In this observational study using standardized UBM criteria
, the overall prevalence of plateau iris was 36.9%. Interestingly, although the ACD was deepest in subgroup 1,
7 however, we noted that the proportion of plateau iris was similar across the three PACG subgroups. Increased peripheral IT was associated with an increased likelihood of plateau iris.
The proportion of plateau iris among our predominantly Chinese subjects is comparable to the estimated prevalence of 30% plateau iris by Kumar et al.
3 in their Singapore PACG cohort (which was different from our subject cohort) and the prevalence of 30% in the Indian population by Kumar et al.
8 Despite the presence of a patent LPI, some eyes with primary angle closure disease display residual angle closure and elevated IOP, suggesting the involvement of mechanisms other than pupil block.
9 In a study evaluating the progress of PACG in a Sino-Mongoloid population, Wang et al.
10 showed that pure pupillary block accounted for only 38% of angle closure in Chinese individuals, whereas 54% had combined mechanisms. This suggests that non–pupil block mechanisms such as plateau iris and/or combined mechanisms play a significant role in angle closure pathogenesis in this population.
Conventionally, plateau iris is defined as the presence of an occludable angle on gonioscopy, with a flat iris plane and a relatively deep central anterior chamber.
4 This is in sync with the definition in the Liwan Eye Study, wherein relatively deep central anterior chamber and a centrally flat iris plane were used to characterize the plateau iris profile gonioscopically.
11 Interestingly, Baek et al.
12 identified two distinct subgroups of PAC/PACG in Korean subjects; they hypothesized that plateau iris configuration or thick peripheral iris may be a contributing factor for angle closure in one of their subgroups that was characterized by relatively deeper ACD. However, they did not specifically examine the presence of plateau iris features. Based on this conventional concept of a relatively deep ACD in plateau iris, we hypothesized that the eyes categorized to subgroup 1,
7 characterized by significantly deeper ACD, would have the highest proportion of plateau iris. However, we noted that the proportion of plateau iris was similar across the subgroups irrespective of the differences in ACD. Our findings confirm the recent studies that have shown that eyes with plateau iris may have relatively shallower ACD.
5,6 Mansoori et al.
5 evaluated the prevalence of plateau iris in 262 PACG patients after LPI using UBM-defined criteria. Plateau iris was present in 83 (31.68%) of 262, and these eyes were noted to have significantly shallower central ACD compared with those with pupillary block and normal eyes. In a study of 91 Japanese angle closure patients using UBM, the estimated prevalence of plateau iris was 17.6%; and, interestingly, plateau iris was noted in patients with an ACD significantly shallower than that in the healthy controls.
13 Likewise, Mandell et al.
6 reported that the plateau iris group showed an ACD significantly shallower than that of the healthy control group and the pupil block group in their cohort of 318 patients diagnosed by UBM as either pupillary block or plateau iris. They postulated that the shallow anterior chamber in plateau iris may be explained by the anterior position of the ciliary processes that produces an anterior position of the lens. However, they further hypothesized that an anteriorly positioned lens may also lead to pupillary block, and it is likely that the two conditions frequently coexist. Mandell and colleagues
6 speculate that the lack of forward bowing of the iris in plateau iris gives a clinical impression of deeper ACD; although the axial distance of the lens from the cornea is relatively small.
Increased peripheral IT has been associated with PACG.
14 As the peripheral iris is in close proximity to the anterior chamber angles, therefore, a thicker peripheral iris may contribute to development of angle closure.
14 We found that eyes with greater peripheral IT were modestly associated with having plateau iris. It is likely that the eyes in which plateau iris and thick peripheral iris coexist are more inclined toward an increased risk for angle closure. However, the presence of PAS may have an effect on the IT750 measurements, and this may in turn impact the modest association of iris thickness with plateau iris. Further, although we had excluded eyes with software delineation errors,
7 there is still a possibility that some eyes may have had errors in measurements due to unclear delineation of the posterior iris surface with ASOCT. This may have also affected the relatively small association of IT with plateau iris.
The relatively high proportion of plateau iris among patients with PACG warrants a need for a more optimum and specific management for the condition. Several treatment approaches have been evaluated, each with varying efficacy. Some studies have described the use of pilocarpine, which produces iris thinning and facilitates opening of the anterior chamber angles by pulling the iris away from the trabecular meshwork.
15 However, pilocarpine-induced myopia and miosis, along with the relatively short duration of action, limits its therapeutic application. Prostaglandin analogues have been found to be effective in lowering IOP in eyes with PACG
16,17; however, their efficacy in plateau iris needs to be elucidated. Ritch et al.
18 demonstrated the long-term effectiveness of argon laser peripheral iridoplasty (ALPI) in eliminating residual appositional closure after LPI for a small cohort of subjects diagnosed with plateau iris by gonioscopy. Nonetheless, reports of ALPI as an effective treatment for plateau iris is still limited. Despite the available management options, there is still a need for more studies to determine the optimal and effective treatment modality for plateau iris.
The high incidence of plateau iris in these PACG eyes also underlies the importance of post-LPI gonioscopy in the clinical assessment of the angle, which can guide further investigation with UBM, if indicated, to accurately categorize these eyes and offer the appropriate management.
One of the limitations of this study is that the occurrence of plateau iris was based on a single cross-sectional image in each quadrant; variations in the quadrant may be missed by a single image representing a quadrant. Furthermore, the presence of PAS cannot be accurately assessed from UBM images, and it may have an effect on IT estimation. Second, UBM images were not gradable/available in approximately 11% of the subjects, and it is not known how they would have influenced the present results. UBM images were taken supine, and the ASOCT was taken sitting. This may have led in predisposed subjects to more movement of the lens. Also, it is unknown whether the ciliary body rotation causes the lens to be more anteriorly placed or whether the more anteriorly positioned lens (due to other factors, including increased vitreous pressure) may make the ciliary body appear anteriorly placed. Additionally, the ASOCT subgroups were categorized from parameters obtained from horizontal scans, whereas categorization of plateau iris was based on the assessment of all four quadrants of UBM images. Image analysis of vertical ASOCT scans (by the customized ZAAP software) is often not possible because simultaneous image acquisition of both quadrants is usually not possible in vertical scans. It is therefore not known whether similar subgroups arise if parameters from vertical scans were obtained. Furthermore, as the UBM may have inadvertently caused some compression, it may have changed the anterior chamber parameters from how they were measured on ASOCT.
In conclusion, the proportion of plateau iris was similar across the three ASOCT-based PACG subgroups and more than one-third of subjects with PACG were diagnosed with plateau iris based on standardized UBM criteria. In addition, we noted that eyes with increased peripheral IT have an increased likelihood of plateau iris.