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Role debated over many years
The crystalline lens has a pivotal role in primary angle closure (PAC), both in the pathogenesis of pupil block1 and by exacerbating the effect of non-pupil block mechanisms such as peripheral iris crowding. Eyes with angle closure tend to have shallow anterior chambers and thick, anteriorly positioned lenses when compared with normal eyes.2–5 Removing the lens creates more space in the anterior chamber and widens the angle, which may be enough to achieve intraocular pressure (IOP) control.6 The role of lens extraction as a treatment for angle closure has been debated for many years. But with the knowledge that the lens is the single most important contributing factor to the angle closure process, and having acquired the technology and skills to perform relatively safe small incision cataract surgery, should we now be thinking about performing early lens extraction in angle closure patients with the aim of preventing the development of glaucomatous optic neuropathy at a later stage?
In this issue of the BJO (p 14) Tan and co-authors highlight some of the controversies and ethical considerations surrounding the role of early lens extraction in patients with acute angle closure. Theoretically, removing the lens at an early stage will deepen the anterior chamber and open the angle, thus hindering the formation of peripheral anterior synechiae (PAS) and improving the prospects for good long term IOP control. In addition, many of these patients will eventually require surgery for visually significant cataract at some stage.7 Tan et al report the corrected visual acuity of patients presenting with acute angle closure soon after resolution of the attack. Just over 50% of patients obtained a visual acuity of 6/12 or better at a mean interval of 1.7 days after the acute angle closure episode. Factors such as corneal oedema may still be contributing to reduced visual acuity so soon after an attack and one would expect the vision to improve even more over a longer follow up period. The authors think that with this degree of improvement in visual acuity following an acute attack it may not be justifiable to propose primary lens extraction instead of iridotomy as treatment for acute angle closure.
Any discussion of early lens extraction does not in any way imply that ophthalmologists should deviate from current protocols for the management of this potentially blinding condition. All patients presenting with acute angle closure should be treated immediately with systemic and topical medications to lower the IOP, followed by laser iridotomy for the affected and fellow eyes. However, Asian patients who present with acute angle closure can take longer to respond to medical treatment and may require additional interventions such as argon laser peripheral iridoplasty to break the acute attack before performing laser iridotomy.8,9 In the follow up period after an acute attack of angle closure a substantial proportion of Chinese Singaporean subjects develop chronic elevation of IOP and glaucomatous optic nerve damage.7 Possible explanations for these findings include delayed initial presentation,10 a greater role for non-pupil block mechanisms in Asian patients with PAC and the presence of pre-existing asymptomatic primary angle closure glaucoma (PACG) before the acute attack.
If we are to attempt to implement prevention of blindness programmes targeted at primary angle closure glaucoma we need evidence that our interventions are effective in preventing disease progression and visual loss
The trial being run by this group in Singapore comparing primary lens extraction with iridotomy in acute angle closure will help determine whether early surgical intervention can benefit the patient by reducing their need for future medical or surgical glaucoma treatment. Until this has been proved, lens extraction, which is often technically demanding and runs the risk of complications when performed in these eyes, should be reserved for cases in which the acute attack is not responding to conventional medical and laser treatment. In these cases removal of the lens is often an effective means of achieving rapid control of the IOP.11
When faced with a patient presenting with acute symptomatic primary angle closure it is difficult to make judgments on how much the lens is contributing to the disease. Assessment of the contribution of lens opacity to poor visual acuity is complicated by the presence of corneal oedema and it is often difficult to obtain a clear gonioscopic view of the angle. In the setting of asymptomatic PAC the picture is less complicated, there is more time to consider the different management options, and the surgery is technically more straightforward. However, the role of lens extraction in this form of the disease is still unclear and there is currently little in the way of evidence to guide us.
For a patient who has residual appositional angle closure following iridotomy and coexisting lens opacity, it is reasonable to have a low threshold for doing cataract surgery at the earliest sign of visual symptoms. Difficulties arise when dealing with cases in which the lens appears to be making a significant contribution to the residual angle closure but there is no significant cataract and visual acuity is good. Does this situation justify a clear lens extraction and can prophylactic surgery prevent future development of PACG in these cases? Angle imaging methods, such as ultrasound biomicroscopy and anterior segment optical coherence tomography, can be useful tools in determining mechanisms underlying post-iridotomy angle closure and guiding the clinician towards the appropriate treatment. Eyes in which imaging demonstrates anterior rotation of ciliary processes and plateau iris configuration may respond to argon laser peripheral iridoplasty.12,13 Otherwise, in cases in which IOP is normal and there are no signs of optic nerve head damage the patient can probably be observed.
The other area of difficulty is in deciding whether cataract surgery alone can control the IOP in more established disease with glaucomatous optic neuropathy, or whether it needs to be combined with trabeculectomy. Some studies suggest that cataract surgery may be as effective as filtering surgery in controlling IOP in PACG cases.14,15 But it may be that stage and chronicity of the angle closure process dictate which surgery should be done to achieve optimum outcomes. In cases in which there is early optic disc cupping and mild visual field loss, lens extraction alone may be enough to achieve adequate IOP control; whereas eyes with advanced glaucomatous optic neuropathy are more likely to have poor residual trabecular meshwork function as a result of PAS or non-synechial damage.16 In such cases phacotrabeculectomy may be necessary to achieve the degree of IOP control required to prevent progression of glaucomatous optic neuropathy. This is a similar theory to that used to explain why laser iridotomy appears to be less effective in controlling IOP in advanced PACG.17–19 It is probably oversimplifying things to extrapolate data from laser studies to the surgical management of PACG and other issues need to be considered. These include the frequency and consequences of IOP spikes following cataract surgery in angle closure patients and whether target pressures aimed for following surgery in POAG patients should be applied to patients with PACG. Studies investigating the effectiveness of surgical interventions for angle closure should be designed with these factors in mind.
A randomised controlled trial is under way in Hong Kong comparing phacoemulsification with phacotrabeculectomy for PACG (CC Tham, personal communication). The results of this and other ongoing trials in Asia investigating the effectiveness of early detection and treatment for primary angle closure are needed to help guide clinicians when making decisions on which interventions are likely to be beneficial to the patient. From a public health perspective PACG has been projected to be one of the commonest causes of irreversible blindness in the populous countries of Asia.20 If we are to attempt to implement prevention of blindness programmes targeted at PACG we need evidence that our interventions are effective in preventing disease progression and visual loss.
Note in Proof
Role debated over many years