Objective: To measure the choroidal thickness of patients with primary angle closure (PAC) before and after Nd:YAG laser peripheral iridotomy (LPI) using enhanced depth imaging spectral-domain optical coherence tomography (EDI-OCT); and to compare the difference between the choroidal thickness of patients with PAC and normal subjects. Methods: This was a prospective cohort study. Thirty patients (48 eyes) who were diagnosed with PAC were selected as the PAC group in the ophthalmology clinic of Shaoxing People's Hospital from October 2015 to February 2017. Thirty patients (50 eyes) undergoing regular physical examination were selected as normal controls in our hospital during the same period. Intraocular pressure and central anterior chamber depth were measured in the normal control group and PAC group before LPI, and 1 week, 1 month, 3 months and 6 months after LPI. Choroidal thickness was measured with EDI-OCT at the subfovea, as well as at 0.5, 1.5 and 3.0 mm from the fovea superiorly,inferiorly, temporally, and nasally. Repeated measures ANOVA was used to compare data between the two groups at different time points, and to plot the choroidal thickness of the receiver operating characteristic curve to determine the best diagnostic limits. Pearson correlation analysis was used to assess the correlation among choroidal thickness, intraocular pressure and central anterior chamber depth. Independent t test was used to compare between the two groups. Results: The central anterior chamber depth of the PAC group was more shallow than that of the normal control group (t=-14.383, P < 0.001). The difference in central anterior chamber depth was statistically significant before LPI and 1 week, 1 month, 3 months and 6 months after LPI (F=10.313, P=0.001). And the central anterior chamber depth of the PAC group became deeper from preoperation to 3 months postoperation (P < 0.01). The choroidal thickness of the PAC group was greater than that of the normal control group at 13 points before LPI (all P < 0.01). The difference in choroidal thickness was statistically significant before and after LPI (F=240.512, P < 0.001). The choroidal thickness of the PAC group at 1 week, 1 month, 3 months and 6 months after LPI was significantly thinner than that at pre-operation (P < 0.001). The choroidal thickness of all subjects in the two groups was negatively correlated with the depth of the central anterior chamber at SF, T1, T2, T3, S2, S3, I1, I2 and I3 (r=-0.249, -0.239, -0.416, -0.330, -0.184, -0.176, -0.189, -0.184, -0.160, P < 0.001). There was no correlation between choroidal thickness, intraocular pressure or axial length. Conclusions: The choroid in patients with PAC is thicker than in the normal group. The choroid in PAC patients became thinner after LPI surgery. Choroidal thickening may be one of the pathogenic factors of PACG. In addition, choroidal thickness plays a role in the early diagnosis of PAC and the disease observation and assessment of the
therapeutic effect of LPI.