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Editors Selection IGR 16-4

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Comment by David Friedman on:

25131 Optical coherence tomography quantitative analysis of iris volume changes after pharmacologic mydriasis, Aptel F; Denis P, Ophthalmology, 2010; 117: 3-10

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The common concept for the pathogenesis of primary angle closure has been a relative pupillary block in eyes in which the proximity between the anterior lens surface and the posterior iris surface generates an increase in trans-pupillary aqueous flow resistance, thus increasing the pressure difference between the posterior chamber and the anterior chamber and forcing the iris to bow anteriorly, particularly in its weakest part, which is located in the iris periphery. This situation usually occurs if the pupil is mid-dilated. In that situation, an unbalance exists between an increase in trans-pupillary flow resistance and relaxation of the peripheral iris tissue necessary to allow a forward bowing of the peripheral iris and leading to an acute blockade of the anterior chamber angle.

This concept for the pathogenesis of primary angle closure has been extended by the study performed by Aptel and Denis. Using anterior segment optical coherence tomography, they examined fellow eyes of patients who had experienced an episode of primary acute angle-closure and 30 eyes of 30 normal age- and gendermatched subjects with open angles. All fellow eyes had a patent peripheral iridotomy. Aptel and Denis found that 30 minutes after medical pupillary dilatation, the mean iris volume increased significantly in the fellow eyes, whereas it decreased significantly in the open-angle eyes of the control group. Interestingly, both groups did not vary in iris volume before papillary dilatation. The authors concluded that the iris volume increased after pupil dilation in narrowangle eyes, and that this biometric change was associated with a narrowing of the angle despite a patent laser peripheral iridotomy. This finding is of profound importance for the understanding of the pathogenesis of primary angle closure.

There may have remained some questions:
1. The control group was significantly more myopic than the study group (P < 0.001). Correspondingly, at baseline of the study (i.e., miotic pupil), the anterior chamber depth was significantly deeper in the study group, axial length was longer, the angle opening distance at 500 μm (AOD 500) smaller and the trabecular-iris space at 500 μm (TISA 500) lower (table 1). The authors may assess whether the differences in the iris structure between study group and control group remain statistically significant, if both groups were matched for refractive error and anterior chamber depth. Refractive error and anterior chamber depth represent clinically the so far most important parameters to assess the risk for primary angle closure.
2. The medical pupillary dilatation was greater in the study group than in the control group (3.30 mm versus 2.78 mm). How did that affect the measurements and the interpretations of the measurements? What is the result of the statistical analysis, if both groups were matched for pupil diameter after medical mydriasis?
3. Just from layman's understanding of geometry, this reviewer does not understand how the iris volume can increase in one group with wider pupil (i.e., the study group) and decrease in a second group with a narrower pupil (i.e., the control group), although the iris surface area is smaller in the group with the wider pupil and although in both groups the cross-sectional iris area decreases?
4. In figure 4, the diagram appears to consist of two clouds of points. The left cloud represents the open-angle eyes (with a steep and negative regression line, if calculated separately as a group), and the right cloud represents fellow eyes (study group) (with a presumably steep positive regression line). Connecting both clouds together leads to a significant regression line, the direction of which does not agree with the direction of the regression lines within each of the two groups? The question additionally arises whether the clear differentiation between both groups as shown in figure 4 is due to the pre-selection of patients (e.g., the significant difference in baseline refractive error or in mydriatic pupil diameter) or whether it is indeed a clear difference between both groups offering a qualitative diagnostic parameter to detect eyes such as those of the study group?
5. For a multivariate analysis, the study population appears to be too small.

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