The main mechanism of PACG is considered as pupillary block. Increased resistance of aqueous humor flow between the iris and anterior lens surface leads to angle closure. A short axial length (AL), thick lens, anteriorly placed lens, are the main risk factors.
Traumatic or spontaneous lens dislocationcan cause acute angle closure. The features of zonular instability include iridodonesis, decentration of the nucleus, phacodonesis, the 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 zonula 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 pupil block, leading to an increase in posterior chamber pressure; consequently, the iris is pushed forward and anterior angle will be closed, resulting in increased IOP. Its clinical manifestations are very similar to those of APAC and, thus, is prone to misdiagnosis.The literature also suggested that the major form of the secondary glaucoma associated with lens subluxation was the open-angle type.
We analyzed 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 APAC, CPACG, and cataract groups, the anterior chamber of patients with acute angle closure due to lens dislocation was significantly shallower, even less than 0.66mm. The result showed that AD was a sensitive indicator, because it wasstatisticlly significant inall groups’ individually comparison. Therefore, whenever shallow anterior chamber is observed during clinical diagnosis of patients with APAC, it is necessary to devote attention to acute secondary angle closure caused by lens factors. Considering the constituent ratio of gender were different in four groups (p<0.05), we used ROC adjusted of gender to distinguish which factor was the most sensitive one.The result showed that RLP was the most sensitive one to distingush ASAC-LS patinets from the other three groups. Relative lens position (RLP)= [ACD +1⁄2LT] /AL ×10.This formula has three parameters, ACD, LT, AL which we should focused on in clinic. According to our results, LT and AL were not sensitive vaue to distinguish these 4 diseases. It has been reported that the anterior chamber depth is significantly different between the involved eye and the contralateral eye in patients with acute angle closure due to LS. The calculated parameters - RLP ,LP, CLP showed significant difference in multiple comparison results and were sensitive indicators of four groups.
The RLP (AUROC: 0.934) , ACD (AUROC: 0.929), AD (AROC: 0.925) , LP (AUROC: 0.892), CLP (AUROC: 0.903), had high power of discrimination. LT in our study was not a sensitive value to distiguish APAC from ASAC-LS. While in primary angle closure patients, LT was a powerful value .
In this study, data from the contralateral eyes were incomplete; therefore, anterior chamber depth was not compared between the two eyes.
Patients with angle-closure glaucoma usually exhibit a shorterocular axial length. However, the ocular axial length in the group of patients with acute angle closure caused by ASAC-LS was not significantly different 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 axial lengthamong the population with acute angle closure. Other causes of acute angle closure include iris bombe, pupil block, and plateau iris [17-18].
Among the four groups of patients, lens thickness in the ASAC-LS group was the greatest, and was significantly different from that in the cataract and the CPACG groups; therefore, lens thickness was not sufficientto diagnose the four diseases.As a result, lens position (LP)(defined as the sum of anterior chamber depth and 1⁄2lens thickness) was introduced in this study. Calculations indicated thatthere was significant difference between any two groups. Some studiesin 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 using UBM as an indicator of lens morphologyand found that lens vault increases in patients with unstable suspensory ligaments of the lens.
When the lens is subluxated, the lens zonule dehisencehas a large effect on the position of the lens. In this group, the dehisence was recorded during surgery and was found to correlate withAD. Therefore, for occult LS, which does not have clear clinical manifestations and does not have a very high UBM diagnosis rate in our data.
The diagnostic accuracy was 98.0% with 25 MHz UBM and slightly subluxated lens eyes could be detected.
AD can be used as one of the indirect determinant indicators.
In summary, we retrospectively analyzed 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 youngerpatients with acute angle-closure glaucoma, it is necessary to exclude lens zonula relaxation caused by abnormal lens development; otherwise, the patients would bemisdiagnosed with APAC rather than acute secondary angle-closure glaucoma due to lens dislocation and undergo peripheral iridotomy or glaucoma filtering surgery, which not only increases the risk for complications, such as intraoperative vitreous herniation, postoperative shallow anterior chamber and even malignant glaucoma, but also reduces 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. The anterior chamber depth should be measured: a short depth (<1.25mm) is highly indicative of abnormality in the lens zonuladehiscence or relaxation, andthe depth should be compared with that of the contralateral eye. Lens thickness should be measuredand, if it is >5.13mm, abnormal suspensory ligament of the lens should be suspected. Meanwhile, LP and CLP can be calculated for differential diagnosis.
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 to measure the patients with no serious opacity of cataracts. The gender difference was a factor which may introduce biases. Due to the small number of samples in our study, we will discuss it in the future study.