randomized to temporal vs superior LPI failed to show a difference in the development of dysphotopsia.6 Over a relatively short time period of 2 weeks, there was no noted significant difference in dysphotopsia symptoms in eyes with superior vs temporal PI. The study also was conducted in South Asian eyes specifically and may not be as generalizable in its results.

Given the equivalent safety profile of horizontal vs superior iridotomy placement, and because an iridotomy anywhere will achieve the goal of eliminating pupillary block, in my practice I recommend a nasal or temporal iridotomy routinely. I will preferentially choose a nasal LPI with the hope that the shadow from the nose will further mitigate dysphotopsia, but this conjecture is purely anecdotal. There are special circumstances where I would preferentially suggest an LPI in an alternative placement. Where a large iridotomy is needed, I still prefer a superior placement; the large size should overcome the prismatic effect and linear coalescence of the dysphotopsia, thus resulting in light scatter. Additionally, if protecting against silicone oil-induced glaucoma, an inferior iridotomy is preferable. Lastly, in uveitic eyes with a history of or concern for seclusio pupillae and iris bombe, it may be advised to place more than 1 large iridotomy, because these eyes tend to rapidly scar.

Ultimately, the most important part of treating angle-closure spectrum disease is identifying it in the first place. Careful dark-room gonioscopy remains paramount. The decision on where to place the LPI can ultimately be made based on the patient’s lid anatomy and individual characteristics, but the best default may be placement along the horizontal meridian far away from upper or lower lid margin. The iridotomy should be of ample size, and gonioscopy should be performed after placement to reassess the angle anatomy and confirm resolution of pupillary block. GP

References

  1. Azuara-Blanco A, Burr J, Ramsay C, et al; EAGLE study group. Effectiveness of early lens extraction for the treatment of primary angle-closure glaucoma (EAGLE): a randomised controlled trial. Lancet. 2016;388(10052):1389-1397.
  2. He M, Jiang Y, Huang S, et al. Laser peripheral iridotomy for the prevention of angle closure: a single-centre, randomised controlled trial. Lancet. 2019;393(10181):1609-1618.
  3. 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):364-367.
  4. Weintraub J, Berke SJ. Blurring after iridotomy. Ophthalmology. 1992;99(4):479-480.
  5. 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):929-935.
  6. 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):345-351.