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Editors Selection IGR 18-3

Anatomical Structures: Iridocorneal angle

Comment by Syril Dorairaj & Rouzbeh Amini on:

53667 Comparison of iris insertion classification among american caucasian and ethnic Chinese using ultrasound biomicroscopy, Wang YE; Li Y; Wang D et al., Investigative Ophthalmology and Visual Science, 2013; 54: 3837-3843

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The study of anatomical features of the anterior segment is a quintessential element in determining risk factors associated with the primary angle closure glaucoma (PACG). Ultrasound biomicroscopy (UBM), and more recently anterior segment optical coherence tomography (ASOCT), have been used as excellent tools to quantify such features. Over the past years, many PACG anatomical risk factors such as anterior chamber depth, lenticular thickness, and iris volume have been identified using UMB and ASOCT imaging modalities. In addition to the anatomical features, age, gender, and ethnic background are also important risk factors for PACG. In particular, PACG is more prevalent in old versus young, female versus male, and Asian versus non-Asian individuals. In their recent study, Wang and colleagues, using UBM, specified another anatomical characteristic of the iris, namely 'iris insertion', as a potential significant difference between the Chinese and non-Chinese populations. The prospective study was conducted in three age- and sex-matched cohorts: American Caucasians, American Chinese, and Mainland Chinese. The iris insertion into the ciliary body was categorized in three groups: basal, middle, and apical insertions, in which irides were inserted near the base of ciliary body, around the middle portion of the ciliary body, and towards the apex of the ciliary body, respectively. UBM imaging enabled Wang and colleagues to visualize iris profile in all four quadrants (nasal, temporal, inferior, and superior).

Wang and colleagues observed that non-basal (ie, middle and apical insertions) are significantly more prevalent in the nasal and temporal quadrants of Chinese volunteers in comparison to those of the Americans. They also noted that non-basal insertion was more common among the older volunteers (regardless of the ethnicity) when compared to the younger ones. When they performed statistical analysis between the basal and non-basal groups, independent of ethnic background, Wang and colleagues found that other metrics of the anterior segment generally associated with PACG were also significantly different for the majority of cases. In all but superior quadrants, a significant difference was found between the two groups for anterior chamber depth, anterior chamber width, anterior chamber angle, and anterior chamber volume (P < 0.05). In addition, iris area and curvature were both significantly different between the basal and non-basal groups in nasal and temporal cases (data obtained using ASOCT).

An interesting outcome of Wang et al.'s study was the close relationship between the iris insertion and PACG risk factors. Clearly, because the study excluded the glaucomatous eyes, a direct correlation between iris insertion and the prevalence of PACG was rendered impossible. However, by quantifying other well-established anatomical and non anatomical risk factors, Wang and colleagues have indicated that iris insertion could potentially be an additional risk factor for PACG. Future studies that compare iris insertion between the normal and glaucomatous narrow/closed anterior chamber angles are necessary. While categorizing iris insertion into basal- and non-basal groups, Wang and colleagues did not specify whether such difference was purely anatomical and semi-permanent or not. In particular, was it possible to change the iris insertion from basal to middle insertion by, for example, indentation gonioscopy? Wang and colleagues also conducted their study in the dark condition when pupil was fully dilated. Recent studies (Quigley et al., Journal of Glaucoma 2009; 18(3):173-179, and Jouzdani et al. Invest Ophthalmol Vis Sci 2013; 54(4): 2977-2984) have shown significant differences in iris metrics during dilation in normal eyes versus PACG cases. Future studies could shed more light on the iris insertion as a useful parameter to evaluate and whether it is a relatively static parameter or whether it changes dynamically with other metrics of the anterior segment, such as iris volume, anterior chamber angle, etc.

The study of Wang and colleagues showed a higher prevalence of non-basal iris insertion in older volunteers. It has been shown that the mechanical stiffness of the corneoscleral tissues, as well as that of the lens capsule and ciliary zonule, increases with age. Alteration in the iris insertion by age could be caused as a response to the changes in the biomechanics of the ocular globe. Study of iris insertion in myopic eyes that undergo significant changes in the scleral extracellular matrix during the growth process could provide some evidence of whether such correlation exists. In addition, Wang and colleagues did not find any significant difference in the superior quadrant of the examined volunteers. Further longitudinal studies have to be done to answer the questions, does the attachment of the iris move with age and why peripheral anterior synechiae occurs more frequently in the superior quadrant.

Interestingly, previous investigations have documented that there is an increased prevalence of plateau iris or lens related cause for persistence of angle closure in Chinese population. These results, taken together, suggest that dynamic anterior segment parameters must be viewed as an important factor to consider in the larger picture of the mechanical environs of the anterior segment, and that more careful study may be necessary to explore how the different structures and parameters (thickness/area/depth) in the anterior segment interact to determine the risk of developing angle closure in older people. It may be that for a given individual, a different iris attachment would lead to elevated risk of developing angle closure. Or, it could be that multiple factors are not significant taken individually, but are significant in combination, and iris attachment, thickness, axial length, curvature, etc. may contribute to development of angle closure synergistically.

The key question that remains unanswered is: if non-basal iris insertion is a PACG risk factor, what is the underlying mechanism? If the crowding of the incompressible irides at the iris root following dilation leads to closure or narrowing of the angle as suggested by Quigley et al. (2009) and Jouzdani et al. (2013), then non-basal insertion seem to prevent such phenomena as there is more room at the iris root. An alternative hypothesis could be posed that the higher prevalence of non-basal iris in the Chinese population, who are at high risk for developing PACG, may be an evolutionary response to reduce crowding of the iris root to prevent PACG. This issue could be more perplexing as there was no significant difference between the iris insertion of female and male volunteers, while it has been well established that women are more prone to developing PACG than men. More studies that carefully examine the mechanism of iris-aqueous-humor interaction in eyes with basal versus non-basal iris insertions are necessary.

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