Primary angle‐closure glaucoma is a multifactorial disease

Primary angle-closure glaucoma is an important cause of potentially preventable blindness; understanding the mechanism of primary angle-closure disease (PACD) is important for directed preventive and curative treatment. Anterior segment optical coherence tomography (ASOCT) is a non-invasive technique for imaging the angle that has potential in the diagnosis and management of PACD. In this issue, Liu reports the use of the AS-OCT in specifically selected cases to illustrate some common mechanisms of PACD. The article has clinical and causal implications. We will address the clinical implications first. While some may consider it one of the clinical ‘dark arts’, gonioscopy is at least ‘art in the dark’. Like any other clinical technique, however, in order to provide accurate and reproducible information, gonioscopy must be performed under standard conditions. Liu correctly cautions that ‘traditional’ gonioscopy – presumably implying a technique that does not deliberately use dim illumination and other measures to avoid pupillary constriction to obtain an in vivo view – can miss PACD. Liu also recommends using the International Society Geographical and Epidemiological Ophthalmology (ISGEO) classification rather than one that uses a presumed ‘predominant’ mechanism. We feel that the classification based on mechanism of angle closure has served us well in inducing thinking about the mechanisms in individual clinical cases, albeit that there is some artificiality in suggesting that each mechanism acts in isolation. The ISGEO classification was originally created for uniform reporting of surveys and subsequently crept into clinical practice; accordingly – and like any classification – it has deficiencies. Some of the issues with the clinical use of this classification have been highlighted, and it may in the future be modified, but the ISGEO classification has a sound basic structure, and there is now a body of data using it. It is currently believed that appositional angle closure – in itself, and also by producing damage caused by iridotrabecular contact (ITC) – may cause glaucoma even in the absence of synechiae formation. Liu’s other clinically important conclusion is that ITC must be eliminated by treating the multiple factors that may contribute to it. Translating this into practice, it is not enough to perform a laser iridotomy and then be content with an intraocular pressure measurement. If we are intervening to alter the angle configuration, then it is mandatory to re-examine the angle after any such procedure. The effect of the procedure must be evaluated and depending on the findings, further clinical intervention by way of laser or surgery may (or may not) be indicated, but subsequent angle re-evaluation is again required. The AS-OCT may have a role to play in this evaluation, particularly if our pre-intervention ability to predict outcomes – such as those shown by Liu – is enhanced. It remains crucial to remember, however, that treatment of ITC is not an end in itself, but is a means to prevent vision loss from glaucoma. Much remains to be discovered about ITC. It is, after all, an inferred finding clinically (with histological correlates that are not routinely accessible); the ITC is not actually visualized. Is all ITC (equally) bad? – the risk of progression may not be the same in all ITC and must surely depend at least on extent. What is the longitudinal risk for glaucoma of those who show ITC only in the dark or only on AS-OCT? Are treatments such as selective laser trabeculoplasty safe in trabecular meshworks that have been subjected to ITC? The case series also highlights some of the deficiencies of the AS-OCT. The angle-closure mechanism has been implied by a single horizontal scan. Extrapolating a single horizontal scan to the three-dimensional, 360 degrees of angle is problematic. The reported case with the peripheral anterior synechiae (PAS) looks impressive – and was selected by the author for this reason – but we must remember that as far as current AS-OCT technology is concerned, it was detected purely by chance; the horizontal scan happened to pass through the PAS. If it had been located in any other quadrant, AS-OCT alone would have missed the PAS possibly leading to a wrong diagnosis and an incorrectly inferred mechanism. Additionally, the localization of the scleral spur is more difficult in PACD, that is, in the very cases where such localization is critical. Finally, AS-OCT provides limited visibility of the structures behind the iris, and we have to almost imagine the ciliary processes holding up the iris following lens extraction. The use of the ultrasound biomicroscopy (UBM) would have highlighted this mechanism more clearly. Future advances in AS-OCT such as swept-source scanning may allow us to achieve reliable 360 degrees imaging and allow us to label angles as open or closed based on other landmarks such as Schlemm’s canal or Schwalbe’s line. At the moment it cannot replace gonioscopy but can be a useful adjunct.