In this study, we evaluated the LPI-induced morphological changes in the anterior segment amongst subgroups of patients with different angle-closure mechanisms in Caucasian eyes. The most common angle-closure mechanism was PB (55%), followed by PIC (31%), TPI (10.9%) and LLV (3.1%) in this cohort. The shortest ACD and greatest LV were seen in the LLV group, and the largest angle width was found in the PIC group. The TPI group showed lesser angle widening and greater iris flattening after LPI when compared to those of the PB group. Since the SSOCT provides the cross-sectional and circumferential assessment of anterior segment, using this information to classify the angle-closure eyes into different mechanisms may provide insights into individualized management for the PACD. Imaging may also help predict which eyes could have the best benefit for a specific treatment such as LPI.
Like in previous studies, we found that the ACD at baseline was the shortest in the LLV group and the longest in the PIC group.(5, 9) The pre-LPI angle width (AOD750) was the smallest in the TPI group and was the largest in the PIC group.(5, 9) However, studies reported that the PB was accounted for 35%, the PIC was 16–22%, the TPI was 14–26% and the LLV was 17–35% in an Asian population.(5, 9, 10) In this cohort of Caucasians, the percentage of the PB was much higher but that of LLV was lower when compared to the published data. The different ethnicity may drive to this difference in angle-closure mechanisms.
It was hypothesized that the angle-closure mechanisms are based on the forces acting at four different anatomic levels: the iris, the ciliary body, the lens and posterior to the lens.(4) Since the pathophysiology of the PACD is mechanistic predominantly, the management of PACD depends highly on correctly identifying the underlying anatomical mechanism.(13) In this study, the angle was widened after the LPI in all groups(14) because the pressure gradient between anterior and posterior chamber is equalized. It leads to a reduction of the iris curvature which is a surrogate for defining pupillary block. Thus, it may have some degree of pupillary block in all the eyes of this cohort. In contrast, Kwon et al also showed that the angle was widened in the PB and TPI groups but not in the PIC groups.(9)
When compared amongst the subgroups, there were significant differences in LPI-induced angle widening and iris flattening between the PB and TPI group. Interestingly, after the LPI, the TPI group had lesser angle widening than the PB group even though their irides flattened more. The peripheral iris thickness was also not significantly different from its baseline in both groups. Therefore, the TPI group, as expected, showed less effect of LPI on angle widening because peripheral iris was thicker and was not changed due to LPI. However, the narrowing of angle or shortening of the anterior chamber depth over time(15) could not be assessed in this study and future study with long-term follow-up is needed.
Several limitations in this study warrant further discussion. First, the sample size of each group in the current study was small especially the LLV group. However, that could be the incidence rate of angle-closure mechanisms in the Polish population and future studies with a large sample size are needed to confirm it. Grading and classification of the SSOCT images were subjective and may lead to a systematic bias. The follow-up period in the current study was short (only 7 days after LPI) so the long-term changes in anterior segment in different angle-closure mechanisms should be assessed in future studies.
In summary, the LPI-induced anterior segment morphology changes were different among the angle-closure mechanisms in a Caucasian population. Subgrouping of angle-closure mechanisms based on the SSOCT images may help to predict the effect of LPI. It provides early evaluation of high-risk groups and adjusts the relevant treatment to the predominant mechanism.