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Editors Selection IGR 8-1

Surgical Treatment: Placement of Iridotomy

Comment by Prin Rojanapongpun on:

74729 Comparison of New Visual Disturbances after Superior versus Nasal/Temporal Laser Peripheral Iridotomy: A Prospective Randomized Trial, Srinivasan K; Zebardast N; Krishnamurthy P et al., Ophthalmology, 2018; 125: 345-351

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Laser peripheral iridotomy (LPI) is widely practice with favorable risk/benefit ratio. Yet, it may produce undesirable complications like dysphotopsia that could be quite disabling without any real benefit to those relatively healthy angle closure suspect subjects (PACS). So, it is the physician's responsibility to avoid any known visual disturbances. As there were some conflicting evidence on where to perform LPI and its relation to post-LPI dysphotopsia, Srinivasan et al. investigated this with a larger scale, well powered study in a RCT fashion. Based on Srinivasan,¹ LPI location either superior, nasal or temporal was not associated with statistically significant differences in new visual disturbances. However, their results contradict another randomized, prospective, single-masked, paired-eye comparative study of Vera et al.² which found that temporal LPI was less likely to result in linear dysphotopsia compared with superior placement. Why the differences? Different study design, subject type and technique detail will definitely dictate the different outcomes and side effects of LPI. There could several missing parts of information specifically the followings:

1. Dynamic lid margin and LPI location: Placing LPI near the lid margin, with good tear meniscus, has been considered an important factor for linear dysphotopsia in Spaeth³ and Vera² studies. However, none of these studies ever designed to study dynamic lid position and LPI location in relation to dysphotopsia. Srinivasan¹ also did not report information on post-laser location of LPI in relation to lid margin. Although study protocol stated 'completely clear from lid margin', it is hard to know whether how many final locations are at any lid margin and any correlation to new dysphotopsia.
2. Size and exact peripheral location of LPI: a large enough size at a more central location may induce stronger straylight through crystalline lens equatorial area while the very far peripheral temporal location will less likely produce any focused rays of light on the retina. Superior LPI is usually more central location as well as in the area of thicker iris and needing more laser energy. This reflects well in the study showing significant higher total laser energy needed at superior location (59.1 mJ vs. 45.1 mJ with p<0.001). If both superior and temporal LPI were performed in a more central iris location, both would produce nearly the same frequency of dysphotopsia. But if temporal LPI was performed in the far periphery while superior LPI was performed naturally more central, the dysphotopsia rate could be different. Srinivasan reported 12.3% of all patients with new-onset of 1 or more dysphotopsia symptoms. There is no detailed information on the dimension of LPI performed by 4 different participating centers. It is not known whether the incidence of dysphotopsia was evenly distributed or skewed. The exact description of LPI in term of size and how peripheral it is made is also needed to understand the different outcome.
3. Data collection and follow-up: Dysphotopsia is subjective and could be variable. Assessment at 2 weeks could be different from a longer follow-up. Questionnaire distribution and data collection process can obtain different results. Self-report symptoms vs direct questioning will provide different outcomes.
4. Study population: Which category of angle closure cases compose into the study subjects will dictate the outcome. It is known that primary angle closure spectrum ranges from primary angle closure suspect (PACS), primary angle closure (PAC) and primary angle closure glaucoma (PACG). The outcome of LPI in preventing IOP rise varies from >90% success in PACS to <10% success in PACG. Likewise, the complications could be quite different from almost nil to corneal decompensation. This could partly explain why the outcomes of different studies looking for dysphotopsia were different or even contradictory. There were 16.4-17.5% of PAC or PACG in Srinivasan study. Besides, all subjects are south Asian population which is quite different from Vera that has only 34.9% of the same south Asian population.

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