The main mechanism of PACG is considered to be a pupillary block. Increased resistance of aqueous humour flow between the iris and anterior lens surface leads to angle closure. A short AL, thick lens, and anteriorly placed lens are the main risk factors .
Traumatic or spontaneous lens dislocation can cause acute angle closure. The features of zonular instability include iridodonesis, decentration of the nucleus, phacodonesis, lens equator exposure, and vitreous prolapse in the AC. In clinic, due to the risk of iatrogenic angle-narrowing and elevated intraocular pressure, angle-closed eyes usually do not undergo pharmacologic pupil dilation . Due to relaxation or lens zonular dehiscence, the anterior capsule of the lens can attach or adhere to the posterior surface of the iris . The lens and/or vitreous hernia can cause a pupillary block, which leads to an increase in the posterior chamber pressure; consequently, the iris is pushed forward, and the anterior angle is closed, which results in increased IOP. Its clinical manifestations are very similar to those of APAC, and thus, it is prone to misdiagnosis. Reports in the literature have also suggested that the major form of secondary glaucoma associated with lens subluxation was the open-angle type .
We analysed the clinical features of a group of patients with acute secondary angle closure due to lens dislocation, monocular onset, and acute anterior chamber shallowing. Compared with the patients in the APAC, CPACG, and cataract groups, the anterior chamber of patients with acute angle closure due to lens dislocation was significantly shallower, even less than 0.66 mm. The result showed that the AD was a sensitive indicator because it was statistically significant in all groups’ individual comparisons. Therefore, whenever a shallow anterior chamber is observed during clinical diagnosis of patients with APAC, it is necessary to consider acute secondary angle closure caused by lens factors. Considering the gender ratio differed among the four groups (p<0.05), we used ROC curves adjusted for gender to distinguish which factor was the most sensitive. The result showed that RLP was the most sensitive to distinguish ASAC-LS patients from the other three groups. Relative lens position (RLP)= [ACD +1⁄2LT] /AL ×10. This formula has three parameters which we should focused on in clinic: ACD, LT, and AL. According to our results, the LT and AL were not sensitive values to distinguish these 4 diseases. It has been reported that the ACD significantly differs between the involved eye and the contralateral eye in patients with acute angle closure due to LS . The calculated parameters, RLP, LP, and CLP, showed a significant difference in the multiple comparison results and were sensitive indicators for the four groups.
The LT in our study was not a sensitive value to distinguish APAC from ASAC-LS. While in primary angle closure patients, the LT was a powerful value .
In this study, data from the contralateral eyes were incomplete; therefore, ACD was not compared between the two eyes.
Patients with angle-closure glaucoma usually exhibit a shorter ocular AL. However, the ocular AL in the group of patients with acute angle closure caused by ASAC-LS did not significantly differ from that in the cataract group but was longer than that in the APAC and CPACG groups. It has been reported that LS patients have the longest ocular AL among those with acute angle closure. Other causes of acute angle closure include iris bombe, pupillary block, and plateau iris [17-18].
Among the four groups of patients, LT in the ASAC-LS group was the greatest and significantly differed from that in the cataract and the CPACG groups; therefore, LT was not sufficient to diagnose the four diseases. As a result, LP (defined as the sum of the ACD and 1⁄2 the LT) was introduced in this study. Calculations indicated that there was a significant difference between any two groups. Some studies  in the literature used lens vault (defined as the perpendicular distance between the anterior lens pole and the horizontal line joining the two scleral spurs) measured by UBM as an indicator of lens morphology and found that lens vault increases in patients with unstable suspensory ligaments of the lens.
When the lens is subluxated, the lens zonular dehiscence substantially affects the position of the lens. In the ASAC-LS group, the dehiscence was recorded during surgery and was found to correlate with AD. Therefore, occult LS does not have clear clinical manifestations and does not have an exceptionally high UBM diagnosis rate in our data. The diagnostic accuracy was 98.0% with 25 MHz UBM, and eyes with a slightly subluxated lens could be detected .
In summary, we retrospectively analysed biometric characteristics of the anterior segment of patients with acute angle closure secondary to occult LS. Several points should be addressed during diagnosis and treatment. For younger patients with acute angle-closure glaucoma, it is necessary to exclude lens zonular relaxation caused by abnormal lens development; otherwise, the patients would be misdiagnosed with APAC rather than acute secondary angle-closure glaucoma due to lens dislocation and undergo peripheral iridotomy or glaucoma filtering surgery, which not only increase the risk for complications, such as intraoperative vitreous herniation, postoperative anterior chamber shallowing, and malignant glaucoma, but also reduce the success rate of the operation. When applicable, UBM should be used to observe whether the suspensory ligament of the lens is severed or simply relaxed.
Biometric parameters ACD, AD were sensitive indicators for ASAC-LS. The calculated parameters RLP,CLP, LP were also sensitive for ASAC-LS.
Limitations of the present study include the absence of a biometric comparison of the lateral eyes in each group of patients. LS900 can only be used for measurements in patients without serious cataract opacities.